Are sensory receptors neurons?

Question

Background

There are many receptor types in the body, with various functions and various mechanisms of transduction. Receptor cells are considered to be part of the peripheral nervous system, as they are the first step in conveying information from the periphery to the brain.

Many receptor cells do not fire action potentials. Action potentials are widely considered to be a key property of neurons. Instead, receptors often operate by graded responses. For example, upon illumination photoreceptors gradually hyperpolarize. Cochlear hair cells gadually depolarize when the stereocilia are bent into one direction and hyperpolarize when they go in the opposite direction. Hence, many receptors are in fact analog sensors, whereas spiking neurons can be considered to be operating in binary (all-or-nothing) mode.

Given their similarities and differences, are receptor cells classified as neurons?

^{Credits: This question was formulated after a discussion with @OneFace on the question "Relationship between nerves and axons"}

Answer 1

Short answer

Receptor cells are specialized neurons


Background

There are, globally, three types of neurons (Eckert's Animal Physiology):

1. Sensory neurons: these cells transmit information from external stimuli (e.g., sound, light, pressure), or from internal stimuli (e.g., blood oxygen level or head orientation);
2. Interneurons: these cells connect other neurons within the central nervous system;
3. Motor neurons: these neurons carry effector signals to effector organs, causing contraction of muscles or secretion by gland cells.

To illustrate these classes of neurons let's consider the visual peripheral system. Fig. 1 shows a schematized representation of the retinal circuitry, connecting the photoreceptors to the ganglion cells of the optic nerve (interneurons):

^{Source: Instituto Balseiro}

The photoreceptor's response is graded. Notably, the bipolar cell, being an interneuron has also a graded response. The bipolar cells convey the signal, in turn, to the ganglion cells that translate the analog, graded signals into a binary, neural spiking code that enters the optic nerve.

Receptor cells are considered neurons, but they are highly specialized. Although interneurons may come in various shapes and sizes, they are not so specialized as receptor cells. The photoreceptor, for example, has a highly specialized dendritic region containing the photosensitive pigment and membrane stacks that capture light.

Another receptor is the Pacinian corpuscle, a vibrotactile mechanoreceptor of the skin:

^{Source: Pixgood}

In this case, the receptor cell is in fact a myelinized and spiking neuron that functionally much resembles an interneuron. Just as the photoreceptor, the dendritic part does not connect to another neuron, but is highly specialized structure designed to sense a physical external stimulus. In case of the Painian corpuscle, it is an onion-like structure consisting of many lamina. These lamellar structures substantially change the receptor's response to mechanical stimuli. Specifically, it allows the corpuscle to respond rapidly to pressure changes and allow it to faithfully transmit vibratory stimuli.

<sub>Reference

Eckert, Animal Phsyiology:
- Chapter 5 - The Physiological basis of Neuronal Function
- Chapter 7 - Sensing the Environment</sub>