Download 22. ANS.Neuroscience

The Autonomic Nervous
System
Dr. Zeenat Zaidi
The Autonomic Nervous System
• Concerned with the innervation and control of
visceral organs, smooth muscle and glands
• Regulates and coordinates visceral functions: heart
rate, blood pressure, respiration, digestion, urination
& reproduction
• The majority of the activities of the autonomic
system do not impinge on consciousness
• The control exerted by the system is extremely rapid
and widespread
• Along with the endocrine system, its primary
function is homeostasis of the internal environment
Organization of the Autonomic Nervous System
• Like the somatic nervous system:
 It is distributed both in the central and
peripheral nervous system
 It has both afferent & efferent components
and contains afferent neurons, efferent
neurons and interneurons
 The visceral receptors include
chemoreceptors, baroreceptors, and
osmoreceptors. Ischemia or stretch can
cause extreme pain
Visceral Sensory System
Like the somatic system:
Somatic
Autonomic
• Afferent impulses originate in the receptors, travel
via afferent pathways to the CNS and terminate on
the interneurons at different levels
• Cell bodies of the afferent neurons are located in the
sensory ganglia
Visceral motor system is different
from somatic motor system in
many respects
Somatic motor system Autonomic motor system
Effector
Skeletal muscle
Cardiac muscle, smooth
muscle, glands
Type of control
Voluntary
Involuntary
Neural pathway
One motor neuron extends
from the CNS to skeletal
muscle
Chain of two motor neurons:
Preganglionic &
Postganglionic neuron
Action on
effectors
Always excitatory
May be excitatory or inhibitory
Neurotransmitter
Acetylcholine
Acetylcholine or
norepinephrine
Rate of
conduction
Rapid due to myelinated
axons
Slower due to thinly
myelinated or unmyelinated
axons
Visceral motor system
• The efferent pathway is
composed of two
neurons:
• Preganglionic neurons,
whose cell bodies are
located in the brain and
spinal cord (CNS)
• Postganglionic neurons
whose cell bodies are
located in the
autonomic ganglia (PNS)
The axons of the
preganglionic neurons
synapse with the
postganglionic neurons
• Based on the anatomical,
physiological and
pharmacological characteristics,
the autonomic nervous system
is divided into:
 Sympathetic: Activated
during exercise, excitement,
and emergencies. “fight,
flight, or fright”
 Parasympathetic: Concerned
with conserving energy. “rest
and digest”
Anatomical Differences in Sympathetic
and Parasympathetic Divisions
• Location within CNS
 Sympathetic division
lodges in all the thoracic
& upper lumbar (L1,2)
segments of spinal cord
(thoracolumbar outflow)
 Parasympathetic division
lodges in brain & sacral
(S2,3,4) segments of
spinal cord (craniosacral
outflow)
• Location, number &
size of ganglia
 Sympathetic:
 Fewer
 Larger
 Located nearer the
CNS
 Parasympathetic:
 Many
 Smaller
 Located nearer the
viscera, sometimes in
the wall of the viscera
• Pre- and postganglionic fibers
 Sympathetic:
 Shorter pre- &
longer
postganglionic fibers
 Parasympathetic:
 Longer pre- &
shorter
postganglionic fibers
• Branching of axons
• Sympathetic axons:
 Highly branched.
 Each preganglionic fiber synapses with many postganglionic
neurons that pass to many visceral effectors
 Influences many organs
• Parasympathetic axons:
 Few branches
 Each preganglionic fiber usually synapses with four or five
postganglionic neurons that pass to a single visceral effector
 Localized effect
• Neurotransmitter
released by:
• preganglionic axons
 Acetylcholine for both
divisions (cholinergic)
• postganglionic axons
 Sympathetic: mostly
norepinephrine
 Parasympathetic:
acetylcholine
Parasympathetic Division
Cranial Outflow
• Emerges from brain
• Preganglionic neurons located
in cranial nerve nuclei
(Edinger-Westphal, superior &
inferior salivatory, lacrimal and
dorsal motor nucleus of vagus
nerve) in the brain stem
• Preganglionic fibers are carried
by Occulomotor, Facial,
Glossopharyngeal and Vagus
nerve and innervate organs of
the head, neck, thorax, and
abdomen
Sacral Outflow
• Emerges from S2-S4
• Preganglionic neurons
located in lateral horn of
spinal gray matter
• Preganglionic fibers
carried by pelvic
splanchnic nerves to
innervates organs of the
pelvis and lower
abdomen
Parasympathetic Ganglia
1. Ganglia related to
innervation of
thoracic, abdominal
& pelvic viscera
• Preganglionic fibers
carried by vagus nerve
and pelvic splanchnic
nerves
2. Ganglia related to innervation of Head & Neck
• Ciliary ganglion:
 Location: Orbit
 Preganglionic fibers carried
by occulomotor nerve
 Postganglionic fibers
carried by short ciliary
nerve
 Targets: Intraocular
muscles
• Otic ganglion:




Location: infratemporal fossa
Preganglionic fibers carried by glossopharyngeal nerve
Postganglionic fibers carried by auriculotemporal nerve
Target: Parotid gland
• Pterygopalatine
ganglion:
 Location:
Pterygopalatine
fossa
 Preganglionic fibers
carried by facial
nerve
 Postganglionic
fibers carried by
maxillary nerve
 Targets: Lacrimal
gland, nasal &
palatine mucosal
gland
• Submandibular ganglion:
 Location: Submandibular region
 Preganglionic fibers carried by facial nerve
 Postganglionic fibers carried by lingual
nerve
 Targets: Submandibular & sublingual glands
Sympathetic Division
• Issues from T1-L2
• Preganglionic neurons
located in the lateral
gray horn.
• Preganglionic fibers
run in the ventral
roots of the spinal
nerve
• Supplies visceral
organs and structures
of superficial body
regions
Sympathetic Ganglia
• Multiple, large in size
• Located nearer the
central nervous system
• Based on their relation
to the vertebral
column, they are
grouped into:
 Paravertebral
 Prevertebral
Paravertebral Ganglia
• Consist of the right and left
sympathetic chains or trunks.
• The chains lie next to the vertebral
column throughout its length,
running across the necks of the ribs
in the thorax and along the
vertebral bodies in the abdomen.
• There is approximately one
ganglion associated with each
spinal cord segment, except in the
cervical and the sacral regions.
• The chains end into a common
‘ganglion impar’ in front of coccyx
• Adjacent ganglia of the
sympathetic chain are
connected to each other
by interganglionic rami
which contain fibers
ascending or descending
between ganglia.
• Each ganglion in chain
connected to ventral rami
of the spinal nerves by
white and/or gray rami
communicantes
Prevertebral Ganglia
• Unpaired, not segmentally
arranged
• Located in abdomen,
anterior to the vertebral
column
• Main ganglia
 Celiac
 Superior mesenteric
 Inferior mesenteric
 Aorticorenal
General Plan of the Sympathetic Nervous System
• Preganglionic fibers arise from
neurons in the lateral horn of
the spinal cord between T1
and L2 levels of the cord
• Fibers leave the spinal cord in
ventral rootlets, ventral roots of
spinal nerve
• Travel through the spinal nerve,
and then enter the first few
millimeters of the ventral primary
ramus
• Join the sympathetic chain via the
white rami communicantes. The
white rami communicantes are so
named because they are
collections of myelinated axons
• Once the preganglionic fibers
have arrived in the chain they
do one of these three things:
1. May synapse immediately in
the ganglion located at the
level it entered.
2. May ascend or descend in the
sympathetic trunk before
synapsing in a ganglion located
at a different spinal cord level.
3. May pass through the
sympathetic chain ganglia
without synapsing, run in
splanchnic nerve to reach &
synapse in a prevertebral
ganglion.
2
1
3
2
Each pregenglionic fiber is
allowed to synapse once
The postganglionic fibers reach their target structures by
any one of these possible routes:
1. Many fibers re-enter
the ventral primary rami of
spinal nerves via gray
rami communicantes
and get distributed to the
body, especially the blood
vessels, sweat glands
and arrector pili muscles,
through the ventral and
dorsal primary rami.
2. Other fibers leave the
ganglia and travel directly to
their target organs. This is how
postsynaptic sympathetic
fibers reach the organs of the
thorax.
3. Some fibers form perivascular plexuses along blood
vessels to reach their targets
e.g. fibers reaching organs in
the head, and in the abdomen
and pelvis.
• T1 to L2 ventral rami are
connected to the
sympathetic chain via white
rami communicantes, which
carry preganglionic
sympathetic fibers to the
sympathetic chain
• All the ventral rami receive
postganglionic sympathetic
fibers from sympathetic
chain by a gray ramus
• The sympathetic chains carry
the preganglionic fibers from
T1-L2 levels up to the head
and neck and down into the
lower abdomen and pelvis.
Sympathetic Pathways
Head & Neck
Thoracic Organs
Sympathetic Pathways
Abdominal Organs
Pelvic Organs
Distribution of Autonomic Fibers
• Both divisions innervate
mostly the same
structures & operate in
conjunction with one
another (have
antagonistic control over
the viscus) to maintain a
stable internal
environment
• The sympathetic system
dominates at some sites
and the parasympathetic
system dominates at
other sites
Exceptions in the sympathetic nervous system
• Some viscera do not possess dual control e.g.
arrector pili muscle is made to contract by the
sympathetic activity, has no parasympathetic supply
• Sweat glands:
 Postganglionic neurons involved with stress-related
excretion release norepinephrine (“sweaty palms”)
 Postganglionic neurons involved with thermoregulation
release acetylcholine
•
Kidneys:
• Postganglionic neurons to the smooth muscle of the renal
vascular bed release dopamine
The Role of the Adrenal Medulla
• Major organ of the
sympathetic nervous
system
• Secretes great amount
of epinephrine & a
little of norepinephrine
• Stimulated by
preganglionic
sympathetic fibers
Enteric Nervous System
• Intrinsic nervous system that directly
controls the gastrointestinal system
• Composed of two plexuses of nerve
cells and fibers located in the wall of
gastrointestinal tract from the
esophagus to the anal canal
• Submucous or Meisner’s plexus lies in
the submucosa, is mainly concerned
with the control of the glands in the
mucous membrane
• Myenteric or Aurbach’s plexus lies
between the circular and the
longitudinal muscle layer, controls the
muscle and movements of the gut wall
Communicates with the CNS through the parasympathetic
(eg, via the vagus nerve) and sympathetic (eg, via the
prevertebral ganglia) nervous systems
Visceral Reflexes
• Parasympathetic reflexes include: gastric and
intestinal reflexes, defecation, micturition, direct
light reflexes, swallowing reflex, coughing reflex,
baroreceptor reflex and sexual arousal.
• Sympathetic reflexes include: cardio-accelaratory
reflex, vasomotor reflex, pupillary reflex and
ejaculation (in males).
• All visceral reflexes are polysynaptic.
• The simplest visceral reflex arc
consists of:
1. a receptor
2. a sensory neuron
3. an interneuron, &
4. Two motor neurons (pre- &
postganglionic)
• Long reflexes: processed in CNS,
similar to polysynaptic somatic
reflex
• Short reflexes : bypass CNS
entirely, processed in ganglia,
e.g. enteric NS in walls of
digestive tract
Central Control of the ANS
• Cortical centers influence via
connections with the limbic system
• Hypothalamic integration centers
interact with both higher and
lower centers to regulate
autonomic, somatic and endocrine
systems to preserve body
homeostasis
• Reflex activity is mediated by spinal cord
and brain stem (medullary centers).
• Reticular formation exerts most direct
influence
Disorders of the Autonomic Nervous System
• Hypertension
 Can result from overactive
sympathetic vasoconstriction
• Raynaud’s disease
 Characterized by constriction
of blood vessels
 Provoked by exposure to cold
or by emotional stress
• Achalasia of the cardia &
Congenital megacolon
 Defect in the autonomic
innervation of the esophagus
and colon respectively
• Primary autonomic failure: A chronic
degenerative disease of the nervous
system leading to fainting attacks,
incontinence of urine and bowel, and
impotence
• Horner's syndrome: Due to damage or
blockage of the path of the
sympathetic fibers to the head & neck.
The symptoms include:
 Sunken eyeball (enophthalmos)
 Constricted pupil (miosis) that does not
react to light
 Drooping upper eyelid (ptosis)
 Pronounced lack of sweating
(anhidrosis) on the forehead above the
affected eye, face, and neck.
 Facial flushing
normal
Horner’s syndrome
• Sympathetic
 Stress reaction
 Fight-or-flight
 Primes body for
intense skeletal
muscle activity
• Parasympathetic
 Maintenance
functions
 Rest-and-repair
 Counterbalances
sympathetic function