Sensory Receptors

Classification of Receptors by Stimulus

Sensory receptors are primarily classified as chemoreceptors, thermoreceptors, mechanoreceptors, or photoreceptors.

Learning Objectives

Differentiate among the types of stimuli to which receptors respond

Key Takeaways

Key Points

  • Chemoreceptors detect the presence of chemicals.
  • Thermoreceptors detect changes in temperature.
  • Mechanoreceptors detect mechanical forces.
  • Photoreceptors detect light during vision.
  • More specific examples of sensory receptors are baroreceptors, propioceptors, hygroreceptors, and osmoreceptors.
  • Sensory receptors perform countless functions in our bodies mediating vision, hearing, taste, touch, and more.

Key Terms

  • photoreceptor: A specialized neuron able to detect and react to light.
  • mechanoreceptor: Any receptor that provides an organism with information about mechanical changes in its environment such as movement, tension, and pressure.
  • baroreceptor: A nerve ending that is sensitive to changes in blood pressure.

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)

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.

A schematic of the classes of sensory receptors: Sensory receptor cells differ in terms of morphology, location, and stimulus.

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 receptors with corresponding stimuli to which they respond.
Receptor Stimulus
Apmullae of Lorenzini (primarily function as electroreceptors) Electric fields, salinity, and temperature
Baroreceptors Pressure in blood vessels
Chemo receptors Chemical stimuli
Electromagnetic radiation receptors Electromagnetic radiation
Electroreceptors Electrofields
Hydroreceptors Humidity
Infrared receptors Infrared radiation
Magnetoreceptors Magnetic fields
Mechanoreceptors Mechanical stress or strain
Nociceptors Damage or threat of damage to body tissues (leads to pain perception)
Osmoreceptors Osmolarity of fluids
Photoreceptors Visible light
Proprioceptors Sense of position
Thermoreceptors Temperature
Ultraviolet receptors 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.

Classification of Receptors by Location

Some sensory receptors can be classified by the physical location of the receptor.

Learning Objectives

Differentiate among sensory receptors by location

Key Takeaways

Key Points

  • Sensory receptors that share a common location often share a related function.
  • Sensory receptors code four aspects of a stimulus: modality (or type), intensity, location, and duration.
  • Cutaneous touch receptors and muscle spindle receptors are both mechanoreceptors, but they differ in location.

Key Terms

  • cutaneous touch receptor: A type of sensory receptor found in the dermis or epidermis of the skin.
  • muscle spindle: Sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle.

Types of Receptors

As we exist in the world, our bodies are tasked with receiving, integrating, and interpreting environmental inputs that provide information about our internal and external environments. Our brains commonly receive sensory stimuli from our visual, auditory, olfactory, gustatory, and somatosensory systems.

Remarkably, specialized receptors have evolved to transmit sensory inputs from each of these sensory systems. Sensory receptors code four aspects of a stimulus:

  1. Modality (or type)
  2. Intensity
  3. Location
  4. Duration

Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor.

Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function. For example, sensory receptors in the retina are almost entirely photoreceptors. Our skin includes touch and temperature receptors, and our inner ears contain sensory mechanoreceptors designed for detecting vibrations caused by sound or used to maintain balance.

Force -sensitive mechanoreceptors provide an example of how the placement of a sensory receptor plays a role in how our brains process sensory inputs. While the cutaneous touch receptors found in the dermis and epidermis of our skin and the muscle spindles that detect stretch in skeletal muscle are both mechanoreceptors, they serve discrete functions.

In both cases, the mechanoreceptors detect physical forces that result from the movement of the local tissue, cutaneous touch receptors provide information to our brain about the external environment, while muscle spindle receptors provide information about our internal environment.

This is a drawing of a mammalian muscle spindle. It shows a typical position in a muscle (left), neuronal connections in spinal cord (middle), and expanded schematic (right). The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers. The sensory endings of a primary afferent and a secondary afferent coil around the non-contractile central portions of the intrafusal fibers.

Muscle spindle: Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle), and an expanded schematic (right). The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers. The sensory endings of a primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile central portions of the intrafusal fibers.