What is the definition of a neurotransmitter?

A neurotransmitter is specifically defined as a chemical messenger that facilitates communication between neurons by transmitting signals across synapses, which are the junctions where one neuron communicates with another. When a neuron is activated, it releases neurotransmitters into the synaptic cleft, the small space between neurons. These chemicals then bind to specific receptors on the surface of the neighboring neuron, leading to various effects, such as depolarization or hyperpolarization, depending on the type of neurotransmitter and receptor involved.

This definition is foundational to understanding how the nervous system operates, as neurotransmitters play a critical role in processes such as mood regulation, response to stimuli, and coordination of movement. Their function is distinct from that of hormones, which are released into the bloodstream and have broader effects on bodily systems, and is also separate from proteins involved in muscle contraction or types of neurons. This clarity illustrates the unique and vital role neurotransmitters have in neural communication and signaling within the nervous system.