393
CHAPTER ELEVEN
Nervous System II
system and end together in some other part. The names
that identify nerve tracts often refl
ect these common ori-
gins and terminations. For example, a
spinothalamic tract
begins in the spinal cord and carries sensory impulses asso-
ciated with the sensations of pain and touch to the thala-
mus of the brain (part of the diencephalon). A
corticospinal
tract
originates in the cortex of the brain (the outer por-
tion of the cerebrum) and carries motor impulses on upper
motor neurons downward through the spinal cord. These
impulses control lower motor neurons whose axons lead to
skeletal muscles.
Ascending and Descending Tracts
The nerve tracts of the spinal cord together with the spinal
nerves provide a two-way communication system between
the brain and body parts outside the nervous system. The
tracts that conduct sensory impulses to the brain are called
ascending tracts;
those that conduct motor impulses from
the brain to motor neurons reaching muscles and glands are
descending tracts.
The ascending and descending tracts are comprised
of axons. Typically, all the axons in a given tract originate
from neuron cell bodies in the same part of the nervous
TABLE
11.2
|
Parts of a ReF
ex Arc
Part
Description
±unction
Receptor
The receptor end of a dendrite or a specialized receptor cell in a
sensory organ
Sensitive to a speciF
c type of internal or external change
Sensory neuron
Dendrite, cell body, and axon of a sensory neuron
Transmits nerve impulse from the receptor into the brain or spinal cord
Interneuron
Dendrite, cell body, and axon of a neuron within the brain or spinal
cord
Serves as processing center; conducts nerve impulse from the sensory
neuron to a motor neuron
Motor neuron
Dendrite, cell body, and axon of a motor neuron
Transmits nerve impulse from the brain or spinal cord out to an e±
ector
ector
A muscle or gland
Responds to stimulation by the motor neuron and produces the re²
ex
or behavioral action
Motor
neurons
= Stimulation
= Inhibition
Interneuron
Motor
neurons
Flexor
relaxes
Flexor contracts
Extensor
contracts
Extensor
relaxes
Sensory neuron
+
+
+
FIGURE 11.10
When the ²
exor muscle on one side is stimulated to contract in a withdrawal re²
ex, the extensor muscle on the opposite side also
contracts. This helps to maintain balance.
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