Excitation

Many smooth muscles show a great deal of spontaneous activity. This is particularly so in intestinal muscles, where the spontaneous contractions serve to mix and move the gut contents. The electrical activity consists of slow waves of variable amplitude and all-or-nothing action potentials (Fig. 11.7). The fibres are depolarized and the frequency of action potentials increases if the muscle is stretched. The spontaneous activity can be modified by the action of extrinsic nerves, by adrenaline, and in uterine muscle by the action of hormones of the reproductive cycle. The smooth muscles of the iris, nictitating membrane and vas deferens are not spontaneously active.

The action potentials last for several milliseconds and are thus much longer than those of nerve axons and skeletal muscle cells. They are insensitive to tetrodotoxin and can often be produced in the absence of sodium ions, but are prevented by calcium channel blocking agents such as nifedipine. This suggests that calcium ions are the main carriers of inward current.

Action potentials can be initiated in sheets or strips of smooth muscle by electrical stimulation; they will then propagate along the axes of the muscle cells from one cell to another. There is also some slower propagation across the axes of the cells.

Only a proportion of the cells receive innervation from the nerves supplying the muscle. The electrical changes produced by nervous action on these cells spread to nearby cells by current flow from cell to cell, probably via the gap junctions by which they are connected. Stimulation of the nerves results in postsynaptic potentials of various types. Excitatory nerves produce depolarizing potentials whereas inhibitory nerves produce hyperpolarizing ones.

There are a number of transmitter substances involved. Acetylcholine, acting on muscarinic receptors, is an excitatory transmitter in much intestinal

ADP ATP

Fig. 11.8. Activation of smooth muscle actomyosin. Calcium ion concentrations in the cell rise as a result of two actions: (1) depolarization of the plasma membrane causes inflow of calcium ions through voltage-gated calcium channels, and (2) combination of a neurotransmitter or hormone with a 7TM receptor activates the phosphatidylinositol signalling system so that calcium ions are released from the sarcoplasmic reticulum via IP3 receptors. The calcium ions combine with calmodulin to activate myosin light chain kinase (MLCK), which then catalyses the phosphorylation of the myosin regulatory light chain MLC20. This leads to cross-bridge formation and movement, the splitting of ATP and the production of force. From Aidley (1998).

ADP ATP

Fig. 11.8. Activation of smooth muscle actomyosin. Calcium ion concentrations in the cell rise as a result of two actions: (1) depolarization of the plasma membrane causes inflow of calcium ions through voltage-gated calcium channels, and (2) combination of a neurotransmitter or hormone with a 7TM receptor activates the phosphatidylinositol signalling system so that calcium ions are released from the sarcoplasmic reticulum via IP3 receptors. The calcium ions combine with calmodulin to activate myosin light chain kinase (MLCK), which then catalyses the phosphorylation of the myosin regulatory light chain MLC20. This leads to cross-bridge formation and movement, the splitting of ATP and the production of force. From Aidley (1998).

muscle and in the iris. Noradrenaline is excitatory at some sites, as in the vas deferens, and inhibitory at others, as in intestinal muscle and the iris of the eye. Other neurotransmitters are active at various sites: candidates include ATP, nitric oxide, and various peptides such as substance P, vasointestinal peptide (VIP) and others.

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