The autonomic (visceral) nervous system (ANS) is concerned with control of target tissues: the cardiac muscle, the smooth muscle in blood vessels and viscera, and the glands. It helps maintain a constant internal body environment (homeostasis). The ANS consists of efferent pathways, afferent pathways, and groups of neurons in the brain and spinal cord that regulate the system's functions. It is modulated by supraspinal centers such as brain stem nuclei and the hypothalamus.
The ANS is divided into two major anatomically distinct divisions with opposing actions: the sympathetic (thoracolumbar) and parasympathetic (craniosacral) divisions (Fig 20–1). The sympathetic and parasympathetic divisions of the ANS are anatomically distinct, and are different in terms of their pharmacological properties, that is, their response to medications. They are sometimes referred to as the sympathetic nervous system and the parasympathetic nervous system. Many commonly used medications (eg, medications for treating high blood pressure, for regulating gastrointestinal function, or for maintaining a regular heart beat) have their major actions on neurons within these systems.
Overview of the sympathetic nervous system and of its sympathetic (thoracolumbar) and parasympathetic (craniosacral) divisions. Inf., inferior; Sup., superior.
Some authorities consider the intrinsic neurons of the gut as forming a separate enteric nervous system.
The efferent components of the autonomic system are organized into sympathetic and parasympathetic divisions, which arise from preganglionic cell bodies in different locations.
The autonomic outflow system is organized more diffusely than the somatic motor system. In the somatic motor system, lower motor neurons project directly from the spinal cord or brain, without an interposed synapse, to innervate a relatively small group of target cells (somatic muscle cells). This permits individual muscles to be activated separately so that motor action is finely tuned. In contrast, a more slowly conducting two-neuron chain characterizes the autonomic outflow. The cell body of the primary neuron (the presynaptic, or preganglionic, neuron) within the central nervous system is located in the intermediolateral gray column of the spinal cord or in the brain stem nuclei. It sends its axon, which is usually a small-diameter, myelinated B fiber (see Chapter 3), out to synapse with the secondary neuron (the postsynaptic, or postganglionic, neuron) located in one of the autonomic ganglia. From there, the postganglionic axon passes to its terminal distribution in a target organ. Most postganglionic autonomic axons are unmyelinated C fibers.
The autonomic outflow system projects widely to most target tissues and is not as highly focused as the somatic motor system. Because the postganglionic fibers outnumber the preganglionic neurons by a ratio of about 32:1, a single preganglionic neuron may control the autonomic functions of a rather extensive terminal area.