The first neurotransmitter to be discovered, about 90 years ago, was acetylcholine (ACh). This is a chemical substance released by neurons to the connected muscles, causing them to contract, and by those neurons regulating the heartbeat. Furthermore, ACh also serves as a transmitter in many regions of the brain.

ACh and muscles

Acetylcholine is formed at the axon terminals. When an action potential arrives here, the electrically charged calcium ion rushes in and ACh is released into the synapse and attaches to its respective receptors. This opens the ion channels and causes the (voluntary) muscle to contract. After this, the chemical is broken down and synthesized anew in the nerve terminal. Myasthenia gravis, a disease characterized by fatigue and muscle weakness, results from antibodies blocking the receptors for this specific neurotransmitter.

ACh and the brain

Much less is known about the functions of ACh in the brain. Recent discoveries seem to suggest that it might be of critical importance for normal attention, memory and sleep. One of the goals of current research is to find a way to restore ACh releasing neurons, since these are the ones that perish in Alzheimer’s patients, leading to a number of symptoms, including problems concerning the memory mechanism.

Amino Acids

Amino acids, the building blocks of proteins, are widely distributed throughout the body and brain. Some of these can also serve as neurotransmitters. Examples of this are:

Glutamate and aspartate, which can serve as excitatory signals.
Glycine and Gamma-aminobutyric acid (GABA) are two amino acids that inhibit the firing of neurons.

Glutamate and aspartate

Glutamate or aspartate activate N-methyl-D-aspartate (NMDA) receptors, one of three major classes of glutamate receptors, which has been implicated in processes ranging from learning and memory to development and specification of nerve contacts in a developing animal. The stimulation of these receptors may promote beneficial changes in the brain, whereas overstimulation may cause cell damage or cell death in trauma or stroke.

There are still many questions about the receptor’s precise structure, regulation, location and function. Developing drugs that block or stimulate activity at NMDA receptors hold promise for improving brain function and treating neurological disorders. This work, however, is still in a very early stage.

GABA

Valium and anticonvulsant drugs increase the activity of GABA. In Huntington’s disease, a hereditary disorder that usually begins during midlife, the GABA producing neurons in the brain centers that coordinate movement degenerate, causing uncontrollable movements.