In all histology books, it is stated that all sensory nerve endings (receptors) consist of dendrites that translate physical stimuli from the environment into neural signals. However, several sensory neurons seem to include axons, such as the merkel and olfactory cells shown below. They act as receptors, but feature axons as well. So is it the pictures, or the texts that are incorrect?
The textbooks are right; the receptor part of sensory neurons is always considered to be the dendrite. Axons are nonetheless an integral part of some receptor types to send the signal to the brain. Other, more specialized sensory cells do not have an axon and therefore rely on secondary neurons to send the signal to the brain.
There are three types of sensory cells, namely those with free nerve endings, those with specialized dendritic structures, and specialized receptor cells that synapse directly onto a secondary neuron (Fig. 1).
Fig. 1. Types of sensory neurons. Source: Premed HQ.
- Sensory neurons with free nerve endings have an exposed dendrite that functions as receptor. Examples are nociceptors (pain receptors) and hair follicle receptors.
- Other sensory neurons are complex cells that feature a dendritic region specialized for a particular receptor function. Examples are the Pacinian corpuscle and olfactory neurons.
- The most common receptor type are specialized receptor neurons that have no axon and need a secondary sensory neuron to send the signal to the brain. Examples are the photoreceptor cells in the retina, the hair cells in the cochlea and Merkel cells.
In all histology books, it is stated that all sensory nerve endings (receptors) are made of dendrites which conduct sensation from environment to body but it doesn't seem to be true.
The phrase "nerve endings" (or "free nerve endings". See Wiki page) is typically associated with fine hairlike structures throughout the skin and organs that act as (one type of) touch, pain, or heat sensors. These are, yes, extensions of the dendrites of sensory neurons whose cell bodies live in the dorsal root ganglia of the spinal cord and who send the signal on via axons, either locally within the spinal cord or to brain structures.
But this is not the same as saying that all sensory structures of any sense modality are dendrites or extensions of them. Each sense has specialized cells to detect that sensory stimulus. The eye has photoreceptors (rods and cones), the ear has inner hair cells, the nose has olfactory receptor neurons, the tongue has taste cells, and the skin and organs have free nerve endings and specialized mechnoreceptors such as pacinian corpuscles. Most of these do the work of transducing the stimulus in a specialized receptor portion of the cell, such as the rod itself, or the cilia of a hair cell or ORN, etc., and then the signal is sent to the next cell in the sensory chain via a synapse, sometimes via a very short axon or just sort of axon "nub".