Neurons generally do not touch each other so despite carrying electrical charges they communicate via neurotransmitters which are proteins released into a small space known as the synaptic cleft between the neurons. There are numerous neurotransmitters but most synapses are mediated by just seven of them. Detailing the neurotransmitters is not within the scope of this course but they are a fascinating area of research and many pharmacologic interventions, especially for mental illness, focus on regulating these compounds. To give one quick example, Parkinson’s disease is due to the inadequate production of one of these proteins, dopamine, in the brain.
Figure 3. Synaptic cleft
Once the neurotransmitter is released into the synaptic cleft it attaches to receptors on the opposing neuron. This interaction between the neurotransmitter and its specific receptor will have one of two effects on the receiving neuron. It will either increase the action potential in the second neuron if it is an excitatory response, or if it is an inhibitory response, it will decrease the action potential. As each neuron has many connections with other neurons the net effect on any specific neuron is the net result of all the excitatory and inhibitory signals it is exposed to at any one time. Once the neurotransmitter is in the cleft it must be rapidly removed from the cleft to keep the response from being persistent. As mentioned previously this is another process where the neuron must use energy to reabsorb and recycle these neurotransmitters.