With special senses, how do receptor cells communicate with associated sensory neurons?

• A. By signaling via hormonal pathways
• B. By direct electrical coupling without chemical signaling
• C. By altering their shape without signaling
• D. By releasing neurotransmitter sufficient to elicit an action potential in the sensory neuron

Answer: (D)

Special senses rely on receptor cells that act as transducers, turning a stimulus into chemical signals that drive the associated sensory neurons. The receptor cell, upon stimulation, releases neurotransmitter at a synapse with the sensory neuron. This neurotransmitter binds to receptors on the sensory neuron, causing it to depolarize and fire action potentials (or adjust its firing rate). This is how vision, hearing, balance, taste, and smell transmit information to the nervous system.

For example, hair cells in the inner ear release glutamate onto afferent fibers that carry the signal to the brain; photoreceptors modulate their neurotransmitter release in response to light onto downstream neurons; taste receptor cells release ATP onto gustatory fibers; olfactory receptor neurons release neurotransmitter onto olfactory bulb neurons. Hormonal signaling, direct electrical coupling, or signaling without neurotransmitter release do not describe the usual local communication between receptor cells and their sensory neurons in these systems.