11 THE NERVOUS SYSTEM: AUTONOMIC AND MOTOR SYSTEMS
The Autonomic Nervous System
The autonomic nervous system innervates most effector organs and tissues in the body.
Its functions occur at a subconscious level when needed. The involuntary nervous system.
DUAL INNERVATION IN THE AUTONOMIC NERVOUS SYSTEM
Both branches of the autonomic system innervate most organs (dual innervation) but their
functions are opposite in nature (opposite effects).
The parasympathetic nervous system is active during resting conditions, when it stimulates the
digestive organs, and inhibits cardiovascular system.
The sympathetic nervous system is most active during periods of excitation or physical activity,
when it coordinates a group of physiological changes (fight-or-flight response) that prepares the
body to cope with threatening situations.
The primary function is to regulate the function of effector organs so as to maintain homeostasis.
At rest, both are active, but the parasympathetic nervous system dominates. When the body is
excited or stressed, sympathetic nervous system dominates.
The existences of both mechanisms allow a push-pull mechanism that permits greater control
over the effector organs (i.e. sympathetic increases heart rate by increasing frequency of action
potentials, and parasympathetic decreases heart rate by decreasing the frequency).
ANATOMY OF THE AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system consists of efferent pathways containing 2 neurons arranged in
series that communicate between the CNS and the effector organ.
The neurons communicate with each other through synapses located in peripheral structures
called autonomic ganglia.
The neurons that travel from the CNS to the ganglia are called preganglionic neurons. The
neurons that travel from the ganglia to the effector organs are called postganglionic neurons.
A single preganglionic neuron synapses with several postganglionic neurons. Also neurons
within the ganglion (intrinsic neurons) modulate the flow of information to the target organs.
AUTONOMIC NEUROTRANSMITTERS AND RECEPTORS
The two primary neurotransmitters in the peripheral nervous system are acetylcholine and
Cholinergic: neurons that release acetylcholine; they are released by preganglionic neurons of
the sympathetic and parasympathetic branches, and by postganglionic neurons of the
The sympathetic preganglionic releases acetylcholine that acts on endocrine cells in the adrenal
medulla to stimulate the release of epinephrine.
Adrenergic: neurons that release norepinephrine. Norepinephrine is released by almost all
sympathetic postganglion neurons.
Acetylcholine and norepinephrine can bind to different classes and subclasses of cholinergic and
adrenergic receptors having different responses.
TYPES OF CHOLINERGIC RECEPTORS
o Two major classes are nicotinic receptors (4) and muscarinic receptors (5).
o Nicotinic cholinergic receptors are located on the cell bodies and dendrites of
sympathetic and parasympathetic postganglion neurons, on chromaffin cells, and on
skeletal muscle cells.
o Muscarinic cholinergic receptors are found on effector organs or the parasympathetic
o All nicotinic cholinergic receptors are involved with Na and K channels. When
acetylcholine binds to these receptors, the channels open allowing Na to flow in and K to
flow out leading to depolarization. 11 THE NERVOUS SYSTEM: AUTONOMIC AND MOTOR SYSTEMS
o Muscarinic cholinergic receptors are coupled to G proteins and 2 ndmessengers.
Acetylcholine can either be excitatory or inhibitory, depending on the target and the
TYPES OF ADRENERGIC RECEPTORS
o They are coupled to G proteins that either activate or inhibit 2 messenger systems.
o Binding of norepinephrine or epinephrine to a receptor activates G protein that activates
phospholipase C, which catalyzes the conversion of phosphatidylinos