Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 38

To emphasize, Fig. 3.5 shows a dendritic pulse for β ¼ 0.05 (If β ¼ 0.05 then

Time(ms) = t Mem ¼ 1.9 s). As the voltage lowers into a threshold region below

55 mV, the STM neuron will lose its ability to self-trigger and thus its memory.
Electrically, a STM neuron is a pulsating version of a “one-shot” in digital
design, presenting true output for a given time and then going false.

Synapses

The brain, it is estimated, contains far more synapses then neurons, perhaps 500
trillion (1 trillion 10 12 ); This averages to about 10 3 synapses per neuron, some

more, some less. Synapses are numerous, so it is reasonable to expect that they are
not all used at once and that they are not all exactly alike.
A synapse is a purposeful gap between a bouton at the terminations of an axon,
and a receptor, usually located on a spine of a dendrite. This gap is called the
synaptic cleft; it is typically about 20 nm wide (20

¼

10 9 m). Neurons generally
do not touch each other physically. Figure 3.6 is an artist's concept of a synapse.
Note that the term presynaptic refers to the side that contains the bouton; postsyn-
aptic refers to the side that contains the receptor.
Pulse bursts can be triggered by excitatory neurotransmitters within excitatory
synapses. Also possible are inhibitory neurotransmitters and inhibitory synapses
that slow down and perhaps stop the propagation of dendritic pulses. Excitatory and
inhibitory neurotransmitters may be fast acting or slow acting.
Excitatory synapses trigger the propagation of one or more pulses, and inhibitory
neurotransmitters have the power to stop the propagation of pulses. The chief
purpose of an excitatory neurotransmitter is to carry a one-way signal from the
bouton of a given neuron to the receptor of another neuron. Neurotransmitters, like
any molecule in an ionic solution, become positive ions, and once released, they
drift and diffuse across the cleft to perform their function.

Intuitive Models for Triggering by Neurotransmitters

Under the simplest of physical models, positive excitatory neurotransmitter ions are
ejected from vesicles in boutons because they are repelled by the positive pulses
that arrive at the tips of an axon. Neurotransmitters then drift and diffuse away from
their presynaptic source to shower a postsynaptic receptor. Neurotransmitter ions
may cling briefly to their receptor, partly because the receptors hold a negative rest
voltage of about 70 mV, which will attract positive ions.

Once a pulse is triggered, it begins to propagate along the dendrite away from the
receptor. The positive pulse also exerts a repulsive force on the positively charged
neurotransmitters and will push them away. Most often they soon return to trigger
another pulse.