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THE AUTONOMIC NERVOUS
SYSTEM
INTRODUCTION
ANS in the Structural Organization
of the Nervous System
COMPARISON
OF
THE
SOMATIC AND AUTONOMIC NERVOUS SYSTEM
• The somatic nervous system stimulates skeletal muscles, while the
ANS innervates cardiac and smooth muscle and glands
• In the somatic nervous system, the cell bodies of the neurons are in
the spinal cord and their axons extend to the skeletal muscles they
innervate
– The ANS consists of a two-neuron chain
• The neurotransmitter released by the somatic motor neurons is
acetylcholine, which always has an excitatory effect; the
neurotransmitters released by the ANS are epinephrine and
acetylcholine, and both may have either an excitatory or an
inhibitory effect
• There is overlap between the somatic and autonomic nervous
systems, and most body responses to changing internal and
external stimuli involve both skeletal muscle activity and visceral
organ responses
SOMATIC VS AUTONOMIC
DIVISIONS
OF
THE AUTONOMIC NERVOUS SYSTEM
• The parasympathetic division keeps body
energy use as low as possible while
directing digestion and elimination
activities
• The sympathetic division prepares the
body to respond to an emergency or
threatening situation (or vigorous exercise)
ANS ANATOMY
ANS SUBDIVISIONS
PARASYMPATHETIC
(CRANIOSACRAL)
DIVISION
• The preganglionic axons extend from the CNS
nearly all the way to the structures to be
innervated where they synapse with ganglionic
neurons in the terminal ganglia
• The cranial outflow consists of preganglionic
fibers that run in the oculomotor, facial,
glossopharyngeal, and vagus cranial nerves
• The rest of the large intestine and the pelvic
organs are served by the sacral outflow, which
arises from neurons located in the lateral gray
matter of spinal cord segments
PARASYMPATHETIC NERVOUS
SYSTEM
SYMPATHETIC
(THORACOLUMBAR)
DIVISION
• The sympathetic division supplies the visceral organs in
the internal body cavities but also all visceral structures
in the somatic part of the body
• When synapses are made in chain ganglia, the
postganglionic axons enter the ventral (or dorsal) ramus
of the adjoining spinal nerves by way of communicating
branches called gray rami communicates
• The preganglionic fibers from T5 down synapse in
collateral ganglia; thus these fibers enter and leave the
sympathetic chains without synapsing
• Some fibers of the thoracic splanchnic nerves terminate
by synapsing with the hormone producing medullary
cells of the adrenal cortex
SYMPATHETIC NERVOUS
SYSTEM
SYMPATHETIC PATHWAYS
The visceral sensory neurons are
the first link in autonomic reflexes
by sending information concerning
chemical changes, stretch, and
irritation of the viscera
VISCERAL REFLEXES
REFERRED PAIN
ANS PHYSIOLOGY
• Neurotransmitters and Receptors
– Cholinergic receptors, such as nicotinic and
muscarinic receptors, bind acetylcholine
– Adrenergic receptors alpha and beta bind to
epinephrine
• Knowing the locations of the cholinergic
and adrenergic receptor subtypes allows
specific drugs to be prescribed to obtain
desired inhibitory or stimulatory effects on
target organs
ANS PHYSIOLOGY
•
Interactions of the autonomic divisions
– Most visceral organs receive dual innervations by both ANS divisions, allowing
for a dynamic antagonism to exist between the divisions and precise control of
visceral activity
– The sympathetic division will increase heart and respiratory rates during a fightor-flight situation and decrease activity of digestive and elimination organs
– Sympathetic tone occurs in the vascular system, and parasympathetic tone
occurs in the digestive and urinary tracts
– The parasympathetic and sympathetic divisions may work together to achieve a
common purpose
• For example, the parasympathetic division controls erection while the sympathetic
division controls ejaculation
– The sympathetic division mediates reflexes that regulate body temperature,
release renin from the kidneys, and promote metabolic effects
– The parasympathetic division exerts short-live, localized control its effectors,
while the sympathetic division responds in a diffuse and interconnected way to
cause a body-wide mobilization
ANS PHYSIOLOGY
• Control of Autonomic Functioning
– The brain stem appears to exert the most
direct influence over autonomic functions
– The hypothalamus is the main integration
center for the autonomic nervous system
– Cortical or voluntary control of the autonomic
nervous system does appear to be possible
LEVELS OF ANS CONTROL
HOMEOSTATIC IMBALANCES
OF
THE ANS
• Hypertension, or high blood pressure, may result
from an overactive sympathetic vasoconstrictor
response due to continuous high levels of stress
• Raynaud’s disease is characterized by
intermittent attacks causing the skin of the
fingers and the toes to become pale, then
cyanotic and painful
• Mass reflex reaction is a life-threatening
condition involving uncontrolled activation of
both somatic and autonomic motor neurons
DEVELOPMENTAL ASPECTS
OF THE ANS
• Embryonic and fetal development of the
autonomic nervous system
– ANS preganglionic neurons and somatic motor
neurons derive from the embryonic neural tube
– ANS structures found in the PNS (ganglionic neurons,
adrenal medulla, and all autonomic ganglia) derive
from the neural crest
– Nerve growth factor is a protein secreted by target
cells of the postganglionic axons
• On old age the efficiency of the ANS begins to
decline, partly due to structural changes of some
preganglionic axonal terminals