If all sensory signals are eventually transduced into the common action potential, then how do receptors encode stimulus modality? Give an example

Question

What will be an ideal response?

Answer 1

Answer: One way in which sensory systems can code for stimulus modality is described by the theory of labeled lines. Since most sensory receptors are maximally sensitive to only one type of stimulus, and a sensory receptor is part of (or synapses with) a particular afferent neuron, signals in that afferent neuron must represent a specific stimulus modality. In other words, nociceptive afferent neurons are active ONLY in response to nociceptive stimuli, and thermal afferent neurons are active ONLY in response to thermal stimuli. In a common example, we know that the optic nerve transmits the signal "light" whenever the eye is stimulated, even if the stimulus is pressure on the eyeball.

The assumption with the labeled-line theory is that there is a discrete pathway from a sensory cell to the integrating center, but sensory connections are not always this simple. For example, polymodal receptors (e.g., ampullae of Lorenzini), which are sensitive to a variety of different stimuli, cannot code in the basic way laid out by the labeled-line theory. In this case, the afferent neuron may discriminate modality based on the firing pattern (e.g., high-frequency bursts instead of tonic firing). Additionally, neighboring polymodal receptors, with slightly different sensitivities to the various stimuli, may work together to send a coded signal to the afferent neuron. This type of "cross-fiber coding" is not well understood.