391
CHAPTER ELEVEN
Nervous System II
brain. The person becomes aware of the experience and may
feel pain.
A withdrawal refl
ex protects because it prevents or limits
tissue damage when a body part touches something poten-
tially harmful.
Table 11.2
summarizes the components of a
refl ex arc. Clinical Application 11.2 discusses some familiar
refl
exes.
PRACTICE
7
What is a nerve pathway?
8
Describe a ref
ex arc.
9
DeF
ne ref
ex.
10
Describe the actions that are part o± a withdrawal ref
ex.
inhibit the action of the antagonistic extensor muscles (recip-
rocal innervation). This inhibition allows the fl exor muscles
to effectively withdraw the affected part
(f
g. 11.10)
.
While flexor muscles on the affected side (ipsilateral
side) contract, the fl
exor muscles of the other limb (contral-
ateral side) are inhibited. Furthermore, the extensor muscles
on the contralateral side contract, helping to support the
body weight shifted to that side. This phenomenon, called a
crossed extensor refl
ex,
is due to interneuron pathways in the
refl ex center of the spinal cord that allow sensory impulses
arriving on one side of the cord to pass across to the other
side and produce an opposite effect (±
g. 11.10).
Concurrent with the withdrawal refl ex, other interneu-
rons in the spinal cord carry sensory impulses upward to the
Effector
(muscle or gland)
Central
Nervous
System
Sensory or
afferent neuron
Motor or
efferent neuron
Receptor
(a)
Receptor
Cell body
of sensory
neuron
Sensory neuron
Motor neuron
Effector
(muscle
or gland)
Ventral
Central
canal
White matter
Gray matter
Spinal cord
Dorsal
Interneuron
4
5
3
2
1
I
m
p
u
ls
e
(b)
FIGURE 11.7
Ref
ex arc. (
a
) Schematic o± a ref
ex arc. (
b
) A ref
ex arc usually includes a receptor (1), a sensory neuron (2), integration within the CNS
involving at least one synapse (3), a motor neuron (4), and an e²
ector (5). In this example o± a spinal ref
ex, the integration center is in the spinal cord.
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