Ganong's Review of Medical Physiology, 23rd Edition

162
SECTION II
Physiology of Nerve & Muscle Cells

antagonist muscles. This example of postsynaptic inhibition is dis-
cussed in Chapter 6, and the pathway is illustrated in Figure 6–6.

INVERSE STRETCH REFLEX

Up to a point, the harder a muscle is stretched, the stronger is
the reflex contraction. However, when the tension becomes
great enough, contraction suddenly ceases and the muscle re-
laxes. This relaxation in response to strong stretch is called the
inverse stretch reflex
or
autogenic inhibition.
The receptor for the inverse stretch reflex is in the
Golgi ten-
don organ
(Figure 9–5). This organ consists of a netlike collec-
tion of knobby nerve endings among the fascicles of a tendon.
There are 3–25 muscle fibers per tendon organ. The fibers from
the Golgi tendon organs make up the Ib group of myelinated,
rapidly conducting sensory nerve fibers. Stimulation of these Ib
fibers leads to the production of IPSPs on the motor neurons
that supply the muscle from which the fibers arise. The Ib fibers
end in the spinal cord on inhibitory interneurons that in turn

terminate directly on the motor neurons (Figure 9–3). They

also make excitatory connections with motor neurons supply-

ing antagonists to the muscle.

Because the Golgi tendon organs, unlike the spindles, are in

series with the muscle fibers, they are stimulated by both pas-

sive stretch and active contraction of the muscle. The threshold

of the Golgi tendon organs is low. The degree of stimulation by

passive stretch is not great because the more elastic muscle

fibers take up much of the stretch, and this is why it takes a

strong stretch to produce relaxation. However, discharge is reg-

ularly produced by contraction of the muscle, and the Golgi

tendon organ thus functions as a transducer in a feedback cir-

cuit that regulates muscle force in a fashion analogous to the

spindle feedback circuit that regulates muscle length.

The importance of the primary endings in the spindles and

the Golgi tendon organs in regulating the velocity of the muscle

contraction, muscle length, and muscle force is illustrated by

the fact that that section of the afferent nerves to an arm causes

the limb to hang loosely in a semiparalyzed state. The organiza-

tion of the system is shown in Figure 9–6. The interaction of

FIGURE 9–5
Golgi tendon organ.
(Reproduced, with permission, from Goss CM [editor]:
Gray’s Anatomy of the Human Body,
29th ed. Lea & Febiger, 1973.)

Organ of Golgi, showing

ramification of nerve fibrils

Muscular fibers

Tendon bundles

Nerve fiber

FIGURE 9–6
Block diagram of peripheral motor control system.
The dashed line indicates the nonneural feedback from muscle that lim-
its length and velocity via the inherent mechanical properties of muscle.
γ
d
, dynamic
γ
-motor neurons;
γ
s
, static
γ
-motor neurons.
(Reproduced,
with permission, from Houk J in:
Medical Physiology,
13th ed. Mount-Castle VB [editor]. Mosby, 1974.)

Inter-

neurons

Force feedback

Efferent signal

Length and velocity

feedback

Tendon

organs

Muscle

Muscular

force

Muscle

length

Load

Spindles

α

−

−

+

+

+

γd

γs

Interneuronal

control

signal

α Control

signal

γ-Dynamic

control

signal

γ-Static

control

signal

Internal

disturbances

External

forces

Length and velocity