The Peripheral Nervous System and Reﬂex Activity
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
. 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
. 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
Action potentials (APs)
are generated at a
constant rate in the
associated sensory fiber.
Stretching activates the
muscle spindle, increasing
the rate of APs.
extrafusal and intrafusal
muscle fibers contract.
Tension is maintained in
the muscle spindle and it
can still signal changes in
extrafusal muscle fibers
contract. The muscle
spindle becomes slack
and no APs are fired. It is
unable to signal further
How muscle stretch is detected
The purpose of
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
makes sure that
the muscle stays at that length. For example, the
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
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
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.