Figure 1138

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Generation of action potentials in a smooth-muscle fiber resulting from spontaneous depolarizations of the membrane (pacemaker potentials).

of smooth-muscle fibers, it divides into numerous branches, each branch containing a series of swollen regions known as varicosities. Each varicosity contains numerous vesicles filled with neurotransmitter, some of which are released when an action potential passes the varicosity. Varicosities from a single axon may be located along several muscle fibers, and a single muscle fiber may be located near varicosities belonging to postganglionic fibers of both sympathetic and parasympathetic neurons (Figure 11-39). Therefore, a number of smooth-muscle fibers are influenced by the neurotransmitters released by a single nerve fiber, and a single smooth-muscle fiber may be influenced by neurotransmitters from more than one neuron.

Whereas some neurotransmitters enhance contractile activity, others produce a lessening of contractile activity. Thus, in contrast to skeletal muscle, which receives only excitatory input from its motor neurons, smooth-muscle tension can be either increased or decreased by neural activity.

Moreover, a given neurotransmitter may produce opposite effects in different smooth-muscle tissues. For example, norepinephrine, the neurotransmitter released from most postganglionic sympathetic neurons, enhances contraction of vascular smooth muscle. In contrast, the same neurotransmitter produces relaxation of intestinal smooth muscle. Thus, the type of response (excitatory or inhibitory) depends not on the chemical messenger per se but on the receptor to which the chemical messenger binds in the membrane.

In addition to receptors for neurotransmitters, smooth-muscle plasma membranes contain receptors for a variety of hormones. Binding of a hormone to its receptor may lead to either increased or decreased contractile activity.

Although most changes in smooth-muscle contractile activity induced by chemical messengers are accompanied by a change in membrane potential, this is not always the case. Second messengers, for example, inositol trisphosphate, can cause the release of calcium from the sarcoplasmic reticulum, producing a contraction, without a change in membrane potential.

Local Factors Local factors, including paracrine agents, acidity, oxygen concentration, osmolarity, and the ion composition of the extracellular fluid, can also alter smooth-muscle tension. Responses to local factors provide a means for altering smooth-muscle contraction in response to changes in the muscle's immediate internal environment, which can lead to regulation that is independent of long-distance signals from nerves and hormones.

Some smooth muscles respond by contracting when they are stretched. Stretching opens mechano-sensitive ion channels, leading to membrane depolarization. The resulting contraction opposes the forces acting to stretch the muscle.

On the other hand, some local factors induce smooth-muscle relaxation. Nitric oxide (NO) is one of

Postganglionic sympathetic neuron

9 Postganglionic parasympathetic

Postganglionic sympathetic neuron

9 Postganglionic parasympathetic

Smooth Muscle Innervation Varicosities

FIGURE 11-39

Innervation of smooth muscle by postganglionic autonomic neurons. Neurotransmitter is released from the varicosities along the branched axons and diffuses to receptors on muscle-fiber plasma membranes.

FIGURE 11-39

Innervation of smooth muscle by postganglionic autonomic neurons. Neurotransmitter is released from the varicosities along the branched axons and diffuses to receptors on muscle-fiber plasma membranes.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems the most commonly encountered paracrine agents that produces smooth-muscle relaxation. NO is released from some nerve terminals as well as a variety of epithelial and endothelial cells. Because of the short life span of this reactive molecule, it acts as a paracrine agent, influencing only those cells that are very near its release site.

It is well to remember that seldom is a single agent acting on a smooth muscle, but rather the state of contractile activity at any moment depends on the simultaneous magnitude of the signals promoting contraction versus those promoting relaxation.

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Responses

  • lachlan
    How can a neurotransmitter to produce opposite effects in different tissues?
    8 years ago
  • Harvey
    How can the same neurotransmitter have opposite effects at different tissues when sympathetic?
    8 years ago

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