Page 36 - Artificial Intelligence in the Age of Neural Networks and Brain Computing

9. The Synapse 23

(A) From (B)
Presynaptic
Neuron

Synaptic
Input
Signal

Neurotransmi er
Synaptic
molecules
Variable
Cleft
Weight
Receptors
Synaptic
Output
Signal
To
Postsynaptic
Neuron
FIGURE 1.16
A synapse corresponding to a variable weight. (A) Synapse. (B) A variable weight.

a protein called a neurotransmitter is injected into the cleft. Each activation pulse
generated by the presynaptic neuron causes a finite amount of neurotransmitter to
be injected into the cleft. The neurotransmitter lasts only for a very short time,
some being reabsorbed and some diffusing away. The average concentration of
neurotransmitter in the cleft is proportional to the presynaptic neuron’s firing rate.
Some of the neurotransmitter molecules attach to receptors located on the
postsynaptic side of the cleft. The effect of this on the postsynaptic neuron is either
excitatory or inhibitory, depending on the nature of the synapse and its neurotrans-
mitter chemistry [20e24]. A synaptic effect results when neurotransmitter mole-
cules attach to the receptors. The effect is proportional to the average amount of
neurotransmitter present and the number of receptors. Thus, the effect of the presyn-
aptic neuron on the postsynaptic neuron is proportional to the product of the presyn-
aptic firing rate and the number of receptors present. The input signal to the synapse
is the presynaptic firing rate, and the synaptic weight is proportional to the number of
receptors. The weight or the synaptic “efficiency” described by Hebb is increased or
decreased by increasing or decreasing the number of receptors. This can only occur
when neurotransmitter is present [20]. Neurotransmitter is essential both as a signal
carrier and as a facilitator for weight changing. A symbolic representation of the
synapse is shown in Fig. 1.16B.
The effect of the action of a single synapse upon the postsynaptic neuron is
actually quite small. Signals from thousands of synapses, some excitatory, some
inhibitory, add in the postsynaptic neuron to create the (SUM) [20,25]. If the
(SUM) of both the positive and negative inputs is below a threshold, the postsynaptic
neuron will not fire and its output will be zero. If the (SUM) is greater than