Alpha motor neuron 5315271 218142034 2008-06-09T10:08:35Z 78.86.9.206 /* Signaling */ {{Infobox Anatomy | Name = Alpha motor neuron | Latin = | GraySubject = 190 | GrayPage = | Image = Gray642.png | Caption = Alpha motor neurons are derived from the [[basal plate (neural tube)|basal plate]] (basal lamina) of the developing [[embryo]]. | Width = 200 | Image2 = | Caption2 = | System = [[Central nervous system]] | CarnegieStage = | Precursor = [[Basal plate (neural tube)|Basal plate]] | GivesRiseTo = | MeshName = Anterior+Horn+Cells | MeshNumber = A08.186.854.080 | DorlandsPre = m_20 | DorlandsSuf = 12542868 }} '''Alpha motor neurons''' ('''α-MNs''') are large [[lower motor neuron]]s of the [[brainstem]] and [[spinal cord]]. They innervate [[extrafusal muscle fiber]]s of [[skeletal muscle]] and are directly responsible for initiating their [[muscle contraction|contraction]]. Alpha motor neurons are distinct from [[gamma motor neuron]]s, which innervate [[intrafusal muscle fiber]]s of [[muscle spindle]]s. While their [[cell body|cell bodies]] are found in the [[central nervous system]] (CNS), alpha motor neurons are also considered part of the [[somatic nervous system]]—a branch of the [[peripheral nervous system]] (PNS)—because their [[axon]]s extend into the periphery to innervate [[skeletal muscle]]s. An alpha motor neuron and the muscle fibers it innervates is a [[motor unit]]. A motor neuron pool contains the cell bodies of all the alpha motor neurons involved in contracting a single muscle. ==Location== Alpha motor neurons innervating the [[head]] and [[neck]] are found in the [[brainstem]]; the remaining α-MNs innervate the rest of the body and are found in the [[spinal cord]]. Because there are fewer muscles in the head and neck than in the rest of the body, there are more α-MNs in the spinal cord than in the brainstem. In general, α-MNs on one side of the brainstem or spinal cord innervate muscles on that same side of body. The one exception is the [[trochlear nucleus]] in the brainstem, which innervates the [[superior oblique muscle]] of the eye on the opposite side of the face. ===Brainstem=== In the brainstem, α-MNs and other [[neuron]]s reside within clusters of cells called ''[[Nucleus (neuroanatomy)|nuclei]]'', some of which contain the cell bodies of neurons belonging to the [[cranial nerve]]s. Not all [[cranial nerve nuclei]] contain α-MNs; those that do are ''motor nuclei'', while others are ''sensory nuclei''. Motor nuclei are found throughout the brainstem—[[medulla oblongata|medulla]], [[pons]], and [[midbrain]]—and for developmental reasons are found near the midline of the brainstem. Generally, motor nuclei found higher in the brainstem (ie, more rostral) innervate muscles that are higher on the face. For example, the [[oculomotor nucleus]] contains α-MNs that innervate muscles of the eye, and is found in the midbrain, the most rostral brainstem component. By contrast, the [[hypoglossal nucleus]], which contains α-MNs that innervate the tongue, is found in the medulla, the most caudal (ie, towards the bottom) of the brainstem structures. ===Spinal cord=== [[Image:Gray764.png|thumb|200px|right|The [[corticospinal tract]] is one of the major descending pathways from the [[brain]] to the α-MNs of the [[spinal cord]].]] In the spinal cord, α-MNs are located within the [[gray matter]] that forms the [[ventral horn]]. These α-MNs provide the motor component of the [[spinal nerve]]s that innervate muscles of the body. [[Image:Medulla spinalis - Substantia grisea - English.svg|thumb|200px|left|Alpha motor neurons are located in lamina IX according to the [[Rexed lamina system]].]] As in the brainstem, higher segments of the spinal cord contain α-MNs that innervate muscles higher on the body. For example, the [[biceps brachii muscle]], a muscle of the arm, is innervated by α-MNs in spinal cord segments C5, C6, and C7, which are found rostrally in the spinal cord. On the other hand, the [[gastrocnemius muscle]], one of the muscles of the leg, is innervated by α-MNs within segments S1 and S2, which are found caudally in the spinal cord. Alpha motor neurons are located in a specific region of the spinal cord's gray matter. This region is designated lamina IX in the [[Rexed lamina system]], which classifies regions of gray matter based on their [[cytoarchitecture]]. Lamina IX is located predominantly in the medial aspect of the ventral horn, although there is some contribution to lamina IX from a collection of motor neurons located more laterally. Like other regions of the spinal cord, cells in this lamina are [[somatotopy|somatotopically]] organized, meaning that the position of neurons within the spinal cord is associated with what muscles they innervate. In particular, α-MNs in the medial zone of lamina IX tend to innervate proximal muscles of the body, while those in the lateral zone tend to innervate more distal muscles. There is similar somatotopy associated with α-MNs that innervate flexor and extensor muscles: α-MNs that innervate [[flexion|flexors]] tend to be located in the dorsal portion of lamina IX; those that innervate [[Extension (kinesiology)|extensors]] tend to be located more ventrally. ==Connectivity== Like other neurons, lower motor neurons have both afferent (incoming) and efferent (outgoing) connections. Alpha motor neurons receive input from a number of sources, including [[upper motor neuron]]s, [[sensory neuron]]s, and [[interneuron]]s. The primary output of α-MNs is to [[extrafusal muscle fiber]]s. This afferent and efferent connectivity is required to achieve coordinated muscle activity. ===Afferent input=== {| class="wikitable" style="float:right; margin-left:10px; margin-top:10px; margin-right:0; clear:both" |+ Selected pathways between [[upper motor neuron]]s and alpha motor neurons ! UMN origin ! α-MN target ! Tract name |- | [[Cerebral cortex]] | [[Brainstem]] | [[Corticonuclear tract]] |- | Cerebral cortex | [[Spinal cord]] | [[Corticospinal tract]] |- | [[Red nucleus]] | Spinal cord | [[Rubrospinal tract]] |- | [[Vestibular nuclei]] | Spinal cord | [[Vestibulospinal tract]] |- | [[Midbrain tectum]] | Spinal cord | [[Tectospinal tract]] |- | [[Reticular formation]] | Spinal cord | [[Reticulospinal tract]] |} [[Upper motor neuron]]s (UMNs) send input to α-MNs via several pathways, including (but not limited to) the [[corticonuclear tract|corticonuclear]], [[corticospinal tract|corticospinal]], and [[rubrospinal tract]]s. The corticonuclear and corticospinal tracts are commonly encountered in studies of upper and lower motor neuron connectivity in the control of voluntary movements. The [[corticonuclear tract]] is so named because it connects the [[cerebral cortex]] to [[cranial nerve nuclei]]. (The corticonuclear tract is also called the ''corticobulbar tract'', as the brainstem is sometimes called the "bulb" of the brain.) It is via this pathway that upper motor neurons from the cortex descend from the cortex and [[synapse]] on α-MNs of the brainstem. Similarly, UMNs of the cerebral cortex are in direct control of α-MNs of the [[spinal cord]] via the [[lateral corticospinal tract|lateral]] and [[ventral corticospinal tract]]s. The sensory input to α-MNs is extensive and has its origin in [[Golgi tendon organ]]s, [[muscle spindle]]s, [[mechanoreceptor]]s, [[thermoreceptor]]s, and other [[sensory neuron]]s in the periphery. These connections provide the structure for the neural circuits that underlie [[reflex]]es. There are several types of reflex circuits, the simplest of which consists of a single synapse between a sensory neuron and a α-MNs. The [[knee-jerk reflex]] is an example of such a monosynaptic reflex. The most extensive input to α-MNs is from local [[interneuron]]s, which are the most numerous type of neuron in the [[spinal cord]]. Among their many roles, interneurons synapse on α-MNs to create more complex reflex circuitry. One type of interneuron is the [[Renshaw cell]], discussed later. ===Efferent output=== Alpha motor neurons send fibers that mainly synapse on [[extrafusal muscle fiber]]s. Other fibers from α-MNs synapse on [[Renshaw cell]]s, inhibitory [[interneuron]]s that synapse on the α-MN and limit its activity in order to prevent muscle damage. ==Signaling== :{{main|Neuromuscular junction}} Like other neurons, α-MNs transmit signals as [[action potential]]s, rapid changes in electrical activity that propagate from the [[cell body]] to the end of the [[axon]]. To increase the speed at which action potentials travel, α-MN axons have large diameters and are heavily [[myelin]]ated by both [[oligodendrocyte]]s and [[Schwann cell]]s. Oligodendrocytes myelinate the part of the α-MN axon that lies in the [[central nervous system]] (CNS), while Schwann cells myelinate the part that lies in the [[peripheral nervous system]] (PNS). The transition between the CNS and PNS occurs at the level of the [[pia mater]], the innermost and most delicate layer of [[meninges|meningeal tissue]] surrounding components of the CNS. The axon of an α-MN connects with its extrafusal muscle fiber via a [[neuromuscular junction]], a specialized type of [[chemical synapse]] that differs both in structure and function from the chemical synapses that connect neurons to each other. Both types of synapses rely on [[neurotransmitter]]s to [[transduction (physiology)|transduce]] the electrical signal into a chemical signal and back. One way they differ is that synapses between neurons typically use [[glutamate]] or [[GABA]] as their neurotransmitters, while the neuromuscular junction uses [[acetylcholine]] exclusively. Acetylcholine is sensed by [[nicotinic acetylcholine receptor]]s on extrafusal muscle fibers, causing their contraction. ==Role in disease== {{main|Motor neuron disease}} [[Image:Polio.jpg|thumb|200px|right|[[Poliomyelitis]], caused by the [[poliovirus]] seen here, is associated with the selective loss of cells within the ventral horn of the [[spinal cord]], where α-MNs are located.]] Injury to α-MNs is the most common type of lower motor neuron [[lesion]]. Damage may be caused by [[Physical trauma|trauma]], [[ischemia]], and [[infection]], among others. In addition, certain diseases are associated with the selective loss of α-MNs. For example, [[poliomyelitis]] is caused by a [[virus]] that specifically targets and kills motor neurons in the ventral horn of the spinal cord. [[Amyotropic lateral sclerosis]] likewise is associated with the selective loss of motor neurons. [[Paralysis]] is one of the most pronounced effects of damage to α-MNs. Because α-MNs provide the only voluntary innervation to [[extrafusal muscle fiber]]s, losing α-MNs effectively severs the connection between the brainstem and spinal cord and the muscles they innervate. Without this connection, voluntary and involuntary (reflex) muscle control is impossible. Voluntary muscle control is lost because α-MNs relay voluntary signals from upper motor neurons to muscle fibers. Loss of involuntary control results from interruption of [[reflex circuit]]s such as the tonic [[stretch reflex]]. A consequence of reflex interruption is that [[muscle tone]] is reduced, resulting in [[flaccid paresis]]. Another consequence is the depression of [[deep tendon reflex]]es, causing [[hyporeflexia]]. Muscle weakness and [[atrophy]] are inevitable consequences of α-MN lesions as well. Because muscle size and strength are related to the extent of their use, denervated muscles are prone to atrophy. A secondary cause of muscle atrophy is that denervated muscles are no longer supplied with trophic factors from the α-MNs that innervate them. Alpha motor neuron lesions also result in abnormal [[electromyography|EMG]] potentials (eg, [[fibrillation potential]]s) and [[fasciculation]]s, the latter being spontaneous, involuntary muscle contractions. Diseases that impair signaling between α-MNs and extrafusal muscle fibers have similar signs to those that occur with α-MN disease. For example, [[myasthenia gravis]] is an [[autoimmune disease]] that prevents signaling across the [[neuromuscular junction]], which results in functional denervation of muscle. ==Development== Alpha motor neurons originate in the [[basal plate]], the ventral portion of the [[neural tube]] in the developing [[embryo]]. [[Sonic hedgehog]] (Shh) is secreted by the nearby [[notochord]] and other ventral structures (eg, the [[floor plate]]), establishing a gradient of highly concentrated Shh in the basal plate and less concentrated Shh in the [[alar plate]]. Under the influence of Shh and other factors, some neurons of the basal plate [[differentiation|differentiate]] into α-MNs. Like other neurons, α-MNs send [[axon]]al projections to reach their target [[extrafusal muscle fiber]]s via [[axon guidance]], a process regulated in part by [[neurotrophic factor]]s released by target muscle fibers. Neurotrophic factors also ensure that each muscle fiber is innervated by the appropriate number of α-MNs. As with most types of neurons in the [[nervous system]], α-MNs are more numerous in early development than in adulthood. Muscle fibers secrete a limited amount of neurotrophic factors capable of sustaining only a fraction of the α-MNs that initially project to the muscle fiber. Those α-MNs that do not receive sufficient neurotrophic factors will undergo [[apoptosis]], a form of [[programmed cell death]]. Because they innervate many muscles, some clusters of α-MNs receive high concentrations of neurotrophic factors and survive this stage of neuronal pruning. This is true of the α-MNs innervating the upper and lower limbs: these α-MNs form large cell columns that contribute to the [[cervical enlargement|cervical]] and [[lumbar enlargement]]s of the spinal cord. In addition to receiving neurotrophic factors from muscles, α-MNs also secrete a number of [[trophic factor]]s to support the muscle fibers they innervate. Reduced levels of trophic factors contributes to the muscle atrophy that follows an α-MN lesion. ==Nomenclature== Like other motor neurons, α-MNs are named after the properties of their [[axon]]s. Alpha motor neurons have [[Aα axon]]s, which are large-[[caliber]], heavily [[myelin]]ated fibers that conduct [[action potential]]s rapidly. By contrast, [[gamma motor neuron]]s have [[Aγ axon]]s, which are slender, lightly myelinated fibers that conduct less rapidly. ==References== *{{cite book | author=John A. Kiernan | title=Barr's the Human Nervous System: An Anatomical Viewpoint | edition=8th edition | year=2005 | publisher=Lippincott Williams & Wilkins | location=Hagerstwon, MD | id=ISBN 0-7817-5154-3}} *{{cite book | author=Duane E. Haines | title=Neuroanatomy: An Atlas of Structures, Sections, and Systems | edition=6th edition | year=2004 | publisher=Lippincott Williams & Wilkins | location=Hagerstwon, MD | id=ISBN 0-7817-4677-9}} {{Nervous tissue}} [[Category:Somatic motor system]] [[Category:CNS neurons]] [[Category:Efferent neurons]] [[fi:Alfamotoneuroni]]