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The Autonomic Nervous System
BIO 137 Anatomy & Physiology
Divisions of the Nervous System
PERIPHERAL NERVOUS SYSTEM
•
CENTRAL NERVOUS SYSTEM
Cranial and Spinal Nerves
– SENSORY
• Afferent neurons (input)
– MOTOR
• Efferent neurons (output)
• SOMATIC
– Effector: skeletal muscle
• AUTONOMIC
– Effectors: smooth muscle, cardiac
muscle, glands
– PARASYMPATHETIC
» maintains homeostasis
– SYMPATHETIC
» Fight or Flight
– Brain and Spinal Cord
– Interneurons
Autonomic Nervous System
• Functions independently and continuously
without conscious effort
• Controls visceral activities
• Effectors are cardiac muscle, smooth muscle
and glands
• Oversees heart rate, breathing rate, body
temperature and other visceral activities that
aid in maintaining homeostasis
Divisions of the PNS: Motor Divisions
• SOMATIC
– Conscious, voluntary control
– Effectors: skeletal muscle
• AUTONOMIC
– Involuntary control, without conscious effort
– Effectors: smooth muscle, cardiac muscle, glands
•
•
•
•
PARASYMPATHETIC (maintains homeostasis)
Active under normal, restful conditions
Prepares body for normal activities
Maintains homeostasis
• SYMPATHETIC (Fight or Flight)
• Prepares body for fight or flight situations
• Active in Stressful situations
Introduction to the ANS
• 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.
ANS Innervation
• Most visceral organs innervated by both
sympathetic and parasympathetic fibers
• Generally exert antagonistic effects
– Sympathetic = ↑heart rate
– Parasympathetic = ↓heart rate
• Both systems are usually partially active
Autonomic Nervous System
• Sensory signals originate in skin and viscera and
travel to CNS
• Motor impulses exit the CNS and lead to ganglia
outside of CNS
• Information is integrated in ganglia and then
travels to an effector
• 2 Neuron Chain
Autonomic Nerve Pathway
• Motor nerve pathway is a 2 neuron chain
– Preganglionic neuron
• Cell body in CNS
– Postganglionic neuron
• Cell body in ganglia
• Preganglionic axon synapses with postganglionic
cell body at a ganglia
• Neurotransmitters are released at the end of each
neuron
2 Neuron Chain in the ANS Pathway
Parasympathetic Division of the ANS
• Long Preganglionic neurons arise from 4 cranial
nerves in the brainstem and the sacral region of
cord and extend to ganglia near effectors
• Short postganglionic neurons lead from ganglia to
effectors
– The vagus nerve (CN X) carries nearly 80% of the total
parasympathetic flow to the organs of the thorax and upper
abdomen.
– Pre & post-ganglionic fibers release acetylcholine (cholinergic)
• Operates under normal conditions to maintain
homeostasis
Sympathetic Division of the ANS
• Axons of motor nerves (from T1-L2) exit through ventral
root of spinal nerves, branch and enter sympathetic
ganglia (trunks) located in chains along vertebral column
– Sympathetic preganglionic neurons exit the spinal cord
only between levels T1-L2
• Short pre-ganglionic fiber releases acetylcholine
(cholinergic)
• Long post-ganglionic fiber releases norepinephrine
(adrenergic)
• Operates under stressful conditions
• Fight or flight response
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.
Autonomic Neurotransmitters
Cholinergic Neurons
• release acetylcholine
•All preganglionic
parasympathetic and
sympathetic neurons
•All postganglionic
parasympathetic neurons
Adrenergic Neurons
• release norepinephrine
• All postganglionic
sympathetic neurons
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ANS Receptors & Actions
Cholinergic Receptors
•bind acetylcholine, ACh
•Nicotinic – usually excitatory & quick
•All post-ganglionic neurons
•Muscarinic – usually
excitatory & slow
•Parasympathetic
effector membranes
ANS Receptors & Actions
Adrenergic Receptors
•bind norepinephine
•Alpha – different responses on
different effectors; usually
excitatory
•Beta - different responses on
different effectors; β1-excites;
β2- inhibits
•Sympathetic effector
membranes
ANS Neurotransmitters
Neurons and Neurotransmitters of the Parasympathetic
Nervous System
Preganglionic
Cell body in brain or spinal
cord
Acetylcholine (ACh)
Postganglionic
Cell body in intramural
ganglion
Acetylcholine (ACh)
ANS Neurotransmitters
Neurons and Neurotransmitters of
the Sympathetic Nervous System
Preganglionic
oCell body in lateral horn of
ospinal cord
oAcetylcholine (ACh)
Postganglionic
Cell body in sympathetico
chain gangliono
(norepinephrine, NE) ol
except sweat glands (Ach) o
Summary of ANS
Control of Autonomic Activity
• Controlled largely by CNS
• Medulla oblongata regulates cardiac, vasomotor
and respiratory activities
• Hypothalamus regulates visceral functions
• Limbic system and cerebral cortex control
emotional responses
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Goals for Autonomic Nervous System
• Name each division and answer the following
questions:
– Where are ganglia located?
– What neurotransmitter is secreted by the pre &
post-ganglionic neurons?
– What type of receptor is found on post-ganglionic
neurons and effectors?
– What is the general function of the division?