Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar

lower motor neuron Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve lower ...

s of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

and

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

. They innervate extrafusal muscle fibers of

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from

gamma motor neuron A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( Aγ) fibers going to the muscle. Like ...

s, which innervate intrafusal muscle fibers of

muscle spindle Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...

s. While their cell bodies are found in the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

(CNS), α motor neurons are also considered part of the

somatic nervous system The somatic nervous system (SNS), also known as voluntary nervous system, is a part of the peripheral nervous system (PNS) that links brain and spinal cord to skeletal muscles under conscious control, as well as to sensory receptors in the skin ...

—a branch of the

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of Bilateria, bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside t ...

(PNS)—because their

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s extend into the periphery to innervate

s. An alpha motor neuron and the muscle fibers it innervates comprise a

motor unit In biology, a motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. Groups of motor units often work tog ...

. A motor neuron pool contains the cell bodies of all the alpha motor neurons involved in contracting a single muscle.

Location

Alpha motor neurons (α-MNs) innervating the

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple ani ...

and

neck The neck is the part of the body in many vertebrates that connects the head to the torso. It supports the weight of the head and protects the nerves that transmit sensory and motor information between the brain and the rest of the body. Addition ...

are found in the

; the remaining α-MNs innervate the rest of the body and are found in the

. There are more α-MNs in the spinal cord than in the brainstem, as the number of α-MNs is directly proportional to the amount of fine motor control in that muscle. For example, the muscles of a single finger have more α-MNs per fibre, and more α-MNs in total, than the muscles of the

quadriceps The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...

, which allows for finer control of the force a finger applies. In general, α-MNs on one side of the brainstem or spinal cord innervate muscles on that same side of the body. An exception is the trochlear nucleus in the brainstem, which innervates the

superior oblique muscle The superior oblique muscle or obliquus oculi superior is a fusiform muscle originating in the upper, medial side of the orbit (anatomy), orbit (i.e. from beside the nose) which abducts, depresses and internally rotates the eye. It is the only e ...

of the eye on the opposite side of the face.

Brainstem

In the brainstem, α-MNs and other

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s reside within clusters of cells called '' nuclei'', some of which contain the cell bodies of neurons belonging to the

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

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,

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of ...

, and

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

—and for developmental reasons are found near the midline of the brainstem. Generally, motor nuclei found higher in the brainstem (i.e., more rostral) innervate muscles that are higher on the face. For example, the

oculomotor nucleus The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. F ...

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 (i.e., towards the bottom) of the brainstem structures.

Spinal cord

In the spinal cord, α-MNs are located within the

gray matter Grey matter, or gray matter in American English, is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and ...

that forms the ventral horn. These α-MNs provide the motor component of the

spinal nerve A spinal nerve is a mixed nerve, which carries Motor neuron, motor, Sensory neuron, sensory, and Autonomic nervous system, autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each s ...

s that innervate muscles of the body. Medulla spinalis - Substantia grisea - English

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 The biceps or biceps brachii (, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both Muscle head, heads of the muscle arise on the scapula and join to form a single ...

, 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 The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It is located superficial to the soleus in the posterior (back) compartment of the leg. It runs from its t ...

, 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 Cytoarchitecture (from Greek κύτος 'cell' and ἀρχιτεκτονική 'architecture'), also known as cytoarchitectonics, is the study of the cellular composition of the central nervous system's tissues under the microscope. Cytoarchit ...

. 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 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 flexors tend to be located in the dorsal portion of lamina IX; those that innervate

extensors In anatomy, extension is a movement of a joint that increases the angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extend ...

tend to be located more ventrally.

Development

Alpha motor neurons originate in the basal plate, the ventral portion of the

neural tube In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, ...

in the developing

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

Sonic hedgehog Sonic hedgehog protein (SHH) is a major signaling molecule of embryonic development in humans and animals, encoded by the ''SHH'' gene. This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organoge ...

(Shh) is secreted by the nearby

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

and other ventral structures (e.g., 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 differentiate into α-MNs. Like other neurons, α-MNs send

al projections to reach their target extrafusal muscle fibers via axon guidance, a process regulated in part by

neurotrophic factor Neurotrophic factors (NTFs) are a family of biomolecules – nearly all of which are peptides or small proteins – that support the growth, survival, and differentiation of both developing and mature neurons. Most NTFs exert their trop ...

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 In biology, the nervous system is the complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...

, 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 Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

, a form of

programmed cell death Programmed cell death (PCD) sometimes referred to as cell, or cellular suicide is the death of a cell (biology), cell as a result of events inside of a cell, such as apoptosis or autophagy. PCD is carried out in a biological process, which usual ...

. 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 and lumbar enlargements of the spinal cord. In addition to receiving neurotrophic factors from muscles, α-MNs also secrete a number of trophic factors to support the muscle fibers they innervate. Reduced levels of trophic factors contributes to the muscle atrophy that follows an α-MN lesion.

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 Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles ...

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduc ...

s, and

interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

s. The primary output of α-MNs is to extrafusal muscle fibers. This afferent and efferent connectivity is required to achieve coordinated muscle activity.

Afferent input

Upper motor neuron Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles ...

s (UMNs) send input to α-MNs via several pathways, including (but not limited to) the corticonuclear, corticospinal, and

rubrospinal tract The rubrospinal tract is one of the descending tracts of the spinal cord. It is a motor control pathway that originates in the red nucleus. It is a part of the lateral indirect extrapyramidal tract. The rubrospinal tract fibers are efferent ne ...

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 The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...

to cranial nerve nuclei. (The corticonuclear tract is also called the ''corticobulbar tract'', as the target in the brainstemwhich is medullais archaically called the "bulb.") It is via this pathway that upper motor neurons descend from the cortex and

synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

on α-MNs of the brainstem. Similarly, UMNs of the cerebral cortex are in direct control of α-MNs of the

via the

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

and ventral corticospinal tracts. The sensory input to α-MNs is extensive and has its origin in Golgi tendon organs,

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into action potential, electrical signals tha ...

thermoreceptor A thermoreceptor is a non-specialised sense Cutaneous receptor, receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian ...

s, and other

s in the periphery. These connections provide the structure for the neural circuits that underlie

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

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

s, which are the most numerous type of neuron in the

. Among their many roles, interneurons synapse on α-MNs to create more complex reflex circuitry. One type of interneuron is the Renshaw cell.

Efferent output

Alpha motor neurons send fibers that mainly synapse on extrafusal muscle fibers. Other fibers from α-MNs synapse on Renshaw cells, i.e. inhibitory

s that synapse on the α-MN and limit its activity in order to prevent muscle damage.

Signaling

Like other neurons, α-MNs transmit signals as

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s, rapid changes in electrical activity that propagate from the cell body to the end of the

. To increase the speed at which action potentials travel, α-MN axons have large diameters and are heavily

myelin Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

ated by both

oligodendrocyte Oligodendrocytes (), also known as oligodendroglia, are a type of neuroglia whose main function is to provide the myelin sheath to neuronal axons in the central nervous system (CNS). Myelination gives metabolic support to, and insulates the axons ...

s and

Schwann cell Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include Satellite glial cell, satellite ...

s. Oligodendrocytes myelinate the part of the α-MN axon that lies in the

(CNS), while Schwann cells myelinate the part that lies in the

(PNS). The transition between the CNS and PNS occurs at the level of the

pia mater Pia mater ( or ),Entry "pia mater"
in
meningeal tissue surrounding components of the CNS. The axon of an α-MN connects with its extrafusal muscle fiber via a

neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

, a specialized type of

chemical synapse Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous syste ...

that differs both in structure and function from the chemical synapses that connect neurons to each other. Both types of synapses rely on

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

s to transduce the electrical signal into a chemical signal and back. One way they differ is that synapses between neurons typically use

glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

GABA GABA (gamma-aminobutyric acid, γ-aminobutyric acid) is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. GA ...

as their neurotransmitters, while the neuromuscular junction uses

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

exclusively. Acetylcholine is sensed by

nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are Receptor (biochemistry), receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the c ...

s on extrafusal muscle fibers, causing their contraction. Like other motor neurons, α-MNs are named after the properties of their

s. Alpha motor neurons have Aα axons, which are large-

caliber In guns, particularly firearms, but not #As a measurement of length, artillery, where a different definition may apply, caliber (or calibre; sometimes abbreviated as "cal") is the specified nominal internal diameter of the gun barrel Gauge ( ...

, heavily

ated fibers that conduct

s rapidly. By contrast,

s have Aγ axons, which are slender, lightly myelinated fibers that conduct less rapidly.

Clinical significance

Injury to α-MNs is the most common type of lower motor neuron

lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by injury or diseases. The term ''Lesion'' is derived from the Latin meaning "injury". Lesions may occur in both plants and animals. Types There is no de ...

. Damage may be caused by trauma,

ischemia Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

, and

infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...

, among others. In addition, certain diseases are associated with the selective loss of α-MNs. For example,

poliomyelitis Poliomyelitis ( ), commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 75% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe ...

is caused by a

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

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 Paralysis (: paralyses; also known as plegia) is a loss of Motor skill, motor function in one or more Skeletal muscle, muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory d ...

is one of the most pronounced effects of damage to α-MNs. Because α-MNs provide the only innervation to extrafusal muscle fibers, 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 circuits such as the tonic

stretch reflex The stretch reflex (myotatic reflex), or more accurately ''muscle stretch reflex'', is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length but ...

. A consequence of reflex interruption is that

muscle tone In physiology, medicine, and anatomy, muscle tone (residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state.O’Sullivan, S. B. (2007) ...

is reduced, resulting in flaccid paresis. Another consequence is the depression of deep tendon reflexes, causing hyporeflexia. Muscle weakness and

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), malnutrition, poor nourishment, poor circulatory system, circulation, loss of hormone, ...

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 EMG potentials (e.g., fibrillation potentials) and

fasciculation A fasciculation, or muscle twitch, is a spontaneous, involuntary muscle contraction and relaxation, involving fine muscle fibers. They are common, with as many as 70% of people experiencing them. They can be benign, or associated with more seriou ...

s, the latter being spontaneous, involuntary muscle contractions. Diseases that impair signaling between α-MNs and extrafusal muscle fibers, namely diseases of the neuromuscular junction have similar signs to those that occur with α-MN disease. For example,

myasthenia gravis Myasthenia gravis (MG) is a long-term neuromuscular junction disease that leads to varying degrees of skeletal muscle weakness. The most commonly affected muscles are those of the eyes, face, and swallowing. It can result in double vision, ...

is an

autoimmune disease An autoimmune disease is a condition that results from an anomalous response of the adaptive immune system, wherein it mistakenly targets and attacks healthy, functioning parts of the body as if they were foreign organisms. It is estimated tha ...

that prevents signaling across the

, which results in functional denervation of muscle.

References

* *

External links

NIF Search - Alpha Motor Neuron
via the

Neuroscience Information Framework The Neuroscience Information Framework is a repository of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/ genomic resources and provides many aut ...

{{Authority control Somatic motor system Central nervous system neurons Efferent neurons