Excitatory neurotransmitters are neurotransmitters that increase the likelihood that a nerve cell will produce an action potential, an electrochemical impulse that nerve cells use to transmit signals. They are distinguished from inhibitory neurotransmitters, which make an action potential in the cell less likely. The most common excitatory neurotransmitter in all vertebrates, including human beings, is called glutamate.
The distinction between excitatory and inhibitory neurotransmitters is a spectrum, not an absolute division. The effects of a neurotransmitter depend on the type of receptor it has bonded with, which means that the same neurotransmitter may be excitatory or inhibitory depending on the circumstances. Thus, neurotransmitters that are primarily excitatory and classified as such may actually be inhibitory in some circumstances. There are also neurotransmitters, such as acetylcholine, that are not predominantly excitatory or inhibitory and so do not fit into either category.
Neurotransmitters are molecules that nerve cells, or neurons, use to communicate. When electrically stimulated, the transmitting, or prensynatptic, neuron releases neurotransmitters into the gap, called a synapse, between itself and an adjacent neuron. These neurotransmitters bond with receptors on the exterior membrane of the receiving, or postsynaptic, neuron. Many different types of receptor exist, bonding with different types of neurotransmitters according to their own chemical properties. When a neurotransmitter binds with a receptor, it activates structures in the postsynaptic cell's membrane called ion channels that allow specific types of electrically charged atoms, or ions, to pass through the membrane.
When the neuron is not transmitting, these channels regulate the movement of ions so that the interior of the cell is positively charged and the exterior is negatively charged, a default state called a rest potential. Excitatory neurotransmitters activate channels that allow the passage of positively charged ions, usually sodium ions, into the atom. If enough excitatory neurotransmitters bond with receptors, the resulting influx of positive ions creates a voltage across the cell membrane, which activates more sodium channels and so on until all sodium channels are open. This sends an electrical impulse through the nerve cell that travels down a cellular structure called an axon until it reaches the next synapse, where the process repeats as the impulse triggers the release of excitatory neurotransmitters for the next neuron.
The most common excitatory neurotransmitter, glutamate, is important to learning and memory. It is also important to long-term potentiation, a process that strengthens signal transmissions between specific neurons and is an important part of how the nervous system adapts itself over time. Excessive accumulations of glutamate in the synapses, a condition called excitotoxicity, can damage or kill neurons and may be linked to diseases of the nervous systemm such as Parkinson's disease, Alzheimer's disease, and multiple sclerosis. Excessive glutamate levels may also be a cause of epileptic seizures.