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Chapter 10
stretched, the greater the rate of receptor fi ring. In contrast,
contraction of the extrafusal fi bers and the resultant shorten-
ing of the muscle remove tension on the spindle and slow the
rate of fi ring in the stretch receptor (
Figure 10–5b
).
When the afferent fi bers from the muscle spindle enter
the central nervous system, they divide into branches that take
different paths. In
Figure 10–6
, path A makes excitatory syn-
apses directly onto motor neurons that go back to the muscle
that was stretched, thereby completing a refl ex arc known as
the
stretch refl
ex.
This refl
ex is probably most familiar in the form of the
knee jerk,
part of a routine medical examination. The exam-
iner taps the patellar tendon (see Figure 10–6), which passes
over the knee and connects extensor muscles in the thigh to
the tibia in the lower leg. As the tendon is pushed in by tap-
ping, the thigh muscles it is attached to are stretched, and all
the stretch receptors within these muscles are activated. This
stimulates a burst of action potentials in the afferent nerve
fi bers from the stretch receptors, and these action potentials
activate excitatory synapses on the motor neurons that control
these same muscles. The motor units are stimulated, the thigh
muscles contract, and the patient’s lower leg extends to give
the knee jerk. The proper performance of the knee jerk tells
the physician that the afferent fi bers, the balance of synaptic
input to the motor neurons, the motor neurons, the neuro-
muscular junctions, and the muscles themselves are function-
ing normally.
Because the afferent nerve fi bers in the stretched muscle
synapse directly on the motor neurons to that muscle without
any interneurons, this portion of the stretch refl ex is called
monosynaptic.
Stretch refl exes have the only known mono-
synaptic refl ex arcs. All other refl
ex arcs are
polysynaptic;
they have at least one interneuron, and usually many, between
the afferent and efferent neurons.
In path B of Figure 10–6, the branches of the afferent
nerve fi bers from stretch receptors end on inhibitory inter-
neurons. When activated, these inhibit the motor neurons
controlling antagonistic muscles whose contraction would
interfere with the refl ex response. In the knee jerk, for exam-
ple, neurons to muscles that fl ex the knee are inhibited. This
component of the stretch refl ex is polysynaptic. The activation
of neurons to one muscle with the simultaneous inhibition of
neurons to its antagonistic muscle is called
reciprocal inner-
vation.
This is characteristic of many movements, not just the
stretch refl ex.
Path C in Figure 10–6 activates motor neurons of
syn-
ergistic muscles
—that is, muscles whose contraction assists
the intended motion. In the example of the knee jerk refl ex,
this would include other muscles that extend the leg.
In path D of Figure 10–6, the axon of the afferent neu-
ron continues to the brainstem and synapses there with inter-
neurons that form the next link in the pathway that conveys
information about the muscle length to areas of the brain
dealing with motor control. This information is especially
important during slow, controlled movements such as the per-
formance of an unfamiliar action. Ascending paths also pro-
vide information that contributes to the conscious perception
of the position of a limb.
Alpha-Gamma Coactivation
As indicated in Figure 10–5b, stretch on the intrafusal fi bers
decreases when the muscle shortens. In this example, slacken-
ing of the spindle stretch receptors greatly reduces the action
potentials along the afferent neuron, and there can be no
indication of any further changes in muscle length the entire
time the muscle is shortening. A mechanism exists to pre-
vent this loss of information: the two ends of each intrafusal
Begin
To brain
Motor neuron to flexor muscles
Motor neuron to other
extensor muscles
Motor neuron to extensor
muscle originally stretched
Flexor muscle
Afferent
nerve
fiber from
stretch
receptor
Spinal
cord
Tibia
Stretch
receptor
Extensor
muscle
Kneecap
(bone)
Point of
physician’s
tap on knee
Patellar
tendon
A
B
C
D
Excitatory
synapse
Inhibitory
synapse
Excitatory
neuromuscular
junction
Neurons ending with:
Figure 10–6
Neural pathways involved in the knee jerk refl ex. Tapping the
patellar tendon stretches the extensor muscle, causing (paths A and
C) compensatory contraction of this and other extensor muscles,
(path B) relaxation of fl exor muscles, and (path D) information
about muscle length to go to the brain. Arrows indicate direction of
action potential propagation.
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