Introduction 
Figure 23- A: The anatomical arrangement of a typical reflex, in this case the knee jerk. Each cell actually represents a population 
of many neurons. Information about stretch of the quadriceps femoris muscle is conveyed by afferent neurons to several loci 
within the central nervous system. In the spinal cord, afferent neurons act directly on the motor neurons to the quadriceps and, 
by means of inhibitory interneurons, indirectly on the motor neurons to the antagonistic muscle, the biceps. Both of these 
actions combine to produce the coordinated expression of the reflex behavior. In addition, information is conveyed to higher 
regions of brain to update them about the information coming into the nervous system and about the behavior that is being 
generated. These higher centers, in turn, can act to modify the reflex 
behavior. 
B: The sequences of signaling changes that produce the reflex action. 
Graded stretch of the muscle produces a graded receptor potential in the 
muscle spindle of the afferent neuron that propagates passively to the trig- 
ger zone at the first node of Ranvier. If the potential is sufficiently large, it 
will trigger an action potential that will propagate actively along the axon 
to the terminal region. At the terminal the change in membrane potential, 
produced by the action potential, gives rise to a secretory potential that 
leads to the release of transmitter substance. The transmitter diffuses 
across the synaptic cleft and interacts with receptor molecules on the mem- 
brane of the postsynaptic motor cell to initiate a synaptic potential. The 
synaptic potential then propagates passively to the initial segment of the 
axon, where it, in turn, initiates an action potential that propagates to the 
terminals of the motor neuron. This action potential leads ultimately to a 
synaptic potential in the muscle, which initiates an action potential that 
causes the contraction of the muscle. 
Reprinted by permission of the publisher from Kandel, E.R., and 
Schwartz, J.H. Principles of Neural Science, 2nd edition, p. 20. Copyright 
1985 by Elsevier Science Publishing Co., Inc. 
Cell body in 
dorsal root ganglion^ 
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Quadriceps 
(extensor) 
Patella 
la afferent fiber 
Pyramidal 
tract 
To contralateral 
spinal cord 
Inhibitory 
Interneuron 
B 
Graded 
receptor 
potential 
Action 
potential 
Action 
potential 
J 
Secretory 
potential 
Synaptic 
potential 
Action 
potential 
Synaptic 
potential 
Action 
potential 
1 1 
Muscle spindle Myelin 
Dorsal root 
ganglion cell 
JL 
1 1 
Receptor — Trigger 
zone 
Input 
Stretch 
-3 E 
Axon 
Integrative Conductile 
Synapse — Trigger 
zone 
Integrative 
Output Input 
Axon 
Conductile 
Action 
potential 
Synaptic 
potential 
Synapse 
Muscle 
Output Input 
Ivii 
