CHAPTER 14: THE AUTONOMIC NERVOUS SYSTEM
OVERVIEW OF THE ANS
PNS
↓ ↓
SENSORY MOTOR
↓
↓
SOMATIC
AUTONOMIC
voluntary
involuntary
↓
↓
sympathetic
parasympathetic
COMPARISON OF SOMATIC AND AUTONOMIC
Both systems have motor fibers, but differ in terms of their effectors, efferent pathways and target
structures.
Effectors: somatic effectors are skeletal muscles. Autonomic effectors are smooth muscle,
cardiac muscle and glands.
Efferent pathways: the somatic extends its axons from cell bodies (which are located in the
CNS) to the effectors. The autonomic axons from the CNS synapse in the PNS’s ganglia. A postganglionic axon continues to the effector.
Target organ responses to their neurotransmitters: the somatic uses ACh and always stimulates
the target. The autonomic can stimulate or inhibit the target. In the autonomic, preganglionic
axons use ACh and postganglionic axons use either ACh or norepinephrine/epinephrine.
The ANS is also called the “involuntary NS” or the general visceral motor system. The function of the
ANS is to work with the CNS, endocrine system and motor neurons that innervate glands, smooth
muscles and cardiac muscles. Signals from the CNS produce adjustments from the ANS to maintain
homeostasis.
Both the somatic and autonomic NS are composed of cranial and spinal nerves that branch from the
brain and spinal cord. The two systems clearly differ in terms of targets and types of control they
exert. The somatic system is voluntary and provides conscious control of sensory nerves and voluntary
(skeletal/striated) muscles. The autonomic nervous system is involuntary and provides unconscious
control of cardiac and smooth muscles, glands and blood vessels.
The autonomic nervous system extends lightly myelinated axons from cell bodies in the CNS to ganglia
in the PNS. From the ganglia in the PNS, unmyelinated axons extend to the effector (target) location.
The somatic system extends a myelinated axon from the CNS into the PNS and to the effector
(target). The ANS is technically a motor (efferent) system. Sensory neurons that monitor internal
conditions such as blood pressure are considered part of the somatic NS.
The divisions of the ANS innervate all major body organs, have antagonistic function. They provide
dual innervation of the major body organs and differ in their origins within the CNS and in their
ganglia location. In the sympathetic NS neurons originate from the thoracic and lumbar regions of the
spine. One ganglion is in chains next to either side of the spinal cord (short preganglionic and long
postganglionic axons). In the parasympathetic NS neurons originate from the sacral region of the
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spine. Ganglia are located on or in the walls of organs that they innervated (long preganglionic and
short postganglionic axons).
SYMPATHETIC AND PARASYMPATHETIC NS EFFECTS
In genera, the sympathetic system stimulates organs and the parasympathetic system inhibits organs.
The actual response depends upon the particular organ(s) involved. Ex: sympathetic stimulates the
heartbeat and depresses stomach contraction. The parasympathetic system is most active in
situations that are nonstressfull. The sympathetic system is the fight or flight system – it operates
when situations are stressful.
PHYSIOLOGY OF THE ANS
All preganglionic axons of the ANS and all parasympathetic axons that are postganglionic release ACh.
Axons that release ACh are called cholinergic fibers. Norepinephrine and epinephrine are released by
most sympathetic postganglionic axons. Fibers that release these compounds are called adrenergic
fibers. Exceptions: sweat glands in the skin, as well as blood vessels in the skeletal muscles and
external genitalia (all secrete ACh). Responses of target organs to ACh and NE/E varies. Can be
stimulating or inhibiting. The response of the target depends upon the type of target organ receptors.
TARGET ORGAN RECEPTORS
Cholinergic receptors: there are two types of cholinergic (named for the drugs that mimic Ach)
receptors: (1) nicotinic receptors, which elicit a stimulating effect that results in excitation of
the neuron or effector and (2) muscarinic receptors, which inhibit or stimulate depending on the
target organ.
Adrenergic receptors: there are also two types: (1) alpha, which is stimulatory and (2) beta,
which is inhibitory.
CONTROL OF THE ANS
This system is involuntary but ultimately is under the control of the CNS. The medulla provides partial
regulation and the hypothalamus handles the rest.
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DISORDERS/DISEASES
Achalasia
Atonic Bladder
Horner’s Syndrome
Hypertension
Mass Reflex Action
Raynaud’s Disease
Can be caused by fibrosis of the esophagus. The gastro-esophageal
sphincter does not relax when swallowing. Vomiting may occur. The
cause of problem still unknown.
Urinary bladder becomes flaccid and therefore overfills and allows
urine to dribble out. Results from loss of autonomic innervation
following a spinal cord injury.
Destruction of the superior sympathetic trunk on one side of the body.
Causes drooping of upper eyelids (ptosis), constriction of the pupils
and the person does not sweat on affected side of head.
A/k/a high blood pressure. Can result from an overactive sympathetic
NS promoted by high stress levels.
A/k/a autonomic hyperreflexia or epilepsy of the spinal cord. An
uncontrolled activation of autonomic and somatic motor neurons.
Occurs in cases of quadriplegia and spinal cord injuries. Is a surge of
nerve output from larger regions of the spinal cord.
Skin of fingers becomes pale, then cyanotic and painful. It can be
provoked by exposure to cold. Is often called an exaggerated
vasoconstriction effect.
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