Nervous System

A. Cranial nerves - twelve pairs of nerves supplying structures of the head and neck and their derivatives

B. Spinal nerves - 31 pairs of nerves supplying those areas not supplied by cranial nerves

• Typical spinal nerve - consists of mixed sensory and motor nerve fiber components
1. dorsal root(lets) - sensory fibers from a spinal nerve to the dorsal horn of the spinal cord
2. ventral root(lets) - motor fibers from the ventral horn of the spinal cord to a spinal nerve
3. dorsal primary rami - mixed sensory and motor branches of spinal nerves which supply the deep back muscles and the skin overlying the deep back muscles (a narrow band of skin on either side of the median plane)

i. medial cutaneous branches (in gross anatomy, cutaneous = sensory [plus sympathetics])

ii. lateral cutaneous branches

4. ventral primary rami - mixed sensory and motor branches of spinal nerves which supply the rest of the trunk wall and the four extremities

i. lateral cutaneous branches

· anterior branches

· posterior branches

ii. anterior cutaneous branches

· medial branches

· lateral branches

1. Plexus - intermingling of nerve fibers from different areas or spinal levels

a. spinal nerve - a plexus of ventral primary rami from different levels

i. cervical - C1-C4

ii. brachial - C5-T1

iii. lumbar - L1-L4

iv. sacral - L4-S4

Remember: there may be variation in contributions to spinal plexuses.

b. autonomic - mixing of autonomic nerve fibers from different areas

i. sympathetic

ii. parasympathetic

2. Ganglia - collections of neuron cell bodies outside the Central Nervous System (CNS)

a. dorsal root - contain sensory neuron cell bodies, but no synapses; a synapse is defined as the transfer of a nerve impulse from one neuron to another, or from a neuron to an effector organ

b. autonomic - contain postganglionic neuron cell bodies, do have synapses with preganglionic neuron fibers

D. Autonomic nervous system - automatic nervous system

1. Characteristics

a. regulates all visceral structures

b. is automatic – involuntary

c. is, by definition, motor, or efferent - even though it is now known that the autonomic nerves carry afferent (sensory) fibers - accounts for visceral pain

d. consists of two neurons (is two neurons long)

i. preganglionic - located within the CNS

ii. postganglionic - located in autonomic ganglia

e. consists of two antagonistic parts which generally innervate the same visceral organs

i. sympathetic

ii. parasympathetic

2. Sympathetic nervous system - found in all 31 pairs of spinal nerves, but outflow from the CNS is T1-L2

a) thoracolumbar - outflow from all 12 pairs of thoracic and lumbar spinal nerves 1 and 2

b) preganglionic neurons - cell bodies located in the spinal cord between the dorsal horn and the ventral horn; fibers enter spinal nerves with the ventral roots

c) rami communicans - means of sympathetic fibers leaving or re-entering spinal nerves

i. white - conducts preganglionic fibers out of spinal nerves and into the sympathetic chain

ii. gray - conducts postganglionic nerve fibers back into spinal nerves

d) ganglia - contain cell bodies of postganglionic neurons (2nd neuron)

i. sympathetic chain (paravertebral) - run on either side of the vertebral column

ii. collateral (prevertebral) - some distance from the origins of their preganglionic fibers; generally around blood vessels; they receive splanchnic nerves

e) splanchnic nerves - preganglionic nerve fibers which leave the sympathetic chain without synapsing; they synapse in collateral ganglia

f) postganglionic nerve fibers - from autonomic ganglia, after synapse; travel to the effector organ

g) preganglionic fibers are relatively short - postganglionic fibers are relatively long

h) functions - generally prepares body for "fight or flight"

i. increases: heart rate, blood pressure, blood flow to somatic muscles, respiration

ii. decreases: peristalsis, blood supply to the viscera

iii. dilates pupils

iv. stimulates sweat glands

v. stimulus is generalized and long-lasting - one preganglionic neuron activates upto 20 postganglionic neurons

3. Parasympathetic nervous system

1 craniosacral - outflow is via cranial nerves and sacral spinal cord

· cranial nerves numbered 3, 7, 9 and 10

· sacral spinal nerves S2-S4

2 preganglionic neurons - cell bodies are located in special ganglia in the brain stem and in the sacral spinal cord

3 ganglia - contain the cell bodies of postganglionic neurons

· special - four in number, synapsing with preganglionic fibers from cranial nerves 3, 7 and 9, but all hang off of CN #5

· intrinsic - in the walls of the organs innervated, associated with cranial nerve 10 and S2-S4

4 postganglionic nerve fibers - from the 2nd neuron cell bodies after synapse, they innervate effector organs

5 preganglionic fibers are long and postganglionic fibers are very short

6 functions - to preserve the body as a vegetative organ

· decreases heart rate

· increases peristalsis

· constricts pupil and accommodates the eye

· empties the bladder and rectum

· stimulates salivary and lacrimal glands

· stimulus is discrete, localized and short-lived; one preganglionic neuron will effect as few as two postganglionic neurons

Category: Anatomy Notes