McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 2 9 Introduction to the autonomic nervous system

T he autonomic nervous system (ANS) is sometimes called the involuntary or visceral nervous system because it mostly functions with the person having little con- scious awareness of its activity. Working closely with the endocrine system, the ANS helps to regulate and inte- grate the body’s internal functions within a relatively narrow range of normal, on a minute-to-minute basis. The ANS integrates parts of the central nervous system (CNS) and peripheral nervous system to automatically react to changes in the internal and external environ- ments (see Figure 29.1). STRUCTURE AND FUNCTION OF THE AUTONOMIC NERVOUS SYSTEM The main nerve centres for the ANS are located in the hypothalamus, the medulla and the spinal cord. Nerve impulses that arise in peripheral structures are carried to these centres by afferent nerve fibres. These integrat- ing centres in the CNS respond by sending out efferent impulses along the autonomic nerve pathways. These impulses adjust the functioning of various internal organs in ways that keep the body’s internal environ- ment constant, or homeostatic.

Nerve impulse transmission Throughout the ANS, nerve impulses are carried from the CNS to the outlying organs by way of a two-neuron system. In most peripheral nervous system activities, the CNS nerve body sends an impulse directly to an effector organ or muscle. The ANS does not send impulses directly to the periphery. Instead, axons from CNS neurons end in ganglia , or groups of nerve bodies that are packed together, located outside of the CNS. These ganglia receive information from the preganglionic neuron that started in the CNS and relay that informa- tion along postganglionic neurons. The postganglionic neurons transmit impulses to the neuroeffector cells— muscles, glands and organs. Functions The ANS works to regulate blood pressure, heart rate, respiration, body temperature, water balance, urinary excretion and digestive functions, among other things. This system exerts minute-to-minute control of body responses, which is balanced by the two divisions of the ANS.

Nervous system

Central nervous system

Peripheral nervous system

Cranial nerves

Spinal cord

Sensory nerves

Somatic nerves

Brain

Automatic nervous system

Sympathetic system

Parasympathetic system

Beta- receptors

Muscarinic receptors

Nicotinic receptors

Alpha- receptors

α

α

β

β

1

2

1

2

FIGURE 29.1  Organisation of the nervous system.