Sensory receptors are dendrites of sensory neurons specialized for receiving specific kinds of stimuli. Sensory receptors are classified by three methods Classification by receptor complexity: Free nerve endings are dendrites whose terminal ends have little or no physical specialization.
A sensory receptor, or sense organ, is the part that responds to a stimulus in the internal or external environment of an organism. It is the input to the sensory system.[rx] In response to stimuli, the sensory receptor fires off a nerve fiber that goes to the central nervous system.
Classification by receptor complexity
- Free nerve endings are dendrites whose terminal ends have little or no physical specialization.
- Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective tissue.
- Sense organs (such as the eyes and ears) consist of sensory neurons with receptors for the special senses (vision, hearing, smell, taste, and equilibrium) together with connective, epithelial, or other tissues.
Classification by location
- Exteroceptors occur at or near the surface of the skin and are sensitive to stimuli occurring outside or on the surface of the body. These receptors include those for tactile sensations, such as touch, pain, and temperature, as well as those for vision, hearing, smell, and taste.
- Interoceptors (visceroceptors) respond to stimuli occurring in the body from visceral organs and blood vessels. These receptors are the sensory neurons associated with the autonomic nervous system.
- Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, ligaments, and joints. These receptors collect information concerning body position and the physical conditions of these locations.
Classification by type of stimulus detected
- Mechanoreceptors respond to physical force such as pressure (touch or blood pressure) and stretch.
- Photoreceptors respond to light.
- Thermoreceptors respond to temperature changes.
- Chemoreceptors respond to dissolved chemicals during sensations of taste and smell and to changes in internal body chemistry such as variations of O 2, CO 2, or H + in the blood.
- Nociceptors respond to a variety of stimuli associated with tissue damage. The brain interprets the pain.
Classification of Receptors by Stimulus
Adequate stimulus
A sensory receptor’s adequate stimulus is the stimulus modality for which it possesses the adequate sensory transduction apparatus. The adequate stimulus can be used to classify sensory receptors:
- Baroreceptors respond to pressure in blood vessels
- Chemoreceptors respond to chemical stimuli
- Electromagnetic radiation receptors respond to electromagnetic radiation[24]
- Infrared receptors respond to infrared radiation
- Photoreceptors respond to visible light
- Ultraviolet receptors respond to ultraviolet radiation[citation needed]
- Electroreceptors respond to electric fields
- Ampullae of Lorenzini respond to electric fields, salinity, and to temperature, but function primarily as electroreceptors
- Hydroreceptors respond to changes in humidity
- Magnetoreceptors respond to magnetic fields
- Mechanoreceptors respond to mechanical stress or mechanical strain
- Nociceptors respond to damage, or threat of damage, to body tissues, leading (often but not always) to pain perception
- Osmoreceptors respond to the osmolarity of fluids (such as in the hypothalamus)
- Proprioceptors provide the sense of position
- Thermoreceptors respond to temperature, either heat, cold or both
Sensory receptors are primarily classified as chemoreceptors, thermoreceptors, mechanoreceptors, or photoreceptors.
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, proprioceptors, 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:
- Chemicals (chemoreceptors)
- Temperature (thermoreceptors)
- Pressure (mechanoreceptors)
- Light (photoreceptors)
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 the vision, rod and cone photoreceptors respond to light intensity and color. During the 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
An 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 |
| Ampullae of Lorenzini (primarily function as electroreceptors) | Electric fields, salinity, and temperature |
| Baroreceptors | Pressure in blood vessels |
| Chemoreceptors | 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:
- Free nerve endings characterize the nociceptors and thermoreceptors.
- 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.
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:
- Modality (or type)
- Intensity
- Location
- 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 a 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.
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