Which of the following defines a neurotransmitter?

A neurotransmitter is best defined as a chemical messenger that transmits signals between neurons. This definition encompasses the primary role of neurotransmitters in the nervous system, where they are released from the presynaptic neuron into the synaptic cleft and bind to specific receptors on the postsynaptic neuron, facilitating communication between these cells.

This process is crucial for various physiological functions, including muscle contraction, mood regulation, and the ability to learn and remember. Neurotransmitters can initiate new electrical impulses in neighboring neurons, modulate existing impulses, and influence a wide array of activities in the brain and body.

The other options do not accurately represent neurotransmitters. A type of neuron refers to the actual cells that transmit signals, rather than the substances that facilitate this communication. A form of electrical impulse in the nervous system pertains to action potentials, which are the actual signals traveling along neurons, rather than the chemical signals that propagate these impulses. A structural component of neurons relates to parts like the axon, dendrites, or cell body, rather than the biochemical substances that communicate signals between these structures. Thus, recognizing the role of neurotransmitters is essential for understanding neural communication and functioning.