C h a p t e r 3 6
Disorders of Neuromuscular Function
885
Brain Center Connections.
Although a spinal reflex can func-
tion independently, its sensitivity
is adjusted by higher centers in the
brain. Both types of muscle fibers
are supplied with motor neurons—
the extrafusal fibers with large alpha
motor neurons, which produce mus-
cle contraction; and the intrafusal
fibers with smaller gamma motor
neurons, which control the sensitiv-
ity of the stretch reflex. Descending
fibers of motor pathways synapse
with both alpha and gamma motor
neurons, and the impulses are sent
simultaneously to the large extra-
fusal fibers and to the intrafusal
fibers to maintain muscle spindle
tension (and sensitivity) during mus-
cle contraction.
The Knee-Jerk Reflex.
The
knee-jerk reflex that occurs when the
knee is tapped with a reflex hammer
tests for the intactness of the stretch
reflex arc in the quadriceps muscle.
Stretching of the extrafusal fibers by
tapping with a reflex hammer leads
to lengthening of the intrafusal fibers
and increased firing of the type Ia
afferent neuron. Impulses from the
Ia fiber enter the dorsal horn of the
spinal cord and make monosynaptic
contact with the anterior horn alpha
motor neuron supplying the extra-
fusal fibers in the quadriceps mus-
cle. The resultant reflex contraction
(shortening) of the quadriceps mus-
cle is responsible for the knee jerk.
These muscle reflexes are called
deep
tendon reflexes
(DTRs). They can be
checked at the wrists, elbows, knees,
and ankles as a means of assessing
the components of the stretch reflex
at different spinal cord segments.
3
4
Tendon of
quadriceps
α
motor neuron
Muscle
spindle
Quadriceps
Ia afferent
α
motor neuron
Descending fibers
from brain centers
Ia afferent
Gamma
motor neuron
Intrafusal
muscle fiber
Extrafusal
muscle fiber
and Muscle Tone
