What does the term "neurotransmitter" refer to in neuroscience?

The term "neurotransmitter" specifically refers to chemical messengers that transmit signals across the synaptic gap between neurons in the nervous system. These chemicals play a crucial role in facilitating communication within the brain and throughout the body, influencing a variety of functions, including mood, behavior, and motor control. When an electrical impulse reaches the end of a neuron, it triggers the release of neurotransmitters into the synapse, where they bind to receptors on the adjacent neuron, thereby propagating the signal.

In contrast, the other options describe different biological functions that do not pertain to neurotransmitters. Chemicals that assist in digestion relate to the digestive system, signals sent directly to muscles describe the neuromuscular interaction (which involves neurotransmitters but isn't the definition of one), and hormones influencing mood are typically related to the endocrine system rather than the specific function of neurotransmission. Thus, the focus of the correct answer clearly aligns with the role of neurotransmitters as pivotal chemical messengers in neuronal communication.