Cardiac Muscle The Autonomic Nervous System

The motor functions we have described so far in this part of the book have been concerned with the control of skeletal muscles. These are the muscles that produce overt movements of the body and give rise to the observable external actions that we normally think of as the "behavior" of an animal. However, even in an animal that appears to an external observer to be quiescent, the nervous system is quite busy coordinating many ongoing motor actions that are as important for survival as skeletal muscle movements. These motor activities include such things as regulating digestion, maintaining the proper glucose balance in the blood, regulating heart rate, and so on. The part of the nervous system that controls these functions is called the autonomic nervous system. The motor targets of the autonomic nervous system include gland cells, cardiac muscle cells, and smooth muscle cells such as those found in the gut. To distinguish it from the autonomic nervous system, the parts of the nervous system we have been discussing up to this point whose motor targets are the skeletal muscles are collectively called the somatic nervous system.

In addition to the differences in their target cells, there are other differences between the autonomic and somatic nervous systems. As we have seen, in the somatic nervous system, the cell bodies of the motor neurons are located within the central nervous system, either in the spinal cord or in the nuclei of cranial nerves in the brainstem. By contrast, the cell bodies of the motor neurons in the autonomic nervous system are located outside the central nervous system altogether, in a system of autonomic ganglia distributed throughout the body. The central nervous system controls these autonomic ganglia by way of output neurons called preganglionic neurons, which are located in the spinal cord and brainstem. This arrangement is illustrated in Figure 12-1. The motor neurons in the autonomic ganglia are also called postganglionic neurons. The axons of the preganglionic neurons entering the ganglia are referred to as the preganglionic fibers, while the axons of the autonomic motor neurons carrying the output to the target cells are called the postganglionic fibers. Thus, in the

Central nervous system

Target cells

Brainstem or spinal cord

Autonomic nervous system

Preganglionic neuron

Brainstem or spinal cord

Somatic nervous system

Central nervous system

Target cells

Brainstem or spinal cord

Preganglionic neuron

Brainstem or spinal cord

Smooth muscle i- Cardiac muscle Gland cells

Skeletal muscle

Somatic motor neuron

Figure 12-1 Differences between autonomic and somatic nervous systems. In the autonomic nervous system, the motor neurons are located outside the central nervous system, in autonomic ganglia. The motor neurons contact smooth muscle cells, cardiac muscle cells, and gland cells. The central nervous system controls the ganglia via preganglionic neurons. In the somatic nervous system, the motor neurons are located within the central nervous system and contact skeletal muscle cells.

Smooth muscle i- Cardiac muscle Gland cells

Skeletal muscle

Somatic motor neuron

Figure 12-1 Differences between autonomic and somatic nervous systems. In the autonomic nervous system, the motor neurons are located outside the central nervous system, in autonomic ganglia. The motor neurons contact smooth muscle cells, cardiac muscle cells, and gland cells. The central nervous system controls the ganglia via preganglionic neurons. In the somatic nervous system, the motor neurons are located within the central nervous system and contact skeletal muscle cells.

somatic nervous system, the motor commands exiting from the central nervous system go directly to the target cells, while in the autonomic nervous system, the motor commands from the central nervous system are relayed via an additional synaptic connection in the peripheral nervous system.

The autonomic and somatic nervous systems also differ in the effects that the motor neurons have on the target cells. In Chapter 8, we discussed in detail the synaptic interaction between motor neurons and skeletal muscle cells at the neuromuscularjunction. All of the somatic motor neurons release ACh as their neurotransmitter, and the effect on the skeletal muscle cells is always excitatory: contraction is stimulated. In the autonomic nervous system, however, some motor neurons release ACh and other motor neurons release the neurotransmitter norepinephrine (see Chapter 9), instead of ACh. Further, an autonomic motor neuron may either excite or inhibit its target cell. In general, if norepinephrine excites the target cells, then ACh inhibits them, and vice versa. For example, norepinephrine increases the rate of beating of the heart, while ACh decreases the heart rate, as we will examine in detail shortly.

The norepinephrine-releasing and ACh-releasing motor neurons are organized into anatomically distinct divisions of the autonomic nervous system, called the sympathetic division (norepinephrine-releasing) and the parasympathetic division (ACh-releasing). The ganglia containing the sympathetic motor neurons are called sympathetic ganglia, and those containing para-sympathetic motor neurons are called parasympathetic ganglia. Most of the sympathetic ganglia are arrayed parallel to the spinal cord, one ganglion on each side just outside the vertebral column. There is one pair of these paravertebral ganglia for each vertebral segment. The ganglia are interconnected by thick, longitudinal bundles of axons containing the preganglionic fibers exiting from the spinal cord. Because of these connectives, the paravertebral ganglia form two long chains parallel to the spinal column, sometimes referred to as the sympathetic chains. In addition to the paravertebral ganglia that make up the chains, there are also sympathetic ganglia called the prevertebral ganglia, located within the abdomen.

The parasympathetic ganglia are distributed more diffusely throughout the body and tend to be located closer to their target organs. In some cases, the parasympathetic ganglia are actually located within the target organ itself. This is the case, for example, in the heart. Because the sympathetic ganglia are located predominantly near the central nervous system while the parasympa-thetic ganglia are located mostly near to their target organs, the preganglionic fibers of the sympathetic nervous system are usually much shorter than the preganglionic fibers of the parasympathetic nervous system, which must extend all the way from the central nervous system to the near vicinity of the target organ in order to reach the postganglionic neurons. Conversely, the postganglionic fibers are typically much longer in the sympathetic nervous system than in the parasympathetic nervous system.

Most target organs receive inputs from both the sympathetic and para-sympathetic divisions of the autonomic nervous system. As noted above, the sympathetic and parasympathetic inputs produce opposing effects on the target. In general, excitation of the sympathetic nervous system has the overall effect of placing the organism in "emergency mode," ready for vigorous activity. The parasympathetic nervous system has the opposite effect of placing the organism in a "vegetative mode." For example, sympathetic activity increases the heart rate and blood pressure, diverts blood flow from the skin and viscera to the skeletal muscles, and reduces intestinal motility, all appropriate preparations for rapid reaction to an external threat. Parasympathetic activity, on the other hand, decreases heart rate and blood pressure, and promotes blood circulation to the gut and intestinal motility. All of these actions are appropriate for resting and digesting, in the absence of any threatening situation in the environment.

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