Question

Topic 6: Skeletal muscle contraction

Describe in detail the entire process of excitation coupling in skeletal muscle, begin with an action potential reaching the synaptic knob of a motor neuron through completion of a cross Bridge cycle. after detailing those events to include the following information

• Define a motor unit describe motor unit recruitment and Explain how these terms relate to excitation-contraction coupling.

• Define and describe muscle twitch and summation and explain how these terms relate to motor unit/motor unit Recruitment and excitation-contraction coupling.

Answer 1

Excitation-Contraction Coupling:

The contraction of of skeletal muscles starts with the propagation of action potential through the synaptic knob of the axon of a motor neuron. This causes release of acetylcholine due to calcium induced exocytosis which generates an action potential in the sarcolemma.

Depolarization of sarcolemma spreads inwards along T tubules into the muscle. This depolarization causes release of Calcium from terminal cisternae increasing sarcoplasmic concentration of calcium. Binding of this calcium to troponin C uncover the myosin binding sites on actin.

This results in the attachment of myosin head to actin initiating a crossbridge formation. This causes sliding of thin filaments over thick filaments towards the centre of the sarcomere. This sliding causes a a decrease in sarcomere length producing muscle shortening thereby resulting in muscle contraction.

Motor unit:

It consists of a single motor neuron, it's axonal branches and all the muscle fibres supplied by them.

Motor unit recruitment:

The process of activating more number of motor units is called motor unit recruitment. When one motor neuron discharges or excitation of one motor neuron causes contraction of all muscle fibres supplied by it.

If a greater tension has to be attained, additional motor neurons also fire along with the concerned motor neuron, resulting in the activation of more number of motor units by recruitment.

Muscle twitch:

A spontaneous contraction without any stimulus for a brief time period is called a muscle twitch, which may occur due to fatigue, altered electrolyte levels, adverse effects of certain medications etc.

Summation:

Summation occurs when a second twitch is produced during the contraction phase of the first twitch, then the contraction phase of second twitch is added to the contraction phase of first twitch.

A twitch is produced when a motor neuron discharges and there will be excitation of motor neuron which results in contraction of all the muscle fibres supplied by it.

Summation results in increased generation of tension than a single twitch, thus shortening the stimulation interval. There will be repeated stimulation of a motor unit, thus resulting in stimulation of all contractions.