Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 27

brought back down by the inhibitory channels, it tends
to overshoot the basic resting potential slightly. This
causes a refractory period following a spike, where it
is unable to fire another spike until the membrane po-
tential climbs back up to the threshold level again. The
refractory period can also be caused by the lingering
inactivation of the voltage-gated channels that initiate
the spike. This refractory period effectively results in a
fixed maximum rate at which a neuron can fire spikes
(more on this later).
This spike impulse is communicated down the length
of the axon by the combination of two different mech-
anisms, active and passive . The active mechanism
amounts to a chain-reaction involving the same kinds
of voltage-gated channels distributed along the length
of the axon. A spike triggered at the start of the axon
will increase the membrane potential a little bit further
down the axon, resulting in the same spiking process
taking place there, and so on (think of the domino ef-
fect). However, this active mechanism requires a rela-
tively large amount of energy, and is also relatively slow
because it requires the opening and closing of channels.
Thus, most neurons also have sections of the axon that
propagate the spike using the passive mechanism. This
mechanism is essentially the same as the one that takes
place in the dendrites, based on the cable properties
of electrical propagation. This passive propagation is
much faster, because it is a purely electrical process.
Because the passive mechanism suffers from atten-
uation (weakening signal strength) over distances, the
neuron has some relay stations along the way where
the active spiking mechanism reamplifies the signal.
These relay stations are called the nodes of Ranvier .
To make the passive conduction in between these relay-
station nodes more efficient, the axon is covered by an
insulating sheath called myelin . With this combination
of propagation mechanisms, the neuron can efficiently
send signals over very long distances in the cortex (e.g.,
several centimeters) in a relatively short amount of time
(on the order of a millisecond).

Node of Ranvier

Action
Potential
Propagation

Myelin

Axon Hillock

Figure 2.3: Illustration of the principal aspects of the axonal
output system, including the axon hillock where the action po-
tential (spike) is initiated, and the myelin separated by nodes
of Ranvier that propagates the spike using a combination of
passive and active properties.

with the others. That said, we will just jump in at the
point where a sending neuron has gone over its thresh-
old, and is firing an output signal to a receiving neuron.
Thus, we will trace in more detail the chain of events as
the signal moves down the axon of the sending neuron,
through the synapse, and to the dendrite of the receiv-
ing neuron. We will also refer to the sending neuron
as the presynaptic neuron (before the synapse), and the
receiving neuron as the postsynaptic neuron (after the
synapse).

2.3.1 The Axon

The action of neural firing is variously called spiking ,
or firing a spike, or triggering an action potential .As
we have said, the spike is initiated at the very start
of the axon, in a place called the axon hillock (fig-
ure 2.3). Here, there is a large concentration of two
kinds of voltage-gated channels that only become acti-
vated when the membrane potential reaches a specific
threshold value. Thus, it is the value of the membrane
potential at this point where the threshold is applied, de-
termining the spiking of the neuron.
When one type of voltage-gated channels at the
hillock opens, they cause the neuron to become even
more excited. This further excitation causes the other
type of channels to open, and these channels act to in-
hibit the neuron. This results in a spike of excitation
followed by inhibition. When the membrane potential is

2.3.2 The Synapse

Now we move from the electrical pulse coursing down
the axon as a result of spiking to the release of neuro-