Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 9

Reflexes 163

spindle discharge, tendon organ discharge, and reciprocal
innervation determines the rate of discharge of
α
-motor neu-
rons (see Clinical Box 9–3).

MUSCLE TONE

The resistance of a muscle to stretch is often referred to as its
tone
or
tonus.
If the motor nerve to a muscle is cut, the muscle
offers very little resistance and is said to be
flaccid.
A
hyper-
tonic (spastic)
muscle is one in which the resistance to stretch
is high because of hyperactive stretch reflexes. Somewhere be-
tween the states of flaccidity and spasticity is the ill-defined
area of normal tone. The muscles are generally
hypotonic
when the rate of
γ
-motor neuron discharge is low and hyper-
tonic when it is high.
When the muscles are hypertonic, the sequence of moderate
stretch
→
muscle contraction, strong stretch
→
muscle relax-

ation is clearly seen. Passive flexion of the elbow, for example,

meets immediate resistance as a result of the stretch reflex in

the triceps muscle. Further stretch activates the inverse stretch

reflex. The resistance to flexion suddenly collapses, and the

arm flexes. Continued passive flexion stretches the muscle

again, and the sequence may be repeated. This sequence of

resistance followed by give when a limb is moved passively is

known as the

clasp-knife effect

because of its resemblance to

the closing of a pocket knife. It is also known as the

lengthen-

ing reaction

because it is the response of a spastic muscle to

lengthening.

POLYSYNAPTIC REFLEXES:

THE WITHDRAWAL REFLEX

Polysynaptic reflex paths branch in a complex fashion (Figure

9–7). The number of synapses in each of their branches varies.

Because of the synaptic delay at each synapse, activity in the

branches with fewer synapses reaches the motor neurons first,

followed by activity in the longer pathways. This causes pro-

longed bombardment of the motor neurons from a single stim-

ulus and consequently prolonged responses. Furthermore,

some of the branch pathways turn back on themselves, permit-

ting activity to reverberate until it becomes unable to cause a

propagated transsynaptic response and dies out. Such

reverber-

ating circuits

are common in the brain and spinal cord.

WITHDRAWAL REFLEX

The withdrawal reflex is a typical polysynaptic reflex that occurs

in response to a usually painful stimulation of the skin or sub-

cutaneous tissues and muscle. The response is flexor muscle

contraction and inhibition of extensor muscles, so that the body

part stimulated is flexed and withdrawn from the stimulus.

When a strong stimulus is applied to a limb, the response in-

cludes not only flexion and withdrawal of that limb but also

CLINICAL BOX 9–3

Clonus

A characteristic of states in which increased

γ

-motor neu-

ron discharge is present is

clonus.

This neurologic sign is

the occurrence of regular, repetitive, rhythmic contractions

of a muscle subjected to sudden, maintained stretch. Only

sustained clonus with five or more beats is considered ab-

normal. Ankle clonus is a typical example. This is initiated

by brisk, maintained dorsiflexion of the foot, and the re-

sponse is rhythmic plantar flexion at the ankle. The

stretch

reflex–inverse stretch reflex sequence

may contribute to

this response. However, it can occur on the basis of syn-

chronized motor neuron discharge without Golgi tendon

organ discharge. The spindles of the tested muscle are hy-

peractive, and the burst of impulses from them discharges

all the motor neurons supplying the muscle at once. The

consequent muscle contraction stops spindle discharge.

However, the stretch has been maintained, and as soon as

the muscle relaxes it is again stretched and the spindles

stimulated. Clonus may also occur after disruption of de-

scending cortical input to a spinal glycinergic inhibitory in-

terneuron called the

Renshaw cell.

This cell receives exci-

tatory input from

α

-motor neurons via an axon collateral

(and in turn it inhibits the same). In addition, cortical fibers

activating ankle flexors contact Renshaw cells (as well as

type Ia inhibitory interneurons) that inhibit the antagonis-

tic ankle extensors. This circuitry prevents reflex stimulation

of the extensors when flexors are active. Therefore, when

the descending cortical fibers are damaged

(upper motor

neuron lesion),

the inhibition of antagonists is absent. The

result is repetitive, sequential contraction of ankle flexors

and extensors (clonus). Clonus may be seen in patients with

amyotrophic lateral sclerosis, stroke, multiple sclerosis, spi-

nal cord damage, and hepatic encephalopathy.

FIGURE 9–7

Diagram of polysynaptic connections between

afferent and efferent neurons in the spinal cord.

The dorsal root fi-

ber activates pathway A with three interneurons, pathway B with four

interneurons, and pathway C with four interneurons. Note that one of

the interneurons in pathway C connects to a neuron that doubles back

to other interneurons, forming reverberating circuits.

Sensory

neuron

A

C

Motor

neuron

B