Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 29

into the axon terminal for reuse ( reuptake ). Indeed, if
NT is allowed to linger in the cleft (e.g., by inhibiting
these reuptake mechanisms), then it will keep activating
receptors, which can cause problems. There are many
drugs that can affect various stages of this process, in-
cluding blocking receptor activation, reuptake, and the
postsynaptic chemical processes activated by receptors.
These are important for studying the components of this
complex biochemical system.
The synapse has a number of dynamic properties that
affect the way it behaves as a function of prior activ-
ity. One commonly noted effect is paired-pulse facili-
tation , where the second of two spikes coming in rea-
sonably rapid succession will be stronger, which can be
caused by residual calcium ions in the terminal or resid-
ual binding of vesicles to the membrane as a result of
the prior release episode. It is also likely that extended
high rates of firing will deplete synaptic resources (e.g.,
NT, Ca ++ ), resulting in increased numbers of release
failures , where NT fails to be released during a spike.
This may contribute to the saturating nonlinearity of the
neural output function, as discussed in a later section.
There are a couple of important features of the biol-
ogy of the synapse that should be emphasized. First,
there are a number of ways in which the different com-
ponents of the synapse can affect the overall efficacy or
strength of transmission of information from the sender
to the receiver. As we have mentioned, the net effect of
these biological components is summarized in the com-
putational model by the weight between the two neu-
rons. Furthermore, the modification of one or more of
these weight factors can produce learning.
The main presynaptic components of the weight are
the number of vesicles released with each action po-
tential, the amount of NT within each vesicle (which
is believed not to vary that much), and the efficacy of
the reuptake mechanism. The main postsynaptic fac-
tors include the total number of channel receptors ex-
posed to the neurotransmitter, the alignment and prox-
imity of these receptors with the presynaptic release
sites, and the efficacy of the individual channels in al-
lowing ions to flow. Various researchers have also ar-
gued that the shape of the dendritic spine may have an
important impact on the conductance of electrical sig-
nals from the synapse to the dendrite as a whole (Shep-

herd, 1990). Exactly which of these factors are modified
during learning is a matter of considerable debate, but it
appears that there may be multiple contributors, on both
the pre- and postsynaptic side of things (Malenka &

Nicoll, 1993).

2.3.3 The Dendrite

To continue the chain of communication, the dendritic
end of the synapse houses the receptors and associated
channels that allow ions to flow through the postsynap-
tic membrane. As the electrical effects of these ions
propagate through the dendrites and up to the cell body,
the process of communication can begin again in the
axon of this receiving neuron. We discuss the electrical
integration of inputs in detail in the next section, so the
main things to point out here are the different types of
dendritic receptors, and the NTs that activate them.
In the cortex, there are two primary types of
NT/receptor combinations (along with many others that
we will not discuss). One such type uses the NT glu-
tamate , which in turn binds to the AMPA , NMDA ,
and mGlu receptors on the dendrite (as illustrated in
figure 2.5). As we see in the next section, the AMPA
receptor activated channel provides the primary excita-
tory input because it allows sodium (Na + ) ions to flow
into the dendrite. These ions elevate (excite) the postsy-
naptic membrane potential and make the receiving neu-
ronmorelikelytofireaspike. Whenexcitatorychan-
nels are opened, the resulting change in the postsynaptic
membrane potential is called an excitatory postsynaptic
potential or EPSP .
The NMDA receptor activated channel is also exci-
tatory, but it is probably more important for its effects
on learning, because it allows calcium (Ca ++ ) ions to
enter, which can then trigger chemical processes that
lead to learning. The mGlu (metabotropic glutamate)
receptor may also be important for learning by activat-
ing various chemical processes in the postsynaptic neu-
ron when NT binds to it. We will discuss these learning
effects in greater detail in chapters 4-6.
The other main type of NT/receptor combination uses
the NT GABA , which in turn binds to and activates
GABA receptors on the dendrite. These GABA re-
ceptors open channels that allow chlorine (Cl ￿ ) ions