Question

How do muscle cells move in response to nerve impulses?

Answer 1

Hint: The neuron is the structural and functional unit of the brain.The neuron that innervates muscles and transmits the signals to the muscle fibre for initiating the muscle contraction is called the motor neuron.

Complete answer:
Muscle cells move in response to nerve impulses. This is with the help of a motor unit.
A motor unit consists of a single motor neuron and the muscle fibres it innervates. A muscle fibre is under the control of a single neuron.
Thus there are many motor units in a muscle that result in contraction of the entire muscle. These motor units together form a motor pool.

Motor unit: A motor unit is classified into two types depending on the number of cells in it.
1. Large motor units (number of cells more than 100)
2. Small motor units (number of cells less than 10)

Neuromuscular junction: The site where the motor neuron meets the muscle fibre is known as Neuromuscular junction. It is Separated by a gap called neuromuscular cleft.

The process of the impulse transmission takes place in following steps :
- The signal is sent by CNS to the motor neuron.
- The motor neuron innervates the sarcolemma of the muscle fibre (this is the plasma membrane). The neuron forms a pocket and is known as a motor end plate.
- The end of the motor neuron releases acetylcholine and causes End-plate formation which depolarises the motor fibre (Action potential is generated). The action potential then spreads from sarcolemma to the T tubules.
- T tubules are perpendicular to the sarcolemma. These are extensions of cell membrane and penetrate the entire muscle. The impulse from T tubules then penetrates the SR i.e.Sarcoplasmic reticulum (this is endoplasmic reticulum). The SR then releases Calcium ions into the sarcoplasm
- The calcium thus reaches the myofibrils and the sarcomere (the main unit of contraction)
Sarcomere consists of dark and light bands. Calcium binds with the Troponin protein of light band.
The binding causes structural modification in light binds and the myosin binding site of Actin is activated.
Thus cross bridge formation takes place that results in contraction of the muscle

Note: Sarcoplasmic reticulum stores calcium ions. On excitation, the calcium channels open and releases calcium into the sarcoplasm. The mechanism of muscle contraction is explained by Sliding filament theory.