Question

Describe in detail as great a detail as possible the mechanism by which skeletal muscle cells contract. Start with

an action potential arriving at an axon terminal of a motor neuron and end with the completion of a single cross-

bridge cycle. Be sure you describe all the events and how the operate, and name all the structures, ions, proteins,

molecules involved and explain the role each has. Use proper terminology and explain what is happening at

each step. Then explain what has to occur in order for a muscle cell to relax. This will require a full page.

Answer 1

Action potential arriving at axon terminal of a motor neuron cause the release of neurotransmitter Acetylcholine(Ach)into the Synapse of neuromuscular Junction.This acetyl choline binds to the nicotinic acetyl choilne receptor in the muscle fibre.The nicotinic acetylcholine receptor, a ligand-gated cation channel, admits both K+ and Na+.The acetylcholine causes opening of a cation channel in the receptor capable of transmitting 15,000 – 30,000 Na+ or K+ ions a millisecond.

Since the resting potential of the muscle plasma membrane is near EK, the potassium equilibrium potential, opening of acetylcholine receptor channels causes little increase in the efflux of K+ ions; Na+ ions, on the other hand, flow into the muscle cell. The simultaneous increase in permeability to Na+ and K+ ions produces a net depolarization to about −15 mV from the muscle resting potential of −85 to −90 mV. This depolarization of the muscle membrane generates an action potential, which — like an action potential in a neuron — is conducted along the membrane surface via voltage-gated Na+ channels ,

When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction and the sliding filament theory.

The reaction, created from the arrival of an impulse stimulates the 'heads' on the myosin filament to reach forward, attach to the actin filament and pull actin towards the centre of the sarcomere. This process occurs simultaneously in all sarcomeres, the end process of which is the shortening of all sarcomeres.

Troponin is a complex of three proteins that are integral to muscle contraction. Troponin is attached to the protein tropomyosin within the actin filaments, When the muscle is relaxed tropomyosin blocks the attachment sites for the myosin cross bridges (heads), thus preventing contraction.

When the muscle is stimulated to contract by the nerve impulse, calcium channels open in the sarcoplasmic reticulum (which is effectively a storage house for calcium within the muscle) and release calcium into the sarcoplasm (fluid within the muscle cell). Some of this calcium attaches to troponin which causes a change in the muscle cell that moves tropomyosin out of the way so the cross bridges can attach and produce muscle contraction.

In summary the sliding filament theory of muscle contraction can be broken down into four distinct stages, these are;

1. Muscle activation: The motor nerve stimulates an action potential (impulse) to pass down a neuron to the neuromuscular junction. This stimulates the sarcoplasmic reticulum to release calcium into the muscle cell.

2. Muscle contraction:  Calcium floods into the muscle cell binding with troponin allowing actin and myosin to bind. The actin and myosin cross bridges bind and contract using ATP as energy (ATP is an energy compound that all cells use to fuel their activity – this is discussed in greater detail in the energy system folder here at ptdirect).

3. Recharging:  ATP is re-synthesised (re-manufactured) allowing actin and myosin to maintain their strong binding state

4. Relaxation:  Relaxation occurs when stimulation of the nerve stops. Calcium is then pumped back into the sarcoplasmic reticulum breaking the link between actin and myosin. Actin and myosin return to their unbound state causing the muscle to relax. Alternatively relaxation (failure) will also occur when ATP is no longer available.

In order for a skeletal muscle contraction to occur;

1. There must be a neural stimulus
2. There must be calcium in the muscle cells
3. ATP must be available for energy.

So, a few things can stop a contraction;

1. Energy system fatigue: There is no more ATP left in the muscle cell so it can’t keep contracting.

2. Nervous system fatigue: The nervous system is not able to create impulses sufficiently or quickly enough to maintain the stimulus and cause calcium to release.

3. Voluntary nervous system control: The nerve that tells the muscle to contract stops sending that signal because the brain tells it to, so no more calcium ions will enter the muscle cell and the contraction stops.

4. Sensory nervous system information: For example, a sensory neuron (nerves that detect stimuli like pain or how heavy something is) provides feedback to the brain indicating that a muscle is injured while you are trying to lift a heavy weight and consequently the impulse to that muscle telling it to contract is stopped.