Neurons communicate at structures called synapses in
a process called synaptic transmission. The synapse consists of the two neurons, one of which is sending information
to the other. The sending neuron is known as the pre-synaptic neuron (i.e. before the synapse) while
the receiving neuron is known as the post-synaptic neuron (i.e. after the synapse). Although the flow of information around the brain is achieved by electrical
activity, communication between neurons is a chemical process. When
an action potential reaches a synapse, pores in the cell membrane are
opened allowing an influx of calcium
ions (positively charged calcium atoms)
into the pre-synaptic terminal. This causes a small 'packet' of a chemical neurotransmitter to
be released into a small gap between the two cells, known as the synaptic
cleft. The neurotransmitter diffuses across
the synaptic cleft and interacts with specialized proteins called receptors that
are embedded in the post-synaptic membrane. These receptors are ion
channels that allow certain types of ions (charged atoms) to pass through
a pore within their structure. The pore is opened following interaction
with the neurotransmitter allowing an influx of ions into the post-synaptic
terminal, which is propagated along the dendrite towards the soma. For an annotated animation,
click here.
Synaptic transmission
can be excitatory or inhibitory
Neurotransmission can be either excitatory, i.e. it increases the possibility
of the post-synaptic neuron firing an action potential, or inhibitory.
In this case, the inhibitory signal reduces the likelyhood of an action
potential being generated following excitation.So how does inhibition
work?
Well, this is where things
get a little more complicated! We have seen that the action potential is propagated by the leading edge of a depolarisation wave activating
sodium channels further down the axon. We have also seen that the
activation of these sodium channels is achieved by a small depolarisation
of the neuronal membrane.
But what would happen if the membrane potential was stabilised?
The depolarisation inside the neuronal axon would dissipate and the
action potential would not be able to propagate any further - i.e.
it would be inhibited. This stabilisation
of the membrane potential is achieved by an influx of negatively charged
chloride ions that is unaffected by the depolarisation wave coming
down the axon. Formerly, this is equivalent to an efflux of positively
charged sodium ions. Thus it is like punching a hole in a hose so that
water will leak out through the puncture and not get to the sprinkler!
Confused? Hmmmm....well we can look at
it this way - the negatively charged chloride ions will cancel out
the positively charged sodium ions, hence no depolarisation and no
action potential propagation!!
