The Autonomic Nervous System

Question 1

Autonomic System (ANS)

Answer 1

Involuntary motor (efferent) branch that innervates smooth and cardiac muscle, viscera, and glands

Sometimes referred to as “visceral motor system” regulating unconscious processes

*maintains homeostasis

Question 2

Sympathetic vs. Parasympathetic Nervous System

Answer 2

Most organs receive both sympathetic and parasympathetic innervation

Sympathetic and parasympathetic systems are functionally and anatomically distinct (different)

Effects generally opposing

Question 3

Functions of Sympathetic and Parasympathetic NS

Answer 3

Sympathetic:
-stimulates metabolism; active during stress, exertion, emergency
-increases HR, force of contraction and BP
-dilates blood vessels in skeletal muscles, downregulates digestive activity

Parasympathetic:
-promotes relaxation, nutrient uptake and energy storage
-Decrease HR, force of contraction and BP
-Stimulates digestive activity
-Dilates blood vessels in GI tract

Question 4

Neurons

Answer 4

Basic functional unit of nervous system; highly specialized cells that carry electric signals

Axons are where signal travels to post-synaptic cell

Question 5

Myelin Sheath

Answer 5

Lipid-rich sheath that surrounds some nerve cell axons; acts as insulating layer and increases velocity of signals

Question 6

Autonomic vs. Somatic

Answer 6

Autonomic= involuntary, 2 neruons b/t CNS and target, innervates cardiac muscle, smooth muscle, glands and viscera

Somatic= voluntary, 1 neuron b/t CNS and target, innervates skeletal muscle

Question 7

Pre-ganglionic vs. Post-ganglionic

Answer 7

Pre: neuronal cell bodies in brainstem or spinal cord, myelinated

Post: neuronal cell bodies in autonomic ganglion, unmyelinated

Question 8

Ganglia

Answer 8

Ganglion= collection of neuronal cell bodies found in PNS, autonomic ganglia part of ANS

Function as relay stations b/t pre-ganglionic and post-ganglionic neurons of ANS

Question 9

Autonomic Outflow (Sympathetic vs. Parasympathetic)

Answer 9

Sympathetic: Thoraco-lumbar division
Regions= T1-L2

Parasympathetic: Cranio-sacral division
Cranial nerves:
CN 3, 7, 9, 10
Sacral regions= S2-S4

Question 10

Sympathetic Ganglia (2)

Answer 10

1. Sympathetic chain (paravertebral ganglia)
2. Prevertebral ganglia
   -Celiac
   -Superior mesenteric
   -Inferior mesenteric

Question 11

Sympathetic Chain

Answer 11

Paired sympathetic ganglia linked together to form a chain along length of vertebral column (C1-S5)

Preganglionic sympathetic neurons limited to T1-L2 (above and below distribution)

Sympathetic chain ganglia are connected to coordinate and spread sympathetic responses to many target tissues at the same time

Question 12

Prevertebral Ganglia

Answer 12

Situated anterior to vertebral column along aorta

3 main ganglion:
-Celiac ganglion
-Superior mesenteric ganglion
-Inferior mesenteric ganglion

Named relative to blood vessels branching from abdominal aorta

Question 13

Sympathetic Outflow Pathways

Answer 13

Preganglionic pathway= short

Postganglionic pathway= long

Question 14

White vs. Grey Rami Communicans

Answer 14

Rami communicans= connection b/t spinal nerve and sympathetic trunk

White rami only from T1-L2
Grey rami at all levels along sympathetic chain

Question 15

Sympathetic Outflow Origin

Answer 15

[Preganglionic neuron] originates in the lateral horn of spinal cord from segments T1-L2

Preganglionic neuron exits spinal cord via ventral horn and root to enter spinal nerve

Preganglionic (myelinated) neruon enters the sympathetic chain via the white ramus communicans “on ramp”

Question 16

Preganglionic Neuron 3 Main Routes (Outflow)

Answer 16

*upon entering sympathetic chain

1. Synapse within the chain at the same level
2. Change level within the chain and then synapse
3. Formation of a splanchnic nerve

Question 17

Same Level Outflow Pathway

Answer 17

Preganglionic Neurons:
-Located in lateral horn between T1-L2
-Myelinated axon exits spinal cord and enters sympathetic trunk via white ramus communicans to synapse on the postganglionic neuron
-Communicating ramus called ‘white’ b/c pre fibers are myelinated

Postganglionic Neurons:
-Unmyelinated post neruons pass through grey ramus communicans to enter spinal nerve
-Communicating ramus called ‘grey’ b/c post fibers are unmyelinated
-Post fiber travels within spinal nerve to innervate target

*Nerves target body wall, abdominal and thoracic viscera, limbs

Question 18

Change Level Outflow Pathway

Answer 18

Preganglionic nerve fibers can ascend or descend within sympathetic chain, above/below/within T1-L2 distribution before synapsing

*grey ramus above and below + post ganglionic, white ramus within + pre ganglionic

Question 19

Ascending vs. Descending Nerves

Answer 19

Ascending:
-Head/neck viscera
-Thoracic viscera
-Upper limbs

Descending:
-Abdominopelvic viscera
-Lower limbs

*thoracic viscera supplied by various plexuses, post ganglionic contribute to autonomic plexuses including cardiac plexus (heart) and pulmonary plexus (lungs)

Question 20

Splanchnic Nerves Outflow Pathway

Answer 20

Also known as visceral nerves (associated with abdominoplenvic viscera)

-Form when autonomic fibers do not re-join the spinal nerve; instead form a splanchnic nerve
-Preganglionic fibers bypasses sympathetic chain completely and synapses in a peripheral (pre-vertebral) ganglion near the target organ (celiac, sup. mesenteric or inf. mesenteric)

Originate from thoracic and lumbar vertebral levels of spinal cord as sets of splanchnic nerves including:

1. Greater splanchnic
2. Lesser splanchnic
3. Least/lumbar splanchnic

Question 21

Thoracic and Lumbar Splanchnic Nerves

Answer 21

Greater (T5-9)- synapse in celiac ganglion
Lesser (T10-11)- synapse in superior mesenteric ganglion
Least/lumbar (T12-L2)- synapse in inferior mesenteric ganglion

Question 22

Sympathetic NS Outflow Pathways Summary

Answer 22

1. Preganglionic neuron synapses in sympathetic ganglion, postganglionic axon exits to re-join spinal nerve at same level to innervate target
2. Preganglionic neruon ascends/descends within sympathetic chain before synapsing, postganglionic axon exits via spinal nerve above or below original spinal level
3. Preganglionic neuron passes through sympathetic chain without synapsing, forming a splanchnic nerve, which synapses at corresponding prevertebral ganglion

Question 23

Parasympathetic Outflow Pathways

Answer 23

Preganglionic= long

Postganglionic= short

Question 24

PNS

Answer 24

“Rest and digest”

Targets:
-Viscera of head and neck
-Abdominal and thoracic viscera
-Pelvic viscera

*sweat glands and limb vasculature only receive sympathetic innervation

Question 25

PNS Cranial Outflow

Answer 25

-Preganglionic cell bodies within brainstem cranial nuclei
-Axons travel via cranial nerves 3, 7, 9, 10
-Synapse on ganglia in or near effector organ

Question 26

Cranial Outflow: Head and Neck

Answer 26

Pre-ganglionic neurons leave brainstem from parasympathetic nuclei, synapse on post-ganglionic neuron within parasympathetic ganglia

CN 3- ciliary ganglion (eye- ciliary muscles and sphincter pupillae control shape of lens and size of pupil)
CN 7- pterygopalatine ganglion (lacrimal gland- tear production) and submandibular ganglion (sublingual/submandibular glands- secretion of saliva)
CN 9- otic ganglion (parotid gland)

Question 27

Cranial Outflow: Thoracic and Abdominal Viscera

Answer 27

Preganglionic neurons leave brainstem via CN 10 which contribute to autonomic plexuses, supplies parasympathetics to pharynx, larynx, thorax, and mid-gut structures (to transverse colon) e.g. synapse on post neuron via intra-mural ganglion

Question 28

Sacral Outflow: Distal GI Tract and Pelvic Viscera

Answer 28

Preganglionic neuronal cell bodies located in spinal segments S2-S4

-Axons travel via pelvic splanchnic nerves which contribute to autonomic plexuses (e.g. inferior hypogastric plexus)

Question 29

Visceral Afferent Fibers and Visceral Reflexes

Answer 29

Visceral afferent fibers conduct sensory info from autonomic targets to the CNS (e.g. organs)

*Visceral afferent fibers are DISTINCT from ANS and not considered sympathetic or parasympathetic

Visceral reflexes are the simplest functional units in ANS
-Visceral reflex arc consists of sensory nerve (visceral afferent) and visceral motor neurons (e.g. shining a light in eye triggers visceral reflex that constricts pupils- dilation= symp, constriction= parasymp)

Question 30

Enteric Nervous System

Answer 30

ANS includes a 3rd division called the enteric nervous system
-Network of neurons located in the walls of the digestive tract

Enteric system receives regulatory signals via sympathetic and parasympathetic divisions (but many complex visceral reflexes are still initiated and coordinated independently)