Answer
Smooth muscles can be classified into multi-unit and single unit smooth muscle based on innervation and contraction characteristics.
Mult-unit Smooth muscles
In multi-unit smooth muscles the supplying nerve fiber forms terminal branches which synapse on individual myofibers and form motor units. Varicosities are assigned to specific myocytes, and each myocyte responds (contracts) independently of all the others. Multi-unit myocytes do not generate action potentials (AP), instead, they respond to graded electrical changes in the sarcolemma. They may also contract in response to stretch, and hormones as well as ANS stimulation. Gap junctions are very few in this type of smooth muscle and myofibers contract separately. Organs and structures with this type of smooth muscle include arrector pili of dermis, pulmonary passages, walls of large arteries and iris of the eye.
Work Step by Step
Single unit smooth muscle is also called visceral muscle. It is found in walls of hallow visceral organs like the urinary bladder, the uterus, and the gastrointestinal organs. The myocytes of this kind of muscle are electrically coupled by gap junctions. This enables them to stimulate one another, and to contract as a unit. The nerve varicosities are not associated with individual myocytes, but stimulate several myocytes simultaneously when they release their neurotransmitter.
