Chapter 15

The Autonomic

Nervous System

Introduction to the ANS In this chapter, we examine the structural and functional

features of the autonomic nervous system (ANS) and compare the organization and actions of its two major parts, the sympathetic and parasympathetic divisions. The autonomic nervous system contributes to homeostasis

by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and many glands.

Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.

The enteric division is a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal (GI) tract.

Introduction to the ANS ANS has many distinctive features which set it apart from

SNS. Perhaps the biggest difference between these two

systems is the involvement of conscious control. The ANS usually operates without conscious control,

though centers in the hypothalamus and brain stem do provide regulation for ANS reflexes. Sensory receptors called interoceptors located in blood

vessels, visceral organs, muscles, and the nervous system monitor conditions in the internal environment.

Examples of interoceptors are chemoreceptors that monitor blood CO2 level and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels.

Introduction to the ANS The anatomy of all autonomic pathways can best be understood

by picturing a double-barrelled neuronal construct consisting of

a preganglionic neuron leading to an intermediate ganglion that

contains the cell bodies of post-ganglionic neurons (that

innervate an effector).

Introduction to the ANS

Most body organs have dual ANS innervation; that is, they receive impulses from both sympathetic and parasympathetic neurons. Usually the nerve impulses from one division

stimulate an organ, while impulses from the other division decrease activity.

Furthermore, the responses of the various organs to

ANS stimulation neatly group into two functional

categories :

Like children on a teeter-totter, the sympathetic

divisions “fight or flight” response is balanced

against the “rest and relax” (or rest and digest)

activities of the parasympathetic division.

Divisions of the ANS

Divisions of the ANS

The Sympathetic Division • The cell bodies of neurons which participate in motor

responses of the sympathetic nervous system are located in

the lateral horns of the gray matter in the 12 thoracic

segments and the first two lumbar segments of the cord.

– Sympathetic preganglionic neurons exit the spinal cord

only between levels T1-L2 (hence the name thoracolumbar

division), though sympathetic ganglia extend in the vicinity

of the cord from the cervical to the sacral region.

Some of the major groups of sympathetic

ganglia include:

The sympathetic trunk

(vertebral chain) ganglia

Prevertebral ganglia

The celiac, superior

mesenteric, inferior

mesenteric, aorticorenal

and renal ganglia

The Sympathetic Division

The Sympathetic Division Axons leave the sympathetic trunk in four

possible ways:

They can enter and travel with spinal nerves.

They can form fine networks of periarterial

preganglionic traveling cephalad to synapse in the

cervical ganglia.

Postganglionic axons exiting the sympathetic trunk

can form sympathetic nerves to the heart and lungs.

Preganglionic axons can leave the sympathetic trunk

without synapsing and form splanchnic nerves.

The Sympathetic Division

Major groups of sympathetic ganglia.

The Parasympathetic Division The cell bodies of preganglionic neurons which participate in

motor responses of the parasympathetic nervous system are located in nuclei of 4 cranial nerves in the brainstem (III, VII, IX and X) and in the lateral gray matter of sacral areas of the spinal cord (S2-S4). Four pairs of cranial parasympathetic ganglia innervate

structures in the head: The ciliary, pterygopalatine, submandibular, and otic ganglia. • The cranial-sacral division also has the ganglia

associated with the vagus (X) nerve and the sacral nerves.

The vagus nerve (CN X) carries nearly 80% of the total parasympathetic flow to the organs of the thorax and upper abdomen. Lower abdominal and pelvic organs are innervated by the sacral output.

• The sacral preganglionic axons branch off of sacral spinal nerves to form pelvic splanchnic nerves which synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of the innervated viscera.

• From the terminal ganglia, postganglionic axons innervate smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs

The Parasympathetic Division

The Parasympathetic Division In contrast to the sympathetic system, the parasympathetic response is more controlled.

Presynaptic parasympathetic neurons

usually synapse with only 4–5 postsynaptic neurons, all of which supply a single visceral effector. Parasympathetic stimulation leads to a narrow, focused action on specific organs.

The division of the sympathetic and parasympathetic

divisions of the ANS are compared in Table 15.3

ANS Neurotransmitters The total number of neurotransmitters used in the entire nervous

system is not known, but is well over 100. Despite the variety of possible chemicals that could be used to

transmit chemical messages in the ANS, only 2, acetylcholine and norepinephrine, are used to any great degree. • Synapses at which ACh is used are termed cholinergic. • Synapses at which norepinephrine or epinephrine are used

are termed adrenergic. The neurotransmitter used in all of the synapses of sympathetic

and parasympathetic ganglia (between the synapses of the preganglionic and postganglionic fibers) is acetylcholine. Receptors that respond to Ach released by these cholinergic

neurons are called cholinergic receptors and there are 2 subtypes: nicotinic receptors (found in the ganglia) and muscarinic receptors (found in the synapses with the effector organs).

ANS Neurotransmitters

Acetylcholine acts on a sub-

type of cholinergic receptor

(called nicotinic receptors) at

ganglia of the ANS.

ANS Neurotransmitters

The neurotransmitter used at

most sympathetic postganglionic

synapses is norepinephrine.

The exception to this rule is

that ACh is used at sympathetic

postganglionic synapses for

sweat glands.

ANS Neurotransmitters

The neurotransmitter used at all parasympathetic postganglionic synapses is Ach. These are all a variety

of cholinergic receptors called muscarinic.

ANS Neurotransmitters

Neurons and Neurotransmitters of the Parasympathetic Nervous System

Preganglionic Postganglionic

Cell body in brain or spinal cord

Cell body in intramural ganglion

Acetylcholine (ACh) Acetylcholine (ACh)

ANS Neurotransmitters

Neurons and Neurotransmitters of the Sympathetic Nervous System

Preganglionic Postganglionic

oCell body in lateral horn of ospinal cord

Cell body in sympathetico chain gangliono

oAcetylcholine (ACh) (norepinephrine, NE) ol

except sweat glands (Ach) o

Physiology of the ANS Sympathetic stimulation leads to secretion of norepinephrine by the adrenal

glands, an increase in the rate and strength of the heartbeat, constriction of blood vessels of non-essential organs, dilation of vessels of essential organs (skeletal muscle and the cerebral cortex), an increase in the rate and depth of breathing, hepatic conversion of glycogen to glucose, and decrease in GI activity.

SLUDD is as an acronym used to describe the responses of the

parasympathetic nervous system:

Salivation (increased)

Lacrimation (increased)

Urination (increased)

Digestion (increased)

Defecation (increased)

• … and 3 decreases (in the rate and force of the heart beat, airway

size and rate of breathing, and pupil size)

Physiology of the ANS

Physiology of the ANS

Physiology of the ANS