Chapter 13
The Peripheral Nervous System and Reflex Activity
515
13
Branches of the afferent fibers also synapse with interneurons
that inhibit motor neurons controlling antagonistic muscles (par-
allel processing), and the resulting inhibition is called
reciprocal
inhibition
. Consequently, the stretch stimulus causes the antag-
onists to relax so that they cannot resist the shortening of the
“stretched” muscle caused by the main reflex arc. While this spinal
reflex is occurring, information on muscle length and the speed
of muscle shortening is being relayed (mainly via the dorsal white
columns) to higher brain centers (more parallel processing).
Te most familiar clinical example of a stretch reflex is the
knee-jerk reflex we have just described. Stretch reflexes can be
elicited in any skeletal muscle by a sudden jolt to the tendon
or the muscle itself. All stretch reflexes are
monosynaptic
and
ipsilateral
. In other words, they involve a single synapse and
motor activity on the same side of the body. Stretch reflexes are
the only monosynaptic reflexes in the body. However, even in
these reflexes, the part of the reflex arc that inhibits the motor
neurons serving the antagonistic muscles is polysynaptic.
A positive knee jerk (or a positive result for any other stretch
reflex test) provides two important pieces of information. First,
it proves that the sensory and motor connections between that
muscle and the spinal cord are intact. Second, the vigor of the
response indicates the degree of excitability of the spinal cord.
When the spinal motor neurons are highly facilitated by impulses
Unstretched muscle.
Action potentials (APs)
are generated at a
constant rate in the
associated sensory fiber.
Muscle
spindle
Intrafusal
muscle fiber
Sensory
fiber
Extrafusal
muscle fiber
Time
Stretched muscle.
Stretching activates the
muscle spindle, increasing
the rate of APs.
Time
But normally
α
-
γ
coactivation occurs.
Both
extrafusal and intrafusal
muscle fibers contract.
Tension is maintained in
the muscle spindle and it
can still signal changes in
length.
Time
If only
α
motor neurons
were activated.
Only the
extrafusal muscle fibers
contract. The muscle
spindle becomes slack
and no APs are fired. It is
unable to signal further
length changes.
Time
(a)
How muscle stretch is detected
(b)
The purpose of
α
-
γ
coactivation
Figure 13.17
Operation of the muscle spindle.
The action potentials generated in the
sensory fibers are shown for each case as black lines in yellow bars.
The Stretch Reflex
By sending commands to the motor neurons, the brain essen-
tially sets a muscle’s length. Te
stretch reflex
makes sure that
the muscle stays at that length. For example, the
patellar
(pah-
tel
9
ar) or
knee-jerk reflex
is a stretch reflex that helps keep your
knees from buckling when you are standing upright. As your
knees begin to buckle and the quadriceps lengthens, the stretch
reflex causes the quadriceps to contract without your having to
think about it.
Focus on the Stretch Reflex
(Figure 13.18)
shows
the stretch reflex and a specific example—the knee-jerk reflex.
Te stretch reflex is important for maintaining muscle tone
and adjusting it reflexively. It is most important in the large
extensor muscles that sustain upright posture and in postural
muscles of the trunk. For example, stretch reflexes initiated first
on one side of the spine and then on the other regulate contrac-
tions of the postural muscles of the spine almost continuously.
Let’s look at how the stretch reflex works. As we’ve just seen
in Figure 13.17, when stretch activates sensory neurons of mus-
cle spindles, they transmit impulses at a higher frequency to the
spinal cord. Tere the sensory neurons synapse directly with
a
motor neurons, which rapidly excite the extrafusal muscle fi-
bers of the stretched muscle (Figure 13.18). Te reflexive muscle
contraction that follows (an example of serial processing) resists
further muscle stretching.
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