The human nervous system can be divided into two major functional areas: the somatic nervous system and the autonomic nervous system (ANS). The somatic division is concerned primarily with voluntary function—that is, control of the skeletal musculature. The ANS is responsible for controlling bodily functions that are largely involuntary, or automatic, in nature. For instance, the control of blood pressure (BP) and other aspects of cardiovascular function are under the influence of the ANS. This system also controls other involuntary functions such as digestion, elimination, and thermoregulation.
Considering the potential problems that can occur in various systems, such as the cardiovascular and digestive systems, the use of therapeutic drugs to alter autonomic function is one of the major areas of pharmacology. The purpose of this chapter is to review some of the primary anatomical and physiological aspects of the ANS. This review is intended to provide you with a basis for understanding the pharmacological effects and clinical applications of the autonomic drugs, which are discussed in subsequent chapters.
ANATOMY OF THE AUTONOMIC NERVOUS SYSTEM: SYMPATHETIC AND PARASYMPATHETIC DIVISIONS
The ANS can be roughly divided into two primary divisions: the sympathetic and parasympathetic nervous systems.1,2 The sympathetic, or thoracolumbar, division arises primarily from neurons located in the thoracic and upper lumbar regions of the spinal cord. The parasympathetic, or craniosacral, division is composed of neurons originating in the midbrain, brainstem, and sacral region of the spinal cord. Some sources also consider the enteric nervous system to be a third ANS division. This system is comprised of an extensive network of neurons in the wall of the gastrointestinal (GI) tract that controls various aspects of GI function.1,3 The enteric nervous system, however, contains both sympathetic and parasympathetic components and is therefore often considered part of the two primary ANS divisions. Hence, this chapter will focus on the sympathetic and parasympathetic systems, and the physiological and functional characteristics that differentiate these two primary ANS divisions.1-4
Preganglionic and Neurons
The somatic nervous system uses one neuron to reach from the central nervous system (CNS) to the periphery. In the somatic motor system, for instance, the alpha motor neuron begins in the spinal cord and extends all the way to the skeletal muscle—that is, it does not synapse until it reaches the muscle cell. In both the sympathetic and parasympathetic divisions, however, two neurons are used in sequence to reach from the CNS (i.e., brain or spinal cord) to the peripheral organ or tissue that is being supplied. The first neuron begins at a specific location in the CNS and extends a certain distance toward the periphery before synapsing with a second neuron, which completes the journey to the final destination. The synapse of these two neurons is usually in one of the autonomic ganglia (see the “Sympathetic Organization” and “Parasympathetic Organization” ...