Alpha motor neuron
This article includes a improve this article by introducing more precise citations. (November 2013) ) |
Alpha motor neuron | |
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Identifiers | |
NeuroLex ID | sao1154704263 |
TH | H2.00.01.0.00008 |
FMA | 83664 |
Anatomical terms of neuroanatomy |
Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar 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. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.
While their
An alpha motor neuron and the muscle fibers it innervates comprise 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 (α-MNs) 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. 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, 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 of the eye on the opposite side of the face.
Brainstem
In the brainstem, α-MNs and other
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 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
As in the brainstem, higher segments of the spinal cord contain α-MNs that innervate muscles higher on the body. For example, the
Alpha motor neurons are located in a specific region of the spinal cord's gray matter. This region is designated lamina IX in the
Development
Alpha motor neurons originate in the
Like other neurons, α-MNs send
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
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 neurons, sensory neurons, and interneurons. 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
UMN origin | α-MN target | Tract name |
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Cerebral cortex | Brainstem | Corticonuclear tract
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Cerebral cortex | Spinal cord | Corticospinal tract |
Red nucleus | Spinal cord | Rubrospinal tract |
Vestibular nuclei | Spinal cord | Vestibulospinal tract |
Midbrain tectum
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Spinal cord | Tectospinal tract |
Reticular formation | Spinal cord | Reticulospinal tract
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The
The sensory input to α-MNs is extensive and has its origin in
The most extensive input to α-MNs is from local interneurons, 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.
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 interneurons 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
The axon of an α-MN connects with its extrafusal muscle fiber via a
Like other motor neurons, α-MNs are named after the properties of their axons. Alpha motor neurons have Aα axons, which are large-caliber, heavily myelinated fibers that conduct action potentials rapidly. By contrast, gamma motor neurons 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
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 EMG potentials (e.g., fibrillation potentials) and fasciculations, 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 is an autoimmune disease that prevents signaling across the neuromuscular junction, which results in functional denervation of muscle.
See also
References
- John A. Kiernan (2005). Barr's the Human Nervous System: An Anatomical Viewpoint (8th ed.). Hagerstown, MD: Lippincott Williams & Wilkins. ISBN 0-7817-5154-3.
- Duane E. Haines (2004). Neuroanatomy: An Atlas of Structures, Sections, and Systems (6th ed.). Hagerstown, MD: Lippincott Williams & Wilkins. ISBN 0-7817-4677-9.
External links
- NIF Search - Alpha Motor Neuron Archived 2016-03-04 at the Wayback Machine via the Neuroscience Information Framework