Control of Body Movement
303
muscle fi ber are stimulated to contract during the shortening
of the extrafusal fi bers. This maintains tension in the central
region of the intrafusal fi ber, where the stretch receptors are
located (see Figure 10–5c). It is important to recognize that
the intrafusal fi bers are not large enough or strong enough to
shorten a whole muscle and move joints; their sole job is to
maintain tension on the spindle stretch receptors.
The intrafusal fi
bers contract in response to activation
by motor neurons, but the motor neurons supplying them
are not the same ones that activate the extrafusal muscle
fi bers. The motor neurons controlling the extrafusal muscle
bers are larger and are classifi ed as
alpha motor neurons,
whereas the smaller motor neurons whose axons innervate
the intrafusal fi bers are known as
gamma motor neurons
(see Figure 10–5c).
The cell bodies of alpha and gamma motor neurons to
a given muscle lie close together. Both types are activated
by interneurons in their immediate vicinity and sometimes
directly by neurons of the descending pathways. In fact,
during many voluntary and involuntary movements, they
are
coactivated
—that is, excited at almost the same time.
Coactivation ensures that information about muscle length
will be continuous
ly availab
le to provide for ad
justment
during ongoing actions and to plan and program future
movements.
Tension-Monitoring Systems
Any given set of inputs to a given set of motor neurons can
lead to various degrees of tension in the muscles they inner-
vate. The tension depends on muscle length, the load on the
muscles, and the degree of muscle fatigue. Therefore, feedback
is necessary to inform the motor control systems of the tension
actually achieved.
Some of this feedback is provided by vision (you can
see whether you are lifting or lowering an object) as well as
by afferent input from skin, muscle, and joint receptors. An
additional receptor type specifi cally monitors how much ten-
sion the contracting motor units are exerting (or is being
imposed on the muscle by external forces if the muscle is
being stretched).
The receptors employed in this tension-monitoring sys-
tem are the
Golgi tendon organs,
which are located in
the tendons near their junction with the muscle (see Figure
10–4). Endings of afferent nerve fi bers wrap around colla-
gen bundles in the tendon, bundles that are slightly bowed
in the resting state. When the muscle is stretched or the
attached extrafusal muscle fi bers contract, tension is exerted
on the tendon. This tension straightens the collagen bundles
and distorts the receptor endings, activating them. The ten-
don is typically stretched much more by an active contrac-
tion of the muscle than when the whole muscle is passively
stretched (
Figure 10–7
). Thus, the Golgi tendon organs
discharge in response to the tension generated by the con-
tracting muscle and initiate action potentials that are trans-
mitted to the central nervous system.
Branches of the afferent neuron from the Golgi tendon
organ cause widespread inhibition of the contracting muscle
and its synergists via interneurons (A in
Figure 10–8
). They
also stimulate the motor neurons of the antagonistic muscles
(B in Figure 10–8). Note that this reciprocal innervation is
the opposite of that produced by the muscle-spindle affer-
ents. This difference refl ects the different functional roles of
the two systems: The muscle spindle provides local homeo-
static control of muscle
length
, while the Golgi tendon organ
provides local homeostatic control of muscle
tension
. In addi-
tion, the activity of afferent fi bers from these two receptors
supplies the higher-level motor control systems with informa-
tion about muscle length and tension, which can be used to
modify an ongoing motor program.
The Withdrawal Refl
ex
In addition to the afferent information from the spindle stretch
receptors and Golgi tendon organs of the activated muscle, other
input is transmitted to the local motor control systems. For
example, painful stimulation of the skin, as occurs from stepping
on a tack, activates the fl exor muscles and inhibits the exten-
sor muscles of the
ipsilateral
(on the same side of the body)
leg. The resulting action moves the affected limb away from the
harmful stimulus, and is thus known as a
withdrawal refl
ex
Figure 10–7
Activation of Golgi tendon organs. Compared to when a muscle
is contracting, passive stretch of the relaxed muscle produces less
stretch of the tendon and fewer action potentials from the Golgi
tendon organ.
Relaxed
muscle
Contracting
muscle
Passive
stretch
Time
Time
Time
Afferent
neuron
Action potentials in afferent neurons
Golgi tendon
organ
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