Amber Benton
Anatomical Organization of Nervous System
A. Central Nervous system – brain and spinal cord
Nucleus and tract are components of the CNS:
-nucleus is a collection of neuronal cell bodies
-tract is a collection of axons, or nerve processes
B. Peripheral Nervous system – everything outside of CNS
-Cranial nerves (attached to brain) – 12 pairs
-Spinal nerves (attached to spinal cord) – 31 pairs (8 cervical, 12
thoracic, 5 lumbar, 5 sacral, 1 coccygeal)
Ganglion and Nerve are components of the PNS:
-ganglion is a collection of neuronal cell bodies
-Nerve is a collection of axons, or nerve processes
Dendrites (highly branched) receive information while axons
(unbranched/singular) deliver signals
Sensory (afferent) transmission: involves pseudounipolar neurons
receptors in skin → spinal cord (dorsal horn) → brain or another
location
Motor (efferent) transmission: involves multipolar neurons (most
common type)
Signal in brain → spinal cord (ventral horn) → peripheral nerves on
skeletal muscles
Clinical Example: patellar-tendon reflex
**Spinal cord is continuous with brainstem and connects brain to the
body - “conduit”
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Spinal Cord Protection- 5 levels (Vertebral column, 3 meninges, CSF)
1. Vertebral column is a bony covering that allows spinal cord to pass
through vertebral foramen, forming a spinal canal with multiple
vertebral foramen stacked on top of each other
2. Meninges (3 connective tissue coverings) – cover brain as well
A) Dura mater is outermost dense CT that continues inferiorly
to 2 sacral vertebra (end of dural sac-pg.14) and forms anchoring
ligament to coccyx (filum terminale externum-pg.14)
**Epidural space is above (only in transverse cross section),
or superficial, to dura mater (between bone and dura mater) and contains
fat and veins which epidural anesthesia is administered
**Subdural space is below (only in transverse cross section),
or deep to, dura mater and above, or superficial to arachnoid mater
(between dura mater and arachnoid mater); described as only potential
space, hemorrhage may occur here
nd
B) Arachnoid mater is a thin, transparent membrane with weblike extensions, or arachanoid trabeculae extending through
subarachnoid space to reach pia mater
**Subarachnoid space (between arachnoid mater and pia
mater) contains cerebrospinal fluid which acts as a defense mechanism,
shock absorber, and maintains chemical environment – clinical
importance: for a lumbar puncture spinal tap or anesthesia, must poke
through dura mater to enter subarachnoid space below L2 to avoid spinal
cord puncture even though may risk puncturing cauda equina (between
L3 & L4 or L4 & L5)
**Traversing the subarachnoid space is the denticulate
ligament, which is a lateral extension of pia mater to dura mater
anchoring spinal cord
C) Pia mater is innermost thin CT layer tightly bound to spinal
cord “invests” every contour of spinal cord
Filum terminale internum (pg. 14) is inferior extension of pia
mater extending from end of conus medullaris (tapered inferior portion
of spinal cord) to base of dural sac at S2 (2nd sacral);
found within
cauda equina, a stretched continuation of dorsal and ventral roots of
spinal nerves inferior to L2 (2nd lumbar level, or below conus
medullaris)
Formation of a Spinal Nerve
Receptor on skin of hand receives sensory stimuli and signal travels
along peripheral process of pseudounipolar (sensory) neuron through
dorsal root ganglion to dorsal root (contains sensory fibers only) where
central process, or axon, enters dorsal horn (within unmyelinated gray
matter) via dorsal root. Signal is sent to CNS, or brain.
Multipolar, or motor, neuron in ventral horn of gray matter sends signal
from brain through ventral root and ends on skeletal muscle in hand
The dorsal and ventral roots come together at the intervertebral foramen
(must have at least two vertebrae) to allow spinal nerve to exit from
spinal cord. The sensory and motor neurons in the same “conduit” form
a spinal nerve, or mixed nerve
Spinal Nerve Branching
Once the spinal nerve has been formed from the dorsal root of the
sensory neuron and ventral root of the motor neuron coming together in
same “conduit”, it can then be branched off into two primary rami
(which are both mixed nerves):
Dorsal primary ramus: contains sensory neurons (originating from
dorsal root and horn) ending on receptors on skin of back region and
motor neurons (originating from ventral root and horn) ending on
intrinsic and extrinsic muscles of back which flex and extend vertebral
column
Ventral primary ramus: contains sensory neurons (originating from
dorsal root and horn) ending on receptors in skin of anterior/lateral trunk
and limbs and motor neurons (originating from ventral root and horn)
ending on skeletal muscle of anterior/lateral trunk and skin
**Since the dorsal primary ramus only accommodates innervation of the
back muscles, the ventral primary ramus is understandably much larger
with more networking than the dorsal primary ramus
Differential Development of Spinal Cord
**Explains why vertebral column and spinal cord are not the same
length (vertebral column is longer) and the presence of cauda equina
As Early fetus or Embryo:
Spinal cord fills bony vertebral column
↓
Dorsal and ventral roots at 1st sacral nerve
come together at intervertebral foramen
↓
Bony vertebral column grows more than
Spinal cord over time
↓
Dorsal and ventral roots stretch to
Accommodate this growth
↓
Cauda equina and filum terminale
internum are formed