Smooth muscle physiology

 

Smooth muscles are innervated by the autonomic nervous system since they are involuntary and therefore autonomous. This system is divided into parasympathetic and sympathetic divisions. Neurotransmitters are agents released at synapses and at myoneural junctions which depolarize postsynaptic membrane and and therefore extends impulses across synaptic clefts to the said membrane. Sympathetic neurotransmitter is commonly catecolamines and hence the system is called adrenergic. Parasympathetic neurotransmitter is mainly acetylcholine and hence the system is called cholinergic. Sometimes the system mediates contraction, and yet at other time, relaxation. The simple rule is this- the smooth muscles of the gut and genitourinary system under the parasympathetic system causes contraction, while sympathetic system causes relaxation. This is the same in the respiratory system and the iris. But in skeletal muscle blood vessels, sympathetic fibers cause vasoconstriction, while parasympathetic fibers cause vasodilatation.

There are two different types of nerve supply to the myoneural junction of smooth muscle. They are the multiple unit and the single unit methods.

In multiple units there is establishment of contact of a neuron to several myocytes. In this method of contact only nerve impulse can cause contraction in the smooth muscle. The method is the mode of contact in iris, ductus deferens and in some large arteries.

In single unit method the nerve endings are scanty and the muscle relies on its own rhythmicity to go on contracting without any impulse arriving from the nervous system or nerve. The muscle can also be stimulate to contract by stretching and the nervous impulse only serve to either increase or decrease rate of its inherent rhythmicity contraction. This method is the method of choice in the uterus (as in parturition), and intestine (peristalsis).

Contraction of smooth muscle

This is performed by actin and myosin filaments anchoring to the cell membrane at the edges and shortening to reduce the size of the entire cytoplasm. This leads to oval shape as opposed to the relaxed spindle shaped cell. The myofilaments in smooth muscle is very different from those of skeletal muscles. Firstly the actin and myosin does not bind to myosin as in skeletal muscle. Binding occurs only to phosphorylated myosin. Actin does not have troponin in smooth muscle. ATP is needed in large amount for skeletal muscle while it is required only in little amount in smooth muscle in keeping with its lesser activity.

Calcium is of considerable use in the contraction of smooth muscles and they enter the cell membrane which has calcium channels which may be blocked specifically by channel blocking drugs.

 

Non smooth muscle contractile cells

These include cells contain myofilament actin and myosin and are actively contractile. They are as follows

  • Myepithelial cells found in close proximity to gland contractile cells called myoepitheliocytes

  • Myofibroblasts

  • Pericytes

Histophysiology of receptors

There are α and β receptors on muscle cells

  • α1 stimulation causes  vasoconstriction in arterioles.

  • α2 agents which act on the central nervous system.

  • β1 stimulation causes positive  cardiac inotropic effect.

  • β2 stimulation causes bronchodilation

 

 

 

 

 

 

 

 

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