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Sensory neurons, also known asafferent neurons, are in thenervous system which convert a specific type ofstimulus, via theirreceptors, intoaction potentials orgradedreceptor potentials.^[1] This process is calledsensory transduction. Thecell bodies of the sensory neurons are located in thedorsal root ganglia of thespinal cord.^[2]

The sensory information travels on theafferent nerve fibers in asensory nerve, to thebrain via thespinal cord. Spinal nerves transmit external sensations via sensory nerves to the brain through the spinal cord.^[3] The stimulus can come fromexteroreceptors outside the body, orinteroreceptors inside the body.^[3]

Sensory neurons invertebrates are predominantlypseudounipolar orbipolar, and different types of sensory neurons have differentsensory receptors that respond to different kinds ofstimuli. There are at least six external and two internal sensory receptors:

External receptors that respond to stimuli from outside the body are calledexteroreceptors.^[4] Exteroreceptors includechemoreceptors such asolfactory receptors (smell),taste receptors,photoreceptors (vision),thermoreceptors (temperature),nociceptors (pain),hair cells (hearing andbalance). There are a number of other differentmechanoreceptors fortouch andproprioception (stretch,distortion andstress).

The sensory neurons involved insmell are calledolfactory sensory neurons. These neurons containreceptors, calledolfactory receptors, that are activated byodor molecules in the air. The molecules in the air are detected by enlargedcilia andmicrovilli.^[5] These sensory neurons produce action potentials. Their axons form theolfactory 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 thelimbic system.

Taste sensation is facilitated by specialized sensory neurons located in the taste buds of the tongue and other parts of the mouth and throat. These sensory neurons are responsible for detecting different taste qualities, such as sweet, sour, salty, bitter, and savory. When you eat or drink something, chemicals in the food or liquid interact with receptors on these sensory neurons, triggering signals that are sent to the brain. The brain then processes these signals and interprets them as specific taste sensations, allowing you to perceive and enjoy the flavors of the foods you consume.^[6] When taste receptor cells are stimulated by the binding of these chemical compounds (tastants), it can lead to changes in the flow of ions, such as sodium (Na+), calcium (Ca2+), and potassium (K+), across the cell membrane.^[7] In response to tastant binding, ion channels on the taste receptor cell membrane can open or close. This can lead to depolarization of the cell membrane, creating an electrical signal.

Similar toolfactory receptors,taste receptors (gustatory receptors) intaste buds interact with chemicals in food to produce anaction potential.

Photoreceptor cells are capable ofphototransduction, a process which converts light (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 arephotoreceptor cells,bipolar cells,ganglion cells,horizontal cells, andamacrine cells. The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor (either arod orcone), 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 are photoreceptors that respond significantly tocolor. 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).^[8]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 isdiurnal ornocturnal. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as thetawny owl, the ratio is closer to 1000:1.^[8]Retinal ganglion cells are involved in thesympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive.^[9]

Issues and decay of sensory neurons associated with vision lead to disorders such as:

1. Macular degeneration – 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.^[10]
2. Glaucoma – loss of retinal ganglion cells which causes some loss of vision to blindness.^[11]
3. Diabetic retinopathy – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.^[12]

Theauditory system is responsible for converting pressure waves generated by vibrating air molecules orsound into signals that can be interpreted by the brain.

This mechanoelectrical transduction is mediated withhair cells within the ear. Depending on the movement, the hair cell can either hyperpolarize or depolarize. When the movement is towards the talleststereocilia, 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.^[13]

Problems with sensory neurons associated with the auditory system leads to disorders such as:

1. Auditory processing disorder – 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.^[14]
2. Auditory verbal agnosia – Comprehension of speech is lost but hearing, speaking, reading, and writing ability is retained. This is caused by damage to the posterior superiortemporal lobes, again not allowing the brain to process auditory input correctly.^[15]

Thermoreceptors are sensory receptors, which respond to varyingtemperatures. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown thatmammals have at least two distinct types of thermoreceptors.^[16]Thebulboid corpuscle, is acutaneous receptor acold-sensitive receptor, that detects cold temperatures. While the other type is a warmth-sensitive receptor.

Mechanoreceptors are sensory receptors which respond to mechanical forces, such aspressure ordistortion.^[17]

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.^[18]

Some types of mechanoreceptors fire action potentials when their membranes are physically stretched.

Proprioceptors are another type of mechanoreceptors which literally means "receptors for self". These receptors provide spatial information about limbs and other body parts.^[19]

Nociceptors 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.^[20]

Problems with mechanoreceptors lead to disorders such as:

1. Neuropathic pain - a severe pain condition resulting from a damaged sensory nerve^[20]
2. Hyperalgesia - an increased sensitivity to pain caused by sensory ion channel,TRPM8, which is typically responds to temperatures between 23 and 26 degrees, and provides the cooling sensation associated with menthol and icillin^[20]
3. Phantom limb syndrome - a sensory system disorder where pain or movement is experienced in a limb that does not exist^[21]

Internal receptors that respond to changes inside the body are known asinteroceptors.^[4]

Theaortic bodies andcarotid bodies contain clusters ofglomus cells –peripheral chemoreceptors that detect changes in chemical properties in the blood such asoxygen concentration.^[22] These receptors arepolymodal responding to a number of different stimuli.

Nociceptors respond to potentiallydamaging stimuli by sending signals to the spinal cord and brain. This process, callednociception, usually causes the perception ofpain.^[23][24] They are found in internal organs as well as on the surface of the body to "detect and protect".^[24] Nociceptors detect different kinds of noxious stimuli indicating potential for damage, then initiate neural responses to withdraw from the stimulus.^[24]

1. Thermal nociceptors are activated by noxious heat or cold at various temperatures.^[24]
2. Mechanical nociceptors respond to excess pressure or mechanical deformation, such as apinch.^[24]
3. 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 inBrassica andAllium plants, which target the sensory neural receptor to produce acute pain and subsequent pain hypersensitivity.^[25]

Information coming from the sensory neurons in the head enters thecentral nervous system (CNS) throughcranial nerves. Information from the sensory neurons below the head enters the spinal cord and passes towards the brain through the 31spinal nerves.^[26] 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.

A sensory receptor'sadequate stimulus is thestimulus modality for which it possesses the adequatesensory transduction apparatus. Adequate stimulus can be used to classify sensory receptors:

1. Baroreceptors respond to pressure in blood vessels
2. Chemoreceptors respond to chemical stimuli
3. Electromagnetic radiation receptors respond toelectromagnetic radiation^[27]
   1. Infrared receptors respond toinfrared radiation
   2. Photoreceptors respond tovisible light
   3. Ultraviolet receptors respond toultraviolet radiation^{[citation needed]}
4. Electroreceptors respond toelectric fields
   1. Ampullae of Lorenzini respond to electric fields, salinity, and to temperature, but function primarily as electroreceptors
5. Hydroreceptors respond to changes in humidity
6. Magnetoreceptors respond tomagnetic fields
7. Mechanoreceptors respond tomechanical stress ormechanical strain
8. Nociceptors respond to damage, or threat of damage, to body tissues, leading (often but not always) to pain perception
9. Osmoreceptors respond to theosmolarity of fluids (such as in the hypothalamus)
10. Proprioceptors provide the sense of position
11. Thermoreceptors respond to temperature, either heat, cold or both

Sensory receptors can be classified by location:

1. Cutaneous receptors are sensory receptors found in thedermis orepidermis.^[28]
2. Muscle spindles contain mechanoreceptors that detect stretch in muscles.

Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:

1. Free nerve endings characterize thenociceptors andthermoreceptors and are called thus because the terminal branches of the neuron are unmyelinated and spread throughout thedermis andepidermis.
2. Encapsulated receptors consist of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning.

1. Atonic receptor is a sensory receptor thatadapts slowly to a stimulus^[29] and continues to produceaction potentials over the duration of the stimulus.^[30] 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 arepain receptors,joint capsule, andmuscle spindle.^[31]
2. Aphasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops.^[32] It does not provide information on the duration of the stimulus;^[30] instead some of them convey information on rapid changes in stimulus intensity and rate.^[31] An example of a phasic receptor is thePacinian corpuscle.

There are many drugs currently on the market that are used to manipulate or treat sensory system disorders. For instance,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.^[20] Some drugs may be used to combat other health problems, but can have unintended side effects on the sensory system. Dysfunction in the hair cell mechanotransduction complex, along with the potential loss of specialized ribbon synapses, can lead to hair cell death, often caused by ototoxic drugs like aminoglycoside antibiotics poisoning the cochlea.^[33] Through the use of these toxins, the K+ pumping hair cells cease their function. Thus, the energy generated by theendocochlear potential which drives the auditory signal transduction process is lost, leading to hearing loss.^[34]

Ever since scientists observedcortical remapping in the brain ofTaub'sSilver Spring monkeys, there has been a large amount of research intosensory system plasticity. Huge strides have been made in treating disorders of the sensory system. Techniques such asconstraint-induced movement therapy developed by Taub have helped patients with paralyzed limbs regain use of their limbs by forcing the sensory system to grow newneural pathways.^[35]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. Themirror box developed by V.S. Ramachandran, has enabled patients withphantom 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.^[36]

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

• 
  Illustration of tactile receptors in the skin
• 
  Illustration of lamellated corpuscle
• 
  Illustration of Ruffini corpuscle
• 
  Illustration of skin Merkel cell
• 
  Illustration of tactile corpuscle
• 
  Illustration of root hair plexus
• 
  Illustration of free nerve endings

• Pseudounipolar neuron
• Neural coding
• Posterior column
• Receptive field
• Sensory system
• List of distinct cell types in the adult human body
• Sensory nerve
• Motor nerve
• Afferent nerve fiber
• Efferent nerve fiber
• Motor neuron

• Media related toSensory neuron at Wikimedia Commons
• Purves D, Augustine GJ, Fitzpatrick D, et al., eds. (2001)."Table 9.1 The major classes of somatic sensory receptors".Neuroscience (2nd ed.). Sunderland MA: Sinauer Associates.ISBN 0-87893-742-0.

• Afferent neurons
• Human cells
• Sensory receptors
• Receptor cells

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