ULTIMATE COMPUTING - Quantum Consciousness Studies

From Brain to Cytoskeleton 63
Figure 4.2: Schematic diagram of brain functional components. By Paul
Jablonka.
The sequence of events liberating neurotransmitter molecules from nerve
endings is remarkably uniform at all synapses. Transmitter molecules are stored
inside presynaptic nerve terminals in small vesicles that are analogous to the
secretory granules of gland cells. The amount of neurotransmitter in one vesicle is
considered a “quantum” and in neuromuscular synapses each quantum consists of
one thousand to five thousand molecules of acetylcholine. Each action potential
reaching the presynaptic terminal releases a number of vesicle quanta ranging
from a few to several hundred. The coupling mechanism between the action
potential and vesicle release involves both calcium and the cytoskeleton. As an
action potential impulse arrives at the presynaptic nerve terminal, calcium ions
enter the cytoplasm through the membrane by way of voltage gated channels. In
presynaptic nerve terminals, inward ionic current of the action potential is thus
carried partially by sodium and partially by calcium. Free calcium in the
cytoplasm of the terminal causes vesicles to fuse with the surface membrane and
to expel their contents. The mechanisms by which calcium triggers the release of
vesicles from the presynaptic terminal is not clearly understood, however
cytoskeletal proteins including contractile actin, myosin, and other filamentous
proteins are involved. Calcium mediates dynamic contractile activities in flagella
and skeletal muscle and appears to trigger cytoskeletal expulsion of
neurotransmitters from nerve terminals.
After release, transmitter molecules diffuse across the synaptic cleft and bind
reversibly with the postsynaptic membrane receptors. The distance between the
two membranes is sufficiently small such that the diffusion takes about a
millisecond-a relatively slow event compared to switching in semiconductors.
Whenever a transmitter molecule binds to the postsynaptic membrane, it causes a
small voltage change in the postsynaptic membrane called the miniature endplate
potential which can be either excitatory or inhibitory depending on the
neurotransmitter molecule and postsynaptic receptor. In the resting state there are