Skeletal Muscle Physiology Flashcards
What sequence does muscle contraction follow?
- Activation: excitation
- Excitation-contraction coupling
- Molecular interaction at filaments
- Sarcomere shortening
- Tension development
Plugging in
- 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).
What are the three parts of the neuromuscular junction?
- Presynaptic terminal: axon terminal with synaptic vesicles.
- Synaptic cleft: fluid space through which transmitter moves.
- Postsynaptic membrane: or motor end-plate
What is the process of neuromuscular transmission?
- Nerve action potential – release of acetylcholine (Ach) into synaptic space.
- Diffusion of Ach onto nicotinic receptors of the motor end plate (MEP)
- Depolarisation (end-plate potential)
- Action potential in the muscle.
- Destruction of Ach by AChE; recycling of choline.
How is excitation turned into contraction?
- 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.
What happens to actin binding sites at rest?
In a resting muscle, the actin binding site is covered by a protein called tropomyosin. This prevents contraction.
Describe the process of cross-bridge cycling.
- Actin sites exposed
- Cross-bridges form
- Binding of myosin heads flexes cross-bridges, pulling thin filament along thick
- Fresh ATP binds to myosin heads
- Cross-bridges release
- Myosin heads return to resting position
- Steps 2-6 repeat while Ca2+, ATP are present.
The power stroke
• 10nm movement of each myosin head = 1nmm length change if 100 000 sarcomeres are end to end
What bands shorten and which stay the same during sarcomere contraction?
- 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
What are the phases of a muscle twitch?
- Latent period before external tension appears.
- Contraction time as the muscle reaches maximum twitch tension.
- Relaxation time during which tension returns to baseline levels.
Latent period
- Excitation of motor neuron
- Neuromuscular transmission
- Excitation-contraction coupling
- Cross-bridge cycling
Contraction period
- Sarcomeres pull on series elastic components
* Muscle shortening
Relaxation period
- Re-uptake of Ca2+ into sarcoplasmic reticulum
- Cessation of cross-bridge cycling
- Lengthening of muscle, depending on tension etc.
Internal and external tensions
- 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.
Passive and active tensions
- 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.
What is the relationship between sarcomere length and active tension?
- 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%
Control of active tension
• 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.
Motor unit
- 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).
Motor unit recruitment
- 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.
Order of motor unit recruitment
- 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.
