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SECTION II
Physiology of Nerve & Muscle Cells
extension of the opposite limb. This
crossed extensor response
is properly part of the withdrawal reflex. Strong stimuli in ex-
perimental animals generate activity in the interneuron pool
that spreads to all four extremities. This is difficult to demon-
strate in normal animals but is easily demonstrated in an animal
in which the modulating effects of impulses from the brain have
been abolished by prior section of the spinal cord
(spinal ani-
mal).
For example, when the hind limb of a spinal cat is
pinched, the stimulated limb is withdrawn, the opposite hind
limb extended, the ipsilateral forelimb extended, and the con-
tralateral forelimb flexed. This spread of excitatory impulses up
and down the spinal cord to more and more motor neurons is
called
irradiation of the stimulus,
and the increase in the num-
ber of active motor units is called
recruitment of motor units.
IMPORTANCE OF THE
WITHDRAWAL REFLEX
Flexor responses can be produced by innocuous stimulation of
the skin or by stretch of the muscle, but strong flexor responses
with withdrawal are initiated only by stimuli that are noxious
or at least potentially harmful to the animal. These stimuli are
therefore called
nociceptive stimuli.
Sherrington pointed out
the survival value of the withdrawal response. Flexion of the
stimulated limb gets it away from the source of irritation, and
extension of the other limb supports the body. The pattern as-
sumed by all four extremities puts the animal in position to run
away from the offending stimulus. Withdrawal reflexes are
prepotent;
that is, they preempt the spinal pathways from any
other reflex activity taking place at the moment.
Many of the characteristics of polysynaptic reflexes can be
demonstrated by studying the withdrawal reflex. A weak nox-
ious stimulus to one foot evokes a minimal flexion response;
stronger stimuli produce greater and greater flexion as the
stimulus irradiates to more and more of the motor neuron pool
supplying the muscles of the limb. Stronger stimuli also cause a
more prolonged response. A weak stimulus causes one quick
flexion movement; a strong stimulus causes prolonged flexion
and sometimes a series of flexion movements. This prolonged
response is due to prolonged, repeated firing of the motor neu-
rons. The repeated firing is called
after-discharge
and is due to
continued bombardment of motor neurons by impulses arriv-
ing by complicated and circuitous polysynaptic paths.
As the strength of a noxious stimulus is increased, the reac-
tion time is shortened. Spatial and temporal facilitation occurs
at synapses in the polysynaptic pathway. Stronger stimuli pro-
duce more action potentials per second in the active branches
and cause more branches to become active; summation of the
EPSPs to the firing level therefore occurs more rapidly.
FRACTIONATION & OCCLUSION
Another characteristic of the withdrawal response is the fact
that supramaximal stimulation of any of the sensory nerves
from a limb never produces as strong a contraction of the flex-
or muscles as that elicited by direct electrical stimulation of the
muscles themselves. This indicates that the afferent inputs
fractionate
the motor neuron pool; that is, each input goes to
only part of the motor neuron pool for the flexors of that par-
ticular extremity. On the other hand, if all the sensory inputs
are dissected out and stimulated one after the other, the sum
of the tension developed by stimulation of each is greater than
that produced by direct electrical stimulation of the muscle or
stimulation of all inputs at once. This indicates that the vari-
ous afferent inputs share some of the motor neurons and that
occlusion
occurs when all inputs are stimulated at once.GENERAL PROPERTIES
OF REFLEXES
It is apparent from the preceding description of the properties
of monosynaptic and polysynaptic reflexes that reflex activity
is stereotyped and specific in terms of both the stimulus and
the response; a particular stimulus elicits a particular re-
sponse. The fact that reflex responses are stereotyped does not
exclude the possibility of their being modified by experience.
Reflexes are adaptable and can be modified to perform motor
tasks and maintain balance. Descending inputs from higher
brain regions play an important role in modulating and adapt-
ing spinal reflexes.ADEQUATE STIMULUS
The stimulus that triggers a reflex is generally very precise. This
stimulus is called the
adequate stimulus
for the particular re-
flex. A dramatic example is the scratch reflex in the dog. This
spinal reflex is adequately stimulated by multiple linear touch
stimuli such as those produced by an insect crawling across the
skin. The response is vigorous scratching of the area stimulated.
If the multiple touch stimuli are widely separated or not in a
line, the adequate stimulus is not produced and no scratching
occurs. Fleas crawl, but they also jump from place to place. This
jumping separates the touch stimuli so that an adequate stimu-
lus for the scratch reflex is not produced. It is doubtful if the flea
population would survive long without the ability to jump.FINAL COMMON PATH
The motor neurons that supply the extrafusal fibers in skeletal
muscles are the efferent side of many reflex arcs. All neural influ-
ences affecting muscular contraction ultimately funnel through
them to the muscles, and they are therefore called the
final
common paths.
Numerous inputs converge on them. Indeed,
the surface of the average motor neuron and its dendrites ac-
commodates about 10,000 synaptic knobs. At least five inputs
go from the same spinal segment to a typical spinal motor
neuron. In addition to these, there are excitatory and inhibito-
ry inputs, generally relayed via interneurons, from other levels
of the spinal cord and multiple long-descending tracts from