HUMB1001 — Module 3 — How Do We Move?

HUMB1001 · Integrated Systems Anatomy and Physiology

Module 3:
How Do We Move?

Muscle tissue, contraction and muscles in action

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In this lecture

Three kinds of muscle
A muscle as an organ
Inside a muscle fibre
From nerve to muscle
Turning a signal into a pull
How hard, how fast?
Muscles at work

FRAG 0: Welcome back to Integrated Systems Anatomy and Physiology. At the end of Module 2, we said we'd ask how we move, building on the bones and joints you'd just met. That's where we are now. Bones and joints give you a framework, but on their own they can't move at all. Something has to pull on them, and that something is muscle. Here's a question to keep in mind as you go. Every movement you make, from a blink to a sprint, comes from countless tiny protein heads inside your muscle cells, gripping and pulling. So why does a body stiffen a few hours after death, and then loosen again a day or two later? By the end of this lecture, you'll be able to explain it. A quick reminder about how this lecture works. The narration you're hearing is an AI-generated voice reading a script written by your teaching team. Everything moves at your pace. Use the controls at the bottom of the screen to pause, or to speed the narration up. Some slides will stop and wait for you: a question to answer, something to sort, a diagram to explore. They aren't marked, so have a genuine go before you check your answer. At the end, there's a short reflection, and it becomes a page of notes you can print or save. Let's begin.

Acknowledgement of Country (Boodja). I would like to acknowledge the Whadjuk Noongar people whose land we are standing on and recognise the strength, resilience and capacity of the Noongar people where Curtin University is situated. I pay my respect to their vibrant and endless culture and the leadership of the Elders past, present and future. This country (boodja) where Curtin University is situated has belonged to the Whadjuk Noongar people for thousands of years and is a place of learning for all people now and Curtin University is very proud to continue on this very long tradition.

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What you'll be able to do

Muscle tissue

Its jobs, its properties, three types

A muscle as an organ

Wrappings, fascicles, blood and nerves

Inside a fibre

Sarcomeres and sliding filaments

Nerve to muscle

Action potentials, the junction, calcium

Muscles at work

Names, attachments and actions

FRAG 0: Here's where we're headed. This module has five learning outcomes, and by the end of the lecture you should be able to do each of them. Some of the cell physiology will be familiar from Human Structure and Function, but we'll build every step from the beginning. FRAG 1: First, summarise what the muscular system does, explain the four properties that let muscle tissue do it, and describe the three types of muscle: skeletal, cardiac and smooth. Skeletal muscle gets most of our attention today, but you'll need cardiac muscle for the heart next week, and smooth muscle for blood vessels, the gut and the bladder later in the unit. FRAG 2: Second, describe a skeletal muscle as an organ. It isn't just a bundle of muscle cells. It's wrapped in layers of connective tissue, its cells are grouped and arranged in particular patterns, and it has its own blood supply and nerve supply. FRAG 3: Third, describe what's inside a single muscle cell, how its protein filaments are lined up, and how they slide past each other to shorten it. We'll also look at how a whole muscle controls how hard it pulls, so that the same muscle can lift a pencil or a suitcase. FRAG 4: Fourth, describe the junction where a nerve meets a muscle cell, and explain how the nerve's signal is turned into a contraction. To do that properly, we'll first look at the electrical charge across a cell membrane and the action potential, the signal that nerves and muscles both use. FRAG 5: And finally, recognise how muscles are named, and describe the actions of the muscles behind everyday movements. The names are more helpful than they look, because many of them tell you a muscle's shape, position or job. In the lab, you'll find these muscles on real specimens and work out their actions for yourself.

Where we left off

The signal has reached the muscle. What happens next?

Inside the muscle → switching it on → grading the force → pulling the right way

FRAG 0: In Module 1, you followed a decision to move from your motor cortex down through the brainstem into the spinal cord. There, it was handed over at a synapse to a motor neuron, and that neuron's axon ran out through a spinal nerve to the effector, which for any conscious movement is skeletal muscle. And that's where we left it: the neuron released its neurotransmitter, and the muscle contracted. In Module 2, you met the other half of the system: the bones, the joints between them, and the tendons that tie muscles to bone. FRAG 1: In this module, we follow that signal into the muscle. First, we'll look at muscle as a tissue and its three types. Then we'll open a skeletal muscle, layer by layer, until we reach the proteins that do the pulling. We'll see how a nerve signal crosses onto a muscle cell and switches those proteins on, how a muscle decides how hard to pull, and finally which way each muscle moves a bone. That's where your terms of movement from last week come back.

Section 1

Three kinds of muscle

What muscle does, and the tissues that do it

Seven jobs of muscle

Moving the body

Pulling on bones at joints

Posture

Small, constant adjustments keep you upright

Breathing

Diaphragm and rib muscles

Heat

A by-product of contraction; shivering

Communication

Speech, expression, gesture, writing

Squeezing organs and vessels

Gut, bladder, blood vessel walls

Pumping blood

Cardiac muscle

FRAG 0: Muscle makes up a large share of your body, and it does far more than move your limbs. Your textbook lists seven jobs. Most belong to skeletal muscle, but the last two belong to the other kinds. FRAG 1: First, moving the body. Skeletal muscles pull on bones through their tendons, and the bones turn at the joints you met last week. Walking, lifting and turning your head all work this way. FRAG 2: Second, posture. Even when you're standing still, muscles in your back, hips and legs are working, making small, constant adjustments so that you don't topple over. FRAG 3: Third, breathing. The diaphragm and the muscles between your ribs change the size of your chest, and that moves air in and out of your lungs. You'll see how in the module on ventilation. FRAG 4: Fourth, heat. Contraction uses a lot of energy, and much of it is released as heat. That heat helps keep your body temperature steady. When you're cold, shivering is your muscles contracting rapidly just to make more of it. FRAG 5: Fifth, communication. Speaking, smiling, frowning, writing and gesturing all depend on skeletal muscle. Some of the muscles of your face pull on skin rather than bone. FRAG 6: Sixth, squeezing organs and vessels. Smooth muscle in the walls of hollow organs and tubes pushes food along your gut, empties your bladder and narrows your blood vessels. FRAG 7: And seventh, pumping blood. Cardiac muscle in the wall of your heart contracts with every heartbeat, about a hundred thousand times a day, to push blood around your body.

Four properties

Contractility

Shortens with force

Excitability

Responds to a stimulus with an electrical signal

Extensibility

Can be stretched and still work

Elasticity

Recoils to its resting length

FRAG 0: To do all those jobs, muscle tissue has four functional properties. All three kinds of muscle share them, and each one will come back later in the lecture. FRAG 1: Contractility is the ability to shorten forcefully. This is what makes muscle special: it pulls, and the force it produces is called tension. Muscle can only pull, never push, so to move a bone back again, a different muscle has to pull the other way. FRAG 2: Excitability is the ability to respond to a stimulus. For skeletal muscle, the stimulus is a signal from a nerve. Cardiac and smooth muscle can also respond to hormones and other chemicals. The response is an electrical signal across the cell's membrane, called an action potential, and we'll see exactly how it works later on. FRAG 3: Extensibility means muscle can be stretched beyond its normal resting length and still contract. When you bend your elbow, the muscle on the back of your arm is stretched, but it's ready to pull as soon as you need it. FRAG 4: And elasticity means that once the stretch is over, muscle recoils to its original resting length, the way a rubber band springs back. Extensibility is about being stretched. Elasticity is about springing back.

You hold a hamstring stretch for thirty seconds, then let go. The muscle returns to its resting length. Which property is that?

Right. Elasticity is the recoil: once the stretch stops, the muscle springs back to its resting length. Being able to lengthen during the stretch in the first place was extensibility.

Not quite. Two of these properties are about stretch, and they're a pair. One describes the muscle lengthening while it's pulled. The other describes what happens once the pull stops. Which one does this question describe?

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Three types of muscle tissue

Skeletal

Attached to bones · voluntary · striated

Cardiac

Heart wall · involuntary · striated and branched

Smooth

Walls of hollow organs and vessels · involuntary · unstriated

FRAG 0: There are three kinds of muscle tissue. Under the microscope, the first thing to look for is striations: fine stripes running across the cells. You'll see where they come from in Section 3. FRAG 1: Skeletal muscle is the one you can control. It's attached to bones, mostly by tendons, and it moves your body, holds your posture and lets you breathe, chew and speak. It's by far the most plentiful kind, at roughly a third to two-fifths of your body weight. Its cells are long, striated and unbranched, with many nuclei pushed out to the edge. Although it's called voluntary muscle, it also takes part in reflexes, like the ones you met in Module 1. FRAG 2: Cardiac muscle is found only in the wall of the heart. Its cells are striated, like skeletal muscle, but they're shorter and branched, each with one or two nuclei near the centre, and they're joined end to end at dark lines called intercalated discs. It contracts without you thinking about it, so it's involuntary. FRAG 3: Smooth muscle is found in the walls of hollow organs and tubes: the gut, the bladder, the uterus, the airways and blood vessels. It's also in the iris of your eye and in the skin, where it makes your hairs stand on end. Its cells are spindle-shaped, with one central nucleus and no striations, which is why it's called smooth. It's involuntary too.

Side by side

FeatureSkeletalCardiacSmooth

Cells

Very long cylinders

Short, branched

Spindle-shaped

Nuclei

Many, at the edge

One or two, central

One, central

Striations

Yes

Joined to neighbours by

Nothing

Intercalated discs

Gap junctions (in many)

Control

Voluntary, plus reflexes

Involuntary

Starts itself?

No – needs a nerve signal

Yes

Some can

FRAG 0: Here are the three side by side. This is the comparison you'll make yourself in the lab, when you look at all three under the microscope, so it's worth getting clear now. FRAG 1: Start with the cells. A skeletal muscle cell is a very long cylinder, often several centimetres long, and some run almost the full length of their muscle. Cardiac cells are much shorter, and they branch. Smooth muscle cells are small and spindle-shaped, fat in the middle and tapering at each end. FRAG 2: Next, the nuclei. Each skeletal muscle cell has many nuclei, because it forms when many embryonic cells fuse, and they sit at the edge of the cell. Cardiac cells have one, or sometimes two, in the centre. Smooth muscle cells have a single nucleus in the centre. FRAG 3: Striations. Skeletal and cardiac muscle are both striated, because their protein filaments are lined up in neat, repeating units. Smooth muscle has the same proteins, but they aren't lined up, so there are no stripes. FRAG 4: How the cells connect. Skeletal muscle cells work independently of one another. Cardiac cells are locked together at intercalated discs. Many smooth muscle cells are linked by gap junctions, small channels that let signals pass straight from cell to cell. FRAG 5: Control. Skeletal muscle is voluntary, though it also responds in reflexes. Cardiac and smooth muscle are involuntary: you can't decide to change them, and they're adjusted by the autonomic nervous system and by hormones. FRAG 6: And finally, can it start a contraction by itself? Skeletal muscle can't. Without a signal from a nerve, it stays relaxed. Cardiac muscle can, and that's how your heart keeps beating. Some smooth muscle can too, especially in the gut.

Cardiac muscle up close

Branched, striated cells

Same contractile units as skeletal muscle

Desmosomes

Hold the cells together as the heart squeezes

Gap junctions

The electrical signal spreads cell to cell

Self-starting

Nerves and hormones only adjust rate and force

FRAG 0: Cardiac muscle belongs to this module, because the heart is a muscle before it's a pump. Next week, in Module 4, you'll see how the heart's electrical system and valves turn this tissue into a pump. Here, we'll look at the tissue itself. FRAG 1: Cardiac muscle cells are striated, because inside they have the same orderly contractile units as skeletal muscle, and they contract in much the same way. But they're short and branched, with one or two central nuclei, and they're packed with mitochondria, because the heart can never stop to rest. FRAG 2: The cells are joined end to end at the intercalated discs, and each disc does two jobs. The first is mechanical. Desmosomes rivet one cell to the next, so the cells don't pull apart as the heart squeezes, beat after beat. FRAG 3: The second job is electrical. Gap junctions are tiny channels that connect the inside of one cell with the inside of the next. Ions flow straight through them, so when one cell is excited, the signal spreads rapidly to its neighbours, and large groups of cells contract together as a single unit. FRAG 4: And cardiac muscle is self-starting, or autorhythmic. Specialised cells in the heart produce their own regular electrical signals, so the heart beats without any signal from a nerve. Nerves and hormones don't start each beat. They only speed it up or slow it down, and make it stronger or weaker, as you'll see in Module 4.

Smooth muscle up close

No sarcomeres

A criss-cross lattice anchored to dense bodies

Calcium → calmodulin → MLCK

No troponin; myosin heads switched on directly

Visceral vs multiunit

Sheets that act together, or cells that act alone

Many controls

Autonomic nerves, hormones, local chemicals, stretch

FRAG 0: Smooth muscle belongs to this module too. You'll meet it again in blood vessels, the bladder, the gut and the reproductive organs, so here's how it differs from the other two kinds. FRAG 1: Smooth muscle has the same contractile proteins, actin and myosin, but they aren't arranged in neat repeating units. Instead, they form a criss-cross lattice, anchored to small dark patches called dense bodies, some inside the cell and some on its membrane. When the filaments slide, they pull the dense bodies together, so the whole cell shortens and twists, a bit like wringing out a cloth. FRAG 2: Calcium is the trigger here too, but the switch is different. Much of the calcium comes in from outside the cell, and smooth muscle has no troponin. Instead, calcium binds to a protein called calmodulin. Together, they activate an enzyme called myosin light-chain kinase, which adds a phosphate to the myosin heads, and that lets them start pulling. It's slower to start and slower to stop than skeletal muscle, but it can hold a contraction for a long time using very little energy. That steady, partial contraction is called tone. FRAG 3: There are two arrangements. In visceral, or single-unit, smooth muscle, sheets of cells are linked by gap junctions, so they contract together as one. That's what you find in the gut, the bladder and the uterus. In multiunit smooth muscle, each cell acts on its own and needs its own nerve signal, as in the iris of the eye and the tiny muscles that make your hairs stand on end. FRAG 4: And smooth muscle answers to many controls. Autonomic nerves speed it up or slow it down. Hormones act on it: remember oxytocin from Module 1, making the uterus contract in labour. Local chemicals act on it too, and visceral smooth muscle often contracts when it's stretched, which helps the gut push its contents along.

Drag each clue onto the type of muscle it describes.

Drag each item, or click it and then click where it goes

Calcium binds calmodulin, not troponin

Joined by intercalated discs

Striated cells that branch

Under voluntary control

Spindle-shaped, with no striations

Many nuclei at the edge of each cell

Skeletal

Cardiac

Smooth

All six in place. Many edge nuclei and voluntary control mean skeletal; branching and intercalated discs mean cardiac; spindle cells without stripes, switched on by calmodulin, mean smooth.

Not quite yet. The ones outlined in red are in the wrong place. Look for one detail only one type has: shape, nuclei, joins or control.

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Section 2

A muscle as an organ

Wrappings, fibre patterns, blood and nerves

Wrapped in connective tissue

1Fascia2Epimysium3Fascicle4Perimysium5Muscle fibre6Endomysium7Tendon

FRAG 0: Here's a skeletal muscle with one end cut across, so you can see inside. The thing to notice is that connective tissue wraps it at every level, from the whole muscle down to each single cell. Learn the three layers by their prefixes: epi means upon, peri means around, and endo means within. FRAG 1: On the outside is the fascia, a sheet of connective tissue that surrounds the muscle, separates it from its neighbours and holds groups of muscles together. FRAG 2: Just beneath the fascia, and wrapped tightly around the whole muscle, is the epimysium, a layer of dense connective tissue. FRAG 3: Inside, the muscle is divided into bundles of muscle cells called fascicles. You can sometimes see them as the grain in a piece of meat. FRAG 4: Each fascicle is wrapped in its own layer of connective tissue, the perimysium. Remember peri, around: it goes around each bundle. FRAG 5: Inside each fascicle are the muscle fibres. A muscle fibre is a single muscle cell, long, cylindrical and with many nuclei, just as you saw in the last section. "Muscle fibre" and "muscle cell" mean the same thing. FRAG 6: And each fibre is wrapped in the thinnest layer of all, the endomysium. It's a fine, loose network that holds neighbouring fibres together and carries the smallest blood vessels and nerve branches right up to each cell. FRAG 7: At the ends of the muscle, all three layers run together and merge into the tendon. The tendon's collagen fibres blend into the periosteum, the membrane around the bone you met last week. So when every fibre pulls, the connective tissue gathers the force and passes it through the tendon to the bone. Some muscles end in a broad, flat sheet instead, called an aponeurosis, like those in the front of your abdomen.

Blood and nerve supply

1Artery2Vein3Capillaries4Nerve5Neuromuscular junction

FRAG 0: The same connective-tissue layers carry the muscle's supplies. An artery, a vein and a nerve usually travel together, and they enter a muscle side by side, through the epimysium. Then they branch along the perimysium between the fascicles, and finally run in the endomysium to reach each fibre. FRAG 1: The artery brings oxygen and nutrients. Muscle has a very high blood supply, because contraction uses a great deal of energy, and much of that energy comes from reactions that need oxygen. FRAG 2: The vein carries blood away, taking with it carbon dioxide, other wastes and much of the heat your muscles produce. FRAG 3: Between the artery and the vein are the capillaries. In the endomysium, a dense network of them runs alongside every muscle fibre, so that no fibre is far from a supply of oxygen. When a muscle is working hard, many more of these capillaries open up. FRAG 4: The nerve carries the commands. In it are the axons of motor neurons, whose cell bodies sit in the spinal cord or the brainstem. A motor neuron's axon leaves the spinal cord and travels in a spinal nerve; this is where it arrives. The nerve also carries sensory fibres back from stretch receptors in the muscle, the receptors behind stretch reflexes like the knee jerk. FRAG 5: Inside the muscle, each motor axon branches, and each branch ends on one muscle fibre, at a neuromuscular junction. Every fibre has its own junction, usually near its middle. Without a signal arriving there, a skeletal muscle fibre won't contract. We'll look at the junction in detail in Section 4.

Fascicle arrangements

Parallel and fusiform

Long range of shortening · sartorius, biceps brachii

Convergent

Fan onto one tendon · pectoralis major

Pennate

Feather-like · more force, shorter range · deltoid

Circular

Rings that close an opening · orbicularis oris

FRAG 0: Inside a muscle, the fascicles can be arranged in different patterns. The pattern matters, because it decides the trade-off between how far a muscle can shorten and how much force it can produce. It also tells you the muscle's line of pull, which you'll use later to work out what a muscle does. FRAG 1: In parallel muscles, the fascicles run straight along the length of the muscle, like the long strap-like muscle that crosses the front of your thigh, the sartorius. A fusiform muscle is a variation: spindle-shaped, thick in the middle and tapering at each end, like the biceps brachii in your arm. Because their fascicles are long and lined up with the pull, these muscles can shorten a long way. FRAG 2: In convergent muscles, the fascicles spread out like a fan from a broad base and come together on a single tendon. Your pectoralis major, across the front of your chest, is one. Different parts of the fan can pull in slightly different directions. FRAG 3: In pennate muscles, which comes from the word for feather, short fascicles attach at an angle to a tendon that runs through the muscle. They can be unipennate, attached on one side, bipennate, on both sides, or multipennate, with several tendons. Packing in many short fascicles at an angle gives more force, but a shorter range of movement. The deltoid, over your shoulder, is multipennate. FRAG 4: And in circular muscles, the fascicles form rings around an opening, so when they contract, the opening closes. The orbicularis oris, around your mouth, is one, and you use it every time you purse your lips.

Looking at a cross-section of muscle, you see bundles of fibres, each bundle wrapped in its own connective-tissue sheath. What is that sheath called?

Correct. The bundles are fascicles, and the perimysium (peri, around) goes around each one. The epimysium wraps the whole muscle, and the endomysium wraps each single fibre.

Not quite. Work out the level first: is this sheath around the whole muscle, around a bundle, or around a single cell? Then use the prefixes: upon, around, or within.

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Section 3

Inside a muscle fibre

From the cell down to its proteins

From muscle to myofilament

1Muscle2Fascicle3Muscle fibre (one cell)4Myofibril5Myofilaments (thick and thin)

FRAG 0: Here are the levels you'll need, from biggest to smallest. Two of the names sound alike, so watch them carefully. FRAG 1: The whole muscle, wrapped in its epimysium, as you saw in the last section. FRAG 2: Inside it, the fascicles, each wrapped in perimysium. FRAG 3: Inside each fascicle, the muscle fibres. Remember, a muscle fibre is one whole cell, a long cylinder with many nuclei, and it can be several centimetres long. FRAG 4: Now look inside that cell. It's packed with long, thin rods called myofibrils, running the full length of the fibre. A myofibril is not a cell. It's a structure inside the cell, and each fibre holds hundreds to thousands of them. So the myofibre is the cell, and the myofibrils are inside it. Each myofibril has its own pattern of light and dark bands, and because the bands of neighbouring myofibrils line up, the whole fibre looks striped. FRAG 5: And each myofibril is made of myofilaments: thin filaments and thick filaments, the proteins that do the pulling. Myo means muscle, so you can read these names as muscle fibre, muscle little-fibre and muscle thread.

Inside a fibre

1Sarcolemma2Nucleus3Mitochondrion4Myofibril5Sarcoplasmic reticulum6T tubule

FRAG 0: Let's cut a muscle fibre open. Many of its parts have special names that start with sarco, from the Greek for flesh. Most of them are just the familiar parts of any cell, renamed. FRAG 1: The cell membrane of a muscle fibre is called the sarcolemma, and the cytoplasm inside it is called the sarcoplasm. The sarcolemma is excitable: it can carry an action potential along the whole length of the fibre. FRAG 2: The nuclei, many of them, lie just under the sarcolemma, pushed to the edge by everything packed inside. FRAG 3: Between the myofibrils are lots of mitochondria. They make most of the ATP the fibre needs to contract, as long as there's oxygen. FRAG 4: Most of the space is taken up by the myofibrils, the bundles of filaments you've just met. FRAG 5: Wrapped around each myofibril, like a lacy sleeve, is the sarcoplasmic reticulum. It's the muscle cell's version of the smooth endoplasmic reticulum, and its job is to store calcium. At intervals it swells into wider sacs called terminal cisternae, which hold the most calcium. Keep that in mind, because calcium is the switch for contraction. FRAG 6: And here's the clever part. At regular intervals, the sarcolemma folds deep into the fibre as narrow tubes called transverse tubules, or T tubules. Each one runs right across the fibre, wrapping around the myofibrils and passing between two terminal cisternae. Together, a T tubule and the two cisternae beside it are called a triad. The T tubules carry the electrical signal from the surface deep into the cell, right next to the calcium stores, so every myofibril gets the message at almost the same moment.

The sarcomere

1Z disc2Thin filament3Thick filament4M line5A band6I band7H zone8Titin

FRAG 0: Each myofibril is a chain of identical repeating units, lined up end to end like carriages in a train. Each unit is called a sarcomere, and it's the basic unit of contraction. When a muscle contracts, it's because thousands of sarcomeres, one after another along every myofibril, are all getting shorter at the same time. FRAG 1: A sarcomere runs from one Z disc to the next. The Z discs are plates of protein that form the boundaries between sarcomeres. FRAG 2: Thin filaments are anchored to each Z disc and reach in towards the middle of the sarcomere, from both ends. They don't meet in the middle; there's a gap between the two sets. FRAG 3: In the middle of the sarcomere are the thick filaments. Their ends overlap the inner ends of the thin filaments. FRAG 4: The thick filaments are held in place at their centre by the M line, which runs down the middle of the sarcomere. FRAG 5: Now the bands. The A band is the dark band, and it's exactly as long as the thick filaments. Where the thick and thin filaments overlap, it looks darkest of all. FRAG 6: The I band is the light band, where there are only thin filaments. Each I band spans the boundary between two sarcomeres, so a Z disc runs through its middle. The A and I bands alternating along every myofibril are the striations you saw under the microscope. FRAG 7: In the centre of the A band is a slightly lighter strip, the H zone, where there are thick filaments but no thin ones. Keep an eye on the I band and the H zone, because they're the parts that change when the muscle contracts. FRAG 8: And finally, titin. It's a huge, springy protein that runs from each Z disc to the thick filaments. It keeps the thick filaments centred, and when a stretched muscle is let go, titin helps it spring back. That's part of the elasticity you met earlier.

Thick and thin filaments

Thick filament

Bundles of myosin, heads pointing out

Myosin heads

Bind ATP and actin · pivot at a hinge

Actin

Two twisted strands of beads, each with an active site

Tropomyosin and troponin

Cover the sites · troponin binds calcium

FRAG 0: Let's zoom in once more, on the filaments themselves. Each kind is built from a few proteins, and each protein has a particular job in contraction. FRAG 1: A thick filament is a bundle of a few hundred myosin molecules. Each one is shaped a bit like a golf club, with a long tail and a head at one end. The tails are bundled together in the middle, and the heads stick out along the filament, pointing away from its centre, towards the thin filaments. FRAG 2: The myosin heads are the parts that do the pulling. Each head has two important binding sites. One binds actin, so the head can latch on to a thin filament and form what's called a cross-bridge. The other binds ATP, the cell's energy molecule, and breaks it down to power the next pull. Where the head joins the tail there's a hinge, so the head can pivot. FRAG 3: A thin filament is mostly actin. Actin molecules are small, round proteins, strung together into two strands that twist around each other, like two strings of pearls. Each actin molecule has an active site, a spot where a myosin head can attach. FRAG 4: But at rest, the myosin heads can't reach those sites, because of two more proteins. Tropomyosin is a long, thin strand that lies along the actin and covers the active sites. Troponin is a small complex attached to the tropomyosin, and it has a binding site for calcium. When calcium binds to troponin, troponin pulls the tropomyosin aside and uncovers the active sites. So, tropomyosin is the cover, and troponin is the switch that calcium flips. You'll see that happen in Section 5.

Sliding filaments

1Relaxed

2Heads grab and pull

3Z discs move closer

4Fully contracted

FRAG 0: Here's the sliding filament model, the explanation for how a muscle shortens. Watch the bands as the filaments move, because what changes, and what doesn't, is the evidence for how it works. FRAG 1: Here's a relaxed sarcomere. The I bands are wide, the H zone is wide, and the Z discs are far apart. The myosin heads aren't attached to the thin filaments. FRAG 2: Contraction begins when the myosin heads grab the thin filaments and pull them towards the M line, from both ends at once. Each head pulls, lets go, re-attaches further along and pulls again, so the thin filaments are dragged inwards in many small steps. FRAG 3: Because the thin filaments are anchored to the Z discs, the Z discs are pulled closer together, and the sarcomere gets shorter. Look at the bands. The I bands and the H zone get narrower, because the filaments overlap more. But the A band stays exactly the same width, because it's the length of the thick filaments, and they haven't changed. Neither filament gets any shorter. They simply slide past each other. FRAG 4: In a full contraction, the thin filaments reach the middle of the sarcomere, the H zone disappears, and the I bands become very narrow. And because thousands of sarcomeres lie end to end along every myofibril, each one shortening a little adds up to the whole fibre, and the whole muscle, shortening a lot.

During contraction, which parts get shorter and which stay the same length? Drag each one.

Drag each item, or click it and then click where it goes

H zone

Sarcomere

A band

Thick filament

I band

Thin filament

Gets shorter

Stays the same length

All six in place. The filaments and the A band (the length of the thick filaments) never change. Only the regions that depend on how much the filaments overlap – the I band and H zone – narrow, so the sarcomere shortens.

Not quite yet. The ones outlined in red are in the wrong place. Filaments slide; they don't shrink. Which bands depend on the overlap?

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Section 4

From nerve to muscle

An electrical signal, and how it crosses the gap

The resting membrane potential

A charge across the membrane

Inside negative: about −85 mV in muscle, −70 mV in neurons

Uneven ions

Sodium outside, potassium inside, kept by the pump

Potassium leaks out

Leaves the inside negative

FRAG 0: Every cell in your body has an electrical charge across its membrane. Nerve and muscle cells are special because they can change that charge very quickly, and use the change as a signal. To see how, start with a cell at rest. FRAG 1: At rest, the inside of the membrane is negative compared with the outside. This difference is called the resting membrane potential, and it's measured in millivolts. In a skeletal muscle fibre, it's about minus eighty-five millivolts. In a neuron, it's about minus seventy. It's a tiny voltage, but across a membrane this thin, it's a lot of electrical pressure. FRAG 2: The charge depends on two ions being unevenly spread. There's much more sodium outside the cell than inside, and much more potassium inside than outside. The sodium–potassium pump in the membrane keeps it that way. It uses ATP to push three sodium ions out and bring two potassium ions in, over and over, keeping both gradients steep. FRAG 3: Now the key step. At rest, the membrane has potassium leak channels that are always open, but very few open sodium channels. So potassium ions diffuse out of the cell, down their gradient, carrying positive charge with them. The large proteins inside the cell are negatively charged and can't follow. So the inside is left negative. That's the resting membrane potential, and it's what makes the cell ready to fire.

The action potential

1At rest

2Stimulus to threshold

3Sodium rushes in: inside reverses to about +30 mV

4Potassium flows out: back to negative

5Reset

FRAG 0: An action potential is a brief, sudden reversal of the charge across the membrane. Neurons use it to send signals along their axons, and muscle fibres use it to trigger contraction. Watch the membrane on the left and the voltage on the right. FRAG 1: We start at rest, at about minus eighty-five millivolts, the value for a muscle fibre. Potassium is slowly leaking out, and two special channels are closed. They're voltage-gated channels: they open and close in response to changes in the membrane's voltage. One lets sodium through, the other lets potassium through. FRAG 2: A stimulus makes the inside of the membrane a little less negative. That's called depolarisation. If it's small, nothing much happens. But if the membrane depolarises as far as a critical level, called threshold, the voltage-gated sodium channels open. FRAG 3: And now sodium rushes in, down its steep gradient, carrying positive charge into the cell. The inside becomes less and less negative, and then, very briefly, it reverses, reaching about plus thirty millivolts. This rapid rise is the depolarisation phase of the action potential. FRAG 4: Almost at once, the sodium channels close, and the voltage-gated potassium channels open. Potassium flows out, and the inside becomes negative again. That's repolarisation. The potassium channels are a little slow to close, so for a moment the membrane dips slightly below its resting value, before it settles back. FRAG 5: The whole thing is over in a few thousandths of a second. Surprisingly few ions actually cross the membrane, so the gradients barely change, and the sodium–potassium pump steadily keeps them topped up. The membrane is ready to fire again.

Two rules of the action potential

Full size or nothing. And it spreads.

Each patch of membrane triggers the next.

FRAG 0: Two rules make the action potential a reliable signal. The first is that it's all-or-none. If a stimulus doesn't reach threshold, there's no action potential at all. If it does, you get a full-size action potential, the same height every time, however strong the stimulus. A stronger stimulus can't make a bigger action potential. It can only make more of them, closer together, and you'll see later that this is one way a muscle pulls harder. Straight after an action potential, there's also a short refractory period, when the sodium channels can't reopen and the membrane can't fire again. FRAG 1: The second rule is that it spreads. When one patch of membrane depolarises, the positive charge flows sideways inside the cell and depolarises the patch next to it to threshold, so that patch fires too, and then the next, like a row of falling dominoes. The patch behind is still refractory, so the signal can only travel forwards. That's how an action potential runs along a motor neuron's axon, and, as you'll see, over the whole surface of a muscle fibre and down its T tubules. Cardiac muscle uses action potentials too, with a few twists you'll meet in Module 4.

What causes the rapid rise of an action potential, when the inside of the membrane becomes positive?

How confident are you?

Correct. At threshold, voltage-gated sodium channels open and sodium rushes in down its gradient, carrying positive charge, so the inside briefly becomes positive. Potassium flowing out comes next, and brings the membrane back down.

Not quite. Think about which ion is concentrated outside the cell, so it would flow inwards, and which channels open first when the membrane reaches threshold. The pump works slowly in the background and doesn't make the spike.

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The neuromuscular junction

1Presynaptic terminal2Synaptic vesicles (acetylcholine)3Synaptic cleft4Motor end-plate5Acetylcholine receptors6Acetylcholinesterase

FRAG 0: Here's where the nerve meets the muscle. You saw in Section 2 that each branch of a motor axon ends on one muscle fibre, near its middle. That meeting point is the neuromuscular junction, a synapse between a motor neuron and a skeletal muscle fibre. Let's zoom in. FRAG 1: The end of the axon branch swells into the presynaptic terminal. Pre means before: this is the nerve's side of the junction. FRAG 2: The terminal is packed with synaptic vesicles, tiny sacs full of the neurotransmitter acetylcholine, or ACh for short. It's the chemical messenger that carries the signal across to the muscle. There are mitochondria here too, supplying the energy to make and recycle it. FRAG 3: Between the nerve and the muscle there's a narrow gap, the synaptic cleft. The nerve and the muscle don't actually touch. The signal has to cross this gap as a chemical, not as electricity. FRAG 4: On the other side of the cleft is the motor end-plate, the specialised patch of sarcolemma under the terminal. It's thrown into deep folds, which give it a much larger surface area. FRAG 5: Packed onto those folds are acetylcholine receptors. Each receptor is also a channel. When acetylcholine binds to it, the channel opens and lets sodium ions into the muscle fibre. Because a chemical opens it, rather than a voltage, it's called a ligand-gated channel. FRAG 6: And in the cleft is an enzyme called acetylcholinesterase. Its job is to break acetylcholine down, quickly, so each signal is short and the muscle doesn't stay switched on. Ase at the end of a word usually means an enzyme, and this one breaks down acetylcholine.

Events at the neuromuscular junction

1Action potential arrives

2Calcium enters the terminal

3ACh released into the cleft

4ACh binds receptors

5Sodium in → muscle action potential

6ACh broken down; choline recycled

FRAG 0: Now let's watch a signal cross the junction, step by step. This is the sequence you'll need to be able to describe, so follow it closely. FRAG 1: An action potential travels down the motor neuron's axon and arrives at the presynaptic terminal. FRAG 2: The change in voltage opens voltage-gated calcium channels in the terminal's membrane. Calcium is much more concentrated outside the cell, so it flows into the terminal. Here, calcium is the trigger for the nerve to release its chemical. FRAG 3: Calcium causes the synaptic vesicles to fuse with the terminal's membrane and empty their acetylcholine into the cleft. Releasing a substance this way, by a vesicle fusing with the membrane, is called exocytosis. FRAG 4: Acetylcholine diffuses across the narrow cleft and binds to the receptors on the motor end-plate. Binding opens the ligand-gated channels. FRAG 5: Sodium ions rush in through the open channels, and the end-plate depolarises. That depolarisation is large enough to bring the neighbouring sarcolemma to threshold, so an action potential fires and spreads out over the whole surface of the muscle fibre, just like the one you saw on the axon. Normally, every action potential in the motor neuron produces one action potential in each muscle fibre it supplies. FRAG 6: Finally, the signal has to stop. Acetylcholinesterase splits acetylcholine into two pieces, acetic acid and choline, so the receptor channels close. The choline is taken back up into the terminal and recycled to make new acetylcholine. All of this happens within a few thousandths of a second, ready for the next action potential.

CaseA drug is given that binds to acetylcholine receptors at the motor end-plate and blocks them, without opening them. The motor neurons still fire normally.

What do you predict will happen to the patient's skeletal muscles?

How confident are you?

Correct. Acetylcholine is still released, but it can't open the blocked receptors. Little or no sodium enters, the end-plate doesn't depolarise enough, and no action potential spreads over the fibre, so the muscle doesn't contract. Drugs like this are used during surgery to relax muscles, which is why the patient needs help to breathe. In myasthenia gravis, the body's own antibodies attack these receptors, and muscles tire and weaken easily.

Not quite. Follow the steps at the junction. Is acetylcholine still released? Can it still open the channels it normally opens? And without sodium entering, can the end-plate reach threshold and start an action potential in the fibre?

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Section 5

Turning a signal into a pull

Calcium, cross-bridges and the energy behind them

Excitation–contraction coupling

1Action potential into the T tubules

2Calcium released from the SR

3Calcium binds troponin

4Active sites exposed; cross-bridges form

5Relaxation: calcium pumped back

FRAG 0: Excitation–contraction coupling is the chain of events that links the excitation, the action potential on the sarcolemma, to the contraction of the filaments deep inside the fibre. Watch the top of the screen for the membranes, and the bottom for the filaments. FRAG 1: The action potential spreads across the sarcolemma from the neuromuscular junction, and it doesn't stay on the surface. It runs down every T tubule, deep into the fibre, alongside every myofibril. FRAG 2: Next to each T tubule are the terminal cisternae of the sarcoplasmic reticulum, the fibre's calcium store. The change in voltage in the T tubule opens calcium channels in the sarcoplasmic reticulum, and calcium floods out into the sarcoplasm around the filaments. FRAG 3: That calcium binds to troponin on the thin filaments. Troponin changes shape. FRAG 4: As troponin changes shape, it pulls tropomyosin away from the active sites on actin. Now the myosin heads can reach those sites. They bind, forming cross-bridges, and the filaments start to slide. Remember the cover and the switch: calcium has flipped the switch, and the cover has moved. FRAG 5: And here's how it stops. When the motor neuron stops firing, acetylcholinesterase clears the acetylcholine, and no new action potentials arrive. The calcium channels close, and pumps in the sarcoplasmic reticulum use ATP to move calcium back inside. Without calcium, troponin lets tropomyosin slide back over the active sites, the heads can't bind, and the muscle relaxes. You may remember from Module 2 that your blood calcium is held in a narrow range partly because nerves and muscles need it. Here's one of the reasons.

The cross-bridge cycle

1Active sites exposed

2Cross-bridge forms

3Power stroke

4ATP binds: head lets go

5ATP split: head re-cocked

6Ready to bind again

FRAG 0: Now let's zoom in on one myosin head and follow it through one full cycle. Every head in every sarcomere goes through this cycle, over and over, for as long as calcium is present. FRAG 1: It begins when calcium binds troponin and tropomyosin moves aside, exposing the active sites on actin. The myosin head is already cocked, like a drawn bow. It's holding ADP and phosphate, the two pieces left over from breaking down an ATP molecule, and the energy from that ATP is stored in the head. FRAG 2: The cocked head binds to an active site. That link between myosin and actin is the cross-bridge. As it forms, the phosphate is released. FRAG 3: Now the power stroke. The head swings, using its stored energy, and drags the thin filament towards the M line. The ADP is released. This is the step that actually moves the filament. FRAG 4: The head is now stuck to actin, and it can only let go when a new ATP molecule binds to it. ATP binds to the myosin head itself, at its ATP-binding site, and the head detaches from the actin. FRAG 5: Next, the head breaks that ATP down into ADP and phosphate. The energy released re-cocks the head, swinging it back into its ready position. Because the thin filament has moved, the head now sits opposite a new active site, further along. FRAG 6: And the cycle repeats: bind, stroke, release, re-cock. Thousands of heads cycle at different moments, so some are always attached, and the filament never slips back. It's a bit like a team pulling a rope hand over hand. As soon as the calcium is pumped away, tropomyosin covers the sites again, and the cycling stops.

Three jobs for ATP

Powering the stroke

Split ATP re-cocks each head

Letting go

A fresh ATP detaches the head from actin

Relaxing

Pumps calcium back into the SR

FRAG 0: ATP has come up again and again, so let's pull it together. A contracting muscle fibre uses ATP for three separate jobs, and if any one of them fails, contraction goes wrong. FRAG 1: The first job is powering the stroke. Each time a myosin head splits an ATP molecule, the energy released re-cocks the head, and that stored energy drives the next power stroke. FRAG 2: The second job is letting go. A myosin head can only detach from actin when a fresh ATP binds to it. Without ATP, the heads stay locked onto the thin filaments, and the filaments can't slide back. FRAG 3: And the third job is relaxing. The pumps that move calcium back into the sarcoplasmic reticulum use ATP, so relaxation needs energy too. And as you saw in Section 4, the sodium–potassium pump uses ATP to keep the membrane ready to fire. So a muscle fibre never stops needing ATP, even while it relaxes.

Put the events from nerve to muscle contraction in order.

Drag the steps into order, first at the top, or use the arrow buttons.

⋮⋮Tropomyosin moves off the active sites
⋮⋮Myosin heads bind actin and pull
⋮⋮The motor end-plate depolarises
⋮⋮The sarcoplasmic reticulum releases calcium
⋮⋮Action potential reaches the presynaptic terminal
⋮⋮An action potential spreads along the sarcolemma and T tubules
⋮⋮Acetylcholine is released into the cleft
⋮⋮Calcium binds troponin

That's the full sequence. The signal crosses the junction as a chemical, travels over the fibre and down the T tubules as an action potential, releases calcium, and calcium uncovers actin so the heads can pull.

Not quite. Split it into two halves: first the nerve-to-muscle junction, then what happens inside the fibre. Within each half, ask what has to happen before the next step is even possible.

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Where the ATP comes from

SourceHow it worksHow fast · how long

Creatine phosphate

Hands its phosphate to ADP (creatine kinase)

Instant · seconds

Anaerobic

Glucose split without oxygen → lactate

Fast · up to a minute or two

Aerobic

Mitochondria use oxygen; glucose and fats

Slower · hours

FRAG 0: A muscle fibre stores only enough ATP for a few seconds of hard work, so it has to keep making more. It has three ways of doing it, and they differ in how fast they work and how long they can keep going. FRAG 1: The quickest is creatine phosphate, a high-energy molecule stored in the fibre. An enzyme called creatine kinase moves its phosphate straight onto ADP, remaking ATP almost instantly. But the store is small, so it lasts only about ten seconds of all-out effort, a short sprint or a heavy lift. When muscle fibres are damaged, creatine kinase leaks into the blood, so it's measured in blood tests as a sign of muscle injury. FRAG 2: Next is anaerobic respiration, which means without oxygen. The fibre breaks glucose down quickly, without using oxygen, and turns the end product into lactate. It's fast, but it makes only a little ATP from each glucose molecule, so it can support hard exercise for only a minute or two. FRAG 3: And then there's aerobic respiration, in the mitochondria, using oxygen brought by the blood. It's slower to get going, but it gets far more ATP out of each glucose molecule, and it can use fats as well, so it can keep a muscle going for hours. That's why muscles have such a rich blood supply. When work outpaces supply, fuel stores run down and by-products build up, and the fibres gradually lose their ability to contract. That's fatigue.

A few hours after death, the skeletal muscles stiffen. What's the best explanation?

How confident are you?

Right. After death, the cells stop making ATP. Calcium leaks out of the sarcoplasmic reticulum and the pumps can't return it, so the active sites stay exposed and myosin heads bind. Without ATP, the heads can't detach, so the muscles lock. The stiffness passes a day or two later, as the muscle proteins themselves break down.

Not quite. Think about the three jobs of ATP. Which of them would leave the muscle locked rigid, rather than simply relaxed, if ATP ran out?

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Section 6

How hard, how fast?

Twitches, motor units and three kinds of contraction

From twitch to tetanus

1One twitch: latent, contraction, relaxation

2Summation: a second stimulus adds on

3Incomplete tetanus

4Complete tetanus

FRAG 0: This is a myogram, a recording of how much tension a muscle produces over time. The small marks along the bottom show each time the muscle is stimulated. FRAG 1: Stimulate the muscle once, and you get a twitch: a single, brief contraction and relaxation. It has three phases. First, a short latent phase, when nothing seems to happen. That's the time it takes for the action potential to spread and for calcium to be released. Then the contraction phase, as cross-bridges form and tension rises. Then the relaxation phase, as calcium is pumped back and tension falls. A single muscle fibre is all-or-none too: one action potential gives one full twitch. FRAG 2: Now stimulate again before the muscle has relaxed. The second twitch adds on top of the first, so tension climbs higher. This is called summation. It happens because calcium hasn't all been pumped back yet, so more of it builds up in the sarcoplasm, and more cross-bridges can form at once. FRAG 3: Stimulate faster still, and the twitches merge. The tension rises to a much higher level, with a small dip between each stimulus. This is incomplete tetanus. FRAG 4: And at a very high rate of stimulation, there's no time to relax at all. The tension rises to its maximum and stays there, smooth and steady, for as long as the stimuli continue. This is complete tetanus. Most of your everyday contractions are smooth like this, because your motor neurons fire in rapid bursts. Don't confuse this with the disease called tetanus, which is caused by a bacterial toxin and produces painful muscle spasms.

Motor units

One neuron + its fibres

That's one motor unit

Scattered

A unit's fibres spread through the muscle

All together

Every fibre in a unit contracts at once

Size matters

A few fibres for fine control; over a thousand for power

FRAG 0: To control force, the nervous system doesn't send signals to individual fibres. It works through motor units. FRAG 1: A motor unit is one motor neuron, plus all the muscle fibres it supplies. Remember that a motor axon branches inside the muscle, and each branch ends at a neuromuscular junction on one fibre. Every fibre is supplied by just one neuron, but one neuron can supply many fibres. FRAG 2: The fibres of one motor unit aren't bunched together. They're scattered through the muscle, mixed in with fibres from other units. So when one unit contracts, it pulls gently on the whole muscle, rather than tugging on one corner. FRAG 3: When a motor neuron fires, every fibre in its unit contracts. The neuron can't pick and choose among its own fibres. So the unit is the smallest piece of a muscle the nervous system can switch on. FRAG 4: And motor units come in very different sizes. In muscles that make fine, precise movements, like the ones that move your eyes or your fingers, each unit has only a few fibres, so switching on one more unit adds just a little force. In large, powerful muscles, like those of your thigh and calf, one unit can have more than a thousand fibres, so each unit adds a lot of force. What matters for precision is the size of each unit: how many fibres one neuron controls.

Turning up the force

Recruitment

More motor units join in, small ones first

Frequency

Faster firing → summation, then tetanus

Taking turns

Units alternate, delaying fatigue · tone at rest

FRAG 0: Put the twitch and the motor unit together, and you have the two ways a muscle grades its force. FRAG 1: The first is recruitment: switching on more motor units. For a light task, only a few units are active. As the load gets heavier, more and more units join in. The small units are usually recruited first, and the large, powerful ones last, so you have fine control at low forces and plenty in reserve. FRAG 2: The second is frequency: making each motor neuron fire faster. You saw this on the myogram. A higher rate of action potentials means more summation, and eventually tetanus, so each active unit pulls harder. Remember that every action potential is the same size, so it's the number of them, not their size, that changes. FRAG 3: And during a long, steady contraction, motor units take turns. Some rest while others work, so the muscle's force stays smooth while each unit gets a chance to recover, which delays fatigue. Even at rest, a few units are active at any moment, keeping a small, constant tension in the muscle. That's muscle tone, and it helps keep you upright and keeps your joints stable.

Which of these muscles would you expect to have the smallest motor units – the fewest fibres per motor neuron?

Correct. Eye movements have to be extremely precise, so the eye muscles have tiny motor units of only a few fibres each. Adding one more unit changes the force very slightly. The buttock, thigh and calf muscles have huge units for power.

Not quite. Small motor units mean each extra unit adds only a little force, which suits movements that need to be precise rather than powerful. Which of these muscles makes the finest, most precise movements?

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Three kinds of contraction

Concentric

Tension greater than load · muscle shortens

Isometric

Tension equals load · no change in length

Eccentric

Load greater than tension · lengthens under control

FRAG 0: A muscle always pulls, but whether it shortens depends on the load it's pulling against. Picture a tug-of-war between your muscle and a weight. The muscle's pull is its tension, and the weight is the load. FRAG 1: If the muscle's tension is greater than the load, the muscle shortens, and the load moves. That's a concentric contraction, like your biceps bending your elbow to lift a dumbbell. Concentric and eccentric contractions are both called isotonic, because the muscle changes length while the tension stays about the same. FRAG 2: If the tension exactly matches the load, nothing moves. The muscle stays the same length, even though it's working hard. That's an isometric contraction, like holding the dumbbell still with your elbow bent. Most of your postural muscles contract isometrically for much of the day, holding you upright. FRAG 3: And if the load is greater than the tension, the muscle is pulled longer, even though it's still contracting. That's an eccentric contraction. It's how you lower the dumbbell slowly instead of letting it drop: the biceps keeps pulling, but a little less than the weight, so the elbow straightens under control. You use eccentric contractions every time you lower something, sit down or walk downstairs.

Drag each action onto the kind of contraction the named muscle is making.

Drag each item, or click it and then click where it goes

Biceps: lifting a cup to your mouth

Back muscles: standing still and upright

Quadriceps: walking down stairs

Triceps: pushing up from the floor in a push-up

Abdominal muscles: holding a plank

Biceps: lowering a heavy box slowly

Concentric

Isometric

Eccentric

All six in place. Shortening against the load is concentric, holding still is isometric, and controlling a load as the muscle lengthens is eccentric.

Not quite yet. The ones outlined in red are in the wrong place. Ask: is the muscle getting shorter, staying the same, or being pulled longer?

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Section 7

Muscles at work

Attachments, actions and names

From Module 2 – drag each movement onto its name.

Drag each item, or click it and then click where it goes

Bending the elbow so the hand moves towards the shoulder

Bringing the arm back down to the side

Shrugging the shoulders

Straightening the knee

Pointing the toes, as when standing on tiptoe

Lifting the arm out to the side

Pulling the toes up towards the shin

Turning the palm to face forwards or up

Flexion

Extension

Abduction

Adduction

Elevation

Plantar flexion

Dorsiflexion

Supination

All eight in place. You'll need these names from now on, because every muscle action is described with them.

Not quite yet. The ones outlined in red are in the wrong place. Start from the anatomical position. Which plane, and which direction?

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How a muscle moves a bone

Attached by tendons

Or a flat sheet; some facial muscles to skin

Pulls the ends together

Usually one bone stays still, the other moves

Proximal and distal attachments

The ends closer to and further from the trunk

FRAG 0: Here's your arm, with three muscles you'll meet again: the biceps brachii and the brachialis on the front, and the triceps brachii on the back. Let's use them to see how a muscle moves a bone. FRAG 1: Most muscles attach to bone through a tendon, a cord of dense connective tissue that merges with the periosteum, as you saw earlier. Some attach through a broad, flat sheet, an aponeurosis, and some attach straight to the periosteum. A few, like many muscles of the face, attach to skin instead of bone. FRAG 2: When a muscle contracts, it pulls its two ends towards each other. Usually one end is held still, and the bone at the other end moves. When your biceps contracts, the shoulder end stays put, and your forearm swings up. A muscle can only pull, so to straighten your elbow again, the triceps on the back has to do the pulling. In this way, bones act as levers, and joints as the pivots they turn around. FRAG 3: To describe where a muscle attaches, we use the directional terms from Module 2. The proximal attachment is the end nearer the trunk, and the distal attachment is the end further away. So the biceps has proximal attachments on the scapula, and a distal attachment on the radius. Your textbook calls these attachments the origin and insertion; in this unit we just say proximal and distal attachment.

Muscles work in teams

Agonists (prime movers)

Do most of the work · biceps brachii, brachialis

Antagonist

Does the opposite movement · triceps brachii

Synergist

Helps the agonists · brachioradialis

Fixator

Holds a bone steady · muscles around the scapula

FRAG 0: Muscles rarely work alone. Almost every movement involves a team, and each member has a role. Let's take bending your elbow to lift a cup. FRAG 1: An agonist, or prime mover, is a muscle that does most of the work for a movement. Bending the elbow has two: the biceps brachii, and the brachialis lying just under it. Both cross the front of the elbow, and they pull together. FRAG 2: The antagonist does the opposite movement. For elbow flexion, that's the triceps brachii, which extends the elbow. Agonists and antagonists sit on opposite sides of a joint. While the agonists work, the triceps relaxes, and it can also contract a little to make the movement smooth and controlled. Next time you straighten your elbow, they swap roles. FRAG 3: A synergist helps the agonists. Here it's the brachioradialis, a muscle that runs from the lower end of the humerus down the thumb side of your forearm to the radius. It crosses the front of the elbow too, and adds its pull, most strongly when your thumb points up, as when you lift a cup. FRAG 4: And a fixator holds a bone steady, so the agonists' pull goes into the right movement. The biceps is attached to the scapula, and the scapula can slide over the back of the rib cage. So muscles around the scapula contract to hold it firm, and the biceps' pull moves your forearm instead of your shoulder blade.

Four clues to a muscle's action

Which joint does it cross?

No crossing, no action · the joint sets the options

Which side of the joint?

Check each joint · the knee breaks the elbow's pattern

Line of pull

Follow the fascicles

Attachments

Which bone moves towards which?

FRAG 0: In the lab, you'll be asked to work out what a muscle does just by looking at it. You don't need to memorise every action. Four clues will get you there, the same four your tutors will use. FRAG 1: First, which joint does it cross? A muscle can only move a joint it crosses. If it doesn't cross a joint, it can't act on it. And the joint decides what's possible. The elbow is a hinge, so a muscle crossing it can only flex or extend it, never abduct it. A muscle that crosses two joints, like the biceps, can act on both. FRAG 2: Second, which side of the joint does it cross? At the elbow, muscles that cross in front, like the biceps and brachialis, flex it, and those behind, like the triceps, extend it. The shoulder and hip follow the same pattern, and at the hip, muscles on the inner side adduct and those on the outer side abduct. But check each joint, because it isn't a universal rule. At the knee, it's the other way round: the big muscle group in front of the thigh extends the knee, and the group behind flexes it. And at the ankle, muscles in front lift the foot, which is dorsiflexion. FRAG 3: Third, the line of pull. A muscle pulls along the direction of its fascicles, so follow them, like the arrow here. Remember the fascicle patterns from earlier: a fan-shaped muscle can pull in several directions, depending on which part contracts. FRAG 4: And fourth, the attachments. A contracting muscle pulls its distal attachment towards its proximal one, or, sometimes, the other way round. In a chin-up, your forearm is fixed on the bar, and the same muscles pull your body up towards your hands. Put the four clues together, and you can predict the action of almost any muscle.

Worked example: the deltoid

1Anterior fibres2Middle fibres3Posterior fibres

FRAG 0: Let's try the four clues on one muscle, the deltoid, the triangular muscle that caps your shoulder. You'll do this same exercise at one of the lab stations. It crosses one joint, the shoulder, and its proximal attachments wrap around the front, top and back of the joint, on the clavicle and scapula. All its fibres pull on one distal attachment, halfway down the outer side of the humerus. FRAG 1: The anterior fibres start on the clavicle and cross the front of the shoulder. Front of the shoulder means flexion: they lift your arm forwards. FRAG 2: The middle fibres start on the acromion and cross the top of the joint, on its outer side. Outer side means abduction: they lift your arm out sideways. This part is multipennate, packed with short fascicles for strength, and it's the main abductor of the shoulder. FRAG 3: And the posterior fibres start on the spine of the scapula and cross the back of the shoulder. Back of the shoulder means extension: they swing your arm backwards. So one muscle has three actions, depending on which part contracts.

The hamstrings attach proximally to the ischial tuberosity and distally to the tibia and fibula. They cross the back of the hip and the back of the knee. What do they do?

How confident are you?

Correct. Behind the hip means extension, as for gluteus maximus. But the knee works the other way round: muscles behind it flex it. So the hamstrings extend the hip and flex the knee, which is exactly what you do as you push off and then swing your leg back in walking.

Not quite. Take one joint at a time. At the hip, what does a muscle crossing the back do? At the knee, remember it doesn't follow the elbow's pattern. Which way round is it there?

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How muscles are named

Named forClueExamples

Shape

What it looks like

Deltoid (triangle) · trapezius · serratus (saw-toothed) · piriformis (pear-shaped)

Size

Bigger or smaller, longer or shorter

Maximus, medius, minimus · major, minor · longus, brevis

Number of heads

Bellies sharing one tendon

Biceps · triceps · quadriceps

Fascicle direction

The line of its fibres

Rectus abdominis · transversus abdominis · external oblique

Location or attachment

Where it sits or attaches

Temporalis · intercostals · sternocleidomastoid

Action

What it does

Flexor · extensor · adductor · levator · supinator

FRAG 0: Muscle names look intimidating, but most of them are descriptions in Latin, and they often hint at the muscle's shape, position or job. There are six common rules, and most names combine two or three of them. A few have other origins: sartorius, for example, comes from the Latin for tailor. FRAG 1: Shape. The deltoid is triangular, like the Greek letter delta. The trapezius is shaped like a trapezium, and the rhomboids like a rhombus. Serratus means saw-toothed, from its jagged edge, and piriformis means pear-shaped. FRAG 2: Size. Maximus, medius and minimus mean largest, middle and smallest, like the three gluteal muscles of the buttock. Major and minor mean larger and smaller, and longus and brevis mean long and short. Magnus simply means large. FRAG 3: Number of heads. A head is a separate belly with its own proximal attachment, joining the others at a shared tendon. Biceps has two heads, triceps three, and quadriceps four. FRAG 4: Fascicle direction. Rectus means straight, transversus means across, and oblique means at an angle. In the wall of your abdomen, you'll find all three: rectus abdominis running straight up and down, transversus abdominis running across, and the external and internal obliques running at an angle. FRAG 5: Location or attachment. Temporalis lies over the temporal bone, and the intercostals lie between the ribs. Sternocleidomastoid tells you its attachments: the sternum, the clavicle, and the mastoid process of the temporal bone. FRAG 6: And action. A flexor flexes, an extensor extends, an adductor adducts, a levator raises something, and a supinator supinates. Put rules together, and a long name becomes readable: extensor digitorum longus is the long muscle that extends the toes.

Each word comes from a muscle's name. Drag it onto the rule it uses.

Drag each item, or click it and then click where it goes

temporalis

rectus

deltoid

levator

triceps

maximus

Size

Fascicle direction

Number of heads

Shape

Location

Action

All six in place. Maximus means largest, rectus straight, triceps three heads, deltoid triangle-shaped, temporalis over the temporal bone, and levator means it lifts something.

Not quite yet. The ones outlined in red are in the wrong place. Translate the word first: what does it mean in plain English?

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Face and jaw

1Frontalis2Orbicularis oculi3Zygomaticus4Orbicularis oris5Buccinator6Masseter7Temporalis

FRAG 0: The face has two groups of muscles, and you'll find both at one of the lab stations. The muscles of facial expression are unusual: they don't move joints. They attach to the skin, and pull on it to change your expression. The muscles of mastication, or chewing, move the jaw. FRAG 1: The frontalis covers your forehead. It's named for the frontal bone underneath, and it raises your eyebrows and wrinkles your forehead, as when you're surprised. FRAG 2: The orbicularis oculi is a ring of muscle around the eye. Like every circular muscle, it closes the opening it surrounds, so it closes your eyelids. Squeeze them tight, and it's what causes crow's feet at the corners of your eyes. FRAG 3: The zygomaticus runs from your cheekbone, the zygomatic bone, down to the corner of your mouth. It pulls the corner of the mouth up and out. It's your smiling muscle. FRAG 4: The orbicularis oris is the ring around your lips. It closes your mouth and purses your lips, as in whistling or a kiss. FRAG 5: The buccinator lies deep in your cheek. It squeezes the cheek against your teeth, which keeps food between them while you chew, and it's how you blow out air or play a trumpet. It's counted as a muscle of facial expression, but it helps with chewing. FRAG 6: Now the muscles of mastication. They act at the temporomandibular joint, where the mandible meets the temporal bone, just in front of your ear. Only the mandible moves. The masseter runs from the zygomatic arch down to the mandible. Clench your teeth, and you can feel it bulge at the angle of your jaw. It elevates the mandible, closing your jaw. FRAG 7: The temporalis is a fan-shaped muscle over the side of your skull. It also elevates the mandible, and it pulls it back. Clench your teeth again, and you'll feel it tighten at your temple. Together, the masseter and temporalis give you a very powerful bite.

Muscle tour

Click a region to see its muscles and what they do.

Muscles and main actions

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Name clue

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0 of 6 explored

Five big ideas

Three muscles, one job

Make tension · built and controlled differently

A muscle is an organ

Wrappings carry the pull, the vessels and the nerves

Filaments slide

Calcium switches it on; ATP powers and releases

Signal → calcium → pull

No ATP, no release: rigor mortis

Grading and directing force

Motor units, firing rate, side of the joint

FRAG 0: We've followed a single command from a motor neuron all the way to a moving bone. Before you reflect on it, here are five big ideas to take away. If these make sense, the details will hang together. FRAG 1: First: there are three kinds of muscle, and they all do the same basic job. They produce tension by pulling. Skeletal muscle is striated, voluntary and needs a nerve signal. Cardiac muscle is striated and branched, joined by intercalated discs, and starts each beat itself. Smooth muscle has no stripes, uses calmodulin instead of troponin, and holds steady tone in the walls of hollow organs. FRAG 2: Second: a skeletal muscle is an organ. Connective tissue wraps the whole muscle, each fascicle and each fibre, and runs on into the tendon, so every fibre's pull reaches the bone. The same layers carry the blood vessels that bring oxygen, and the nerve that brings the commands. FRAG 3: Third: contraction means filaments sliding. Inside each sarcomere, myosin heads pull the thin filaments towards the M line, the Z discs move closer, and the I bands and H zone narrow while the filaments themselves stay the same length. Calcium switches the process on, and ATP powers each stroke and lets each head go. FRAG 4: Fourth: a nerve signal becomes a pull in a fixed sequence. The motor neuron's action potential releases acetylcholine at the junction, the fibre fires its own action potential, which runs down the T tubules and releases calcium, and calcium uncovers actin. And that's the answer to the question at the start. After death, calcium leaks out, but there's no ATP, so the myosin heads bind and can't let go, and the muscles lock until their proteins break down. FRAG 5: And fifth: force and direction are both controlled. A muscle pulls harder by recruiting more motor units and firing them faster, and it can shorten, hold or lengthen against a load. Which way it moves a bone depends on which joint it crosses, on which side, and along which line of pull.

Reflect

1. Pick one movement you made today, such as reaching for a cup. Trace it from the decision in your brain to the cross-bridges in your muscle, and back to relaxation.

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2. Choose one idea from this lecture that will matter in your future profession, and explain how.

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Your notes

HUMB1001 · Module 3 · How Do We Move?

Five big ideas

Three muscles, one job — Make tension · built and controlled differently
A muscle is an organ — Wrappings carry the pull, the vessels and the nerves
Filaments slide — Calcium switches it on; ATP powers and releases
Signal → calcium → pull — No ATP, no release: rigor mortis
Grading and directing force — Motor units, firing rate, side of the joint

My reflections

Pick one movement you made today, such as reaching for a cup. Trace it from the decision in your brain to the cross-bridges in your muscle, and back to relaxation.

Choose one idea from this lecture that will matter in your future profession, and explain how.

Before your lab this week

Seeley chapters 9 and 10
Pre-lab “Muscles”; Revise Terms of Movement sheet
Print lab notes: Part 1 and Part 2
Lab coat, safety glasses, closed-in shoes; long hair tied back
Bring your anatomy authorisation sticker
Wet specimens: handle with care, point with a probe
After the lab: Module 3 revision quizzes and FeedForward questions

FRAG 0: Here's your notes page. It brings together the five big ideas and your own reflections, and you can print it or save it as a PDF. Before your lab this week, read chapters nine and ten of your textbook. Complete the pre-lab activity on muscles, and work through the sheet revising the terms of movement. Print both parts of your lab notes: the muscles of the body, and contraction at the cellular level. Bring your lab coat, safety glasses and closed-in shoes, tie long hair back, and make sure you have your anatomy authorisation sticker. This lab uses many wet muscle specimens, so handle them gently, and never point at them with a pen. Afterwards, the revision quizzes and FeedForward questions are a good way to check your understanding. They're not marked, so use them as often as you like. That's the end of Module 3. Next week, in Module 4, we'll look at how cardiac function is controlled, and you'll see cardiac muscle again, this time at work in the heart. See you in the lab.