umn, the lumbar, sacral, and coccygeal nerves do not exit
the cord at the same levels where they exit the column.
Furthermore, the spinal cord is enlarged in the cervical
and lumbar areas where nerves from the upper and lower
limbs connect.
Like the brain, the spinal cord is covered by menin-
ges (Figure 7.12b) and bathed in CSF. The nerve fibers
of the spinal nerves enter and exit the cord at each seg-
ment. The cell bodies of the sensory neurons lie outside
the cord, in ganglia. The cell bodies of the motor neurons
lie in the butterfly-shaped gray area (Figure 7.12c) at
the center of the spinal cord. Surrounding the gray mat-
ter is the white matter. The white matter contains bun-
dles of myelinated nerve fibers ascending and descending
through the spinal cord in separate tracts. The shape and
amount of white and gray matter varies with the spinal
cord segment. The lower segments have less white mat-
ter than upper segments because there are fewer ascend-
ing and descending pathways at the lower levels of the
spinal cord.
The spinal cord is a relay and processing center for
neural input from the periphery. Nerve fibers from recep-
tors of sensory neurons throughout the body enter the spi-
nal cord at various levels depending upon the receptor’s
location. The pathway from there to the brain can involve
several synapses with interneurons prior to the message
reaching its destination, where analysis of the sensation
will occur (Figure 7.13). There are a number of ascend-
ing spinal tracts that conduct information through the spi-
nal cord. Some of the tracts will cross to opposite sides and
some remain on the same side of the body as they make
their way to the brain.
A scending and d esce n d in g p ath w ay s th ro u g h
a.
Sensory information is carried from outside the
body through the spinal cord to the brain.
Ascending (sensory) pathway
1. Incoming sensory neuron (first neuron) has axon that
extends up to medulla.
2. Axon of second neuron crosses midline and extends to
thalamus.
3. Third neuron extends to primary somatosensory cortex.
th e brain and spinal cord
• Figure 7.13
b. Motor pathways descend from the brain through
the spinal cord to the outside.
Descending (motor) pathway
1. Upper motor neuron extends from primary motor cortex into
spinal cord.
2. Upper motor neuron synapses on lower motor neuron in
spinal cord. If the neuron decussates (crosses over) as
shown, the synapse will be on the opposite side of the spinal
cord. Note that some motor neurons do not cross over.
SECOND
NEURON
Primary somatosensory
area of cerebral cortex
Thalamus
THIRD
NEURON
Receptors for
touch,
pressure,
vibration, and
proprioception
Spinal cord
Spinal nerve
FIRST NEURON
Posterior root
ganglion
Medulla
RIGHT SIDE
OF BODY
LEFT SIDE
OF BODY
RIGHT SIDE
Primary motor area of cerebral cortex
LEFT SIDE
The Central Nervous System Coordinates All Nervous Activity
207
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