Human Physiology, 14th edition (2016)

The Nervous System 199

7.7 Synaptic Integration

The summation of many EPSPs may be needed to pro-

duce a depolarization of sufficient magnitude to stimulate

the postsynaptic cell. The net effect of EPSPs on the post-

synaptic neuron is reduced by hyperpolarization (IPSPs)

produced by inhibitory neurotransmitters.

When a presynaptic neuron is experimentally stimulated

at a high frequency, even for just a few seconds, the excit-

ability of the synapse is enhanced—or potentiated—when this

neuron pathway is subsequently stimulated. The improved

efficacy of synaptic transmission may last for hours or even

weeks and is called long-term potentiation (LTP). Long-

term potentiation may favor transmission along frequently

used neural pathways and thus may represent a mechanism of

neural “learning.” LTP has been observed in the hippocampus

of the brain, which is an area implicated in memory storage

(chapter 8; see fig. 8.15).

Most of the neural pathways in the hippocampus use glu-

tamate as a neurotransmitter that activates NMDA receptors.

This implicates glutamate and its NMDA receptors in learning

and memory, and indeed, in a recent experiment, genetically

altered mice with enhanced NMDA expression were smarter

when tested in a maze. The association of NMDA receptors

with synaptic changes during learning and memory is dis-

cussed more fully in chapter 8, section 8.2. Interestingly, the

street drug known as PCP or angel dust blocks NMDA recep-

tors; this suggests that the aberrant schizophrenia-like effects

of this drug are produced by the reduction of glutamate stimu-

lation of NMDA receptors.

LEARNING OUTCOMES

After studying this section, you should be able to:

16. Explain the nature of spatial and temporal
    summation at the synapse.


17. Describe long-term potentiation and depression, and
    explain the nature of postsynaptic and presynaptic
    inhibition.

Because axons can have collateral branches (see fig.  7.1 ),
divergence of neural pathways can occur. That is, one neu-
ron can make synapses with a number of other neurons, and
by that means either stimulate or inhibit them. By contrast,
a number of axons can synapse on a single neuron, allowing
convergence of neural pathways. Figure  7.33 shows conver-
gence of two neurons on a single postsynaptic neuron, which
can thereby integrate the input of the presynaptic neurons.
Spatial summation ( fig. 7.33 ) occurs due to the conver-
gence of axon terminals from different presynaptic axons (up
to a thousand in some cases) on the dendrites and cell body
of a postsynaptic neuron. Because of the long distances (up
to hundreds of micrometers) between the distal ends of the
dendrites and the initial segment of the axon, there is ample
opportunity for synaptic potentials that originate in different
locations to summate. In temporal summation, successively
rapid bursts of activity of a single presynaptic axon can cause
corresponding bursts of neurotransmitter release, resulting
in successive waves of EPSPs (or IPSPs) that summate with
each other as they travel to the initial segment of the axon.
Spatial and temporal summation are important in determining
the strength of the depolarization stimulus at the initial seg-
ment of the axon and thereby the frequency of action poten-
tial production.

Synaptic Plasticity

Repeated use of a particular synaptic pathway can either
increase or decrease the strength of synaptic transmission for
extended periods of time. This is primarily due to the inser-
tion or removal of the AMPA type of glutamate receptors at
the post-synaptic membrane. Insertion of AMPA receptors
promotes long-term potentiation, whereas removal of AMPA
receptors promotes long-term depression of synaptic transmis-
sion. These and other functional changes, as well as structural
changes at synapses, result in synaptic plasticity —the ability
of synapses to change in response to activity.

Figure 7.33 Spatial summation. When only one

presynaptic neuron releases excitatory neurotransmitter, the

EPSP produced may not be sufficiently strong to stimulate action

potentials in the postsynaptic neuron. When more than one

presynaptic neuron produces EPSPs at the same time, however,

the EPSPs can summate at the axon hillock to produce action

potentials. The EPSPs are recorded at the axon hillock; the

action potential is recorded at the initial segment of the axon.

+30 mV

–55 mV

–70 mV

Release of neurotransmitter

from neuron 1 only

Release of

neurotrans-

mitter from

neurons

and

Threshold

EPSP EPSP

EPSP

Action potential

1

2

(^12)