Sensory Receptors – Anatomy, Types, Functions

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 tendon?. সহজ বাংলা: মাংসপেশি/টেনডনে টান।" data-rx-term="strain" data-rx-definition="A strain is injury to a muscle or tendon. সহজ বাংলা: মাংসপেশি/টেনডনে টান।">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.

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)

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.

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.

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.

References