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Nerve cell that converts environmental stimuli into corresponding internal stimuli
The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor (either a rod or 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 are photoreceptors that respond significantly to 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).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 or nocturnal. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the tawny owl, the ratio is closer to 1000:1.Retinal ganglion cells are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive.
Problems and decay of sensory neurons associated with vision lead to disorders such as:
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.
Glaucoma - loss of retinal ganglion cells which causes some loss of vision to blindness.
The auditory system is responsible for converting pressure waves generated by vibrating air molecules or sound into signals that can be interpreted by the brain.
This mechanoelectrical transduction is mediated with hair cells within the ear. Depending on the movement, the hair cell can either hyperpolarize or depolarize. When the movement is towards the tallest 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 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 - 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, again not allowing the brain to process auditory input correctly.
Thermoreceptors are sensory receptors, which respond to varying [temperature]s. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that mammals have at least two distinct types of thermoreceptors.
The bulboid corpuscle, is a cutaneous receptor a cold-sensitive receptor, that detects cold temperatures. The other type is a warmth-sensitive receptor.
Mechanoreceptors are sensory receptors which respond to mechanical forces, such as pressure or 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 are another type of mechanoreceptors which literally means "receptors for self". These receptors provide spatial information about limbs and other body parts.
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.
Problems with mechanoreceptors lead to disorders such as:
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 
Nociceptors respond to potentially damaging stimuli by sending signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain. 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 and 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 (CNS) through cranial nerves. Information from the sensory neurons below the head enters the spinal cord and passes towards the brain through the 31 spinal nerves. 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 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.
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. Some drugs may be used to combat other health problems, but can have unintended side effects on the sensory system. Ototoxic drugs are drugs which affect the cochlea through the use of a toxin like aminoglycoside 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 potential which drives the auditory signal transduction process is lost, leading to hearing loss.
Ever since scientists observed cortical remapping in the brain of Taub'sSilver Spring monkeys, there has been a lot of research into 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.
^Foster, R. G.; Provencio, I.; Hudson, D.; Fiske, S.; Grip, W.; Menaker, M. (1991). "Circadian photoreception in the retinally degenerate mouse (rd/rd)". Journal of Comparative Physiology A 169. doi:10.1007/BF00198171
^Stefanatos, Gerry A.; Gershkoff, Arthur; Madigan, Sean (2005-07-01). "On pure word deafness, temporal processing, and the left hemisphere". Journal of the International Neuropsychological Society: JINS. 11 (4): 456-470, discussion 455. doi:10.1017/S1355617705050538. ISSN1355-6177. PMID16209426.