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Sensory neurons, also known as afferent neurons, are
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
s in the
nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals ...

nervous system
, that convert a specific type of
stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity generally. For example, "The press provides stimulation of political discourse." ...
, via their
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ne ...

receptors
, into
action potential In physiology Physiology (; ) is the scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the are ...

action potential
s or graded potentials. This process is called sensory transduction. The
cell bodies Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
of the sensory neurons are located in the dorsal ganglia of the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
. The sensory information travels on the
afferent nerve fiber Afferent nerve fibers refer to axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from th ...
s in a
sensory nerve A sensory nerve, or afferent nerve, is a general anatomic term for a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and ...
, to the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
via the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
. The stimulus can come from ''exteroreceptors'' outside the body, for example those that detect light and sound, or from ''interoreceptors'' inside the body, for example those that are responsive to
blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motio ...

blood pressure
or the sense of body position.


Types and function

Different types of sensory neurons have different sensory receptors that respond to different kinds of stimuli. There are at least six external and two internal sensory receptors:


External receptors

External receptors that respond to stimuli from outside the body are called exteroreceptors. Exteroreceptors include
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is ...
s (smell),
taste receptor A taste receptor is a type of Receptor (biochemistry), cellular receptor which facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity a ...
s,
photoreceptors
photoreceptors
(vision),
hair cell Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cel ...
s (hearing),
thermoreceptorsA thermoreceptor is a non-specialised sense Sense relates to any of the systems and corresponding organs involved in sensation, i.e. the physical process of responding to Stimulus (physiology), stimuli and providing data for perception. During sensa ...
(temperature), and a number of different
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an ...
s (stretch, distortion).


Smell

The sensory neurons involved in smell are called
olfactory sensory neuron Olfaction, or the sense of smell, is the special sense through which smells (or odors) are perceived. It occurs when an odor binds to a receptor within the nasal cavity, transmitting a signal through the olfactory system. Olfaction has many ...
s. These neurons contain
receptor
receptor
s, called
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is ...
s, that are activated by
odor An odor () or odour (; ) is caused by one or more volatilized s that are generally found in low concentrations that humans and animals can perceive by their of . An odor is also called a "smell" or a "scent", which can refer to either a plea ...
molecules in the air. The molecules in the air are detected by enlarged
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
and
microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption (chemistry), abs ...

microvilli
. These sensory neurons produce action potentials. Their axons form the olfactory nerve, and they synapse directly onto neurons in the cerebral cortex (olfactory bulb). They do not use the same route as other sensory systems, bypassing the brain stem and the thalamus. The neurons in the olfactory bulb that receive direct sensory nerve input, have connections to other parts of the olfactory system and many parts of the limbic system.


Taste

Similar to
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is ...
s,
taste receptor A taste receptor is a type of Receptor (biochemistry), cellular receptor which facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity a ...
s (gustatory receptors) in
taste bud Taste buds contain the taste receptor cells, which are also known as gustatory cells. The taste receptor A taste receptor is a type of Receptor (biochemistry), cellular receptor which facilitates the sensation of taste. When food or other su ...

taste bud
s interact with chemicals in food to produce an
action potential In physiology Physiology (; ) is the scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the are ...

action potential
.


Vision

Photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cellNeuroepithelial cells, or neuroectodermal cells, form the wall of the closed neural tube in early embryonic development ''Embryonic'' is the twelfth studio album by experimenta ...

Photoreceptor cell
s are capable of
phototransduction Visual phototransduction is the sensory transduction of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic ...

phototransduction
, a process which converts light (
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
) into electrical signals. These signals are refined and controlled by the interactions with other types of neurons in the retina. The five basic classes of neurons within the retina are
photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cellNeuroepithelial cells, or neuroectodermal cells, form the wall of the closed neural tube in early embryonic development ''Embryonic'' is the twelfth studio album by experimenta ...

photoreceptor cell
s,
bipolar cells A bipolar neuron, or bipolar cell, is a type of neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims to develop e ...
,
ganglion cells {{stack, Image:Ganglion very high mag.jpg, upGanglion cells within a ganglion. H&E stain. A ganglion cell is a Cell (biology), cell found in a ganglion. Examples of ganglion cells include: * retinal ganglion cell (RGC) found in the ganglion cell l ...
,
horizontal cells Horizontal cells are the laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina of vertebrate eyes. They help integrate and regulate the input from multiple photoreceptor cells. Among their functions, horizon ...
, and
amacrine cells
amacrine cells
. The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor (either a rod or
cone A cone is a three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Greek wikt:πα ...

cone
), bipolar cell, and the ganglion cell. The first action potential occurs in the retinal ganglion cell. This pathway is the most direct way for transmitting visual information to the brain. There are three primary types of photoreceptors:
Cones A cone is a three-dimensional space, three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the Apex (geometry), apex or vertex (geometry), vertex. A cone is fo ...

Cones
are photoreceptors that respond significantly to
color Color (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Engli ...

color
. In humans the three different types of cones correspond with a primary response to short wavelength (blue), medium wavelength (green), and long wavelength (yellow/red)."eye, human." Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010. Rods are photoreceptors that are very sensitive to the intensity of light, allowing for vision in dim lighting. The concentrations and ratio of rods to cones is strongly correlated with whether an animal is
diurnal Diurnal ("daily Daily or The Daily may refer to: Journalism * Daily newspaper A newspaper is a Periodical literature, periodical publication containing written News, information about current events and is often typed in black ink with a ...
or
nocturnal Nocturnality is an animal behavior Ethology is the scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was original ...
. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the
tawny owl The tawny owl (also called the brown owl) (''Strix aluco'') is commonly found in woodlands across much of Eurasia and North Africa, and has 11 recognized subspecies. It is a stocky, medium-sized owl, whose underparts are pale with dark streaks, ...

tawny owl
, the ratio is closer to 1000:1.
Retinal ganglion cell A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the human eye, eye. It receives visual information from photoreceptor cell, photoreceptors via two intermediate neuron typ ...
s are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive. Issues and decay of sensory neurons associated with vision lead to disorders such as: *
Macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred vision, blurred or vision loss, no vision in the center of the visual field. Early on there are often no sympto ...
– degeneration of the central visual field due to either cellular debris or blood vessels accumulating between the retina and the choroid, thereby disturbing and/or destroying the complex interplay of neurons that are present there. *
Glaucoma Glaucoma is a group of eye diseases which result in damage to the optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-l ...

Glaucoma
– loss of retinal ganglion cells which causes some loss of vision to blindness. *
Diabetic retinopathy Diabetic retinopathy, also known as diabetic eye disease (DED), is a medical condition in which damage occurs to the retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates ...
– poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.


Auditory

The
auditory system The auditory system is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neur ...
is responsible for converting pressure waves generated by vibrating air molecules or
sound In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...

sound
into signals that can be interpreted by the brain. This mechanoelectrical transduction is mediated with
hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Be ...
within the ear. Depending on the movement, the hair cell can either hyperpolarize or depolarize. When the movement is towards the tallest
stereocilia Stereocilia (or stereovilli) are non-motile apical modifications of the cell. They are distinct from cilia The cilium (; the plural is cilia) is an organelle found on eukaryotic cells in the shape of a slender protuberance that projects from the ...

stereocilia
, the Na+ cation channels open allowing Na+ to flow into cell and the resulting depolarization causes the Ca++ channels to open, thus releasing its neurotransmitter into the afferent auditory nerve. There are two types of hair cells: inner and outer. The inner hair cells are the sensory receptors . Problems with sensory neurons associated with the auditory system leads to disorders such as: *
Auditory processing disorder Auditory processing disorder (APD), rarely known as King-Kopetzky syndrome or auditory disability with normal hearing (ADN), is an umbrella term for a variety of disorders that affect the way the brain processes auditory information. Individuals w ...
– Auditory information in the brain is processed in an abnormal way. Patients with auditory processing disorder can usually gain the information normally, but their brain cannot process it properly, leading to hearing disability. *
Auditory verbal agnosia Auditory verbal agnosia (AVA), also known as pure word deafness, is the inability to comprehend speech. Individuals with this disorder lose the ability to understand language, repeat words, and write from dictation. Some patients with AVA describe ...
– Comprehension of speech is lost but hearing, speaking, reading, and writing ability is retained. This is caused by damage to the posterior superior
temporal lobes The temporal lobe is one of the four major lobes of the cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain A brain is an organ (anatomy), organ that serv ...
, again not allowing the brain to process auditory input correctly.


Temperature

ThermoreceptorA thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system ...
s are sensory receptors, which respond to varying emperature. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s have at least two distinct types of thermoreceptors.Krantz, John.
Experiencing Sensation and Perception
''. Pearson Education, Limited, 2009. p. 12.3
The bulboid corpuscle, is a
cutaneous receptorThe ''cutaneous receptors are the types of sensory receptor found in the dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: ...
a ''cold-sensitive'' receptor, that detects cold temperatures. The other type is a warmth-sensitive receptor.


Mechanoreceptors

Mechanoreceptors are sensory receptors which respond to mechanical forces, such as
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

pressure
or
distortion Distortion is the alteration of the original shape (or other characteristic) of something. In communications Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an apparent answer to the painful di ...

distortion
. Specialized sensory receptor cells called mechanoreceptors often encapsulate afferent fibers to help tune the afferent fibers to the different types of somatic stimulation. Mechanoreceptors also help lower thresholds for action potential generation in afferent fibers and thus make them more likely to fire in the presence of sensory stimulation. Some types of mechanoreceptors fire action potentials when their membranes are physically stretched.
Proprioceptors Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense Sense relates to any of the systems and corresponding organs involved in sensation, i.e. the physical process of responding to Stimulus (physiology), stimuli and ...
are another type of mechanoreceptors which literally means "receptors for self". These receptors provide spatial information about limbs and other body parts.
Nociceptors A nociceptor ("pain receptor") is a sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates w ...
are responsible for processing pain and temperature changes. The burning pain and irritation experienced after eating a chili pepper (due to its main ingredient, capsaicin), the cold sensation experienced after ingesting a chemical such as menthol or icillin, as well as the common sensation of pain are all a result of neurons with these receptors. Problems with mechanoreceptors lead to disorders such as: *
Neuropathic pain Neuropathic pain is pain caused by damage or disease affecting the Somatosensory system, somatosensory nervous system. Neuropathic pain may be associated with abnormal sensations called dysesthesia or pain from normally non-painful stimuli (allody ...
- a severe pain condition resulting from a damaged sensory nerve *
Hyperalgesia Hyperalgesia ( or ; 'hyper' from Greek ὑπέρ (huper, “over”), '-algesia' from Greek algos, ἄλγος (pain)) is an abnormally increased sensitivity to pain Pain is a distressing feeling often caused by intense or damaging stimuli ...
- an increased sensitivity to pain caused by sensory ion channel,
TRPM8 Transient receptor potential cation channel subfamily M (melastatin) member 8 (TRPM8), also known as the cold and menthol receptor 1 (CMR1), is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This ...
, which is typically responds to temperatures between 23 and 26 degrees, and provides the cooling sensation associated with menthol and icillin * Phantom limb syndrome - a sensory system disorder where pain or movement is experienced in a limb that does not exist


Internal receptors

Internal receptors that respond to changes inside the body are known as interoceptors.


Blood

The
aortic bodies The aortic bodies are one of several small clusters of peripheral chemoreceptors located along the aorta, aortic arch. They are important in measuring Partial pressure, partial pressures of oxygen and carbon dioxide in the blood, and Acid–base hom ...

aortic bodies
and
carotid bodies The carotid body is a small cluster of chemoreceptor cells, and supporting sustentacular cells. The carotid body is located in the adventitia, in the bifurcation (fork) of the common carotid artery, which runs along both sides of the neck. The ...
contain clusters of
glomus cell Glomus cells are the cell type mainly located in the carotid bodies and aortic bodies. Glomus type I cells are peripheral chemoreceptors which sense the oxygen, carbon dioxide and pH levels of the blood. When there is a decrease in the blood's p ...
s –
peripheral chemoreceptorsPeripheral chemoreceptors (of the carotid and aortic bodies) are so named because they are sensory neuron, sensory extensions of the peripheral nervous system into blood vessels where they detect changes in chemical concentrations. As transducers o ...
that detect changes in chemical properties in the blood such as
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
concentration. These receptors are polymodal responding to a number of different stimuli.


Nociceptors

Nociceptors respond to potentially damaging stimuli by sending signals to the spinal cord and brain. This process, called
nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a ...
, usually causes the perception of
pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain The International Association for the Study of Pain (IASP) is an international learned society A learned societ ...
. They are found in internal organs as well as on the surface of the body to "detect and protect". Nociceptors detect different kinds of noxious stimuli indicating potential for damage, then initiate neural responses to withdraw from the stimulus. * ''Thermal'' nociceptors are activated by noxious heat or cold at various temperatures. * ''Mechanical'' nociceptors respond to excess pressure or mechanical deformation, such as a pinch. * ''Chemical'' nociceptors respond to a wide variety of chemicals, some of which signal a response. They are involved in the detection of some spices in food, such as the pungent ingredients in
Brassica ''Brassica'' () is a genus of plants in the cabbage and mustard Mustard may refer to: Food and plants * Mustard (condiment) Mustard is a condiment made from the seeds A seed is an Plant embryogenesis, embryonic plant enclosed in a te ...
and
Allium ''Allium'' is a genus of monocotyledonous flowering plants that includes hundreds of species, including the cultivated onion, garlic, scallion, shallot, leek, and chives. The Generic name (biology), generic name ''Allium'' is the Latin word for ...

Allium
plants, which target the sensory neural receptor to produce acute pain and subsequent pain hypersensitivity.


Connection with the central nervous system

Information coming from the sensory neurons in the head enters the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
(CNS) through
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
. Information from the sensory neurons below the head enters the spinal cord and passes towards the brain through the 31
spinal nerve A spinal nerve is a , which carries motor, sensory, and autonomic signals between the and the body. In the there are 31 pairs of spinal nerves, one on each side of the . These are grouped into the corresponding , , , and regions of the spine. ...

spinal nerve
s. The sensory information traveling through the spinal cord follows well-defined pathways. The nervous system codes the differences among the sensations in terms of which cells are active.


Classification


Adequate stimulus

A sensory receptor's adequate stimulus is the
stimulus modality Stimulus modality, also called sensory modality, is one aspect of a stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity gen ...
for which it possesses the adequate sensory transduction apparatus. Adequate stimulus can be used to classify sensory receptors: *
BaroreceptorBaroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that a ...
s respond to pressure in blood vessels *
Chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology) ...
s respond to chemical stimuli * Electromagnetic radiation receptors respond to
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
** Infrared receptors respond to
infrared radiation Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...
** respond to
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
** Ultraviolet receptors respond to
ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...
* Electroreceptors respond to
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ' ...

electric field
s **
Ampullae of Lorenzini The ampullae of Lorenzini are special sensing Organ (anatomy), organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fish (sharks, Batoidea, rays, and chimaeras); howe ...

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 field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
s *
Mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an ...
s respond to
mechanical stress In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighbouring particles of a continuous material exert on each other, while deformation (mechanics)#Strain, strain is the measure of the deformation of ...
or mechanical strain *
Nociceptor A nociceptor ("pain receptor") is a sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates w ...
s respond to damage, or threat of damage, to body tissues, leading (often but not always) to pain perception *
Osmoreceptor An osmoreceptor is a sensory receptor primarily found in the hypothalamus The hypothalamus (from Ancient Greek wikt:ὑπό, ὑπό, "under", and wikt:θάλαμος, θάλαμος, "chamber") is a portion of the brain that contains a number o ...
s respond to the
osmolarityOsmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L (pr ...
of fluids (such as in the hypothalamus) *
Proprioceptor Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons located w ...
s provide the sense of position *
ThermoreceptorA thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system ...
s respond to temperature, either heat, cold or both


Location

Sensory receptors can be classified by location: *
Cutaneous receptor A cutaneous receptor is the type of sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates w ...
s are sensory receptors found in the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
or
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. *
Muscle spindle Muscle spindles are stretch receptors within the body of a muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed ...
s 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 A free nerve ending (FNE) or bare nerve ending, is an unspecialized, afferent nerve fiber sending its signal to a sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as c ...
characterize the
nociceptor A nociceptor ("pain receptor") is a sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates w ...
s and
thermoreceptorA thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system ...
s and are called thus because the terminal branches of the neuron are unmyelinated and spread throughout the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
and
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. * 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 and continues to produce
action potential In physiology Physiology (; ) is the scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the are ...

action potential
s over the duration of the stimulus.mentor.lscf.ucsb.edu/course/fall/eemb157/lecture/Lectures%2016,%2017%2018.ppt In this way it conveys information about the duration of the stimulus. Some tonic receptors are permanently active and indicate a background level. Examples of such tonic receptors are
pain receptorsNociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding noxious stimuli. In nociception, intense chemical (e.g., cayenne powder), mechanical (e.g., cutting, cr ...
,
joint capsule In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.muscle spindle Muscle spindles are stretch receptors within the body of a muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed ...
.http://frank.mtsu.edu/~jshardo/bly2010/nervous/receptor.html * A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops. It does not provide information on the duration of the stimulus; instead some of them convey information on rapid changes in stimulus intensity and rate. An example of a phasic receptor is the
Pacinian corpuscle Pacinian corpuscles (or lamellar corpuscles; discovered by Italian anatomist Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anato ...

Pacinian corpuscle
.


Drugs

There are many drugs currently on the market that are used to manipulate or treat sensory system disorders. For instance,
Gabapentin Gabapentin, sold under the brand name Neurontin among others, is an anticonvulsant Anticonvulsants (also commonly known as antiepileptic drugs or as antiseizure drugs) are a diverse group of pharmacological agents used in the treatment of epil ...

Gabapentin
is a drug that is used to treat neuropathic pain by interacting with one of the voltage-dependent calcium channels present on non-receptive neurons. Some drugs may be used to combat other health problems, but can have unintended side effects on the sensory system.
Ototoxic Ototoxicity is the property of being toxicity, toxic to the ear (''oto-''), specifically the cochlea or vestibulocochlear nerve, auditory nerve and sometimes the vestibular system, for example, as a side effect of a drug. The effects of ototoxicity ...
drugs are drugs which affect the
cochlea The cochlea is the part of the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opene ...

cochlea
through the use of a toxin like
aminoglycoside Aminoglycoside is a medicinal chemistry, medicinal and bacteriology, bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside ( ...
antibiotics, which poison hair cells. Through the use of these toxins, the K+ pumping hair cells cease their function. Thus, the energy generated by the
endocochlear potentialThe endocochlear potential (EP; also called endolymphatic potential) is the positive voltage of 80-100mV seen in the cochlear endolymphatic spaces. Within the cochlea the EP varies in the magnitude all along its length. When a sound is presented, the ...
which drives the auditory signal transduction process is lost, leading to hearing loss.


Neuroplasticity

Ever since scientists observed
cortical remapping Cortical remapping, also referred to as cortical reorganization, is the process by which an existing cortical map is affected by a stimulus resulting in the creating of a 'new' cortical map. Every part of the body is connected to a corresponding ...
in the brain of Taub's Silver Spring monkeys, there has been a large amount of research into neuroplasticity, sensory system plasticity. Huge strides have been made in treating disorders of the sensory system. Techniques such as constraint-induced movement therapy developed by Taub have helped patients with paralyzed limbs regain use of their limbs by forcing the sensory system to grow new neural pathways. Phantom limb syndrome is a sensory system disorder in which amputees perceive that their amputated limb still exists and they may still be experiencing pain in it. The mirror box developed by V.S. Ramachandran, has enabled patients with phantom limb syndrome to relieve the perception of paralyzed or painful phantom limbs. It is a simple device which uses a mirror in a box to create an illusion in which the sensory system perceives that it is seeing two hands instead of one, therefore allowing the sensory system to control the "phantom limb". By doing this, the sensory system can gradually get acclimated to the amputated limb, and thus alleviate this syndrome.


Other animals

Hydrodynamic reception is a form of mechanoreception used in a range of animal species.


Additional images

File:Blausen 0809 Skin TactileReceptors.png, Illustration of Tactile Receptors in the Skin File:Blausen 0804 Skin LamellatedCorpuscle.png, Illustration of Lamellated Corpuscle File:Blausen 0807 Skin RuffiniCorpuscle.png, Illustration of Ruffini Corpuscle File:Blausen 0805 Skin MerkelCell.png, Illustration of Skin Merkel Cell File:Blausen 0808 Skin TactileCorpuscle.png, Illustration of Tactile Corpuscle File:Blausen 0806 Skin RootHairPlexus.png, Illustration of Root Hair Plexus File:Blausen 0803 Skin FreeNerveEndings.png, Illustration of Free Nerve Endings


See also

* Sensory nerve * Pseudounipolar neuron * Efferent nerve * Neural coding * Posterior column * Receptive field * Sensory system


References


External links

*
The major classes of somatic sensory receptors
{{Somatosensory system Afferent neurons Human cells Sensory receptors Receptor cells