The Peripheral Nervous System and Reﬂex Activity
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
, 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
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
. As a result, the contracting muscle relaxes as its
antagonist is activated
±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.
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
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
Quadriceps strongly contracts.
Tendon organs are activated.
Afferent ﬁbers synapse with
interneurons in the spinal cord.
impulses to muscle
tendon are damped.
to antagonist muscle
cause it to contract.
The tendon reﬂex.