Chapter 10
Quantitative and Thought Questions
10.1 None. The gamma motor neurons are important in preventing the muscle-spindle stretch receptors
from going slack, but when this reflex is tested, the intrafusal fibers are not flaccid. The test is performed
with a bent knee, which stretches the extensor muscles in the thigh (and the intrafusal fibers within the
stretch receptors). The stretch receptors are therefore responsive.
10.2 The efferent pathway of the reflex arc (the alpha motor neurons) would not be activated, the
effector cells (the extrafusal muscle fibers) would not be activated, and there would be no reflexive
response.
10.3 The drawing must have excitatory synapses on the motor neurons of both ipsilateral extensor and
ipsilateral flexor muscles.
10.4 A toxin that interferes with the inhibitory synapses on motor neurons would leave unbalanced the
normal excitatory input to these neurons. Thus, the otherwise normal motor neurons would fire
excessively, which would result in increased muscle contraction. This is exactly what happens in lockjaw
as a result of the toxin produced by the tetanus bacillus.
10.5 In mild cases of tetanus, agonists (stimulators) of the inhibitory interneuron neurotransmitter
gamma-aminobutyric acid (GABA) can shift the balance back toward the inhibition of alpha motor
neurons. In more severe cases, paralysis can be induced by administering long-lasting drugs that block
the nicotinic acetylcholine receptors at the neuromuscular junction.
10.6 Having a person pull the arms apart during the performance of the knee-jerk test is called the
Jendrassik maneuver (named for a Hungarian physician who first described it). The exact mechanism by
which the reflex increases is unknown. It is most commonly thought that performing a task with the
arms simply distracts the person so that there is less descending inhibition of the motor neurons to the
leg muscles. In other words, a person may be more able to truly relax the leg muscles (thus allowing the
reflex to more fully occur) when her attention is not focused on the legs or anticipating the strike of the
reflex hammer. Another possible explanation is that voluntarily flexing arm muscles produces a general
activation and increase in excitability of interneurons in the spinal cord that integrate synchronized arm
and leg movements; those inputs add to those from the stretch receptors to enhance activation of the
motor neurons to leg extensors.
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