Skeletal muscles are stimulated to contract by nerves and act as effectors

In a nutshell

Skeletal muscle is the effector that moves the skeleton. It contracts when it is stimulated, and because a muscle can only pull, muscles are arranged in antagonistic pairs.

This subtopic covers the structure of a muscle down to the sarcomere, how the sliding filament mechanism shortens it, the roles of actin, myosin, calcium ions, tropomyosin and ATP, how ATP is supplied (including phosphocreatine), and the two types of muscle fibre.

Assumed knowledge: Proteins, ATP, Respiration.

Core content

Muscles work in antagonistic pairs

A muscle can only exert a force by contracting (shortening). It cannot actively lengthen or push. To move a bone one way and then back, muscles are therefore arranged in antagonistic pairs: as one muscle contracts, the other relaxes, and to reverse the movement the two swap roles.

• The muscle that contracts to produce the movement is the agonist; the one that relaxes is the antagonist.
• The muscles pull against an incompressible skeleton (the bones do not squash), so the force is transmitted to move the bone at a joint.

For example, at the elbow the biceps contracts to bend (flex) the arm while the triceps relaxes. To straighten the arm the triceps contracts while the biceps relaxes.

An antagonistic pair can also both partly contract at the same time to hold a joint steady at a fixed angle, for example to keep the body upright.

Gross and microscopic structure of skeletal muscle

Skeletal muscle is built in a hierarchy: a whole muscle is made of many muscle fibres, and each fibre contains many myofibrils.

A muscle fibre is a single, specialised muscle cell:

Ultrastructure of a myofibril: the sarcomere

A myofibril is made of repeating units called sarcomeres. One sarcomere runs from one Z-line to the next, and it is the functional (contractile) unit of the muscle.

Each sarcomere holds two kinds of protein filament:

• thick filaments made of myosin, which have projecting heads;
• thin filaments made of actin, which carry the myosin binding sites.

Their overlap produces the striped (striated) banding seen under the microscope:

A quick way to remember it: the dark A-band is the length of the myosin, and the light I-band is the region where there is only actin.

The sliding filament mechanism

When a muscle contracts the filaments do not get shorter. Instead the actin and myosin filaments slide past one another, so each sarcomere shortens, and the whole muscle shortens with it.

What happens to one sarcomere as it contracts:

Still don't get it? · why the A-band stays the same length

Picture two hair combs pushed teeth-first towards each other. Each comb keeps its own length however far you push them together. What changes is how much the teeth overlap, and how big the gap left in the middle is.

Now the muscle version, one step at a time. The A-band is defined as the region taken up by the myosin (thick) filaments. Myosin filaments never change length, so the A-band never changes length. When the sarcomere shortens, it is the actin (thin) filaments that slide inwards: the H-zone (the myosin-only gap in the middle) closes up as actin moves in, the I-band (the actin-only region at the edges) shrinks as actin is pulled towards the centre, and the Z-lines are pulled closer.

So the exam answer is: the A-band stays the same because it is the length of the myosin filament, while the I-band and H-zone shorten and the Z-lines move closer together. Writing that the A-band widens or shortens loses the mark.

The actinomyosin bridge cycle

In a relaxed muscle, tropomyosin (a protein wound along the actin) covers the myosin binding sites on the actin, so the myosin heads cannot attach.

When the muscle is stimulated, the wave of depolarisation triggers the sarcoplasmic reticulum to release calcium ions (Ca²⁺) into the sarcoplasm. The cycle then runs:

1. Calcium ions cause tropomyosin to move, uncovering the binding sites on the actin.
2. Myosin heads bind to the exposed sites, forming actinomyosin cross-bridges.
3. Each myosin head bends, pulling the actin filament towards the centre of the sarcomere (the power stroke). ADP and P_i are released.
4. A new ATP molecule binds to the myosin head, making it detach from the actin.
5. Calcium ions also activate the enzyme ATPase on the myosin head, which hydrolyses the ATP. The energy released moves the head back to its upright position, ready to bind again further along the actin.

The cycle repeats many times per second as long as calcium ions and ATP are present, and many heads pulling together produce a large force.

When stimulation stops, the calcium ions are actively transported back into the sarcoplasmic reticulum (using ATP). Tropomyosin then blocks the binding sites again, and the muscle relaxes.

The roles at a glance:

On the specification, the roles of calcium ions and tropomyosin are what is required. The role of troponin is not required by AQA, so you can give a full answer without it (you may see it credited as an alternative, but you do not need to learn it).

Still don't get it? · the actinomyosin cross-bridge cycle

Imagine a heavy rope (the actin) being hauled in by a team of hands (the myosin heads). Each hand grabs the rope, pulls it in a little, lets go, reaches forward, and grabs again further along. No single hand pulls far, but hundreds of them grabbing and re-grabbing haul the rope a long way. Crucially, a hand has to let go before it can reach forward again.

Building it up one step at a time: (1) a myosin head grabs the actin, forming a cross-bridge; (2) it pulls, the power stroke, sliding the actin along; (3) to reach forward again it must first let go, and it can only let go when a new ATP binds to it, so ATP is what breaks the grip; (4) hydrolysing that ATP (by ATPase) is what cocks the head back, ready to grab further along. That is why ATP is needed twice in the cycle: once (by binding) to detach the head, and again (by hydrolysis) to re-set it. The calcium ions are the switch that starts the whole thing, by moving tropomyosin off the binding sites.

So in the exam: ATP binds to the myosin head, causing it to detach from actin; hydrolysis of ATP by ATPase provides the energy to move the myosin head back; the head then re-attaches further along the actin and the cycle repeats while calcium ions and ATP are present.

The roles of ATP and phosphocreatine

Every energy-requiring step of contraction runs on ATP:

• ATP binding causes the myosin head to detach from the actin (breaking the cross-bridge);
• hydrolysis of ATP (by ATPase) provides the energy to move the myosin head back for the next power stroke;
• ATP is also used to actively transport calcium ions back into the sarcoplasmic reticulum during relaxation.

Muscle stores only enough ATP for a couple of seconds of contraction, so it must be regenerated continuously. One very fast route is the phosphocreatine (PCr) system:

• Phosphocreatine is a store of high-energy phosphate held in the muscle.
• It regenerates ATP almost instantly by transferring its phosphate group to ADP: ADP + phosphocreatine → ATP + creatine.
• It is anaerobic (no oxygen is needed) and extremely fast, so it supplies ATP for the first few seconds of intense activity, such as the start of a sprint.
• The store is small and runs out within seconds. It is rebuilt during recovery using ATP made by aerobic respiration.

ATP is also regenerated by aerobic respiration in the mitochondria and by anaerobic respiration (glycolysis), which you meet in Respiration.

Slow and fast skeletal muscle fibres

Skeletal muscle contains two fibre types, suited to different kinds of activity.

• Slow-twitch (type 1) fibres contract slowly and less powerfully, but resist fatigue, so they suit endurance activity such as maintaining posture and long-distance running. They are adapted for aerobic respiration and are found in postural muscles such as those of the calf and back.
• Fast-twitch (type 2) fibres contract rapidly and powerfully but fatigue quickly, so they suit short, intense bursts such as sprinting and weightlifting. They rely on anaerobic respiration and phosphocreatine, and are found in muscles used for rapid movement such as the biceps and the muscles that move the eye.

Worked examples

Model 5-mark answer, the roles of calcium ions, tropomyosin and ATP in the contraction of a myofibril.

A good answer gives distinct, linked points in order. Six creditable points are written here so that any five score:

1. Calcium ions cause tropomyosin to move on the actin filament.
2. This uncovers the binding sites on the actin.
3. Myosin heads bind to the actin, forming actinomyosin cross-bridges.
4. The myosin heads bend, pulling the actin filaments towards the centre of the sarcomere (the power stroke).
5. ATP binds to the myosin head, causing it to detach from the actin.
6. Calcium ions activate ATPase, which hydrolyses ATP; the energy released moves the head back to re-attach further along.

The two points students most often drop are number 5 (ATP is needed to detach the head, not only to form the bridge) and saying the binding site is on the actin.

Calculation, sarcomere length and number from a micrograph.

A sarcomere is measured as 48 mm long on a photomicrograph taken at a magnification of ×20 000. Find the actual length of one sarcomere, then the number of sarcomeres in a myofibril 12 mm long.

Rearrange the magnification equation to get the actual size:

$\text{magnification} = \frac{\text{image size}}{\text{actual size}} \quad\Rightarrow\quad \text{actual size} = \frac{\text{image size}}{\text{magnification}}$

\text{actual length} = \frac{48\ \text{mm}}{20\,000} = 0.0024\ \text{mm} = 0.0024 \times 1000 = 2.4\ \text{µm}

Now put both lengths in the same unit before dividing. The myofibril is 12 mm = 12 000 µm long, so:

\text{number of sarcomeres} = \frac{12\,000\ \text{µm}}{2.4\ \text{µm}} = 5000

The step that most often goes wrong is the unit conversion: mixing millimetres and micrometres leaves the answer several powers of ten out. Convert everything to micrometres first, then divide.

Common exam mistakes

• Writing that the myosin head binds to the active site on actin. It is a binding site; "active site" is rejected.
• Saying the actin and myosin filaments shorten. They do not change length; they slide past one another, and it is the sarcomere that shortens.
• Claiming the A-band widens or shortens during contraction. The A-band stays the same (it is the length of the myosin); only the I-band and H-zone shorten and the Z-lines move closer.
• Describing the binding sites without saying they are on the actin, or leaving actin out of the answer altogether.
• Giving ATP a role only in forming the cross-bridge. ATP is needed to detach the myosin head (its binding breaks the cross-bridge) and, when hydrolysed, to move the head back for the next stroke.
• Saying ATP or respiration produces energy, or that ATPase acts directly on the myosin. Energy is released by hydrolysing ATP, and ATPase hydrolyses ATP; ATP does not "attract" or "pull" myosin to actin.
• Telling the wrong story: describing calcium ions in synaptic transmission (vesicles and neurotransmitter) instead of their role inside the sarcomere. Keep calcium ions on tropomyosin.
• Calling the phosphocreatine system "anaerobic respiration". It is a separate, direct transfer of a phosphate to ADP, not respiration. It phosphorylates ADP / provides a phosphate, so write phosphate, never "phosphorus".
• Comparing fibres without a comparison, or using the wrong word for size. Fast fibres have a wider diameter (not "thicker" or "bigger"), and you must say slow fibres have more mitochondria and more capillaries than fast fibres, not just "has mitochondria".

Key definitions

• Sarcomere: the region of a myofibril from one Z-line to the next; the functional (contractile) unit of the muscle.
• Actinomyosin cross-bridge: the attachment formed when a myosin head binds to a binding site on the actin filament.
• Sliding filament mechanism: contraction in which the actin and myosin filaments slide past one another to shorten the sarcomere, while the filaments themselves do not change length.
• Tropomyosin: a protein that covers the myosin binding sites on the actin filament in a relaxed muscle.
• Antagonistic pair: a pair of muscles that pull in opposite directions, so that one contracts while the other relaxes.
• Phosphocreatine: a store of phosphate in muscle that rapidly regenerates ATP by transferring its phosphate group to ADP.
• Myoglobin: a red, oxygen-storing pigment found in muscle fibres.
• Slow-twitch fibre: a muscle fibre adapted for aerobic respiration that contracts slowly and is resistant to fatigue.
• Fast-twitch fibre: a muscle fibre adapted for anaerobic respiration that contracts rapidly and powerfully but fatigues quickly.

Specification

• I can explain why skeletal muscles act in antagonistic pairs against an incompressible skeleton.
• I can describe the gross and microscopic structure of skeletal muscle and the ultrastructure of a myofibril (the sarcomere, and the A-band, I-band, H-zone, Z-line and M-line).
• I can explain the roles of actin, myosin, calcium ions and ATP in the contraction of a myofibril.
• I can explain the roles of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation.
• I can state how the sarcomere bands change when a muscle contracts.
• I can explain the roles of ATP and phosphocreatine in muscle contraction.
• I can compare the structure, location and general properties of slow and fast skeletal muscle fibres.

Related notes

• Proteins
• ATP
• Respiration
• Nervous coordination
• Homeostasis