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At the neuromuscular junction, presynaptic motor axons terminate 30 nanometers from the cell membrane or sarcolemma of a muscle fiber. The sarcolemma at the junction has invaginations called postjunctional folds , which increase its surface area facing the synaptic cleft. [ 4 ]
Visceral efferent neurons innervate smooth muscle, cardiac muscle, and glands, and have the ability to be either excitatory or inhibitory in function. Neuroeffector junctions are known as neuromuscular junctions when the target cell is a muscle fiber. Non-synaptic transmission is characteristic of autonomic neuroeffector junctions.
The neuromuscular junction is the synapse that is formed between an alpha motor neuron (α-MN) and the skeletal muscle fiber. In order for a muscle to contract, an action potential is first propagated down a nerve until it reaches the axon terminal of the motor neuron.
In biology, a motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. [1] Groups of motor units often work together as a motor pool to coordinate the contractions of a single muscle.
The neuromuscular junction diseases present within this subset are myasthenia gravis, and Lambert-Eaton syndrome.(reference 26) In each of these diseases, a receptor or other protein essential to normal function of the junction is targeted by antibodies in an autoimmune attack by the body. [citation needed]
MuSK (for Muscle-Specific Kinase) [1] is a receptor tyrosine kinase required for the formation and maintenance of the neuromuscular junction. [2] It is activated by a nerve-derived proteoglycan called agrin, [3] which is similarly also required for neuromuscular junction formation. [4]
It is an enzyme that catalyzes the breakdown of acetylcholine and some other choline esters that function as neurotransmitters: acetylcholine + H 2 O = choline + acetate. It is found at mainly neuromuscular junctions and in chemical synapses of the cholinergic type, where its activity serves to terminate cholinergic synaptic transmission.
Agrin functions by activating the MuSK protein (for Muscle-Specific Kinase), [10] which is a receptor tyrosine kinase required for the formation and maintenance of the neuromuscular junction. [11] Agrin is required to activate MuSK. [12] Agrin is also required for neuromuscular junction formation. [13]