Neurotransmitters are small chemical molecules released at the presynaptic axonal membrane of one neuron into the synaptic cleft to facilitate signal transduction from the pre-synaptic neuron to the postsynaptic target cell.
Different types of neurotransmitters have been identified. Neurotransmitters can be classified based on different functions, including excitatory neurotransmitters, inhibitory neurotransmitters and modulatory neurotransmitters. Excitatory neurotransmitters have excitatory effects on the neuron, meaning they increase the likelihood that the neuron will fire an action potential. Some of the major excitatory neurotransmitters include epinephrine and norepinephrine. Inhibitory neurotransmitters have inhibitory effects on the neuron, including serotonin and gamma-aminobutyric acid (GABA). Modulatory neurotransmitters are capable of affecting a larger number of neurons at the same time. Note that some neurotransmitters, such as acetylcholine and dopamine, can create both excitatory and inhibitory effects depending upon the type of receptors that are present.
Based on chemical and molecular properties, the major classes of neurotransmitters include amino acids, such as glutamate and glycine; monoamines, such as dopamine and norepinephrine; peptides, such as somatostatin and opioids; and purines, such as adenosine triphosphate (ATP). In some instances, they can also be categorized into one of six types, involving amino acids, peptides, monoamines, purines, gasotransmitters and acetylcholine.
The main function of neurotransmitters is to inhibit or excite the activity of the postsynaptic cell. In order to send messages via neurotransmitters, neurons need to communicate with each other, which they do through synapses. When signals travel through a neuron and reach the end of that neuron, they cannot simply travel through to the next one. Instead, the neuron must trigger the release of neurotransmitters, which then carry signals across the synapses with the goal of reaching the next neuron. As the Figure 1 shows, during synaptic transmission, the action potential (an electrical impulse) triggers the synaptic vesicles of the pre-synaptic neuron to release neurotransmitters (a chemical message). These neurotransmitters diffuse across the synaptic gap (the gap between the pre and post-synaptic neurons) and bind to specialized receptor sites on the post-synaptic neuron.
Figure 1. The Diagram of Interaction between Neurotransmitters and Their Receptors
*This diagram is derived from Encyclopædia Britannica, Inc.
Neurotransmitters are synthesized by neurons and are stored in vesicles. These vesicles typically are located in the axon's terminal end. The neuron which released the neurotransmitters is called the presynaptic neuron. The neuron which receives the neurotransmitters is called the postsynaptic neuron. The end of each neuron has presynaptic endings and vesicles, which are sacks containing neurotransmitters. When a nerve impulse (or action potential) triggers the release of neurotransmitters, these chemicals are then released into the synapse and then is taken up by the receptors on the next neuron. This process is known as neurotransmission.
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