Connection lost. Please refresh the page.
Online
Get help How to study Login Register
Ready to learn?
Pick your favorite study tool

Sensory receptors

From smelling freshly baked cookies, sensing the warmth of a cup of tea, or the sharp pain of a stubbed toe, our daily experience is shaped by a wide range of sensory stimuli. Sensory receptors are the getaway through which we perceive the world around us. These specialized excitable cells convert physical stimuli into electrical signals that the nervous system can interpret. This process is called sensory transduction.
It is important to understand the distinction between the terms 'sensation' and 'perception': sensation is used to describe the stimulation of sensory receptors; while perception refers to the central processing of sensory stimuli. Sensation is a prerequisite for perception, but not all sensations are perceived.
Sensory receptors can be classified structurally based on the cell type and position as well as functionally based on the type of stimuli they transduce.

Key facts about sensory receptors
Definition Excitable cells specialized in converting physical stimuli into electrical signals, which are interpretable by the nervous system through sensory transduction.
Structural classification Nonencapulsated sensory receptors (free nerve endings, hair follicle endings, epithelial tactile (Merkel) complexes;
Encapsulated sensory receptors
(bulbous (Ruffini) corpuscle, tactile (Meissner) corpuscle etc.);
Specialized receptor cells
(hair cells (internal ear), gustatory cells).
Location-based classification Proprioceptors (sense body position and movement);
Interoceptors
(detect internal conditions);
Exteroceptors
(detect external/environmental stimuli).
Functional classification Photoreceptors (detect light stimuli);
Thermoreceptors
(respond to temperature changes);
Mechanoreceptors
(detect mechanical pressure);
Chemoreceptors
(sense changes in chemical composition);
Nociceptors
(respond to noxious/‘damage causing’ stimuli).
Adaptation Decline in receptor potential upon continuous activation:
Tonic receptors
(slowly-adapting; signaling the presence of a stimuli);
Phasic receptors
(rapidly-adapting; information about the stimulus timing).
Contents
  1. What are sensory receptors?
  2. Types of sensory receptors
    1. Structure
    2. Location
    3. Function/modality
  3. Adaptation
  4. Sources
+ Show all

What are sensory receptors?

Sensory receptors are specialized cells (neuronal or nonneuronal) which detect and respond to specific stimuli from the external environment and/or within the body, converting them into electrical signals for interpretation by the nervous system.

Different types of stimuli are sensed by different types of receptors. This selectivity is conserved along the neural pathways of the central nervous system (CNS), which are formed by a series of neurons from the sensory receptor to secondary and third-order neurons. This series of neurons synapsing in sequence to transmit a sensory stimulus is sometimes referred to as a labeled line. The brain effectively distinguishes and processes various types of stimuli because the identity of each stimulus is preserved by the neurons in a labeled line. These neurons receive synaptic input from specific sensory neurons, creating dedicated pathways for different sensory modalities.

Sensory modalities can concern general senses, stimulating receptors throughout the body, or special senses, having a specific sense organ dedicated to them; examples include the nose for smell/olfaction, the tongue for taste/gustation, the internal ear for hearing and the eye for vision. Somatosensation is considered a general sense as it refers to excitation by various stimuli including temperature, pain, pressure, vibration, light touch, tickle, itch and proprioception.

Types of sensory receptors

Sensory receptors can be broadly classified according to the general structure or location. Alternatively, a more detailed method of classifying sensory recepters is by their function/type of stimuli which causes them to generate a receptor potential. 

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.

Summary of functional classification of sensory receptors
Photoreceptors Located in the retina; detect visible light stimuli.

Rod cells
(contain rhodopsin, highly sensitive to light);
Cone cells
(contain iodopsins, sensitivity for colour vision);
Photosensitive retinal ganglion cells
(sensitive to blue light for nonvisual functions).
Thermoreceptors Respond to temperature changes
Warm signals: maximum sensitivity at ~45 °C
Cold signals: maximum sensitivity at ~27 °C
Mechanoreceptors Epithelial tactile complex (Merkel cells/discs, low-frequency vibrations);
Lamellar (Pacinian) corpuscles
( high-frequency vibrations);
Tactile (Meisssner) corpuscles
(light touch and low frequency stimuli);
Bulbous (Ruffini) corpuscles
(respond to stretch);
Hair follicle endings
(hair movement);
Muscle spindles
(changes in muscle length and velocity);
Tendon organs
(changes in muscle tension);
Hair cells
(internal ear, hearing and equilibrium).
Chemoreceptors Gustatory receptor cells (stimulated by tastants, chemicals dissolved in saliva);
Olfactory sensory neurons
 (stimulated by odorants/airborne chemical molecules);
Central chemoreceptors
(detect changes in cerebrospinal fluid pH (O2/CO2 levels);
Peripheral chemoreceptors
(carotid arteries and aortic arch, sensing O2, H+/pH);
Osmoreceptors
(located in the hypothalamus; respond to changes in solute concentrations of body fluids).
Nociceptors Respond to noxious stimuli e.g., extremes in pressure, temperature or chemical concentration (‘damage/pain causing’ stimuli)

Adaptation

When constantly exposed to a stimulus, most sensory receptors exhibit a decline in receptor potentials, a phenomenon described as adaptation. Adaptation can be mediated by intracellular signal cascades, alterations in the response of accessory structures, as well as changes in the threshold for action potential initiation. The adaptation rate varies among the different sensory receptors, with some adapting slowly and others rapidly.

Slowly-adapting receptors are called tonic receptors, as their sensory neurons maintain a level of discharge as long as the stimulus persists, thus signaling the continuous presence of a stimulus. On the other hand, rapidly-adapting receptors are called phasic receptors, as they discharge at the onset and/or the offset of the stimulus, thus conveying information about the stimulus timing.

Sensory receptors: want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

What do you prefer to learn with?

“I would honestly say that Kenhub cut my study time in half.” – Read more.

Kim Bengochea Kim Bengochea, Regis University, Denver
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

Register now and grab your free ultimate anatomy study guide!