518
UNIT 3
Regulation and Integration of the Body
13
Te crossed-extensor reflex is obvious when you step bare-
foot on broken glass. Te ipsilateral response causes you to
quickly liF your cut foot, while the contralateral response ac-
tivates the extensor muscles of your opposite leg to support
the weight suddenly shiFed to it. Te crossed-extensor reflex
also occurs when someone unexpectedly grabs your arm. Te
grasped arm is withdrawn as the opposite arm pushes you away
from the attacker (±igure 13.20).
Superficial Reflexes
Superficial reflexes
are elicited by gentle cutaneous stimula-
tion, such as that produced by stroking the skin with a tongue
depressor. Tese clinically important reflexes depend both on
functional upper motor pathways and on cord-level reflex arcs.
Te best known are the plantar and abdominal reflexes.
Plantar Reflex
Te
plantar reflex
tests the integrity of the spinal cord from L
4
to S
2
and indirectly determines if the corticospinal tracts are
functioning properly. ²o elicit the plantar reflex, draw a blunt
object downward along the lateral aspect of the plantar surface
The Flexor and Crossed-Extensor Reflexes
A painful stimulus initiates the
flexor
, or
withdrawal
,
reflex
,
which causes automatic withdrawal of the threatened body part
from the stimulus (
Figure 13.20
, leF). Tink of the response
that occurs when you prick your finger. ±lexor reflexes are ip-
silateral and polysynaptic, the latter a necessity when several
muscles must be recruited to withdraw the injured body part.
Because flexor reflexes are protective and important to our
survival, they override the spinal pathways and prevent any
other reflexes from using them at the same time. However, like
other spinal reflexes, descending signals from the brain can
override flexor reflexes. Tis happens when you are expecting
a painful stimulus, for example a skin prick as a lab technician
prepares to draw blood from a vein.
Te
crossed-extensor reflex
oFen accompanies the flexor
reflex in weight-bearing limbs and is particularly important in
maintaining balance. It is a complex spinal reflex consisting of
an ipsilateral withdrawal reflex and a contralateral extensor re-
flex. Incoming afferent fibers synapse with interneurons that
control the flexor withdrawal response on the same side of the
body and with other interneurons that control the extensor
muscles on the opposite side.
Afferent
fiber
Extends
Flexes
Efferent
fibers
Extensor
inhibited
Flexor
stimulated
Site of stimulus:
A noxious stimulus
causes a
flexor
reflex
on the same
side, withdrawing
that limb.
Site of reciprocal
activation:
At the
same time, the
extensor muscles
on the opposite
side are activated.
Arm movements
Interneurons
Efferent
fibers
Flexor
inhibited
Extensor
stimulated
+
+
+
+
+
Excitatory synapse
Inhibitory synapse
Figure 13.20
The crossed-extensor reflex.
In this example, a stranger suddenly grasps the
right arm, which is withdrawn reflexively while the opposite (left) arm reflexively extends and
pushes the stranger away.
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