ULTIMATE COMPUTING - Quantum Consciousness Studies

From Brain to Cytoskeleton 61
“gray matter” refers to clumps of cell bodies and dendrites known as “nuclei.” A
schematic diagram of brain functional organization is shown in Figure 4.2.
4.2.2 Neuronal Signaling
Signals which transmit information among nerve cells consist of electrical
potential changes produced by ionic currents flowing across their surface
membranes. The currents are carried by ions such as sodium, potassium, calcium,
and chloride and occur due to the opening and closing of membrane protein ion
channels. The nerve maintains an electrical polarization across its membrane by
actively pumping sodium ions out, and potassium ions in. Thus when the ion
channels are opened (by voltage change, neurotransmitters, drugs, etc.) sodium
and potassium rapidly flow through the channel creating a depolarization.
Depending on the spatial location and temporal sequence of channels, activation
can result in waves used as signals.
Neurons carry only two obvious types of signals: localized “gated” potentials
which are older on the evolutionary scale and analog, and propagating “all or
none” action potentials which are newer and digital. Localized gated potentials
can spread only one to two millimeters, are attenuated and distorted by local
resistivity, and are essential where spatial and temporal summation (“integration”)
is required. This occurs at sensory nerve endings (“receptor potentials”), neuronal
synaptic junctions where both excitatory and inhibitory potentials are integrated
(“synaptic potentials”), and as slow waves arising as rhythmic depolarizations in
dendrites. Gated potentials in dendrites are also integrated at the cell body to
initiate (when appropriate) propagating action potentials along axons. A primary
role for localized dendritic potentials in cerebral neuron information processing
has been emphasized by several authors including Alwyn Scott (1977) and Ross
Adey (1966) who feel dendritic slow wave potentials allow cerebral neurons to
“whisper together.”
Action potentials (“nerve impulses”) propagate as membrane depolarization
waves along axons. They occur due to sequential opening of membrane channels
which allow passive diffusion of ions. Gaps in myelinization (“nodes of Ranvier”)
along axons contain abundant ion channels so that impulses propagate rapidly
between nodes where they are slowed and susceptible to modulation (“saltatory
conduction”). With this exception, action potentials occur on an “all or none”
basis (i.e. digital) from integration of dendritic input (i.e. analog) at the cell body
region of the neuron. The frequency of firing is related to the stimulus intensity; a
sensory nerve responding to muscle stretch fires at a rate proportional to the
degree of stretch. Action potential velocity is fixed for given axons dependent on
axon diameter, degree of myelinization, and distribution of ion channels. Typical
action potential velocities of about 100 meters per second allow effective
communication within relatively large nervous systems.
4.2.3 Interneuronal Synapses
Action potentials and axons terminate at synaptic connections with other
neurons or effector cells such as muscle or gland. Final branch portions of axons
are thin with swollen synaptic terminals known as boutons. Some axons may have
multiple boutons, each one forming a synapse. Generally, synapses form between
the axon terminal and another neuron’s dendrite, although axon-cell body, axonaxon,
and dendrite-dendrite synapses also occur. Many, even most, dendritic
synapses occur on dendritic “spines”knobby dendritic protuberances.
Two modes of synaptic signaling have been recognized: electrical and
chemical. At electrical synapses, currents generated by an impulse of the
presynaptic nerve terminal spread directly to the next neuron through a low