 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Upper motor neuron" – news ·newspapers ·books ·scholar ·JSTOR(May 2010) (Learn how and when to remove this message) |
| Upper motor neuron | |
|---|---|
 The motor tract | |
| Identifiers | |
| FMA | 84631 |
| Anatomical terminology | |
Upper motor neurons (UMNs) is a term introduced byWilliam Gowers in 1886. They are found in thecerebral cortex andbrainstem and carry information down to activateinterneurons andlower motor neurons, which in turndirectly signalmuscles tocontract or relax. UMNs represent the major origin point for voluntary somatic movement.
Upper motor neurons represent the largestpyramidal cells in the motor regions of the cerebral cortex. The major cell type of the UMNs is theBetz cells residing inlayer V of theprimary motor cortex, located on theprecentral gyrus in the posterior frontal lobe. Thecell bodies of Betz cellneurons are the largest in the brain, approaching nearly 0.1 mm indiameter. The axons of the upper motor neurons project out of the precentral gyrus travelling through to the brainstem, where they will decussate (intersect) within the lowermedulla oblongata to form the lateralcorticospinal tract on each side of thespinal cord. The fibers that do notdecussate 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 or interneurons within theventral horn of the spinal cord, each of whose axons innervate a fiber of skeletal muscle.[1][2]
These neurons connect thebrain to the appropriate level in the spinal cord, from which pointnerve signals continue to the muscles by means of thelower motor neurons. Theneurotransmitterglutamate transmits the nerve impulses from upper to lower motor neurons, where it is detected byglutamate receptors.
Upper motor neurons travel in severalneural pathways through thecentral nervous system (CNS):
| Tract | Pathway | Function |
|---|---|---|
| corticospinal tract | from themotor cortex tolower motor neurons in theventral 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 themotor cortex to several nuclei in thepons andmedulla oblongata | Involved in control of facial and jaw musculature, swallowing and tongue movements. |
| colliculospinal tract (tectospinal tract) | from thesuperior colliculus to lower motor neurons | Involved in involuntary adjustment of head position in response to visual information. |
| rubrospinal tract | fromred 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 fromsemicircular canals | It is responsible for adjusting posture to maintain balance. |
| reticulospinal tract | fromreticular formation | Regulates various involuntary motor activities and assists in balance. |
Anyupper motor neuron lesion, also known as pyramidal insufficiency, occurs in the neural pathway above theanterior horn of the spinal cord. Such lesions can arise as a result ofstroke,multiple sclerosis,spinal cord injury or otheracquired brain injury. The resulting changes in muscle performance that can be wide and varied are described overall asupper motor neuron syndrome. Symptoms can include muscleweakness, decreasedmotor control including a loss of the ability to performfine movements, increased vigor (and decreased threshold) of spinalreflexes includingspasticity,clonus (involuntary, successive cycles of contraction/relaxation of a muscle), and anextensorplantar response known as theBabinski sign.[3]
{{cite journal}}:Cite journal requires|journal= (help)