Sensory neuron

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Structure of human sensory system

Sensory neurons (also called sensory receptor cells) are neurons that convert a specific type of stimulus into action potentials or graded potentials in the same cell or in an adjacent one.[citation needed] This process is called transduction.

This sensory information travels along a sensory nerve to the brain or spinal cord. The stimulus can be coming from outside of the body, for example light and sound, or from inside the body, for example blood pressure or the sense of body position. Different types of sensory neurons respond to different kinds of stimuli.

Types and function[edit]

Smell[edit]

The sensory receptor cells involved in smell are called olfactory receptor neurons. These cells contain odor receptors, receptor molecules that are activated by interacting with molecular structures on the odor molecule.

Taste[edit]

Similarly to olfactory receptor neurons, taste receptors (gustatory receptors) in taste buds interact with chemicals in food to produce an action potential.

Somatosensory system[edit]

The somatic sensory system includes the sensations of touch, pressure, vibration, limb position, heat, cold, and pain.

The cell bodies of somatic sensory afferent fibers lie in ganglia throughout the spine. These neurons are responsible for relaying information about the body to the central nervous system. Neurons residing in ganglia of the head and body supply the central nervous system with information about the aforementioned external stimuli occurring to the body. Pseudounipolar cell bodies are located in the dorsal root ganglia.[1]

Mechanoreceptors[edit]

Specialized 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.[2]

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.[3]

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.[4]

Problems with mechanoreceptors lead to disorders such as:

  • Neuropathic pain - a severe pain condition resulting from a damaged sensory nerve [4]
  • 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 [4]
  • Phantom limb syndrome - a sensory system disorder where pain or movement is experienced in a limb that does not exist [5]

Vision[edit]

Photoreceptor cells contain specialized proteins such as rhodopsin to transduce the energy in light into electrical signals. Vision is one of the most complex sensory systems. The eye has to first "see" via refraction of light. Then, light energy has to be converted to electrical signals by photoreceptor cells and finally these signals have to be refined and controlled by the synaptic interactions within the neurons of the retina. The five basic classes of neurons within the retina are photoreceptor cells, bipolar cells, ganglion cells, horizontal cells, and amacrine cells.

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.

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.[6]
  • Glaucoma – loss of retinal ganglion cells which causes some loss of vision to blindness.[7]
  • Diabetic retinopathy – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.[8]

Auditory[edit]

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 K+ cation channels open allowing K+ 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 while the outer hair cells are usually from efferent axons originating from cells in the superior olivary complex.[9]

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.[10]
  • 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.[11]

Classification[edit]

Adequate stimulus[edit]

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

Location[edit]

Sensory receptors can be classified by location:

Morphology[edit]

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

Rate of adaptation[edit]

  • A tonic receptor is a sensory receptor that adapts slowly to a stimulus[citation needed] and continues to produce action potentials over the duration of the stimulus.[14] 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 receptors, joint capsule, and muscle spindle.[15]
  • A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops.[citation needed] It does not provide information on the duration of the stimulus;[14] instead some of them convey information on rapid changes in stimulus intensity and rate.[15] An example of a phasic receptor is the Pacinian corpuscle.

Drugs[edit]

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.[4] 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.[16]

Neuroplasticity[edit]

Ever since scientists observed cortical remapping in the brain of Taub's Silver 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.[17] 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.[18]

See also[edit]

Additional images[edit]

References[edit]

  1. ^ Purves, Dale; Augustine, George; Fitzpatrick, David; Hall, William; LaMantia, Anthony-Samuel; McNamara, James; White, Leonard (2008). Neuroscience (4 ed.). Sinauer Associates, Inc. p. 207. ISBN 978-0878936977. 
  2. ^ Purves, Dale; Augustine, George; Fitzpatrick, David; Hall, William; LaMantia, Anthony-Samuel; McNamara, James; White, Leonard (2008). Neuroscience (4 ed.). Sinauer Associates, Inc. p. 209. ISBN 978-0878936977. 
  3. ^ Purves, Dale; Augustine, George; Fitzpatrick, David; Hall, William; LaMantia, Anthony-Samuel; McNamara, James; White, Leonard (2008). Neuroscience (4 ed.). Sinauer Associates, Inc. pp. 215–216. ISBN 978-0878936977. 
  4. ^ a b c d Lee, Y; Lee, C; Oh, U (2005). "Painful channels in sensory neurons". Molecules and Cells. 20 (3): 315–324. 
  5. ^ Halligan, Peter W; Zeman, Adam; Berger, Abi (1999-09-04). "Phantoms in the brain". BMJ: British Medical Journal. 319 (7210): 587–588. doi:10.1136/bmj.319.7210.587. ISSN 0959-8138. PMC 1116476Freely accessible. PMID 10473458. 
  6. ^ de Jong, Paulus T.V.M. (2006-10-05). "Age-Related Macular Degeneration". New England Journal of Medicine. 355 (14): 1474–1485. doi:10.1056/NEJMra062326. ISSN 0028-4793. PMID 17021323. 
  7. ^ Alguire, Patrick; Dallas, Wilbur; Willis, John; Kenneth, Henry (1990). "Chapter 118 Tonometry". Clinical methods : the history, physical, and laboratory examinations (3 ed.). Butterworths. ISBN 040990077X. OCLC 15695765. 
  8. ^ "NIHSeniorHealth: Diabetic Retinopathy - Causes and Risk Factors". nihseniorhealth.gov. Retrieved 2016-12-19. 
  9. ^ Purves, Dale; Augustine, George; Fitzpatrick, David; Hall, William; LaMantia, Anthony-Samuel; McNamara, James; White, Leonard (2008). Neuroscience (4 ed.). Sinauer Associates, Inc. pp. 327–330. ISBN 978-0878936977. 
  10. ^ "Auditory Processing Disorder (APD)" (PDF). British Society of Audiology APD Special Interest Group MRC Institute of Hearing Research. 
  11. ^ 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. ISSN 1355-6177. PMID 16209426. 
  12. ^ Michael J. Gregory. "Sensory Systems". Clinton Community College. Retrieved 2013-06-06. 
  13. ^ http://medical-dictionary.thefreedictionary.com/Cutaneous+receptor
  14. ^ a b mentor.lscf.ucsb.edu/course/fall/eemb157/lecture/Lectures%2016,%2017%2018.ppt[dead link]
  15. ^ a b http://frank.mtsu.edu/~jshardo/bly2010/nervous/receptor.html Archived August 3, 2008, at the Wayback Machine.
  16. ^ Priuska, E.M.; Schacht, J. (1997). "Mechanism and prevention of aminoglycoside ototoxicity: Outer hair cells as targets and tools". Ear, Nose, Throat Journal. 76: 164–171. PMID 9086645. 
  17. ^ Schwartz and Begley 2002, p. 160; "Constraint-Induced Movement Therapy", excerpted from "A Rehab Revolution," Stroke Connection Magazine, September/October 2004. Print.
  18. ^ Blakeslee, Sandra; Ramachandran, V. S. (1998). Phantoms in the brain : probing the mysteries of the human mind. William Morrow & Company, Inc. ISBN 0688152473. OCLC 43344396. 

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