Regulation and Integration of the Body
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grasped arm is withdrawn as the opposite arm pushes you away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).
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.
tests the integrity of the spinal cord from L
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
which causes automatic withdrawal of the threatened body part
from the stimulus (
, 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.
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.
Site of stimulus:
A noxious stimulus
on the same
Site of reciprocal
same time, the
on the opposite
side are activated.
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right arm, which is withdrawn reﬂexively while the opposite (left) arm reﬂexively extends and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.