A motor neuron (or motoneuron), also known as efferent neuron is a neuron that allows for both voluntary and involuntary movements of the body through muscles and glands. Its cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma 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. Innervation takes place at a neuromuscular junction and twitches can become superimposed as a result of summation or a tetanic contraction. Individual twitches can become indistinguishable, and tension rises smoothly eventually reaching a plateau.
Although the word "motor neuron" suggests that there is a single kind of neuron that controls movement, this is not the case. Indeed, upper and lower motor neurons—which differ greatly in their origins, synapse locations, routes, neurotransmitters, and lesion characteristics—are included in the same classification as "motor neurons." Essentially, motor neurons, also known as motoneurons, are made up of a variety of intricate, finely tuned circuits found throughout the body that innervate effector muscles and glands to enable both voluntary and involuntary motions. Two motor neurons come together to form a two-neuron circuit. While lower motor neurons start in the spinal cord and go to innervate muscles and glands all throughout the body, upper motor neurons originate in the cerebral cortex and travel to the brain stem or spinal cord. It is essential to comprehend the distinctions between upper and lower motor neurons as well as the routes they follow in order to effectively detect these neuronal injuries and localise the lesions.
Disorders causing selective injury to motor neurons are classified as motor neuron diseases.
Development
Motor neurons begin to develop early in embryonic development, and motor function continues to develop well into childhood. In the neural tube cells are specified to either the rostral-caudal axis or ventral-dorsal axis. The axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis (the basal plate). This homeodomain is known as the motor neural progenitor domain (pMN). Transcription factors here include Pax6, OLIG2, Nkx-6.1, and Nkx-6.2, 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, 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.
|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
|-
|Lateral motor column
|Brachial and lumbar region (both regions are further divided into medial and lateral domains)
|Muscles of the limbs
|-
|Phrenic motor column
|Cervical region
|Diaphragm
|}
Anatomy and physiology
thumb|upright=2.4|Spinal cord tracts
thumb|Location of lower motor neurons in spinal cord
Upper motor neurons
Upper motor neurons originate in the motor cortex located in the precentral gyrus. The cells that make up the primary motor cortex are Betz cells, which are giant pyramidal cells. The axons of these cells descend from the cortex to form the corticospinal tract. Corticomotorneurons project from the primary cortex directly onto motor neurons in the ventral horn of the spinal cord. Their axons synapse on the spinal motor neurons of multiple muscles as well as on spinal interneurons. Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas.
- Lateral corticospinal tract
- Rubrospinal tract
- Lateral reticulospinal tract
- Vestibulospinal tract
- Medial reticulospinal tract
- Tectospinal tract
- Anterior corticospinal tract
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 of 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 three categories:
- 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.
- 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.
See also
- Betz cell
- Central chromatolysis
- Motor dysfunction
- Motor neuron disease
- Nerve
- Sensory nerve
- Motor nerve
- Afferent nerve fiber
- Efferent nerve fiber
- Sensory neuron