What is defined as a brief electrical charge that travels down an axon?

The correct answer is action potential. This term refers to a rapid and temporary change in the electrical charge that travels along the membrane of a neuron, specifically down its axon. In simpler terms, when a neuron is activated by an adequate stimulus, an action potential occurs, characterized by a sequence of electrical impulses that trigger cascades of cellular responses. This process is essential for the transmission of signals throughout the nervous system, enabling communication between neurons.

Other options refer to different concepts within neurobiology. A neurotransmitter is a chemical messenger released from the axon terminals that transmits signals between neurons at synapses. The term threshold refers to the minimum level of stimulus needed to generate an action potential. Lastly, the synaptic gap (also known as the synaptic cleft) is the tiny space between neurons where neurotransmitters cross to facilitate communication between them. Understanding these distinctions highlights the role of action potential as the fundamental means by which electrical signals propagate along neurons.