Which type of chemical messenger travels a short distance across a synaptic cleft?

• A. Paracrine
• B. Neurotransmitter
• C. Hormone
• D. Autocrine

Answer: (B)

The correct choice, a neurotransmitter, is a chemical messenger specifically designed to transmit signals across a synaptic cleft, which is the small gap between neurons. When an electrical impulse (action potential) reaches the end of a neuron, it triggers the release of neurotransmitters from synaptic vesicles into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic neuron, facilitating communication between the two neurons.

Neurotransmitters operate over short distances, enabling rapid signaling essential for processes like reflexes, muscle contraction, and various brain functions. This quick and localized action distinguishes neurotransmitters from other chemical messengers that may act over more extensive distances or different mechanisms.

In contrast, paracrine messengers typically affect neighboring cells but are not restricted to the synaptic cleft; they can diffuse over short distances to target adjacent cells. Hormones are chemical signals secreted into the bloodstream, traveling long distances to distant target organs or tissues. Autocrine signals act on the same cell that created them, which is also distinct from the transmission across a synaptic cleft. Understanding these differences clarifies why neurotransmitters are the primary signaling molecules in synaptic communication.