Sensory receptors can be classified by the type of stimulus that generates a response in the receptor. Broadly, sensory receptors respond to one of four primary stimuli:

1. Chemicals (chemoreceptors)
2. Temperature (thermoreceptors)
3. Pressure (mechanoreceptors)
4. Light (photoreceptors)

A schematic of the classes of sensory receptors

Sensory receptor cells differ in terms of morphology, location, and stimulus.

This is a a schematic drawing of the classes of sensory receptors. Sensory receptor cells differ in terms of morphology, location, and stimulus. This drawing shows four different receptors—free nerve endings, encapsulated nerve ending, a sensory cell, and peripheral processes. These are shown to be connected to the sensory ganglion and central nervous system in different ways.

All sensory receptors rely on one of these four capacities to detect changes in the environment, but may be tuned to detect specific characteristics of each to perform a specific sensory function. In some cases, the mechanism of action for a receptor is not clear. For example, hygroreceptors that respond to changes in humidity and osmoreceptors that respond to the osmolarity of fluids may do so via a mechanosensory mechanism or may detect a chemical characteristic of the environment.

Sensory receptors perform countless functions in our bodies. During vision, rod and cone photoreceptors respond to light intensity and color. During hearing, mechanoreceptors in hair cells of the inner ear detect vibrations conducted from the eardrum. During taste, sensory neurons in our taste buds detect chemical qualities of our foods including sweetness, bitterness, sourness, saltiness, and umami (savory taste). During smell, olfactory receptors recognize molecular features of wafting odors. During touch, mechanoreceptors in the skin and other tissues respond to variations in pressure.

Classification of Sensory Receptors

Adequate Stimulus

Adequate stimulus can be used to classify sensory receptors. A sensory receptor's adequate stimulus is the stimulus modality for which it possesses the adequate sensory transduction apparatus.

Sensory receptor table

Sensory receptors with corresponding stimuli to which they respond.

This is a list of sensory receptors with the corresponding stimuli to which they respond. The ampullae of Lorenzini receptor responds to stimulus from electric fields, salinity, and to temperature; baroreceptors respond to pressure in blood vessels; chemoreceptors respond to chemical stimuli; electromagnetic radiation receptors respond to electrofields; hydroreceptors respond to humidity; infrared receptors respond to infrared radiation; magnetocreceptors respond to magnetic fields; mechanoreceptors respond to mechanical stress or strain; nociceptors respond to damage to body tissues; osmoreceptors respond to the osmolarity of fluids; photoreceptors respond to visible light; proprioceptors respond to the sense of position; thermoreceptors respond to temperature; and ultraviolet receptors respond to ultraviolet radiation.

Location

Sensory receptors can be classified by location:

• Cutaneous receptors are sensory receptors found in the dermis or epidermis.
• Muscle spindles contain mechanoreceptors that detect stretch in muscles.

Morphology

Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:

1. Free nerve endings characterize the nociceptors and thermoreceptors.
2. Encapsulated receptors consist of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning.

Rate of Adaptation

A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus.