Dr.Hand
9.9.96

Peripheral Nervous system  (PNS)
 
I. DEFINITION

	A. Everything superficial to CNS
	B. Spinal (spinal nerves) and cranial (cranial nerves) portions

II. Functional Importance of PNS

	A. Sensory Importance: 
    
    B. Motor commands:
				
The motor and sensory components work together, providing linkage to CNS. 
Sensory info taken in about what's going on in the periphery, and 
messages from brain sent out. 

III. Basic PNS subdivisions

	A. Autonomic: visceral/smooth muscle related
	B. Somatic (somite related)
		1. basic tissue types innervated: skin, subcutis, striated muscle, 				
tendons ligaments, synovial sheaths, joint capsules, and bone.

IV: today's lecture

	A. somatic subdivision of spinal portion of PNS.

V. PNS components, somatic
	A. gross anatomical parts.
		1. roots, rootlets, ganglia, spinal nerve, primary rani, plexi, and 			
named peripheral nerves.

under the dorsal root ganglion is the ventral root ganglion. the dorsal 
and ventral roots beyond the ganglion come together to form the spinal 
nerves. the dorsal root ganglion is a sensory "into the cns" pathway. the 
ventral root ganglion takes motor messages out to skeletal muscle.

spinal nerve is a mixed pathway, carrying the fibers from the dorsal and 
ventral root ganglia. 

these roots and rootlets come out in segmental fashion between vertebrae, 
in the vertebral canals. The nerves are numbered like the vertebrae. the 
spinal nerves supply skin and muscle for that segment of the body. All 
the way from head to tail....

Epaxial muscles - dorsal to the transverse processes.
Hypaxial muscles - ventral to transverse processes.

dorsal lamina covers spinal cord, can be opened to relieve pressure 
-->laminectomy.

Intervertebral foramen allows nerve roots to get out of spinal column.

dorsal branch small compared to ventral branch. Goes to skin. Ventral 
ramis and branch very large. Involved not only in inervating arm, but 
digits etc, where really good sensory innervation needed. So, more 
hypaxial innervation exists.

communicating ramis/visceral branch - part of ANS, not going to deal 
w/today.

must differentiate betw./root and ramis. dorsal branches and ventral 
branches are both sensory and motor. ventral root -just motor, dorsal 
root- just sensory.

primitive pattern of nerve pathways easy to see along chest/intercostals, 
from cord out to anterior cutaneous branch. but, in animals with limbs, 
you end up with rami from adjacent segments joining into plexi - brachial 
or lumbosacral plexi.  off of those plexi come the named peripheral 
nerves. - radial, femoral, sciatic, etc.

in lower cervical vertebrae and lower thoracic vertebrae, you can see 
intertwining of the ventral rami, which come together and fuse into 
plexi.

[slide- shows limb bud in fetus, segemented areas...]


[slide - typical thoracic nerve and its branches.]

	B. Basic histologic organization.
		1. MOTOR component
			a. motorneurons - location
			b. motor end plates

motor neurons have cell body, long processes w/recieving dendrites and 
long axon to carry messages away from body, which connects to muscle or 
skin or whatever.

[slide- myelin sheath over motor nerve axon, adipose tissue around.]
		2. SENSORY component
			a. cells of origin (psueudounipolar neurons, because only one pole 				
comes off.) and location

cell body is round, with one process that comes off and then splits 
itself into a process which synapses with the cord and a process which 
innervates the periphery (skin, joints, muscle, etc)

			b. receptors/types/locations

specialized cell types which respond to stimuli eg pain, light, sound, 
etc
[slide- various receptor types which act as transducers ]

			c. innervation pattern (punctate, density).

receptors are not spread out uniformly across surfaces. they are 
punctate, and scattered. there is a huge density of sensory receptors, 
for example, on the tips of the human fingers; fewer over palm, and fewer 
still on the doral trunk.

so, sensory info travels to cord via dorsal root, and axons send off side 
branches called collaterals which connect to lots of neurons, and message 
can be sent right back out via motor nerve in a reflex arc, or it can go 
all the way up to the brain via the ascending pathways in the gray matter 
of the cord.