At the origin of every thought or action, there are the nerve impulses
that travel through the body’s neurons. These impulses
are nothing more than the movement of electrically charged inorganic
molecules through the neural membrane.
But this movement of electrical charges would never allow us to
be self-aware if their coordination were not determined by the
properties of the channels in the neural membrane. These channels
are actually large proteins embedded in this membrane, and their
properties are determined by the human genome, itself the result
of evolution.
The fundamental property of this membrane is that it is semi-permeable.
In other words, it allows some charged molecules, known as ions,
to pass through more easily than others.
Among these ions that play an important
role in the nervous system, potassium (K+), which is positively
charged, is the one that passes most easily through a neural
membrane in its resting state. Sodium (Na+), which also has a
positive charge, and chloride (Cl-), which has a negative one,
have more difficulty passing through the membrane. Large, negatively
charged molecules inside the neuron cannot get out but do also
influence the membrane’s electrical potential.
To complete the picture, we should note that the calcium ion (Ca++)
also plays an important role, but in the process of synaptic
transmission.
The resting potential is the equilibrium that
results from the distribution of these ions on either side of the
membrane. In this base state, which is modified by the passage of
a nerve impulse, the inside of the neuron is negatively charged
relative to the outside. This resting potential measures approximately
-70 millivolts.
(click on 4. Resting Membrane Potential)
Click on step numbers 1, 2, and 3 in the diagram below to see
how these various kinds of ions are distributed and how they move
when a nerve impulse passes down the neuron.
During this “refractory” period,
the membrane cannot generate a new impulse, which forces the
cycle of opening ion channels to shift down to the neighbouring
region of the membrane. In this way, the nerve impulse is propagated
down the length of the axon.
The process that enables
a nerve impulse to pass from one neuron to another is called
synaptic transmission. This transmission is effected by chemical
molecules, called neurotransmitters,
which bind to receptors.
It is through variations in the amount of neurotransmitters
released, the receptors available, or the affinity between
the two that the synapses undergo changes and enable us to
learn.
Synaptic transmission is thus an omnipresent mechanism that
is the source of the brain’s
great plasticity. Dozens of times per second, at the ends
of each of our billions of neurons, the following sequence of
events takes place (click the numbers to see each step).
Of course, each of these four major steps
in synaptic transmission actually comprises other
steps.
