Skeletal Muscle Physiology

Question 1

What sequence does muscle contraction follow?

Answer 1

1. Activation: excitation
2. Excitation-contraction coupling
3. Molecular interaction at filaments
4. Sarcomere shortening
5. Tension development

Question 2

Plugging in

Answer 2

• The point of control for muscle fibres is a synapse – an axon terminal resting on infolded sarcolemma
• Near the muscle, the motor nerve branches out
• Each branch ends in a terminal swelling (bouton), the presynaptic part of a neuromuscular junction (NMJ).

Question 3

What are the three parts of the neuromuscular junction?

Answer 3

• Presynaptic terminal: axon terminal with synaptic vesicles.
• Synaptic cleft: fluid space through which transmitter moves.
• Postsynaptic membrane: or motor end-plate

Question 4

What is the process of neuromuscular transmission?

Answer 4

1. Nerve action potential – release of acetylcholine (Ach) into synaptic space.
2. Diffusion of Ach onto nicotinic receptors of the motor end plate (MEP)
3. Depolarisation (end-plate potential)
4. Action potential in the muscle.
5. Destruction of Ach by AChE; recycling of choline.

Question 5

How is excitation turned into contraction?

Answer 5

• Ap propagates along the sarcolemma, passing into the interior via transverse (T) tubules open to the surface.
• The sarcoplasmic reticulum (SR) consists of longitudinal sarcotubules with terminal cisternae close to T tubules.
• The SR, particularly the cisternae, stores Ca2+ ions bound to the protein calsequestrin.
• As an action potential propagates into the T-tubule, it activates voltage gated calcium channels.
• Opening of these channels releases Ca2+ into the cytosol.
• Free Ca2+ then binds to troponin complexes spaced at intervals along the thin filaments.
• This lefts tropomyosin away from the active sites of the actin fibres.

Question 6

What happens to actin binding sites at rest?

Answer 6

In a resting muscle, the actin binding site is covered by a protein called tropomyosin. This prevents contraction.

Question 7

Describe the process of cross-bridge cycling.

Answer 7

1. Actin sites exposed
2. Cross-bridges form
3. Binding of myosin heads flexes cross-bridges, pulling thin filament along thick
4. Fresh ATP binds to myosin heads
5. Cross-bridges release
6. Myosin heads return to resting position
7. Steps 2-6 repeat while Ca2+, ATP are present.

Question 8

The power stroke

Answer 8

• 10nm movement of each myosin head = 1nmm length change if 100 000 sarcomeres are end to end

Question 9

What bands shorten and which stay the same during sarcomere contraction?

Answer 9

• This narrows the H zone and the I band; the A band )determined by the length of the thick filemants) is not changed
• Like hairbrushes

Question 10

What are the phases of a muscle twitch?

Answer 10

1. Latent period before external tension appears.
2. Contraction time as the muscle reaches maximum twitch tension.
3. Relaxation time during which tension returns to baseline levels.

Question 11

Latent period

Answer 11

• Excitation of motor neuron
• Neuromuscular transmission
• Excitation-contraction coupling
• Cross-bridge cycling

Question 12

Contraction period

Answer 12

• Sarcomeres pull on series elastic components

* Muscle shortening

Question 13

Relaxation period

Answer 13

• Re-uptake of Ca2+ into sarcoplasmic reticulum
• Cessation of cross-bridge cycling
• Lengthening of muscle, depending on tension etc.

Question 14

Internal and external tensions

Answer 14

• Cross bridge activity creates internal tension in the muscle
• Before the muscle can perform work, sarcomeres must take up any slack in series elastic components to generate external tension
• Like lifting a brick using a rubber band to pull it.

Question 15

Passive and active tensions

Answer 15

• As well as active tension from contraction, whole muscle also has passive tension: the stretching of series elastic components when muscles are pulled longer than their slack length (by configuration of joints, gravity, etc.)
• Active plus passive tension given total tension.

Question 16

What is the relationship between sarcomere length and active tension?

Answer 16

• The amount of active tension that can be generated is also influenced by muscle length
• Maximum when filaments can interact optimally (A)
• Overstretching (B, C) or compression (D) reduce the active tension (fewer bridges)
• Resting muscle length is usually around optimal.
• Most muscles operate at optimal length +/- 30%

Question 17

Control of active tension

Answer 17

• Tension produced by an active muscle is affected, or controlled, by changing:
o The length of the muscle prior to contraction (initial length)
o The number of active motor units
o The frequency of action potentials activating the motor units
• Every active movement we make uses a combination of these control mechanisms.

Question 18

Motor unit

Answer 18

• A motor unit is a single alpha motor neuron (alphaMN) and the muscle fibres it innervates. Fire the alphaMN, all fibres contract,
• The number of muscle fibres in a motor unit varies greatly.
• Innervation ratio )average muscle fibres per neuron) naries from 1:3 (eye muscles) to 1:700 (major limb muscles).

Question 19

Motor unit recruitment

Answer 19

• Each muscle fibre receives an input from only one alphaMN
• Motor unit activation thresholds vary depending on alphaMN sensitivity
• Small control stimuli excite only the most sensitive alphaMNs, so only those motor units contract,
• AS the stimulus strengthens, more units activate; tension increases
• This is called recruitment
• When all units have been activated, maximum twitch tension is reached.

Question 20

Order of motor unit recruitment

Answer 20

• In large muscles with many motor units, the smaller units tend to be recruited earliest, whereas larger motor units are recruited later (size principle).
• Motor units comprising slower muscle fibre types also appear to be recruited ahead of those with faster fibre types.