NERVOUS SYSTEM II
NERVOUS SYSTEM II
I. CRANIAL NERVES AND GANGLIA
A. Amniotes - 12 pairs of cranial nerves; fish and amphibians have 10 pairs (pairs are numbered with Roman numerals).
B. Most likely derived from segmental structures, they have lost segmental arrangement and are specialized.
C. In mammals, they represent, for the most part, either the primitive dorsal or primitive ventral root (no union between dorsal and ventral roots).
D. First two cranial nerves are extensions of brain tracts (not true nerves).
E. Cranial nerves V, VII, VIII, IX, and X have associated ganglia.
F. Cranial ganglia develop differently than spinal ganglia.
1. Neural crest component.
2. Epidermal placode component.
II. SPINAL NERVES
A. Segmentally arranged connections between spinal cord and parts of body.
B. Dorsal roots:
1. Outgrowths of spinal ganglia; afferent fibers (in-coming).
2. Spinal ganglia appear as compact masses of cells arranged segmentally along the length of the spinal cord; derived from neural crest cell which give rise to sensory neurons.
3. Cell bodies of ganglia send out processes to spinal cord and to peripheral organs.
a. Process to spinal cord connect to dorsolateral (alar) plate.
b. Longer processes grow out to skin, muscles, organs, etc.
c. Fibers are sensory if connected to skin. eg. skin -> sensory fiber -> spinal ganglion -> dorsal root -> spinal cord.
C. Ventral roots:
1. Efferent fibers (out-going); motor nerves.
2. Grow out of ventrolateral (basal) plate and join up with sensory fibers growing out of ganglia.
D. Innervation of limbs:
1. Most vertebrate limbs are supplied by spinal nerves from several ganglia.
2. Plexus = interconnected network of spinal nerves at the base of a limb.
a. Brachial plexus - forelimbs.
b. Lumbar plexus - hind limbs.