C H A P T E R 2 9
Introduction to the autonomic nervous system
445
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
Cranial
nerves
Brain
Spinal
cord
Peripheral nervous system
Automatic nervous system
Parasympathetic
system
Muscarinic
receptors
Nicotinic
receptors
Alpha-
receptors
α
1
α
2
β
2
β
1
Beta-
receptors
Sympathetic
system
Sensory
nerves
Somatic
nerves
FIGURE 29.1
Organisation of the nervous system.
