Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 6

Synaptic & Junctional Transmission 123

cells are excited by the same parallel-fiber excitatory input.
This arrangement, which has been called feed-forward inhibi-
tion, presumably limits the duration of the excitation pro-
duced by any given afferent volley.

SUMMATION & OCCLUSION

As noted above, the axons of most neurons discharge onto
many other neurons. Conversely, any given neuron receives
input from many other neurons (convergence).
In the hypothetical nerve net shown in Figure 6–12, neurons
A and B converge on X, and neuron B diverges on X and Y. A
stimulus applied to A or to B will set up an EPSP in X. If A and
B are stimulated at the same time and action potentials are
produced, two areas of depolarization will be produced in X
and their actions will sum. The resultant EPSP in X will be
twice as large as that produced by stimulation of A or B alone,
and the membrane potential may well reach the firing level of
X. The effect of the depolarization caused by the impulse in A
adds to that due to activity in B, and vice versa; spatial summa-
tion has taken place. In this case, Y has not fired, but its excit-
ability has been increased, and it is easier for activity in neuron
C to fire Y during the EPSP. Y is therefore said to be in the
sub-
liminal fringe
of X. More generally stated, neurons are in the
subliminal fringe if they are not discharged by an afferent vol-
ley (not in the
discharge zone
) but do have their excitability
increased. The neurons that have few active knobs ending on
them are in the subliminal fringe, and those with many are in
the discharge zone. Inhibitory impulses show similar temporal
and spatial facilitation and subliminal fringe effects.
If action potentials are produced repeatedly in neuron B, X
and Y will discharge as a result of temporal summation of the
EPSPs that are produced. If C is stimulated repeatedly, Y and
Z will discharge. If B and C are fired repeatedly at the same
time, X, Y, and Z will discharge. Thus, the response to stimu-

lation of B and C together is not as great as the sum of

responses to stimulation of B and C separately, because B and

C both end on neuron Y. This decrease in expected response,

due to presynaptic fibers sharing postsynaptic neurons, is

called

occlusion.

NEUROMUSCULAR

TRANSMISSION:

NEUROMUSCULAR JUNCTION

ANATOMY

As the axon supplying a skeletal muscle fiber approaches its

termination, it loses its myelin sheath and divides into a num-

ber of terminal boutons, or endfeet (Figure 6–13). The endfeet

contain many small, clear vesicles that contain acetylcholine,

the transmitter at these junctions. The endings fit into

junc-

tional folds,

which are depressions in the

motor end plate,

the thickened portion of the muscle membrane at the junc-

tion. The space between the nerve and the thickened muscle

membrane is comparable to the synaptic cleft at synapses. The

whole structure is known as the

neuromuscular,

or

myoneu-

ral, junction.

Only one nerve fiber ends on each end plate,

with no convergence of multiple inputs.

SEQUENCE OF EVENTS

DURING TRANSMISSION

The events occurring during transmission of impulses from

the motor nerve to the muscle are somewhat similar to those

occurring at neuron-to-neuron synapses (Figure 6–14). The

impulse arriving in the end of the motor neuron increases the

permeability of its endings to Ca2+. Ca2+ enters the endings

and triggers a marked increase in exocytosis of the acetylcho-

line-containing vesicles. The acetylcholine diffuses to the

FIGURE 6–11 Negative feedback inhibition of a spinal
motor neuron via an inhibitory interneuron (Renshaw cell).

Motor

neuron

Motor

neuron

Inhibitory

interneuron

Axon FIGURE 6–12 Simple nerve net. Neurons A, B, and C have exci-

tatory endings on neurons X, Y, and Z.

A

B

C

X

Y

Z