What is neuromuscular junction?
Neuromuscular junction can be defined as a synapse the motor neuron and the muscle fiber. Actually the synapse occurs between the “synaptic end bulbs” of the motor neuron and “motor end plate” of muscle fiber. The motor end plate is in fact the highly excitable region of muscle fiber plasma membrane and it is responsible for initiating action potentials across the muscle surface. This effect ultimately results in muscle contraction.
Anatomy of neuromuscular junction:
The neuromuscular junction is composed of three parts:
Synaptic end bulbs:
As the axon of the motor neuron enters the skeletal muscle, it forms many branches called axon terminals. At the end of each axon terminal, there is a bulbous swelling called “synaptic end bulb”. Each synaptic end bulb contains many synaptic vesicles. These vesicles contain the all-important neurotransmitter substances such as acetylcholine. These neurotransmitter substances are responsible for transmission of impulse from axon to muscle fiber through the synapse.
Motor end plate:
It is the part of the sarcolemma of muscle cell, which is in closest proximity to the synaptic end bulb. It shows certain specific features different than those of other regions of muscle cell sarcolemma, including:
Synaptic Gutter: It is the invaginated membrane, which forms space for the synaptic end bulbs to reach close to the muscle fiber sarcolemma.
Subneural Clefts: These are small folds of the muscle membrane present at the bottom of the synaptic gutter. They greatly increase the surface area at which the neurotransmitter can act.
Increased number of mitochondria: The area of the muscle fiber surrounding the motor end plate shows a considerable increase in the number of mitochondria. The obvious reason for this is the energy demand of the neuromuscular junction.
The Synaptic cleft:
Synaptic cleft is the space between the motor end plate (muscle fiber part) and synaptic end bulb (motor neuron part) of the neuromuscular junction. It is 20 to 30 nanometers wide. Because of this cleft, the connection between the motor neuron and the muscle fiber is not continuous and there is a break. This break is traversed by the neurotransmitters. This shows the importance of these substances in the activity of muscles (and all other nervous control mechanisms).
Difference between synapse and neuromuscular junction:
Synapses and neuromuscular junctions are physiologically the same, however, the neuromuscular junction is a specific type of synapse that occurs between motor neuron and muscle fiber. At other places in the body, such as the Central Nervous System, the structure with similar role is known as synapse.