What best describes neurotransmitters in the context of cellular communication?

• A. Long-range signals released into blood
• B. Local signaling molecules diffusing across synaptic cleft
• C. Large proteins transported through epithelial cells
• D. Cells communicating directly through junctions

Answer: (B)

Neurotransmitters are best described as local signaling molecules that diffuse across the synaptic cleft. In the context of cellular communication, neurotransmitters play a crucial role in transmitting signals from one neuron to another or from a neuron to a target effector cell, such as a muscle or gland cell. When an action potential reaches the end of a neuron, neurotransmitters are released from vesicles into the synaptic cleft, which is the small gap between the presynaptic neuron and the postsynaptic cell. The neurotransmitters then bind to specific receptors on the postsynaptic membrane, leading to a variety of responses, such as the generation of a new action potential in a neuron or muscle contraction.

This mechanism is characterized by its localized effect, as neurotransmitters act specifically on nearby cells rather than affecting distant sites in the body. The diffusion across the synaptic cleft is a rapid process, ensuring quick communication between neighboring cells, which is essential for functions like muscle movement and response to stimuli.