Different types of smooth muscles::
Depending on the membrane properties smooth muscles are conveniently divided into three groups (1) spontaneously active smooth muscle or unitary smooth muscle (2) Electrically inexcitable or multi unit smooth muscle (3) intermediate
spontaneously active smooth muscle::
this type of muscle fibres contract rhythmically and the activity is co ordinated by smooth muscle cell (myogenic) the activity of these muscles depend on the spontaneous generation of action potentials and the presence of a conducting system (the gap junctions) two types of mechanism are responsible for the spontaneous generation of action potentials pacemaker potentials and slow waves
pacemaker potentials can be seen in uterus and taenia coli of the colon whereas slow waves are seen in rhythmic activity of stomach and intestine the spontaneous activity of these muscles can be increased or decreased by parasympathetic or sympathetic nerves respectively
Electrically inexcitable smooth muscles :: this type of muscle is found in bronchi trachea and some arterial smooth muscles these muscles do not generate action potentials each muscle fibre has own nerve fiber (multi unit type ) the membrane potential remains stable unless the tissue is stimulated stimulation may be the result of neurotransmitter release or the activity of local or blood borne agents (eg histamine and bradykinin ) the excitation spreads through gap junctions
Intermediate smooth muscle:: this category is widely distributed it is found in iris pilo erector muscle blood vessels vas deferens and seminal vesicles they have a stable resting membrane potential and when stimulated they exhibit spike like action potentials the cells are linked by gap junction and so the contraction fail to spread throughout the tissue the force of contraction is proportional to the frequency of action potentials and is usually under neural control
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