Motor neuron
Encyclopedia
In vertebrate
s, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system
(or CNS) that project their axons outside the CNS and directly or indirectly control muscle
s. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neuron
s.
According to their targets, motor neurons are classified into three broad categories:
Somatic motor neurons, which directly innervate skeletal muscles, involved in locomotion (such as the muscles of the limbs, abdominal, and intercostal muscles).
Special visceral motor neurons, also called branchial motor neurons, which directly innervate branchial muscles
(that motorize the gills in fish and the face and neck in land vertebrates).
General visceral motor neurons (visceral motor neurons for short) which 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 other words:
It could be argued that, in the command of visceral muscles, the ganglionic neuron, parasympathetic or sympathetic
, is the real motor neuron, being the one that directly innervates the muscle (whereas the general visceral motor neuron is, strictly speaking, a preganglionic neuron). But, for historical reasons, the term motor neuron is reserved for the CNS neuron.
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)
called the neuromuscular junction
. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitter
s that bind to postsynaptic receptor
s and triggers a response in the muscle fiber.
(CNS) to the periphery
.)
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 neuron
s 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
.
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.
.
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
(or CNS) that project their axons outside the CNS and directly or indirectly control muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neuron
Alpha motor neuron
Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
s.
Anatomy and physiology
| 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 gland Sweat gland Sweat glands, or sudoriferous glands, are small tubular structures of the skin that produce sweat. There are two kinds of sweat glands:... s and certain blood vessel Blood vessel The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and... s Motoneuron neurotransmitters |
||
According to their targets, motor neurons are classified into three broad categories:
Somatic motor neurons, which directly innervate skeletal muscles, involved in locomotion (such as the muscles of the limbs, abdominal, and intercostal muscles).
Special visceral motor neurons, also called branchial motor neurons, which directly innervate branchial muscles
Branchiomeric musculature
Branchiomeric muscles are striated muscles of the head and neck. Unlike skeletal muscles that developmentally come from somites, branchiomeric muscles are developmentally formed from the branchial arches....
(that motorize the gills in fish and the face and neck in land vertebrates).
General visceral motor neurons (visceral motor neurons for short) which indirectly innervate cardiac muscle
Cardiac muscle
Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...
and smooth muscles of the viscera ( the muscles of the arteries): they synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
onto neurons located in ganglia of the autonomic nervous system
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
(sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
and parasympathetic), located in the peripheral nervous system
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
(PNS), which themselves directly innervate visceral muscles (and also some gland cells).
In other words:
- the motor command of skeletalSkeletal muscleSkeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
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).
It could be argued that, in the command of visceral muscles, the ganglionic neuron, parasympathetic or sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
, is the real motor neuron, being the one that directly innervates the muscle (whereas the general visceral motor neuron is, strictly speaking, a preganglionic neuron). But, for historical reasons, the term motor neuron is reserved for the CNS neuron.
All vertebrate motor neurons are cholinergic
Cholinergic
The word choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation. Found in most animal tissues, choline is a primary component of the neurotransmitter acetylcholine and functions with inositol as a basic constituent of lecithin...
, that is, they release the neurotransmitter acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
. Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons are noradrenergic
Norepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
, that is, they release the neurotransmitter noradrenaline. (see Table)
Function
The interface between a motor neuron and muscle fiber is a specialized synapseSynapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
called the neuromuscular junction
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s that bind to postsynaptic receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
s and triggers a response in the muscle fiber.
- 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 neurotransmitterNeurotransmitterNeurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
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. Although muscle innervation may eventually play a role in the maturation of motor activity. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiologicalElectrophysiologyElectrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
activity) or on cholinergicAcetylcholineThe chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
neuromuscular junctionNeuromuscular junctionA neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
s, rather than on the muscles themselves.
Somatic motor neurons
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 systemCentral nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
(CNS) to the periphery
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
.)
- Alpha motoneurons innervate extrafusal muscle fiberExtrafusal muscle fiberExtrafusal muscle fiber is a term given to standard muscle fibers as to distinguish them from intrafusal muscle fibers. Extrafusal muscle fibers are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement...
s (typically referred to simply as muscle fibers) located throughout the muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells. - Gamma motoneuronsGamma motoneuronsGamma motoneurons , also called gamma motor neurons, are the efferent component of the fusimotor system, the system by which the central nervous system controls and modifies muscle spindle sensitivity. The fusimotor system refers to the combination of muscle spindles and γ-motoneurons...
innervate intrafusal muscle fiberIntrafusal muscle fiberIntrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen...
s found within the muscle spindleMuscle spindleMuscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
.
In addition to voluntary skeletal muscle contraction, alpha motoneurons also contribute to muscle tone
Muscle tone
In physiology, medicine, and anatomy, muscle tone is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state. It helps maintain posture, and it declines during REM sleep.-Purpose:Unconscious nerve impulses maintain the...
, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neuron
Sensory neuron
Sensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
s within the muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
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
Stretch reflex
The stretch reflex is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length....
.
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 motor neuron may synapse with one or more muscle fibers. The motor neuron and all of the muscle fibers to which it connects is a motor unitMotor unit
”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type . When a motor unit is activated, all of its fibers contract...
.
See also
- Alpha motor neuronAlpha motor neuronAlpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
- Central chromatolysisCentral chromatolysisCentral chromatolysis is a histopathologic change seen in the cell body of a neuron, where the chromatin and cell nucleus is pushed to the cell periphery, in response to axonal injury....
- Gamma motor neuron
- Beta motor neuronBeta motor neuronA beta motor neuron is a kind of lower motor neuron, along with alpha motor neuron and gamma motor neuron. These motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers . Axons of beta motor neurons are myelinated...
- Muscle spindleMuscle spindleMuscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
- Nerve fiber Nerve fiberA nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system , myelin...
- Neuromuscular junctionNeuromuscular junctionA neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
- Motor dysfunction
- Motor unitMotor unit”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type . When a motor unit is activated, all of its fibers contract...
- Motor neuron diseaseMotor neurone diseaseThe motor neurone diseases are a group of neurological disorders that selectively affect motor neurones, the cells that control voluntary muscle activity including speaking, walking, breathing, swallowing and general movement of the body. They are generally progressive in nature, and can cause...
- Efferent nerveEfferent nerveIn the nervous system, efferent nerves, otherwise known as motor or effector neurons, carry nerve impulses away from the central nervous system to effectors such as muscles or glands...
- Nerves
External links
- NIF Search - Motor Neuron via the Neuroscience Information FrameworkNeuroscience Information FrameworkThe 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.-Description:...