Summary

Motor systems of the nervous system can be divided into two parts, based on the motor targets that are innervated. The somatic nervous system is responsible for the control of the skeletal musculature, and thus for most of what we normally think of as the behavior of the organism. The autonomic nervous system is responsible for controlling other important organ systems, involved in maintaining the internal homeostasis of the organism. The autonomic nervous system controls the cardiovascular system, the respiratory system, the digestive system, etc. The autonomic nervous system is organized differently from the somatic nervous system. The motor neurons of the autonomic nervous system are located outside the central nervous system, in autonomic ganglia. The somatic motor neurons, by contrast, are located within the spinal cord and are thus part of the central nervous system. The autonomic nervous system is divided into the parasympathetic and the sympathetic divisions. The parasympathetic autonomic ganglia are located close to or in the target organs themselves. The sympathetic ganglia are typically located close to the central nervous system, and most of them found in two chains of ganglia, called the paravertebral ganglia, that parallel the spinal column on each side of the spinal cord.

The nerve terminals of the parasympathetic postganglionic neurons release the neurotransmitter ACh in the target organ. Acetylcholine typically acts on the target cells by activating muscarinic cholinergic receptors, which exert their postsynaptic actions by altering the level of internal second messengers such as cyclic AMP in the postsynaptic cell. The nerve terminals of the sympathetic postganglionic neurons release the transmitter norepinephrine, which also exerts its postsynaptic effect by altering the levels of internal second messengers. In organs that receive both sympathetic and parasympathetic innervation, the actions of ACh and norepinephrine on the target cells are usually opposite. In the heart, for example, ACh decreases heart rate and reduces cardiac output, while norepinephrine increases heart rate and cardiac output.

The muscle fibers making up the heart are specialized in a number of ways to carry out their function of efficiently pumping blood through the vessels of the circulatory system. These specializations lead to a number of differences between cardiac muscle fibers and skeletal muscle fibers, which are summarized in Table 12-1. In addition, the heart as an organ contains specific structures whose function is to coordinate the pumping activity. These structures include the SA node, the AV node, and the Purkinje fibers. The SA node is the

Table 12-1 Comparison of some properties of skeletal and cardiac muscle fibers.

Property

Striated

Electrically coupled Spontaneously contract in absence of nerve input Duration of contraction controlled by duration of action potential Action potential is similar to that of neurons Calcium ions make an important contribution to the action potential Effect of neural input Division of nervous system that provides neural control

Neurotransmitter released onto muscle fibers by neurons

Effect of neurotransmitter on postsynaptic ion channels

Skeletal muscle Cardiac muscle

Yes Yes

No Yes

No Yes

No Yes

Yes No

No Yes

Excite Excite or inhibit Somatic Autonomic

(parasympathetic and sympathetic) ACh ACh (parasympathetic)

or Norepinephrine (sympathetic) Direct Indirect (via G-proteins)

master pacemaker region of the heart, which controls the heart rate during normal physiological functioning of the heart. The AV node provides a path for electrical conduction between the atria and the ventricles and is responsible for the delay between atrial and ventricular contractions. The Purkinje fibers provide a rapidly conducting pathway for distributing excitation throughout the ventricles during the power stroke of a single heartbeat.

The activity of the heart is controlled by both the sympathetic and parasym-pathetic divisions of the autonomic nervous system. Acetylcholine released by the parasympathetic nerve terminals in the heart causes slowing of the heart rate by opening potassium channels. Norepinephrine released by the sympathetic nerve terminals increases the response of voltage-dependent calcium channels to depolarization, which increases the rate ofbeating and the strength of contraction. Both effects of neurotransmitters are indirect, mediated via receptors that act via GTP-binding proteins. These receptors are muscarinic receptors in the case of ACh and P-adrenergic receptors in the case of norepinephrine. The effect of the P-adrenergic receptors is to increase the levels of cyclic AMP inside the cardiac cells, which in turn promotes phosphorylation of calcium channels by protein kinase A.

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