Sensory receptors

Structure

Sensory receptors are often found as neurons which are specialized for the detection of stimuli (e.g., most mechanoreceptors in the skin are pseudounipolar neurons with peripheral endings adapted for detecting changes in pressure, vibration, touch etc.; rod and cone cells (photoreceptors) are specialised unipolar neurons which contain photosensitive pigments necessary for detecting light stimuli). Alternatively, sensory receptors can also present as ‘cell-neuron complexes’ where stimuli are detected by nonneuronal sensory receptor cells which then communicate with sensory neurons (e.g., gustatory sensory epithelial cells found in the tongue/soft palate respond to tastants in saliva and relay this information to gustatory neurons which carry the signals to the brain).

This varied architecture of sensory receptors allows for the recognition of three primary groupings of sensory receptors:

1. Nonencapsulted receptors which have a relatively simple structure where the peripheral terminals of a sensory neuron are directly embedded into the tissue. They include free nerve endings, like those detecting pain and temperature in the dermis of the skin and hair follicle plexuses which surround the base of hair follicles, detecting hair movement. Epithelial tactile (Merkel) complexes also fall under this category; here a tactile epithelial (Merkel) cell communicates with specialized peripheral endings of a sensory neuron known as tactile meniscus (Merkel disc), conveying formation on light touch/pressure.
2. Encapsulated receptors found on neurons whose peripheral terminals are enclosed in connective tissue that enhances their sensitivity to stimuli. Examples include lamellar (Pacinian) corpuscles in the skin, which respond to pressure and touch; and [Golgi] tendon organs which detect changes in muscle tenson and force.
   
3. Specialized receptor cells/neurons with special structural components adapted to interpret a particular type of stimulus, such as photoreceptors in the retina responding to light stimuli.

When sensory receptors are stimulated, either directly through their membrane proteins or through their accessory structures, a localized depolarization, described as a receptor potential, is initiated. The receptor potential, if sufficiently large to reach the threshold, causes voltage-gated Na+ channels to open, triggering the generation of action potentials. These are propagated down the sensory neuron axon until its terminal where they stimulate neurotransmitter release at the synapse, activating second-order neurons. These carry the signal for further processing to the CNS.

Specialized receptor cells can operate in a slightly different way and do not necessarily generate action potentials upon adequate stimulation. Nevertheless, they still undergo changes in membrane potential that lead to either an increase or decrease in the release of neurotransmitters to the sensory neuron.

Location

Sensory receptors can alternatively be classified as proprioceptors, interoceptors and exteroceptors based on their location relative to the stimuli.

1. Proprioceptors (a.k.a. muscular and articular sensory receptors), including muscle spindles and [Golgi] tendon organs (GTOs), are located near moving parts of the body, such as joints and muscles and they interpret the position of the tissues for the coordination of motor activity.
2. Exteroceptors (a.k.a. cutaneous sensory receptors) are located on or near the body surface to detect stimuli from the external environment. Examples of exteroceptors include thermoreceptors responding to temperature changes and receptors in the skin for pressure and touch and photoreceptors.
3. Interoceptors (a.k.a. visceral sensory receptors (visceroceptors), are found in internal organs and tissues and detect changes in the internal state of the body such as in blood pressure, tissue stretch, pain and chemical composition of body fluids.

Function/modality

Photoreceptors

Photoreceptors detecting light stimuli for vision. There are two most common types of photoreceptors are rod and cone cells, both located in the retina. They contain photosensitive proteins, called photopigments. Rod cells are sensitive to low light and are responsible for night vision (scotopic vision) while cones are responsible for color vision and visual acuity in bright light (photopic vision). Visible light energy stimulates these receptors by biochemically altering their photopigments and consequently their membrane potential. This causes a change in neurotransmitter release to bipolar cells, which then communicate with retinal ganglion cells. The axons of these ganglion cells form the optic nerve.

A lesser discussed type of photoreceptor are photosensitive retinal ganglion cells. These respond to light and play a key role in regulating both circadian rhythms and pupil constriction. Unlike rod and ceon cells, they directly influence non-visual light responses.

Thermoreceptors

Thermoreceptors are free nerve endings which respond to changes in temperature and are primarily located in skin and mucous membranes.

• Thermoreceptors responding to innocuous (nonharmful) warm signals are found on dendritic branches of unmyelinated fibers and respond to temperatures between 30 °C and 45 °C.
• Thermoreceptors responding to innocuous cold signals are dendritic branches of lightly myelinated fibers and respond to subnormal body temperatures above 17 °C, showing maximum sensitivity at ~27 °C.

Mechanoreceptors

Mechanoreceptors are a class of sensory receptors stimulated by a range of physical stimuli including pressure, vibration, stretch, hair follicle position, body position, proprioception and sound.

Tactile epithelial (Merkel) cells are found in the basal layer of the epidermis and respond to low-frequency vibrations (5-15 Hz). Lamellar (Pacinian) corpuscles are mechanoreceptors encapsulated by concentric layers of connective tissue, resembling an onion-like structure. They are found in the dermis and subcutaneous tissue and act by filtering and amplifying deep-pressure stimuli, responding to rapid changes in pressure/ high-frequency vibrations (~250 Hz). Tactile (Meissner) corpuscles located in the papillary dermis are concentrated in skin areas sensitive to light touch such as the fingertips, the lips and the genital skin, as they specialize in detecting shape and textural changes in discriminatory touch and low-frequency stimuli (<50 Hz) such as flutter. Bulbous (Ruffini) corpuscles are spindle-shaped stretch receptors in the skin dermis and joint capsules. The hair follicle plexus (peritricheal receptors) is wrapped around hair follicles of the dermis and detects hair movement or displacement across the skin surface. Muscle spindles and [Golgi] tendon Organs (GTOs) are proprioceptors detecting changes in muscle length-velocity and level of tension respectively.

Mechanoreceptors are also the basis of audition and equilibrium. Cochlear hair cells in the spiral organ (of Corti) of the internal ear have apical hair-like structures called stereocilia which are tethered together by proteins that open or close ion channels depending on the direction they are bent upon stimulation by pressure waves. Vestibular hair cells with stereocilia within the vestibule and semicircular canals of the internal ear sense head position/movement within three-dimensional space and body motion, thus contributing to the sense of balance.

Chemoreceptors

Chemoreceptors detect chemical stimuli in the environment or within the body.

Gustatory sensory epithelial cells are responsible for initiating taste sensations (gustation) and located within the taste buds of the tongue and soft palate. They are stimulated by different chemicals from ingested substances dissolved in the saliva, releasing neurotransmitters that activate sensory neurons in the facial, glossopharyngeal and vagus nerves. Olfactory sensory neurons are bipolar neurons located in the olfactory epithelium of the superior parts of the nasal cavity. These neurons have cilia which detect odorants dissolved in the mucus lining of the nasal cavity.

Other chemoreceptors monitor internal chemical conditions:

• Central chemoreceptors are located in the brainstem and are responsible for sensing changes in carbon dioxide and pH levels/hydrogen ion concentration changes of cerebrospinal fluid, providing input into respiratory regulation.
• Peripheral chemoreceptors are found in the carotid body and aortic arch and are responsive to low levels of oxygen, excess carbon dioxide and low pH (increased H+ concentration), affecting respiratory rate and cardiac function.
• Osmoreceptors are primarily located in the hypothalamus and are stimulated by changes in osmotic pressure (which is proportional to solute concentrations in blood and extracellular fluid etc.). Osmotic pressure is the pressure needed to counteract the osmotic movement of water, ensuring equilibrium between different fluid compartments in the body.
  When solute concentrations increase (indicating dehydration), osmoreceptors trigger actions to conserve water e.g., releasing of antidiuretic hormone (ADH) to reduce urine output. When solute concentrations drop and osmotic pressure decreases, osmoreceptors help to promote water excretion to maintain fluid balance.

Nociceptors

Nociceptors respond to noxious or potentially harmful stimuli, which are often interpreted as pain. They are located across much of the body (both externally (e.g., skin) and internally (e.g., digestive tract)) and can detect a wide range of noxious stimuli such as extreme pressure, temperature (above ~40°C or below ~15°C), or chemically-mediated injury which can cause pain when they exceed a threshold.