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I was recently asked the following question:

Compare the following pairs of receptors in the same sensory system. Include in your discussion:

  • The distribution in the sensory epithelium;
  • What kind of stimulus is effective in activating them;
  • Their physiological response properties, function and central connections.
    1. Rods and cones
    2. Pacinian corpuscles and free nerve endings

In particular, I was not sure what the sensory epithelium is. Is it getting at the fact that Pacinian corpsucles are subcutaneous, while free nerve endings are cutaneous and that cones are centralized at the fovea?

Alos, I am not sure what their "physiological response properties" refers to. Since they are receptors, would their responses just be the output (i.e. peripheral/foveal vision and vibration/pain sense)?

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The distribution in the sensory epithelium indeed refers to the location of the different receptors. And indeed, you are also right that cones are concentrated in the fovea, and rods are mostly found more eccentrically in the retina (Kolb, 2012. I would say the free nerve endings are situated just below the epidermis, and the PCs in deeper layers of the dermis (Fig. 1). But check your notes on what your prof. wishes to hear about this.

![skin
Fig. 1. Skin receptors. source: Neupsy Key

The physiological response properties of receptors is not their output per se. It is the way they respond to stimuli, that includes their output, but also includes the sensation of physical stimuli through second messenger systems or otherwise, and eventually the output. Kolb (2012 goes into detail on the physiology of photoreceptors in quite some detail and for the physiology of touch receptors I would refer to Dougherty in Neuroscience online.

References
- Dougherty, Somatosensory Systems. in: Neuroscience online. University of Texas
- Kolb, Photoreceptors. In: Webvision. The Organization of the Retina and Visual System (2012)

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