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12.2A: Classification of Receptors by Stimulus

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    7653
  • 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.