The Autonomic Nervous System

The Autonomic Nervous System
Visceral sensory
Visceral motor
&

Autonomic nervous system
The autonomic nervous system is the subdivision of the peripheral nervous system that regulates body activities that are generally not under conscious control
Visceral motor innervates non-skeletal (non-somatic) muscles
Visceral sensory will be covered later

ANS is the subdivision of the peripheral nervous system that regulates body activities that are generally not under conscious control
Visceral motor innervates non-skeletal (non-somatic) muscles
Composed of a special group of neurons serving:
Cardiac muscle (the heart)
Smooth muscle (walls of viscera and blood vessels)
Internal organs
Skin
To repeat…

Basic anatomical difference between the motor pathways of the voluntary somatic nervous system (to skeletal muscles) and those of the autonomic nervous system
Somatic division:
Cell bodies of motor neurons reside in CNS (brain or spinal cord)
Their axons (sheathed in spinal nerves) extend all the way to their skeletal muscles
Autonomic system: chains of two motor neurons
1st = preganglionic neuron (in brain or cord)
2nd = gangionic neuron (cell body in ganglion outside CNS)
Slower because lightly or unmyelinated
(see next diagram)

Axon of 1st (preganglionic) neuron leaves CNS to synapse with the 2nd (ganglionic) neuron
Axon of 2nd (ganglionic) neuron extends to the organ it serves
Diagram contrasts somatic (lower) and autonomic:
autonomic
somatic
Note: the autonomic ganglion is motor
this dorsal root ganglion is sensory

Divisions of the autonomic nervous system (visceral motor part of it)
Parasympathetic division
Sympathetic division

Divisions of the autonomic nervous system
Parasympathetic division
Sympathetic division
Serve most of the same organs but cause opposing or antagonistic effects
Parasysmpathetic: routine maintenance
“rest &digest”
Sympathetic: mobilization & increased metabolism “fight, flight or fright” or “fight, flight or freeze”

Where they come from
Parasympathetic:
craniosacral
Sympathetic:
thoracolumbar

Parasympathetic nervous system
“rest & digest”
Also called the craniosacral system because all its preganglionic neurons are in the brain stem or sacral levels of the spinal cord
Cranial nerves III,VII, IX and X
In lateral horn of gray matter from S2-S4
Only innervate internal organs (not skin)
Acetylcholine is neurotransmitter at end organ as well as at preganglionic synapse: “cholinergic”

Parasympathetic continued
Cranial outflow
III - pupils constrict
VII - tears, nasal mucus, saliva
IX – parotid salivary gland
X (Vagus n) – visceral organs of thorax & abdomen:
Stimulates digestive glands
Increases motility of smooth muscle of digestive tract
Decreases heart rate
Causes bronchial constriction
Sacral outflow (S2-4): form pelvic splanchnic nerves
Supply 2nd half of large intestine
Supply all the pelvic (genitourinary) organs

Parasympathetic
(only look at this if it helps you)

Sympathetic nervous system
“fight, flight or fright”
Also called thoracolumbar system: all its neurons are in lateral horn of gray matter from T1-L2
Lead to every part of the body (unlike parasymp.)
Easy to remember that when nervous, you sweat; when afraid, hair stands on end; when excited blood pressure rises (vasoconstriction): these sympathetic only
Also causes: dry mouth, pupils to dilate, increased heart & respiratory rates to increase O2 to skeletal muscles, and liver to release glucose
Norepinephrine (aka noradrenaline) is neurotransmitter released by most postganglionic fibers (acetylcholine in preganglionic): “adrenergic”

Sympathetic nervous system continued
Regardless of target, all begin same
Preganglionic axons exit spinal cord through ventral root and enter spinal nerve
Exit spinal nerve via communicating ramus
Enter sympathetic trunk/chain where postganglionic neurons are
Has three options…

Options of preganglionic axons in sympathetic trunk
Synapse on postganglionic neuron in chain ganglion then return to spinal nerve and follow its branch to the skin
Ascend or descend within sympathetic trunk, synapse with a posganglionic neuron within a chain ganglion, and return to spinal nerve at that level and follow branches to skin
Enter sympathetic chain, pass through without synapsing, form a splanchnic nerve that passes toward thoracic or abdominal organs
These synapse in prevertebral ganglion in front of aorta
Postganglionic axons follow arteries to organs
(see next slides for drawing examples)

Synapse in chain ganglia
at same level or different level

Pass through ganglia and synapse in prevertebral ganglion

Sympathetic

Adrenal gland is exception
On top of kidneys
Adrenal medulla (inside part) is a major organ of the sympathetic nervous system

Adrenal gland is exception
Synapse in gland
Can cause body-wide release of epinephrine aka adrenaline and norepinephrine in an extreme emergency
(adrenaline “rush” or surge)

Summary

Visceral sensory system
Gives sensory input to autonomic nervous system

Visceral sensory neurons
Monitor temperature, pain, irritation, chemical changes and stretch in the visceral organs
Brain interprets as hunger, fullness, pain, nausea, well-being
Receptors widely scattered – localization poor (e.g. which part is giving you the gas pain?)
Visceral sensory fibers run within autonomic nerves, especially vagus and sympathetic nerves
Sympathetic nerves carry most pain fibers from visceral organs of body trunk
Simplified pathway: sensory neurons to spinothalamic tract to thalamus to cerebral cortex
Visceral pain is induced by stretching, infection and cramping of internal organs but seldom by cutting (e.g. cutting off a colon polyp) or scraping them

Referred pain: important to know
Pain in visceral organs is often perceived to be somatic in origin: referred to somatic regions of body that receive innervation from the same spinal cord segments
Plus left shoulder,
from spleen
Anterior skin areas to which pain is referred from certain visceral organs

Visceral sensory and autonomic neurons participate in visceral reflex arcs
Many are spinal reflexes such as defecation and micturition
reflexes
Some only
involve peripheral
neurons: spinal
cord not involved
(not shown)*
*e.g. “enteric” nervous system: 3 neuron reflex arcs entirely within the wall of the gut