Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 26

like cables , and analyzing their cable properties .
Thus, when ions flow through channels in the den-
drites, these signals are integrated as they propagate up
to the cell body. At the point where the cell body transi-
tions into the axon, the membrane potential determines
whether the neuron will fire. The thresholded property
of this firing process is due to the sensitivity of a set
of special channels to the membrane potential — these
channels open up only when the membrane potential is
sufficiently elevated. Such channels are called voltage-
gated channels, of which there are many different types.
Although neural information processing is based fun-
damentally on electricity and diffusion, many chemical
processes are necessary to make everything work. In-
deed, many types of neurons rely on chemicals to send
their outputs to other neurons, instead of directly pass-
ing an electrical signal (but other types of neurons do
send electrical signals directly). The cortical neurons
that we are primarily interested in use this chemical
signaling. These chemicals, called neurotransmitters ,
are released at the parts of the axon that are connected
to the dendrites of other neurons (i.e., at the synapses).
Their release is triggered by an electrical pulse coming
down the axon (called the action potential ), and after
being released, they diffuse over to the dendrites, and
chemically bind to receptors on the dendritic part of
the synapse, resulting in the opening of channels.
Thus, inputs are transmitted in the cortex when the
neurotransmitters open particular types of channels on
the receiving neurons, which then allow specific types
of ions to flow, which then triggers the electrical prop-
agation and integration in the receiving neuron as de-
scribed previously. The great chain of neural commu-
nication thus contains alternating links of electrical and
chemical processes. At a longer time scale, the con-
struction, maintenance, and adaptation of the neuron is
based on a complex interaction between genetics, cel-
lular chemistry, and electrical signals, which is beyond
the scope of this topic.
In the next sections, we cover the biology of the axon,
dendrite, and the synaptic connection between them in
somewhat more detail, and then we explore the elec-
trical mechanisms after that. It is difficult to describe
the chain of neural communication one link at a time,
inasmuch as each piece is so intimately interconnected

Figure 2.2: Image of a cortical pyramidal neuron, show-
ing the major structures.

Reproduced from Sejnowski and

Churchland (1989).

difference is known as the membrane potential (as we
mentioned previously), because it is the cell membrane
that separates the inside and outside of the neuron, and
thus it is across this membrane that the difference in
electrical charge exists. As ions flow into and out of the
neuron through channels, this changes the membrane
potential. These changes in potential in one part of the
neuron will propagate to other parts of the neuron, and
integrate with the potentials there. This propagation and
integration can be understood by treating the dendrites