Motor neuron
Section through the spinal cord. Motor neuron projection through ventral root is shown in red.
Location	Ventral horn of the spinal cord
Function	Excitatory projection (to NMJ)
Neurotransmitter	ACh
Morphology	Projection neuron
Presynaptic connections	M1 via the Corticospinal tract
Postsynaptic connections	Extrafusal muscle fibers via the ventral root

In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (CNS) which project their axons outside the CNS and directly or indirectly control muscles. The term is synonymous with efferent neurons. Image File history File links Medulla_spinalis_-_Section_-_English. ... In anatomy and neurology, the ventral root is the efferent motor root of a spinal nerve. ... The anterior horn is the anterior division of the lateral ventricle of the brain. ... The Spinal cord nested in the vertebral column. ... ACH is a three letter abbreviation that can refer to: Acetylcholine (ACh), a neurotransmitter Achondroplasia, a type of genetic disorder that is a common cause of dwarfism Achnashellach railway station (United Kingdom), as the National Rail code for the station Acholi language (ach), as the ISO 639 code for the... Comparative anatomy is the study of similarities and differences in organisms. ... The primary motor area is a group of networked cells in mammalian brains that controls movements of specific body parts associated with cell groups in that area of the brain. ... The corticospinal or pyramidal tract is a massive collection of axons that travel between the cerebral cortex of the brain and the spinal cord. ... Extrafusal muscle fibers are served by axons of the alpha motor neurons. ... Classes and Clades See below Vertebrates are members of the subphylum Vertebrata (within the phylum Chordata), specifically, those chordates with backbones or spinal columns. ...

Anatomy and physiology

According to their targets, motoneurons are classified into three broad categories: Sweating (also called perspiration or sometimes transpiration) is the loss of a watery fluid, consisting mainly of sodium chloride and urea in solution, that is secreted by the sweat glands in the skin of mammals. ... The arterial system The blood vessels are part of the circulatory system and function to transport blood throughout the body. ...

"Somatic motoneurons", which directly innervate skeletal muscles, involved in locomotion (such as muscles of the limbs, abdominal and intercostal muscles).

“Special visceral motoneurons” — also called “branchial motoneurons”— which directly innervate branchial muscles (that motorize the gills in fish and the face and neck in land vertebrates).

“General visceral motoneurons” — "visceral motoneurons" for short— which indirectly innervate smooth muscles of the viscera (like the heart, or 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). This article or section is in need of attention from an expert on the subject. ...

In other words:

• the motor command of skeletal and branchial muscles is monosynaptic (involving only one motoneuron —respectively “somatic “ and “branchial”— which synapses onto the muscle).
• the command of visceral muscles is disynaptic (involving two neurons: the “general visceral motoneuron” located in the CNS, which synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle).

It could be argued that, in the command of visceral muscles, the ganglionic neuron —parasympathetic or sympathetic— is the real “motoneuron”, being the one that directly innervates the muscle (while the “general visceral motoneuron” is, strictly speaking, a “preganglionic” neuron). But, for historical reasons, the term motoneuron is reserved for the CNS neuron. A top-down view of skeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton. ... Anatomy and Physiology of the A.N.S. In contrast to the voluntary nervous system, the involuntary or autonomic nervous system is responsible for homeostasis, maintaining a relatively constant internal environment by controlling such involuntary functions as digestion, respiration, and metabolism, and by modulating blood pressure. ... The word sympathetic means different things in different contexts. ... In the autonomic nervous system, fibers from the CNS to the ganglion are known as preganglionic fibers. ...

All motoneurons are cholinergic (that is, release the neurotransmitter acetylcholine). Parasympathetic ganglionic neurons are also cholinergic, while most sympathetic ganglionic neurons are noradrenergic (that is, release the neurotransmitter noradrenaline). (see Table) A synapse is cholinergic if it uses acetylcholine as its neurotransmitter. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... Norepinephrine (INN) or noradrenaline (BAN) is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... Norepinephrine, known as noradrenaline outside the USA, is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ...

Function

The interface between a motoneuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motoneuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber. Illustration of the major elements in a prototypical synapse. ... A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ... Chemical structure of D-Aspartic Acid, a common Amino Acid neurotransmitter. ... In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ...

• In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be 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 verterbrates is obtained only by inhibition of the motoneuron itself. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.

Invertebrate is a term coined by Chevalier de Lamarck to describe any animal without a backbone or vertebra, like insects, squids and worms. ... Typical classes Petromyzontidae (lampreys) Placodermi - extinct Chondrichthyes (cartilaginous fish) Acanthodii - extinct Actinopterygii (ray-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Amphibia (amphibians) Reptilia (reptiles) Aves (birds) Mammalia (mammals) Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... Chemical structure of D-Aspartic Acid, a common Amino Acid neurotransmitter. ... In medicine, a muscle relaxant is a drug that causes skeletal muscle contraction to cease. ... Electrophysiology is the study of the electrical properties of biological cells and tissues. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ...

Somatic motoneurons

Somatic motoneurons are further subdivided into two types: alpha efferent neurons and gamma efferent neurons. (Both types are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.) Efferent nerve fibers carry information away from the brain. ... A diagram showing the CNS: 1. ... The peripheral nervous system or PNS, is part of the nervous system, and consists of the nerves and neurons that reside or extend outside the central nervous system (the brain and spinal cord) to serve the limbs and organs, for example. ...

• Alpha motoneurons innervate extrafusal muscle fibers (typically referred to simply as muscle fibers) located throughout the muscle.
• Gamma motoneurons innervate intrafusal muscle fibers found within the muscle spindle.

In addition to voluntary skeletal muscle contraction, alpha motoneurons 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 motoneurons in the spinal cord which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex. Extrafusal muscle fibers are served by axons of the alpha motor neurons. ... γ-motoneurons (gamma-motoneurons or motor neurons) are a component of the fusimotor system, the system by which the CNS controls muscle spindle sensitivity. ... Intrafusal fibers are muscle fibers that comprise the muscle spindle. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... Muscle tone is the continuous and passive partial contraction of the muscles. ... The mechanism of the reflex arc Sensory neurons (or neurones) are nerve cells within the nervous system responsible for converting external stimuli from the organisms environment into internal [[electrical poop is responsible for it aright underlie motor reflex loops and several forms of involuntary behavior, including pain avoidance. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... It is reflex muscle contraction in response of its stretch. ...

Gamma motoneurons 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.

Motor units

A single motoneuron may synapse with one or more muscle fibers. The motoneuron and all of the muscle fibers to which it connects is a motor unit. A motor unit is a group of cells under the control of a single motor neuron; groups of motor units work together, as a single muscle. ...

See also

• neuromuscular junction
• muscle spindle
• motor unit
• Motor neurone disease
• Motor nerve

A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... A motor unit is a group of cells under the control of a single motor neuron; groups of motor units work together, as a single muscle. ... The motor neurone diseases (MND) are a group of progressive neurological disorders that destroy motor neurones, the cells that control voluntary muscle activity such as speaking, walking, breathing, and swallowing. ... Motor nerves enable the brain to stimulate muscle contraction. ...

References

• Sherwood, L. (2001). Human Physiology: From Cells to Systems (4 ed.). California: Brooks/Cole.
• Marieb, E. N., Mallatt, J. (1997). Human Anatomy (2 ed.). California: Benjamin/Cummings.