2.3 Neurotransmitters

The neurotransmitters are chemicals that play an essential role in transmitting neural signals. To be considered a neurotransmitter, there must be a demonstrable excitatory or inhibitory effect of the chemical on its target. Some of these chemicals have a second role as a hormone outside the central nervous system.

Neurotransmitters fall into three major classes: small-molecule transmitters, peptide transmitters, and transmitter gases. The small-molecule neurotransmitters are synthesized at the axon terminal and are created from building blocks from the food we eat. The chemicals from the food are absorbed into the bloodstream and then pumped into the cells via transporter systems. These neurotransmitters are quick acting. The peptide transmitters are created in the cell body according to instructions contained within our DNA and transported via the microtubules in axon to the axon terminal. These neurotransmitters are slower acting at the post-synaptic neuron, working via a secondary messenger system. The third class of neurotransmitters is the transmitter gases. These are synthesized in the cell as needed.

Scientists don’t agree on the exact number of neurotransmitters in the human body. Estimates range from 30-100. One of the key neurotransmitters that we will encounter in our discussions of learning and memory is acetylcholine. Acetylcholine (Ach) is a small-molecule neurotransmitter. It is synthesized from choline that is found in fats like egg yolk and acetate that is found in acidic foods like vinegar. It is broken down using the enzyme acetylcholinesterase (AChE). The brain pathway of neurons involving Ach is active during the wakeful states. It plays an important role in learning and memory. A decrease in Ach is related to Alzheimer’s disease. Ach is the major neurotransmitter in the autonomic nervous system. Two important peptide transmitters are glutamate and GABA. Glutamate is the main excitatory neurotransmitter. It is found in high concentrations in the forebrain and cerebellum. An chemical with a similar structure to glutamate, domoic acid, when taken in excess can damage the hippocampus and impair memory. GABA is the major inhibitory neurotransmitter. Like glutamate, it is found in the forebrain and cerebellum, as well as the amygdala and thalamus. Increases in GABA are associated with sleep and reduction of anxiety. Anti-anxiety drugs like valium, act on GABA receptors.