AUTONOMIC NERVOUS SYSTEM

I.   INTRODUCTION

A.  Somatic Nervous System: innervates skeletal muscle and receives sensory information from the senses
*  “voluntary” nervous system
* CNS and PNS components

B.  Autonomic Nervous System: innervates the viscera (organs)
* influences smooth muscle, cardiac muscle, and glands of skin and viscera
*  “involuntary” nervous system
* CNS and PNS components

–  Most cranial and spinal nerves contain both somatic and autonomic axons
-- A.N.S. contains both motor (efferent) and sensory (afferent) axons

II.  COMPARISON OF SOMATIC VS. AUTONOMIC MOTOR SYSTEMS

A.  Somatic Motor:

*  uses a single neuron to go from spinal cord to skeletal muscle

B.  Autonomic Motor:

*  it uses two neurons to go from the spinal cord to the target tissue:

a) 1st neuron is preganglionic neuron:
* cell body located in brain or spinal cord
* axon will synapse with postganglionic neuron

 

b) 2nd neuron is a postganglionic (ganglionic) neuron:
* cell body in an autonomic ganglion
* axon goes to target tissue (effector)

 

What do you think?  Why does the autonomic nervous system use two neurons in a chain to an organ?

III. DIVISIONS OF THE A.N.S.

A.  Sympathetic Division: (Thoracolumbar)
–  originates in lateral horns of T1-L2 region of spinal cord
--“emergency” situations (flight or fight response)
-- “mass action” effect

B.  Parasympathetic Division: (Craniosacral)
–  some cranial nerves and parts of the sacral region of the spinal cord
-- maintains a homeostatic environment

–  “resting and digesting” system
-- parasympathetic activity is usually discrete and localized

--  Table 18.2 in text compares both divisions

IV.  PARASYMPATHETIC DIVISION

A.  Location & characteristics
-- cranial components are part of CN III (Oculomotor), VII (Facial), IX (Glossopharyngeal), and X (Vagus)
-- sacral components from S2 - S4 segments of the spinal cord
*  pelvic splanchnic nerves

B.  Components

1) Cell bodies of preganglionic motor neurons located w/certain cranial nerves, or in the sacral part of the spinal cord

2) Preganglionic axons

synapse in terminal ganglia which are located close to or on the organ being innervated

3)  Parasympathetic postganglionic axons

travel from the terminal ganglia to the target organ

V.   SYMPATHETIC DIVISION

A.  Components:

1) Cell bodies of preganglionic motor neurons in lateral horns of T1-L2 segments of spinal cord

* the preganglionic axons exit to spinal nerve, and then go into the:

2) White Rami (singular: white ramus):
* runs from spinal nerve to sympathetic trunk

3) Sympathetic Trunks:
* long chains of axons  on either side of vertebral column

* Sympathetic trunk (Paravertebral) ganglia

4) Gray Rami (singular: gray ramus):
*  runs from the sympathetic trunk TO a spinal nerve
* sympathetic postganglionic axons

5) Splanchnic Nerves:
* preganglionic axons that do not synapse in the sympathetic trunk ganglia

 

6) Prevertebral (Collateral) Ganglia:
* splanchnic nerves synapse here
* located in front of the vertebral column

 7) Sympathetic postganglionic axons
* travel directly to target organs

B. Pathways for preganglionic sympathetic axons:

Spinal Nerve Pathway (for innervating blood vessels, glands in skin):

• preganglionic axon synapses in sympathetic trunk ganglion

• postganglionic axon leaves sympathetic trunk in gray ramus, travels in spinal nerve to skin

Postganglionic Sympathetic Nerve Pathway (for innervating internal thoracic organs)

• preganglionic axon synapses in sympathetic trunk ganglion

• postganglionic axon does NOT go through a gray ramus

• postganglionic axon extends anteriorly from the trunk and goes to thoracic organ

Splanchnic Nerve Pathway (for innervating abdominal and pelvic organs)

* preganglionic axon does NOT synapse in sympathetic trunk ganglion

* preganglionic axon leaves sympathetic trunk as a splanchnic nerve

* splanchnic nerve travels to prevertebral ganglion, synapses with postganglionic neuron

* postganglionic axon goes to abdominal/pelvic organ

VI.  FUNCTIONS OF THE ANS
–  also review CRANIAL NERVES lecture

VII. HIERARCHY OF THE A.N.S.
–  hypothalamus
*  oversees ALL ANS activity

–  “middle management” are the autonomic nervous centers in the brainstem

–  the “ordinary workers” are the preganglionic and postganglionic neurons

Parasympathetic innervation causes which of the following?

a. increases heart rate

b. bronchodilation

c. increased activity in the GI tract

d. dilates pupil

e. inhibits salivary gland secretion

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