Motor neuron - wikipedia Motor neuron Motor neuron Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ( `` tigroid '' cytoplasm ). H&E - LFB stain. Details Location Ventral horn of the spinal cord, some cranial nerve nuclei Shape Projection neuron Function Excitatory projection ( to NMJ ) Neurotransmitter UMN to LMN : glutamate ; LMN to NMJ : ACh Presynaptic connections Primary motor cortex via the Corticospinal tract Postsynaptic connections Muscle fibers and other neurons Identifiers MeSH D009046 NeuroLex ID nifext_103 TA A14. 2.00. 021 FMA 5867 Anatomical terms of neuroanatomy ( edit on Wikidata ) A motor neuron ( or motoneuron ) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon ( fiber ) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron -- upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons. A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or a tetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau. Contents 1 Development 2 Anatomy and physiology 2.1 Upper motor neurons 2.2 Nerve tracts 2.3 Lower Motor Neurons 2.4 Somatic motor neurons 2.4. 1 Neuromuscular Junction 2.5 Special visceral motor neurons 2.6 General visceral motor neurons 3 See also 4 References 5 Sources Development Motor neurons begin to develop early in embryonic development, and motor function continues to develop well into childhood. In the neural tube cells are specified to either the rostral - caudal axis or ventral - dorsal axis. The axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral - dorsal axis ( the basal plate ). This homeodomain is known as the motor neural progenitor domain ( pMN ). Transcription factors here include Pax6, OLIG2, Nkx - 6.1, and Nkx - 6.2, which are regulated by sonic hedgehog ( Shh ). The OLIG2 gene being the most important due to its role in promoting Ngn2 expression, a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development. Further specification of motor neurons occurs when retinoic acid, fibroblast growth factor, Wnts, and TGFb, are integrated into the various Hox transcription factors. There are 13 Hox transcription factors and along with the signals, determine whether a motor neuron will be more rostral or caudal in character. In the spinal column, Hox 4 - 11 sort motor neurons to one of the five motor columns. Motor columns of spinal cord Motor column Location in spinal cord Target Median motor column Present entire length Axial muscles Hypaxial motor column Thoracic region Body wall muscles Preganglionic motor column Thoracic region Sympathetic ganglion Lateral motor column Brachial and lumbar region ( both regions are further divided into medial and lateral domains ) Muscles of the limbs Phrenic motor column Cervical region Diaphragm Anatomy and Physiology Spinal cord tracts Location of lower motor neurons in spinal cord Upper motor neurons Upper motor neurons originate in the motor cortex located in the precentral gyrus. The cells that make up the primary motor cortex are Betz cells, which are a type of pyramidal cell. The axons of these cells descend from the cortex to form the corticospinal tract. Nerve tracts Nerve tracts are bundles of axons as white matter, that carry action potentials to their effectors. In the spinal cord these descending tracts carry impulses from different regions. These tracts also serve as the place of origin for lower motor neurons. There are seven major descending motor tracts to be found in the spinal cord : Lateral corticospinal tract Rubrospinal tract Lateral reticulospinal tract Vestibulospinal tract Medial reticulospinal tract Tectospinal tract Anterior corticospinal tract Lower motor neurons Lower motor neurons are those that originate in the spinal cord and directly or indirectly innervate effector targets. The target of these neurons varies, but in the somatic nervous system the target will be some sort of muscle fiber. There are three primary categories lower motor neurons, which can be further divided in sub-categories. According to their targets, motor neurons are classified into three broad categories : Somatic motor neurons Special visceral motor neurons General visceral motor neurons Somatic motor neurons Somatic motor neurons originate in the central nervous system, project their axons to skeletal muscles ( such as the muscles of the limbs, abdominal, and intercostal muscles ), which are involved in locomotion. The three types of these neurons are the alpha efferent neurons, beta efferent neurons, and gamma efferent neurons. They are called efferent to indicate the flow of information from the central nervous system ( CNS ) to the periphery. Alpha motor neurons innervate extrafusal muscle fibers, which are the main force - generating component of a muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells. A single motor neuron may synapse with 150 muscle fibers on average. The motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories : Main Article : Motor Unit Slow ( S ) motor units stimulate small muscle fibers, which contract very slowly and provide small amounts of energy but are very resistant to fatigue, so they are used to sustain muscular contraction, such as keeping the body upright. They gain their energy via oxidative means and hence require oxygen. They are also called red fibers. Fast fatiguing ( FF ) motor units stimulate larger muscle groups, which apply large amounts of force but fatigue very quickly. They are used for tasks that require large brief bursts on energy, such as jumping or running. They gain their energy via glycolytic means and hence do n't require oxygen. They are called white fibers. Fast fatigue - resistant motor units stimulate moderate - sized muscles groups that do n't react as fast as the FF motor units, but can be sustained much longer ( as implied by the name ) and provide more force than S motor units. These use both oxidative and glycolytic means to gain energy. In addition to voluntary skeletal muscle contraction, alpha motor neurons also contribute to muscle tone, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neurons within the muscle spindle detect the degree of stretch and send a signal to the CNS. The CNS activates alpha motor neurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex. Beta motor neurons innervate intrafusal muscle fibers of muscle spindles, with collaterals to extrafusal fibres. There are two types of beta motor neurons : Slow Contracting - These innervate extrafusal fibers. Fast Contracting - These innervate intrafusal fibers. Gamma motor neurons innervate intrafusal muscle fibers found within the muscle spindle. They regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex. There are two types of gamma motor neurons : Dynamic - These focus on Bag1 fibers and enhance dynamic sensitivity. Static - These focus on Bag2 fibers and enhance stretch sensitivity. Regulatory factors of lower motor Neurons Size Principle -- this relates to the soma of the motor neuron. This restricts larger neurons to receive a larger excitatory signal in order to stimulate the muscle fibers it innervates. By reducing unnecessary muscle fiber recruitment, the body is able to optimize energy consumption. Persistent Inward Current ( PIC ) -- recent animal study research has shown that constant flow of ions such as calcium and sodium through channels in the soma and dendrites influence the synaptic input. An alternate way to think of this is that the post-synaptic neuron is being primed before receiving an impulse. After Hyper - polarization ( AHP ) -- we are all familiar with the idea of hyperpolarization after an action potential. A trend has been identified that shows slow motor neurons to have more intense AHPs for a longer duration. One way to remember this is that slow muscle fibers can contract for loner, so it makes since that their corresponding motor neurons fire at a slower rate. Neuromuscular junction The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of acetylcholine ( Ach ) neurotransmitters from the axon terminals from synaptic vesicles bind with the plasma membrane. The acetylcholine molecules bind to postsynaptic receptors found within the motor end plate. Once two acetylcholine receptors have been bound, an ion channel is opened and sodium ions are allowed to flow into the cell. The influx of sodium into the cell causes depolarization and triggers a muscle action potential. T tubules of the sarcolemma are then stimulated to elicit calcium ion release from the sarcoplasmic reticulum. It is this chemical release that causes the target muscle fiber to contract. In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory. For vertebrates, however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile. Muscle relaxation and inhibition of muscle contraction in vertebrates is obtained only by inhibition of the motor neuron itself. Muscle innervation may eventually play a role in the maturation of motor activity. This is how muscle relaxants work by acting on the motor neurons that innervate muscles ( by decreasing their electrophysiological activity ) or on cholinergic neuromuscular junctions, rather than on the muscles themselves. Special visceral motor neurons These are also known as branchial motor neurons, which are involved in facial expression, mastication, phonation, and swallowing. Associated cranial nerves are the oculomotor, abducens, trochlear, and hypoglossal nerves. Branch of NS Position Neurotransmitter Somatic n / a Acetylcholine Parasympathetic Preganglionic Acetylcholine Parasympathetic Ganglionic Acetylcholine Sympathetic Preganglionic Acetylcholine Sympathetic Ganglionic Norepinephrine * * Except fibers to sweat glands and certain blood vessels Motor neuron neurotransmitters General visceral motor neurons These motor neurons indirectly innervate cardiac muscle and smooth muscles of the viscera ( the muscles of the arteries ) : they synapse onto neurons located in ganglia of the autonomic nervous system ( sympathetic and parasympathetic ), located in the peripheral nervous system ( PNS ), which themselves directly innervate visceral muscles ( and also some gland cells ). In consequence : the motor command of skeletal and branchial muscles is monosynaptic ( involving only one motor neuron, respectively, somatic and branchial, which synapses onto the muscle ). the command of visceral muscles is disynaptic ( involving two neurons : the general visceral motor neuron located in the CNS, which synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle ). All vertebrate motor neurons are cholinergic, that is, they release the neurotransmitter acetylcholine. Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons are noradrenergic, that is, they release the neurotransmitter noradrenaline. ( see Table ) See also Betz cell Central chromatolysis Motor dysfunction Nerve References Jump up ^ Tortora, Gerard ; Derrickson, Bryan ( 2014 ). Principles of Anatomy & Physiology ( 14th ed. ). New Jersey : John Wiley & Sons, Inc. pp. 406, 502, 541. ISBN 978 - 1 - 118 - 34500 - 9. Jump up ^ Pocock, Gillian ; Richards, Christopher D. ( 2006 ). Human physiology : the basis of medicine ( 3rd ed. ). Oxford : Oxford University Press. pp. 151 -- 153. ISBN 978 - 0 - 19 - 856878 - 0. Jump up ^ Schacter D.L., Gilbert D.T., and Wegner D.M. ( 2011 ) Psychology second edition. New York, NY : Worth Jump up ^ Russell, Peter ( 2013 ). Biology - Exploring the Diversity of Life. Toronto : Nelson Education. p. 946. ISBN 978 - 0 - 17 - 665133 - 6. Jump up ^ Tortora, Gerard ; Derrickson, Bryan ( 2011 ). Principles of Anatomy Physiology ( 14th ed. ). New Jersey : John Wiley & Sons, Inc. pp. 1090 -- 1099. ISBN 978 - 1 - 118 - 34500 - 9. Jump up ^ Sadler, T. ( 2010 ). Langman 's medical embryology ( 11th ed. ). Philadelphia : Lippincott William & Wilkins. pp. 299 -- 301. ISBN 978 - 0 - 7817 - 9069 - 7. ^ Jump up to : Davis - Dusenbery, BN ; Williams, LA ; Klim, JR ; Eggan, K ( February 2014 ). `` How to make spinal motor neurons ''. Development. 141 ( 3 ) : 491 -- 501. doi : 10.1242 / dev. 097410. PMID 24449832. Jump up ^ Edgar R, Mazor Y, Rinon A, Blumenthal J, Golan Y, Buzhor E, Livnat I, Ben - Ari S, Lieder I, Shitrit A, Gilboa Y, Ben - Yehudah A, Edri O, Shraga N, Bogoch Y, Leshansky L, Aharoni S, West MD, Warshawsky D, Shtrichman R ( 2013 ). `` LifeMap Discovery TM : The Embryonic Development, Stem Cells, and Regenerative Medicine Research Portal ''. PLoS ONE. 8 ( 7 ) : e66629. doi : 10.1371 / journal. pone. 0066629. ISSN 1932 - 6203. Jump up ^ Philippidou, Polyxeni ; Walsh, Carolyn ; Aubin, Josée ; Jeannotte, Lucie ; Dasen, Jeremy S. `` Sustained Hox5 Gene Activity is Required for Respiratory Motor Neuron Development ''. Nature Neuroscience. 15 ( 12 ) : 1636 -- 1644. doi : 10.1038 / nn. 3242. ISSN 1097 - 6256. PMC 3676175. PMID 23103965. Jump up ^ Fitzpatrick, D. ( 2001 ) The Primary Motor Cortex : Upper Motor Neurons That Initiate Complex Voluntary Movements. In D. Purves, G.J. Augustine, D. Fitzpatrick, et al. ( Ed. ), Neuroscience. Retrieved from ( ( cite web url = https://www.ncbi.nlm.nih.gov/books/NBK10962/ title = Archived copy accessdate = 2017 - 11 - 30 deadurl = no archiveurl = https://web.archive.org/web/20180605025217/https://www.ncbi.nlm.nih.gov/books/NBK10962/ archivedate = 2018 - 06 - 05 df = ) ) Jump up ^ Tortora, G.J., Derrickson, B. ( 2011 ). The Spinal Cord and Spinal Nerves. In B. Roesch, L. Elfers, K. Trost, et al. ( Ed. ), Principles of Anatomy and Physiology ( pp. 443 - 468 ). New Jersey : John Wiley & Sons, Inc. Jump up ^ Fitzpatrick, D. ( 2001 ) Lower Motor Neuron Circuits and Motor Control : Overview. In D. Purves, G.J. Augustine, D. Fitzpatrick, et al. ( Ed. ), Neuroscience. Retrieved from ( ( cite web url = https://www.ncbi.nlm.nih.gov/books/NBK10979/ title = Archived copy accessdate = 2017 - 11 - 30 deadurl = no archiveurl = https://web.archive.org/web/20180605025217/https://www.ncbi.nlm.nih.gov/books/NBK10979/ archivedate = 2018 - 06 - 05 df = ) ) ^ Jump up to : `` CHAPTER NINE ''. www.unc.edu. Archived from the original on 2017 - 11 - 05. Retrieved 2017 - 12 - 08. Jump up ^ Silverthorn, Dee Unglaub ( 2010 ). Human Physiology : An Integrated Approach. Pearson. p. 398. ISBN 978 - 0 - 321 - 55980 - 7. ^ Jump up to : Tortora, G.J., Derrickson, B. ( 2011 ). Muscular Tissue. In B. Roesch, L. Elfers, K. Trost, et al. ( Ed. ), Principles of Anatomy and Physiology ( pp. 305 - 307, 311 ). New Jersey : John Wiley & Sons, Inc. ^ Jump up to : Purves D, Augustine GJ, Fitzpatrick D, et al., editors : Neuroscience. 2nd edition, 2001 `` Archived copy ''. Archived from the original on 2018 - 06 - 05. Retrieved 2017 - 09 - 05. CS1 maint : Archived copy as title ( link ) ^ Jump up to : Manuel, Marin ; Zytnicki, Daniel ( 2011 ). `` Alpha, Beta, and Gamma Motoneurons : Functional Diversity in the Motor System 's Final Pathway ''. Journal of Integrative Neuroscience. 10 ( 03 ) : 243 -- 276. doi : 10.1142 / S0219635211002786. ISSN 0219 - 6352. Sources Sherwood, L. ( 2001 ). Human Physiology : From Cells to Systems ( 4th ed. ). Pacific Grove, CA : Brooks - Cole. ISBN 0 - 534 - 37254 - 6. Marieb, E.N. ; Mallatt, J. ( 1997 ). Human Anatomy ( 2nd ed. ). Menlo Park, CA : Benjamin / Cummings. ISBN 0 - 8053 - 4068 - 8. Nervous tissue CNS Tissue Types Grey matter White matter Projection fibers Association fiber Commissural fiber Lemniscus Nerve tract Decussation Commissure Neuropil Meninges Cell Types Neuronal Pyramidal Purkinje Granule Glial insulating : Myelination : Oligodendrocyte other Astrocyte Radial glial cell Ependymal cells Tanycyte Microglia PNS General Dorsal Root Ganglion Ramus Ventral Root Ramus Ramus communicans Gray White Autonomic ganglion ( Preganglionic nerve fibers Postganglionic nerve fibers ) Nerve fascicle Funiculus Connective tissues Epineurium Perineurium Endoneurium Neuroglia Myelination : Schwann cell Neurilemma Myelin incisure Node of Ranvier Internodal segment Satellite glial cell Neurons / nerve fibers Parts Soma Axon hillock Axon Telodendron Axon terminals Axoplasm Axolemma Neurofibril / neurofilament Dendrite Nissl body Dendritic spine Apical dendrite / Basal dendrite Types Bipolar Unipolar Pseudounipolar Multipolar Interneuron Renshaw Afferent nerve fiber / Sensory neuron GSA GVA SSA SVA fibers Ia or Aα Ib or Golgi or Aα II or Aβ and Aγ III or Aδ or fast pain IV or C or slow pain Efferent nerve fiber / Motor neuron GSE GVE SVE Upper motor neuron Lower motor neuron α motorneuron β motorneuron γ motorneuron Termination Synapse Electrical synapse / Gap junction Chemical synapse Synaptic vesicle Active zone Postsynaptic density Autapse Ribbon synapse Neuromuscular junction Sensory receptors Meissner 's corpuscle Merkel nerve ending Pacinian corpuscle Ruffini ending Muscle spindle Free nerve ending Nociceptor Olfactory receptor neuron Photoreceptor cell Hair cell Taste bud Retrieved from `` https://en.wikipedia.org/w/index.php?title=Motor_neuron&oldid=863410704 '' Categories : Efferent neurons Motor system Hidden categories : CS1 maint : Archived copy as title Talk Contents About Wikipedia Català Dansk Deutsch Eesti Español Esperanto فارسی Français Hrvatski Italiano Latviešu Lietuvių Nederlands 日本 語 Polski Português Русский کوردی Српски / srpski Svenska Türkçe Українська Tiếng Việt 15 more Edit links This page was last edited on 10 October 2018, at 16 : 22 ( UTC ). 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Question: where are the cell bodies of efferent motor neurons found
Answer: spinal cord