Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 9

Reflexes 161

CONTROL OF

γ

-MOTOR

NEURON DISCHARGE

The

γ

-motor neurons are regulated to a large degree by de-

scending tracts from a number of areas in the brain. Via these

pathways, the sensitivity of the muscle spindles and hence the

threshold of the stretch reflexes in various parts of the body can

be adjusted and shifted to meet the needs of postural control.

Other factors also influence

γ

-motor neuron discharge. Anxi-

ety causes an increased discharge, a fact that probably explains

the hyperactive tendon reflexes sometimes seen in anxious

patients. In addition, unexpected movement is associated with a

greater efferent discharge. Stimulation of the skin, especially by

noxious agents, increases

γ

-motor neuron discharge to ipsilateral

flexor muscle spindles while decreasing that to extensors and

produces the opposite pattern in the opposite limb. It is well

known that trying to pull the hands apart when the flexed fingers

are hooked together facilitates the knee jerk reflex (Jendrassik’s

maneuver), and this may also be due to increased

γ

-motor neu-

ron discharge initiated by afferent impulses from the hands.

RECIPROCAL INNERVATION

When a stretch reflex occurs, the muscles that antagonize the ac-

tion of the muscle involved (antagonists) relax. This phenome-

non is said to be due to

reciprocal innervation.

Impulses in the

Ia fibers from the muscle spindles of the protagonist muscle

cause postsynaptic inhibition of the motor neurons to the antag-

onists. The pathway mediating this effect is bisynaptic. A collat-

eral from each Ia fiber passes in the spinal cord to an inhibitory

interneuron that synapses on a motor neuron supplying the

FIGURE 9–4
Effect of various conditions on muscle spindle
discharge.
When the whole muscle is stretched, the muscle spindle is
also stretched and its sensory endings are activated at a frequency pro-
portional to the degree of stretching (“loading the spindle”). Spindle af-
ferents stop firing when the muscle contracts (“unloading the spindle”).
Stimulation of
γ
-motor neurons cause the contractile ends of the in-
trafusal fibers to shorten. This stretches the nuclear bag region, initiating
impulses in sensory fibers. If the whole muscle is stretched during stim-
ulation of the
γ
-motor neurons, the rate of discharge in sensory fibers is
further increased.

Increasedγ efferent

discharge—muscle stretched

Increasedγ efferent discharge

Muscle contracted

Muscle stretched

Muscle at rest

Tendon

Spindle

Extrafusal fiber

Sensory nerve

Impulses in

sensory nerve

CLINICAL BOX 9–2

Physiological Tremor

The response of the Ia sensory fiber endings to the dynamic

(phasic) as well as the static events in the muscle is impor-

tant because the prompt, marked phasic response helps to

dampen oscillations caused by conduction delays in the feed-

back loop regulating muscle length. Normally a small oscilla-

tion occurs in this feedback loop. This

physiologic tremor

has

a low amplitude (barely visible to the naked eye) and a fre-

quency of approximately 10 Hz. Physiological tremor is a nor-

mal phenomenon which affects everyone while maintaining

posture or during movements. However, the tremor would be

worse if it were not for the sensitivity of the spindle to velocity

of stretch. It can become exaggerated in some situations such

as when we are anxious or tired or because of drug toxicity.

Numerous factors contribute to the genesis of physiological

tremor. It is likely dependent on not only central

(inferior ol-

ive)

sources but also from peripheral factors including motor

unit firing rates, reflexes, and mechanical resonance.