VPB 121: Veterinary Physiology - II (2+1)

NEUROMUSCULAR JUNCTION

• The motor neuron branches at its end and each branch comes into a close opposition with the skeletal muscle at a specialised area called the neuromuscular junction or motor end plate. This synapse has a presynaptic membrane, a narrow space between the nerve and muscle, called synaptic cleft and a post synaptic membrane sarcolemma.
• The presynaptic knob is the terminal portion of the axon of the motor neuron; the extends from the CNS to the muscle cell. The axoplasm contains a large number of vesicles called synaptic vesicles; contain the excitatory neurotransmitter substance acetylcholine. These vesicles are in clustered located very close to the presynaptic membrane. Acetylcholine is synthesised in the cytoplasm of the nerve terminal and stored in the synaptic vesicles.
• The presynaptic terminal also contains many mitochondria, required for the synthesis of neurotransmitter substance.
• Synaptic cleft is a narrow space of 20 to 30nm wide separates the presynaptic membrane and postsynaptic sarcolemma membranes. The synaptic cleft is filled with extracellular fluid and spongy reticular filaments called basal lamina. The synaptic cleft contains an enzyme, acetylcholinesterase, to hydrolyse acetylcholine.

• The postsynaptic cell membrane has a series of invaginations called junctional folds that increase the surface area of the postsynaptic membrane. Acetylcholine receptors are located on the postsynaptic membrane at the entrance of these junctional folds.

Transmission of impulse across the neuromuscular junction When the action potential reaches the presynaptic terminal of the motor neuron, the depolarisation opens voltage-gated Ca⁺⁺ channels ; and extracellular Ca ⁺⁺ diffuses into the presynaptic nerve cytoplasm. The increased intracellular Ca⁺⁺ level causes the synaptic vesicles to fuse with the presynaptic membrane; the fused synaptic vesicle opens and releases the acetylcholine into the synaptic cleft by exocytosis.