How do neurons communicate with each other?

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!!