Question

Identify and describe the parts of the neuromuscular junction (synapse): synaptic cleft, synaptic knob (also called axon terminal), synaptic vesicles,

Answer 1

Neuromuscular transmission is the transmission of an impulse from the motor neuron to the skeletal muscle fiber supplied by the neuron.

Structure of neuromuscular junction:

Neuromuscular junction is divided into three portions: presynaptic, synaptic, and postsynaptic portions.

Presynaptic portion (Axon Terminal):

The neurons innovating skeletal muscle are known as motor fibers. Their axons are myelinated. The axon of the motor neuron approaches the muscle fiber, it loses its myelin sheath and divides into several fine branches called axon terminals.

Each terminal is expanded at its end to form a knobby structure, called the synaptic knob (terminal button). It contains plenty of mitochondria and neurotransmitter vesicles. The terminal button lies in the groove (synaptic trough) in the surface of the muscle fiber, but outside the muscle cell membrane.

Synaptic vesicles:

The cytoplasm present inside the axons is called the axoplasm. The synaptic knobs possess the axoplasm. In the axoplasm present inside the synaptic knob, tiny, numerous, rounded, sac-like structures called the synaptic vesicles are present. Each synaptic vesicle contains neurotransmitters.

Synaptic cleft:

This is the gap between the terminal button and the muscle fiber. The arrival of action potential to the acts on terminal results in the fusion of the membrane of the synaptic vesicle plasma membrane of the synaptic knob. This causes the rupture of the synaptic vesicles and the release of neurotransmitters in the synaptic cleft.

Postsynaptic portion:

It is known as the motor endplate. The neurotransmitters present in the synaptic cleft binds to the specific receptors present on the postsynaptic membrane resulting in the opening of ion channels. The center of ion generates new potential in the postsynaptic neuron.