Question

What action causes a muscle to contract?

Answer 1

Muscle contracts when a motor message is delivered to it by efferent nerve. Axonic terminal of neuron forms a motor end plate on the surface of muscle fibre that it innervates.

Explanation:

A single motor neuron may send terminals to a number of muscle fibres. Axonic terminal at neuro-muscular junction (motor end plate) releases neurohumor substances. Such molecules get attached to membrane receptors present on muscle fibre. This causes depolarisation on muscle fibre.

Depolarisation opens Na channels, i.e. Na flows from extracellular fluid into the cells (muscle fibres). That causes a series of changes inside the muscle fibre to achieve contraction: most important of which is release of calcium ions from endoplasmic reticulum in the cytoplasm .