394
UNIT THREE
N
ormal reflexes require and reflect nor-
mal neuron functions, so reflexes are
commonly used to assess the condition
of the nervous system. An anesthesiologist, for
instance, may try to initiate a reflex in a patient
being anesthetized to determine how the anes-
thetic drug is affecting nerve functions. In the
case of injury to some part of the nervous system,
observing reflexes may reveal the location and
extent of damage.
Injury to any component of a reF
ex arc alters
its function. ±or example, stroking the sole of
the foot normally initiates a
plantar ref
ex,
which
F
exes the foot and toes. Damage to certain nerve
pathways (corticospinal tract) may trigger an
abnormal response called the
Babinski reFlex,
a
dorsiflexion, extending the great toe upward
and fanning apart the smaller toes. If the injury is
minor, the response may consist of plantar F
ex-
ion with failure of the great toe to F
ex, or plantar
flexion followed by dorsiflexion. The Babinski
reF
ex is normally present in infants up to the age
of twelve months and may reF
ect immaturity in
their corticospinal tracts.
Other reflexes that may be tested during a
neurological examination include the following:
1.
Biceps-jerk ref
ex.
Extending a person’s
forearm at the elbow elicits this reF
ex. The
examiner places a ²
nger on the inside of
the extended elbow over the tendon of
the biceps muscle and taps the ²
nger. The
biceps contracts in response, F
exing at the
elbow.
2.
Triceps-jerk ref
ex.
±lexing a person’s forearm
at the elbow and tapping the short tendon
of the triceps muscle close to its insertion
near the tip of the elbow elicits this
reF
ex. The muscle contracts in response,
extending the elbow.
3.
Abdominal ref
exes.
These reF
exes are
a response to stroking the skin of the
abdomen. ±or example, a dull pin drawn
from the sides of the abdomen upward
toward the midline and above the
umbilicus contracts the abdominal muscles
underlying the skin, and the umbilicus
moves toward the stimulated region.
4.
Ankle-jerk ref
ex.
Tapping the calcaneal
tendon just above its insertion on the
calcaneus elicits this reF
ex. Contraction
of the gastrocnemius and soleus muscles
causes the response of plantar F
exion.
5.
Cremasteric ref
ex.
This reF
ex is elicited in
males by stroking the upper inside of the
thigh. In response, contracting muscles
elevate the testis on the same side.
11.2
CLINICAL APPLICATION
Uses of Refl
exes
At the base of the brain in an area called the medulla
oblongata most of the fasciculus gracilis and fasciculus
cuneatus F bers cross (decussate) from one side to the
other—that is, those ascending on the left side of the
spinal cord pass across to the right side, and vice versa.
As a result, the impulses originating from sensory
receptors on the left side of the body reach the right side
of the brain, and those originating on the right side of the
body reach the left side of the brain
(f g. 11.12)
.
Ascending Tracts
Among the major ascending tracts of the
spinal cord are the following:
1.
Fasciculus gracilis
(fah-sik
u-lus gras
il-is) and
fasciculus cuneatus
(ku
ne-at-us). These tracts are in
the posterior funiculi of the spinal cord
(f
g. 11.11)
.
Their F bers conduct sensory impulses from the skin,
muscles, tendons, and joints to the brain, where they
are interpreted as sensations of touch, pressure, and
body movement.
Posterior spinocerebellar tract
Lateral corticospinal tract
Lateral reticulospinal tract
Rubrospinal tract
Anterior spinocerebellar tract
Lateral spinothalamic tract
Anterior reticulospinal tract
Medial reticulospinal tract
Fasciculus cuneatus
Fasciculus gracilis
Dorsal column
Anterior
corticospinal
tract
Anterior spinothalamic tract
Anterolateral
system
FIGURE 11.11
Major ascending and
descending tracts in a cross section of the
spinal cord. Ascending tracts are in pink,
descending tracts in light brown. (Tracts
are shown only on one side.) The pattern
varies with the level of the spinal cord. This
pattern is representative of the midcervical
region.
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