Download chapter 15 - Victoria College

CHAPTER 15
THE AUTONOMIC NERVOUS SYSTEM
I. REVIEW OF SOMATIC & AUTONOMIC NERVOUS SYSTEMS
SOMATIC
--Sensory neurons relay info for
special/somatic senses (consciously
perceived)
--Motor neurons innervate skeletal
muscles  voluntary movement
**always excitatory
**damage results in lack of stimulation
 lack of muscle tone
--Axon of single motor neuron extends all
the way from CNS to fibers of motor unit
--All motor neurons release ACh only
AUTONOMIC
--Sensory receptors (interoceptors) in blood
vessels, viscera, and muscles monitor
internal changes
**chemo/mechano receptors
**not consciously perceived
--Motor neurons regulate visceral activities
by increasing or decreasing activities in
effectors
**can still function if damaged
**cannot consciously change responses
--Motor pathways consist of 2 motor
neurons in series
**1st has cell body in CNS &
myelinated axon extends to
autonomic ganglion
**2nd has cell body in ganglion & axon
extends to effector organ
**in some pathways, 1st neuron
terminates on adrenal medulla
--Motor neurons release either ACh or NE
--2 motor divisions
**sympathetic
**parasympathetic
**most effectors receive dual
innervation  nerve impulses from
one division increase activity of an
organ and from other division,
decrease activity
II. ANATOMY OF AUTONOMIC MOTOR PATHWAYS
A. Functional Divisions
1. Sympathetic division = “accelerator”
2. Parasympathetic division = “brakes”
B. Neuron Characteristics
1. Sympathetic division
a. preganglionic neuron has its cell body in gray matter of
thoracolumbar region (T1-L2) of spinal cord
i. synapse on autonomic ganglion outside spinal cord
ii. axons usually short
iii. sympathetic trunk ganglia = collections of ganglia
lateral to vertebral column (form sympathetic chains—
Fig 15.2)
iv. collateral ganglia are located about ½ way to effector
organs (outside chain)
b. postganglionic neuron continues from ganglion to effector
organ
i. much longer than preganglionic fibers
ii. terminate in varicosities
c. single preganglionic fiber has many branches (20 or more
postganglionic fibers extending from single ganglia)  allows
for widespread, simultaneous response
2. Parasympathetic division
a. preganglionic neurons originate in brain/brain stem or sacral
spinal cord
i. axons = craniosacral outflow
ii. longer axons than in sympathetic division
iii. synapse a terminal ganglion on or near effector organ
b. postganglionic axons are short & terminate in varicosities on
effector organ
c. preganglionic neuron synapses with only 4 or 5 postganglionic
neurons  supply single effector & allow localized responses
C. Sympathetic structure
1. Preganglionic neurons exit spinal cord thru anterior roots along with
somatic motor neurons
2. Preganglionic neurons can synapse w/ postganglionic neurons in
sympathetic trunk either at the level of entry or at a higher/lower point
in sympathetic chain
3. Sympathetic trunk ganglia can extend length of body, but preganglionic
axons always originate in thoracolumbar region (thoracolumbar
outflow)
4. Some preganglionic neurons extend to adrenal medullae
a. stimulate adrenal medulla to release hormones (NE, Epi)
b. no postganglionic neurons
D. Parasympathetic structure
1. Preganglionic axons exit brain or sacral region as part of cranial or
spinal nerve
2. 80% of craniosacral outflow leaves brains as part of Vagus (CN X)
nerve  axons innervate heart, airways, liver, stomach, gallbladder,
pancreas, intestines
3. Preganglionic neurons end interminal ganglia near or in effector
organs, so postganglionic axons are short
III. AUTONOMIC NT & RECEPTORS
A. NT
1. Sympathetic preganglionic neurons secrete ACh (cholinergic)
2. Sympathetic postganglionic neurons secrete NE or ACh (primarily NE)
3. All parasympathetic fibers are cholinergic
B. Receptors
1. Cholinergic—bind ACh
a. nicotinic receptors respond to ACh from preganglionic neurons
i. found in ALL autonomic ganglia (symp & PS) & in
motor end plate of NMJ
ii. binds nicotine
iii. activation causes depolarization & excitation of
postsynaptic cell (postganglionic fiber, visceral
effector, skel muscle)
b. muscarinic receptors respond to ACh released from PS
postganglionic fibers
i. found in all PS effectors (smooth/cardiac muscle, &
glands)
ii. bind muscarine
iii. activation can cause either depolarization or
hyperpolarization depending on cell type
**in GI tract, binding relaxes smooth muscle
sphincters  increases movement thru tract
**in iris of eye, causes contraction of smooth
muscle  constriction of pupil
iv. anti-muscarinic agents generally relax smooth muscle
**atropine blocks muscarinic receptors & decreases
digestion/secretions/urination during surgeries;
dilation of pupils for ophtho procedures
c. ACh binds both receptor types but nicotinic receptors do not
bind muscarine & vice versa  allows for pharmacological
intervention by activating/inhibiting specific receptors
d. AChE quickly inactivates ACh (short-lived effects)
2. Adrenergic receptors
a. bind NE and epinephrine (Epi)
b. found on visceral effectors innervated by most sympathetic
postganglionic neurons
c. α receptors
i. bind both NE & Epi (NE binds more strongly)
ii. found in sphincters of digestive tract, blood vessels,
pancreas, sweat glands
iii. stimulation usually results in contraction of smooth
muscle
d. β receptors
i. β1 binds NE, Epi
**found primarily in heart
**activation increases heart rate & force of
contractions  increases blood flow
ii. β2 primarily bind epinephrine
**found in blood vessels of heart, skeletal muscles
& smooth muscle of lungs, in urinary bladder,
liver, adipocytes, digestive organs
**activation relaxes smooth muscle
iii. β3 found only in brown adipose tissues
**in infants only
**activation results in thermogenesis
e. NE activation @ synapse is terminated by:
i. reuptake by secreting neuron
ii. inactivation by MAO or COMT
f. NE effects are longer lasting than cholinergic effects
C. Agonists/Antagonists
1. Agonists bind & activate receptor which mimics effect of naturally
occurring NT or hormone
a. parasympathomimetic or sympathomimetic agents
b. phenylephrine = α agonist which causes constriction of blood
vessels in nasal passages  ingredient in decongestant
medications
2. Antagonists bind & block a receptor which prevents the binding of NT
& prevents exertion of NT/hormonal effects
a. antisympathetic & antiparasympathetic agents
b. atropine: anti PS (muscarinic) agent (see III-B-1 above)
c. β blockers are used to treat high blood pressure (can be selective
or non-selective)
IV. PHYSIOLOGICAL RESPONSES OF ANS
A. Autonomic Tone
1. Regulated by hypothalamus
a. increase sympathetic tone & decrease PS tone simultaneously
b. different effects on body organs because different NT released
from postganglionic fibers
i. ACh (PS)
ii. NE (sympathetic)
2. In normal conditions, PS division dominates sympathetic
3. Sweat glands, adipose, skeletal muscles, blood vessels, kidneys, skin,
adrenal medulla and the spleen receive sympathetic innervation only
a. increasing sympathetic stimulation has one effect
b. decreasing sympathetic stimulation has opposite effect
B. Sympathetic Tone
1. Dominates during emotional/physical stress and during exercise
2. Increased sympathetic tone favors functions that support vigorous
activities & synthesis of ATP (energy) (reduces functions that favor
energy storage)
3. Effects on visceral organs
a. heart: increased rate & force of contraction  increased BP
b. blood vessels: to digestive organs/skin/kidneys—constriction;
to muscles/heart/adipose tissue/liver—dilation
c. lungs: dilation of airways, decreased mucus synthesis
d. bladder/digestive tract: relaxation/filling; decreased motility
e. adipose: increased lipolysis to provide energy
f. exocrine: increased sweating, decreased pancreatic secretions
g. endocrine: decreased insulin, increased glucagon, release of
NE/Epi from adrenal medulla
h. brain: increased alertness
i. eye: contraction of radial muscle (dilation of pupils)
4. Effects are longer lasting/more widespread than PS
a. postganglionic neurons diverge
b. NE lingers longer than ACh (several seconds vs. 20 msec)
c. Epi & NE in blood intensify response  blood/tissues do not
contain MAO/COMT
C. Parasympathetic Tone
1. Dominates symp. during normal, resting conditions
2. Increased PS tone favors functions that conserve/restore energy stores
during rest/recovery
3. Effects on visceral organs
a. heart: decreased rate/force of contraction
b. blood vessels: only ones innervated by PS are associated w/
genitalia
c. lungs: constriction of airways, increased mucus production
d. digestive tract: increased motility, relaxation of sphincters,
stimulation of digestive secretions
e. gallbladder, urinary bladder—emptying
f. eye: constriction of pupils, accommodation for near vision
g. exocrine: pancreas releases digestive enzymes
h. endocrine: pancreas increases insulin secretion
V. INTEGRATION/CONTROL OF AUTONOMIC FUNCTION
A. Autonomic Reflexes
1. Integration @ spinal cord, input from higher levels of consciousness
2. Components similar to somatic reflex arc
3. Regulate controlled body functions
a. blood pressure: adjust heart rate/contraction strength/vessel
diameter
b. digestion: adjust muscle tone/motility within GI tract
c. defecation/urination: regulates opening/closing of sphincters
B. Control by Higher Centers
1. Hypothalamus integrates ANS/SNS
a. input: sensory from viscera, special senses, changes in
temp/osmolarity; emotional input from limbic system
b. output
i. influences brain stem: cardiovascular, salivation,
swallowing, vomiting centers
ii. influences spinal cord: urination/defecation
c. posterior/lateral regions control sympathetic responses
d. anterior/medial regions control PS responses
2. Medulla is directly responsible for autonomic motor output & is
regulated by the HT
3. Higher brain functions w/in frontal cortex can also impact autonomic
activity thru HT-medullary pathways