Autonomic Nervous System Flashcards by Troy Santos

The ANS (Autonomic Nervous System) is
a visceral motor neuron system
that innervates

-Smooth muscle in organs and blood vessels
•  Cardiac muscle
•  Glands

How well did you know this?

Not at all

Perfectly

Autonomic Motor Systems with
chain of two motor neurons
compared to single somatic motor axon

Autonomic motor system
•  Preganglionic neuron: thinly myelinated
•  Post-ganglionic neuron: unmyelinated
•  Preganglionic and postganglionic neurons
synapse in a ganglion
Somatic motor system
•  Axons are well myelinated and conduct
impulses more rapidly
•  Have one axon that extends from CNS to the
muscle

How well did you know this?

Not at all

Perfectly

Sympathetic and

parasympathetic divisions

  Innervate mostly the same structures

*   Cause opposite effects

How well did you know this?

Not at all

Perfectly

Sympathetic Nervous System

•  Fight, flight, or fright response
•  Increase heart rate and breathing rate
•  Increases blood and oxygen to skeletal muscles
•  Dilates pupils and airways
•  Inhibits motility of the digestive tract and urinary
tracts
•  Stimulates sweat glands
•  Stimulates arrector pili muscles which causes
hairs to stand up vertically

How well did you know this?

Not at all

Perfectly

Parasympathetic Nervous
System is active when person
is relaxed and body is at rest

-Conserves energy 
•  Heart rate and breathing are at normal 
levels 
•  Gastrointestinal tract digests food 
•  Pupils are constricted

How well did you know this?

Not at all

Perfectly

Anatomical Differences in Sympathetic and

Parasympathetic Divisions

Issue from different 
regions of the CNS 
– Sympathetic—also 
called the 
thoracolumbar 
division 
– Parasympathetic—
also called the 
craniosacral 
division

How well did you know this?

Not at all

Perfectly

Anatomical Differences in Sympathetic

and Parasympathetic Divisions

•  Length of postganglionic axon fibers 
– Sympathetic—long because ganglia are 
typically close to CNS 
– Parasympathetic—short because ganglia are 
close to viscera 
•  Branching of postganglionic axons 
– Sympathetic axons—may be highly branched 
– Parasympathetic axons—few branches

How well did you know this?

Not at all

Perfectly

Anatomical Differences in Sympathetic
and Parasympathetic Divisions:
Neurotransmitter released by
postganglionic axons

Sympathetic: release norepinephrine
(adrenergic)
•  Parasympathetic: release acetylcholine
(cholinergic)

How well did you know this?

Not at all

Perfectly

The Parasympathetic Division

Cranial outflow
– Comes from cranial nerves in brainstem
– Innervates: organs of the head, neck,
thorax, and most of abdomen
•  Sacral outflow
– Comes from visceral motor region of ventral
gray matter in spine
– Innervates: distal half of large intestine,
reproductive organs and erectile tissue in
external genitalia

How well did you know this?

Not at all

Perfectly

Cranial Outflow (Parasympathetic)

•  Preganglionic fibers run via 
– Oculomotor nerve (III) 
– Facial nerve (VII) 
– Glossopharyngeal nerve (IX) 
– Vagus nerve (X) 
•  Cell bodies of cranial nerves located in 
cranial nerve nuclei in the brainstem 
•  Ganglionic cell bodies located near 
viscera/organs/tissues being innervated 1

How well did you know this?

Not at all

Perfectly

Path of Vagus Nerve (CN X)
sends branches through
parasympathetic autonomic
nerve plexuses:

  Nerve plexus = network of nerves
  Cardiac plexus
  Pulmonary plexus
  Esophageal plexus
  Celiac plexus
  Superior mesenteric plexus

How well did you know this?

Not at all

Perfectly

Sacral Outflow

•  Emerges from S2–S4
•  Innervates organs of the pelvis and lower
abdomen and stimulates defecation,
urination, and erection
•  Preganglionic cell bodies are located in
visceral motor region of spinal gray matter
•  Axons run in ventral roots to ventral rami
and form pelvic nerves
•  Run through the inferior hypogastric (or
pelvic) plexus

How well did you know this?

Not at all

Perfectly

The Sympathetic Division

Issues from T1–L2 
•  Preganglionic fibers emerge from the 
LATERAL GRAY HORN in spinal cord 
•  Supplies visceral organs and structures 
of superficial body regions 
•  Contains more ganglia than the 
parasympathetic division

How well did you know this?

Not at all

Perfectly

Sympathetic Trunk Ganglia

Located on both sides of the vertebral 
column 
•  Linked by short nerves into 
sympathetic trunks 
•  Sympathetic trunk ganglia are also 
called chain ganglia or paravertebral 
ganglia

How well did you know this?

Not at all

Perfectly

Sympathetic neurons in the
thoracolumbar spinal cord send motor
axons through trunk ganglia
via the following sequence

Preganglionic axon (lightly myelinated) 
•  Ventral root 
•  Ventral ramus of spinal nerve 
•  White ramus communicans 
•  Associated sympathetic trunk ganglion 
Postganglionic axon (unmyelinated) 
•  Gray ramus communicans 
•  Ventral ramus of spinal nerve

How well did you know this?

Not at all

Perfectly

White & gray rami communicans

•  White rami communicans—contain lightly
myelinated preganglionic fibers traveling to
sympathetic trunk ganglia
•  Gray rami communicans—contain
unmyelinated postganglionic fibers traveling
from ganglia to peripheral structures

Sympathetic preganglionic axons follow

one of three pathways

Synapses with a postganglionic neuron at
the same level and exit on a spinal nerve at that
level
•  # 2. Axon ascends or descends in the
sympathetic trunk to synapse in another ganglion
•  # 3. Axon does not synapse in the sympathetic
trunk but instead passes through and exits in a
splanchnic nerve and synapses in a collateral
ganglion from which a postganglionic axon
extends to the visceral organ

Collateral (prevertebral) ganglia
(#3) differ from sympathetic trunk
ganglia in three ways

Unpaired, not segmentally arranged
•  Occur only in abdomen and pelvis
•  Lie anterior to the vertebral column

Splanchnic nerves

•  Splanchnic nerves carry preganglionic

neurons to the collateral ganglia

Main sympathetic

collateral ganglia

  Celiac
  Superior mesenteric
  Inferior mesenteric
  Inferior hypogastric

Main sympathetic ganglia

Superior cervical 
•  Middle cervical 
•  Inferior cervical 
•  Celiac 
•  Superior mesentery 
•  Inferior mesentery 
•  Hypogastric

Sympathetic Pathways to the

Head

Preganglionic fibers originate in spinal 
cord at T1–T4 
•  Fibers ascend in the sympathetic 
trunk and synapse in superior cervical 
ganglion 
•  Postganglionic fibers associate with 
eye, lacrimal gland, nasal mucosa, 
salivary glands

Sympathetic Pathways to

Thoracic Organs

Preganglionic fibers originate at spinal levels T1–T6
•  Some fibers synapse in nearest sympathetic trunk
ganglion and the postganglionic fibers run to heart,
lungs and esophagus
•  Functions
– Increase heart rate
– Dilate blood vessels to the heart wall
– Dilate bronchioles
– Inhibit muscles and glands in the esophagus and
digestive system

Sympathetic Pathways to

Abdominal Organs

•  Preganglionic fibers originate in spinal cord
(T5–L2)
•  Pass through adjacent sympathetic trunk
ganglia then travel in splanchnic nerves
•  Synapse in collateral ganglia (celiac and
superior mesenteric ganglia) in abdomen

Sympathetic Pathways to the | Pelvic Organs

Preganglionic fibers originate in the spinal cord from T10–L2 •  Some preganglionic fibers pass directly to autonomic plexuses and synapse in collateral ganglia (inferior mesenteric or inferior hypogastric ganglia) •  Postganglionic fibers go from these plexuses to the bladder, reproductive organs, and distal large intestine

Sympathetic Pathways to the | Pelvic Organs

Some preganglionic fibers descend in the sympathetic trunk to lumbar and sacral ganglia where they synapse •  Postganglionic fibers run in lumbar and sacral nerves to inferior mesenteric, aortic and hypogastric plexuses

The Role of the Adrenal Medulla in | the Sympathetic Division

•  Adrenal medulla is major organ of the sympathetic nervous system •  Constitutes largest sympathetic ganglia •  Secretes large quantities of norepinephrine and epinephrine (=adrenaline) to produce a widespread excitatory response, the “surge of adrenaline” •  Stimulated to secrete by preganglionic sympathetic fibers carried in thoracic splanchnic nerve

Central Control of the ANS

-- Reticular formation exerts most direct influence •  Medulla oblongata e.g., cardiac center, vasomotor center, and digestive activities •  Periaqueductal gray matter in midbrain exerts sympathetic fear response –  Control by the hypothalamus and amygdala •  Hypothalamus—the main integration center of the ANS •  Amygdala—main limbic region for emotions –  Control by the cerebral cortex: for example calm feelings achieved during meditation are associated with cerebral cortex influence on the parasympathetic centers of hypothalamus via various limbic structures

Central nervous system control of | autonomic nervous system

*   Cerebral cortex *   Amygdala *   Hypothalamus *   Brainstem *   Spinal cord

Spinal cord control of autonomic nervous system influences visceral spinal reflexes

*   Urination reflex *   Defecation reflex *   Erection reflex *   Ejaculation/orgasm reflex

Brainstem control of | autonomic nervous system

Reticular formation exerts most direct influence over autonomic functions •  Medulla oblongata e.g., cardiac center, vasomotor center, and digestive activities •  Periaqueductal gray matter in midbrain exerts sympathetic fear response

Hypothalamus control of | autonomic nervous system

•  Hypothalamus is main integration center of the autonomic nervous system by influencing preganglionic autonomic neurons in brainstem and spinal cord through direct connections and relays in reticular formation and periaqueductal gray matter •  Medial and anterior parts direct parasympathetic functions •  Lateral and posterior parts direct sympathetic functions

Amygdala control of | autonomic nervous system

•  Amygdala is main limbic region for emotions •  Amygdala communications with the hypothalamus and the periaqueductal gray matter can stimulate sympathetic activity, especially previously learned fear-related behavior

Cerebral frontal cortex control | of autonomic nervous system

People can exert some conscious control over some autonomic functions by developing control over their thoughts and emotions •  Feelings of calm achieved during meditation are associated with cerebral cortex enhancing the parasympathetic centers in the hypothalamus via limbic structures •  Voluntary recall of a frightful experience can activate the sympathetic system via the amygdala

Disorders of the Autonomic | Nervous System

``` •  Raynaud’s disease—characterized by constriction of blood vessels – Provoked by exposure to cold or by emotional stress •  Hypertension—high blood pressure – Can result from overactive sympathetic vasoconstriction ```