The neuromuscular junction of smooth muscles is slightly different and very much simple as compared to neuromuscular junction of skeletal muscles.
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There are two basic types of smooth muscles, the multi unit smooth muscles and unitery smooth muscles.similarly there are two basic types of smooth muscle junctions. The smooth muscle junction of Multi unit smooth muscles is Known as Contact junction while the junction of UNITARY type smooth muscle is known as Diffuse junction.
In diffuse type of junction the nerve fibers form Varicosities just above a connective tissue sheet which is present above the smooth muscle fiber membrane, the neurotransmitters in Varicosities are secreted in sheet and they reach deep inside the smooth muscle fibers with diffusion and excite the lower layers of the muscle fibers.
In contact type of junctions , the Varicosities of nerve fibers secret the neurotransmitters just above the membrane fiber, rather than secreting it into the connective tissue sheet, so the distance is as low as a SYNAPTIC CLEFT of a skeletal muscle. Which leads to quick response and rapid contraction of multi unit smooth muscles.
The neurotransmitters in Varicosities of smooth muscles are of two main types, acetylcholine and norepinephrine, as compared to skeletal muscles where only acetylcholine was secreted. Whether acetylcholine or norepinephrine will cause excitaion of muscle fibers. Or inhibition of muscle fibers, depends upon the type of receptors in smooth muscles.
