Chapter 13
The Peripheral Nervous System and Reflex Activity
517
13
produce a large degree of motion (i.e., circumduct your pitch-
ing arm). Other athletes who require movements of maximum
force learn to stretch muscles as much and as quickly as possible
just before the movement. Tis advantage is demonstrated by
the crouch that athletes assume just before jumping or running.
As we have seen, muscle tone and smooth coordination of
movement depend upon intact stretch reflex pathways. Both
afferent and efferent fibers to the muscle spindle are vitally im-
portant. If either afferent or efferent fibers are cut, the muscle
immediately loses its tone and becomes flaccid.
The Tendon Reflex
Stretch reflexes cause muscle contraction in response to in-
creased muscle length (stretch). Te polysynaptic
tendon re-
flexes
, on the other hand, produce exactly the opposite effect:
Muscles relax and lengthen in response to tension.
When muscle tension increases substantially during contrac-
tion or passive stretching, high-threshold
tendon organs
may be
activated. Afferent impulses are transmitted to the spinal cord,
and then to the cerebellum, where the information is used to
adjust muscle tension. Simultaneously, motor neurons in spinal
cord circuits supplying the contracting muscle are inhibited and
antagonist muscles are activated, a phenomenon called
recipro-
cal activation
. As a result, the contracting muscle relaxes as its
antagonist is activated
(Figure 13.19)
.
±endon organs help to prevent muscles and tendons from
tearing when they are subjected to possibly damaging stretching
force. ±endon organs also function at normal muscle tensions.
In the normal range, tendon organs help to ensure smooth on-
set and termination of muscle contraction.
descending from higher centers, just touching the muscle tendon
produces a vigorous reflex response. On the other hand, when in-
hibitory signals bombard the lower motor neurons, even pound-
ing on the tendon may fail to trigger the reflex response.
Homeostatic Imbalance
13.9
Stretch reflexes tend to be hypoactive or absent in cases of periph-
eral nerve damage or ventral horn injury involving the tested area.
Tese reflexes are absent in those with chronic diabetes mellitus
or neurosyphilis and during coma. However, they are hyperactive
when lesions of the corticospinal tract reduce the inhibitory effect
of the brain on the spinal cord (as in stroke patients).
Adjusting Muscle Spindle Sensitivity
Te motor supply to the muscle spindle allows the brain to vol-
untarily modify the stretch reflex response and the firing rate
of
a
motor neurons. When the γ neurons are vigorously stimu-
lated by impulses from the brain, the spindle is stretched and
highly sensitive, and muscle contraction force is maintained or
increased. When the γ motor neurons are inhibited, the spindle
resembles a loose rubber band and is nonresponsive, and the
extrafusal muscles relax.
Te ability to modify the stretch reflex is important in many
situations. As the speed and difficulty of a movement increase,
the brain increases γ motor output to make the muscle spindles
more sensitive. Tis sensitivity is highest when balance reflexes
must be razor sharp, as for a gymnast on a balance beam. On
the other hand, if you want to wind up to pitch a baseball, it is
essential to suppress the stretch reflex so that your muscles can
+
Excitatory synapse
-
Inhibitory synapse
Quadriceps strongly contracts.
Tendon organs are activated.
Afferent fibers synapse with
interneurons in the spinal cord.
Efferent
impulses to muscle
with stretched
tendon are damped.
Muscle relaxes,
reducing tension.
Efferent impulses
to antagonist muscle
cause it to contract.
Interneurons
Spinal cord
1
2
3a
3b
+
+
+
Quadriceps
(extensors)
Tendon organ
Hamstrings
(flexors)
Figure 13.19
The tendon reflex.
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