Upper motor neuron
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|Upper motor neuron|
|The motor tract|
Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the lower motor neurons. The main effector or motor neurons (efferent neurons) for voluntary movement lie within layer V of the primary motor cortex and are called Betz cells. The cell bodies of these neurons are the largest in the brain, approaching nearly 100μm in diameter.
The primary motor cortex or precentral gyrus, is one of the most important areas in the frontal lobe. The precentral gyrus is the most posterior gyrus of the frontal lobe and it lies anterior to the central sulcus. The pyramidal cells of the precentral gyrus are also called upper motor neurons. The fibers of the upper motor neurons project out of the precentral gyrus ending in the brainstem where they will decussate (intersect) within the lower medulla oblongata to form the lateral corticospinal tract on each side of the spinal cord. The fibers that do not decussate will pass through the medulla and continue on to form the anterior corticospinal tracts. The upper motor neuron descends in the spinal cord to the level of the appropriate spinal nerve root. At this point the upper motor neuron synapses with the lower motor neuron, each of whose axons innervates a fiber of skeletal muscle.
These neurons connect the brain to the appropriate level in the spinal cord, from which point nerve signals continue to the muscles by means of the lower motor neurons. The neurotransmitter, glutamate transmits the nerve impulses from upper to lower motor neurons where it is detected by glutamatergic receptors.
Upper motor neurons travel in several pathways through the central nervous system (CNS):
|corticospinal tract||from the motor cortex to lower motor neurons in the ventral horn of the spinal cord||The major function of this pathway is fine voluntary motor control of the limbs. The pathway also controls voluntary body posture adjustments.|
|corticobulbar tract||from the motor cortex to several nuclei in the pons and medulla oblongata||Involved in control of facial and jaw musculature, swallowing and tongue movements.|
|Colliculospinal tract||from the superior colliculus to lower motor neurons||Involved in involuntary adjustment of head position in response to visual information.|
|rubrospinal tract||from red nucleus to lower motor neurons||Involved in involuntary adjustment of arm position in response to balance information; support of the body.|
|vestibulospinal tract||from vestibular nuclei, which processes stimuli from semicircular canals||It is responsible for adjusting posture to maintain balance.|
|reticulospinal tract||from reticular formation||Regulates various involuntary motor activities and assists in balance.|
Damage to the descending motor pathways anywhere along the trajectory from the cerebral cortex to the lower end of the spinal cord gives rise to a set of symptoms called the "upper motor neuron syndrome". Signs may appear over days or weeks following brain or spinal cord damage, including muscle weakness, decreased motor control including a loss of the ability to perform fine movements, increased vigor (and decreased threshold) of spinal reflexes including spasticity, and an extensor plantar response known as the Babinski sign.
- Saladin, Kenneth S. Anatomy & Physiology: the Unity of Form and Function. Dubuque: McGraw-Hill, 2010. Print.1
- "Frontal Lobe." Rice University Web Calendar. 26 June 2000. Web. 06 Dec. 2010. <http://www.ruf.rice.edu/~lngbrain/cglidden/frontal.html>.