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 Morphology 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 NeuroLex ID Motor Neuron TA A14. 2.00. 021 FMA 5867 Anatomical terminology ( edit on Wikidata ) A motor neuron ( or motoneuron ) is a neuron whose cell body is located in the spinal cord and whose fiber ( axon ) projects outside the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. Motor neurons ' axons are efferent nerve fibers that carry signals from the spinal cord to the effectors to produce effects. Types of motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons. There are upper motor neurons and lower motor neurons, with the cell type described above being a lower motor neuron. Upper motor neurons are cortico - spinal interneurons that arise from the motor cortex and descend to the spinal cord where they activate the lower motor neurons through synapses. The term ' motor neuron ' is usually restricted to the efferent nerves that actually innervate muscles ( the lower 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 Overview 2.1. 1 Upper Motor Neurons 2.1. 2 White Matter Tract 2.1. 3 Lower Motor Neurons 2.2 Somatic motor neurons 2.2. 1 Neuromuscular Junction 2.3 Special visceral motor neurons 2.4 General visceral motor neurons 3 Motor neurons in regenerative medicine 4 See also 5 References 6 Sources Development Development of motor function occurs well into childhood, but the development of actual motor neurons begins before we are born. About 16 days into embryonic development, gastrulation occurs and certain cells are fated to become neural cells. A few days after this the neural tube begins to develop. Within this region, cells are specified to either the rostral - caudal axis or ventral - dorsal axis. Motor neuron development initiates from the ventral progenitor domain, which falls along the ventral - dorsal axis. This homeodomain is known as the motor neural progenitor domain ( pMN ). Transcription factors genes here include the Pax6, Olig2, Nkx6. 1, and Nkx6. 2 factors, 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 - these 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. These columns are further explained in the table below. 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 Laterl Motor Column Brachial and Lumbar region ( both regions are further divided into medial and lateral domains ) Muscles of the limbs Phrenic Motor Coulmn Cervical Region Diaphragm Anatomy and Physiology Overview Upper motor neurons Upper motor neurons are those that originate in the front lobe of the cerebral cortex. The designated area for these neurons is called the primary motor cortex and is located in the precentral gyrus. The cells that make up this cortex are Betz cells, which are a type of pyramidal cell. The axons of these cells descend from the cerebrum to form the corticospinal nerve tract. White matter Tract White matter tracts are bundles of axons and serve as means to propagate nerve impulses on a large scale. These are found in the spinal cord and appear white from the myelin surrounding axons. While these tracts exist for both efferent and afferent nerves, efferent nerves will carry signals down the spinal cord toward their effector target. These tracts also serve as the place of origin for lower motor neurons. There are seven major white matter motor tracts to be found in the spinal cord and are listed below : Lateral Corticospinal Tract Rubrospinal Tract Lateral Reticulospinal Tract Vestibulospinal tract Medial Reticulospinal Tract Tectospinal Tract Anterior Corticospinal Tract It is important to note that these do not account for all the white motor tracts found in the body. An example of another tract is the anterior corticospinal tract, which originates in the cerebral cortex and terminates in the spinal cord. 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 of 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 ) Motor neurons in regenerative medicine Human lower motor neurons can be generated in vitro from embryonic stem cells and induced pluripotent stem cells. They are currently being evaluated as experimental therapies in animal models of motor neuron disease or spinal cord injury. See also Betz cell Central chromatolysis Motor dysfunction Nerve References Jump up ^ Tortora, Jerry ; 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 ^ 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 to : Davis - Dusenbery, B.N., Williams, L.A., Klim, J.R., & Eggan, K. ( 2014 ). How to make spinal motor neurons. Development, 141 ( 3 ), 491 - 501. Jump up ^ LifeMap Discovery TM : the embryonic development, stem cells, and regenerative medicine research portal. 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. PLoS One. 2013 Jul 17 ; 8 ( 7 ) 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 https://www.ncbi.nlm.nih.gov/books/NBK10962/ ^ Jump up to : 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 https://www.ncbi.nlm.nih.gov/books/NBK10979/ ^ Jump up to : `` CHAPTER NINE ''. www.unc.edu. 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 ( 1 ) ^ Jump up to : Manuel, M., Zytnicki, D., ( 2011 ). Alpha, Beta, and Gamma Motoneurons : Functional Diversity in the Motor System 's Final Pathway. Journal of Integrative Neuroscience, 10 ( 3 ), doi : 10.1142 / S0219635211002786 Jump up ^ 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 ^ Steinbeck, JA ; Studer, L ( 8 April 2015 ). `` Moving stem cells to the clinic : potential and limitations for brain repair ''. Neuron. 86 ( 1 ) : 187 -- 206. doi : 10.1016 / j. neuron. 2015.03. 002. PMC 4443446. PMID 25856494. 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 Funiculus Fasciculus 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 ) Connective tissues Epineurium Perineurium Endoneurium Nerve fascicle Neuroglia Myelination : Schwann cell Neurolemma 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=814440983 '' Categories : Efferent neurons Motor system Talk Contents About Wikipedia Català Dansk Deutsch Eesti Español Esperanto فارسی Français Hrvatski Italiano Latviešu Lietuvių Nederlands 日本 語 Polski Русский کوردی Српски / srpski Svenska Türkçe Українська Edit links This page was last edited on 8 December 2017, at 21 : 12. 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Question: what is the main function of motor neurons
Answer: to directly or indirectly control effector organs, mainly muscles and glands