BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. ANIMAL COORDINATION AND REGULATION

17.2. The Nervous System (CNS and PNS)

17.2.3. The Autonomic Nervous System

The Autonomic Nervous system (derived from the Greek autós meaning 'self' and nomós meaning 'law', i.e., 'self-governing'), also known as the vegetative nervous system, is a component of the Peripheral Nervous System that regulates The activity of Internal Organs, including Heart rate, peristalsis, and sweating. It consists of motor Neurons that innervate the smooth Muscle of visceral organs. Consequently, these Muscles are also involuntary—we generally cannot control their contractions by will. The operation of the autonomic nervous system is based on Reflexes called visceral reflexes, the arcs of which pass through the Spinal Cord or Brain and are largely independent of conscious control. However, there are exceptions. For instance, the functioning of the anal and urinary sphincter muscles, associated with defecation and urination respectively, is under conscious control, and these Functions must be learned. It is believed that many other autonomic activities can also be controlled through willpower and training; for example, the Physiological Basis of Various Forms of meditation and relaxation involves The regulation of autonomic functions, and some individuals have achieved notable success in consciously controlling their heart rate and lowering Blood pressure using their mind or "willpower." Nevertheless, the overall operation of this system is essentially involuntary, directed by centers in the Medulla Oblongata (part of the Hindbrain) and the Hypothalamus (another brain Structure) (see Section 17.2.4). These centers receive and integrate sensory information and, by coordinating it with inputs from other regions of The Nervous System, generate appropriate responses.

The autonomic nervous system comprises Two Types of neurons—preganglionic and postganglionic. The Cell bodies of preganglionic neurons reside in the brain or spinal cord, and their unmyelinated axons leave the Central Nervous System within the ventral roots of segmental nerves to form synapses with the dendrites of postganglionic neurons. The cell bodies of postganglionic neurons are located within ganglia, and their unmyelinated axons extend to the effector organ (Fig. 17.18).

The autonomic nervous system is divided into two branches—the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). These systems differ primarily in the Organization of their neurons, as illustrated in Fig. 17.18.

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Fig. 17.18. Simplified diagram of the main elements of the parasympathetic (A) and sympathetic (B) nervous systems. Sensory neurons are not part of the autonomic nervous system.

In the SNS, cellular synapses and the cell bodies of postganglionic neurons are located mainly in paired ganglia situated close to the spinal cord. They are linked into two parallel sympathetic trunks that merge into an unpaired ganglion at the coccygeal level. In the mid-trunk region, each sympathetic ganglion is connected directly to the spinal cord by a white ramus communicans, and to the nearest spinal nerve by a gray ramus communicans (Fig. 17.19). The ganglia of the PNS and its postganglionic neurons are located near or within the effector organs themselves (Fig. 17.18).

Other differences between the two systems concern The Nature of the neurotransmitter released by postganglionic neurons, their effects on effectors, and the conditions under which they become active. These differences are summarized in Table 17.4.

Fig. 17.19. Simplified diagram showing the Location of a sympathetic ganglion and its connections with the spinal cord and spinal nerve.

Table 17.4. Differences between the sympathetic (SNS) and parasympathetic (PNS) nervous systems

As a rule, the SNS and PNS exert opposite (antagonistic) effects on the organs they innervate, enabling the body to regulate internal organ activity rapidly and precisely, thereby maintaining a stable internal environment. For instance, an increase in heart rate resulting from the release of noradrenaline by sympathetic neurons is counterbalanced by the release of acetylcholine from parasympathetic neurons. This prevents The Heart from beating at an excessively rapid pace and ultimately restores a normal heart rhythm once the secretion of both Neurotransmitters reaches a balance. The antagonistic effects of both systems are summarized in Table 17.5. A careful study of this table provides a comprehensive Overview of the Functions of the SNS and PNS. Their structural layout is shown in Fig. 17.20.

Table 17.5. Effects of the sympathetic (SNS) and parasympathetic (PNS) nervous systems on the body

Fig. 17.20. Schematic diagram of the sympathetic and parasympathetic nervous systems. Together, they form the autonomic nervous system, which mediates a portion of the body's involuntary (automatic or unconditioned reflex) responses. Solid lines represent preganglionic nerve fibers, and dashed lines represent postganglionic fibers.