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Németh Orsolya Y8GLY7
2016.11.29.
Upper and Lower Motor neurons
Motor cortex: region of cortex in the frontal lobe that is involved with voluntary movement.
Upper motor neurons: cells that carry motor commands from primary motor cortex (pyramidal
cells of precentral gyrus) to the spinal cord, through descending motor pathways to lower motor
neurons. The fibers usually decussate in the medulla oblongata, in this case they form the lateral
corticospinal tract. There are fibers that do not decussate, these form the ventral corticospinal tract.
This type of decussation called the pyramidal decussation, because the fibers interconnect through
the pyramids of the medulla.
There are a cupple of upper motor neuron pathways, the most important is the corticospinal tract and
the corticobulbar tract. The corticospinal tract originates from the motor cortex to the ventral horn of
spinal cord, the main function of this tract is the control of the limbs. The corticobulbar tract is
originate in motor cortex and end in the pons and medulla, its function is to control of facial and jaw
musculature, swallowing and tongue movements.
Lower motor neurons: innervate skeletal muscle fibers, this is the final common pathway between
CNS and skeletal muscles, traveling from ventral horn of spinal cord. Get motor information from
upper motor neurons.
There are three types of lower motor neurons:
alpha motor neurons – innervate extrafusal muscle fibers,cause muscle contraction
beta motor neurons – innervate intrafusal fibers of muscle spindles with collateral to extrafusal fibers
gamma motor neurons – innervate intrafusal muscle fibers , witch compose th muscle spindles with
sensory afferents, these are proprioceptors – sensing body position.
Deseases:
Upper motor neuron lesions: „pyramidal insufficiency” – lesion above the ventral horn of spinal
cord or motor nuclei. It can be caused by stroke, multiple sclerosis (like Amyotrophic lateral
sclerosis (ALS)), traumatic brain injury ,cerebral palsy .
The sympthoms are muscle weakness ( weakness of extensors in upper limbs or flexors in lower
limbs), decreased motor controll, spasticity (velocity- depend change in muscle tone), Clasp-knife
response (initial higher resistance to movement is followed by lesser resistance), Babinski sign ( big
toe is extended rather then flexed – ponting downwards) and increased deep tendon reflex.
Lower motor neuron lesions: affect nerve fibers from ventral horn of SC or the motor nuclei to
relevant muscles.
The sympthoms are muscle paralysis ( loss of function), fasciculation (involuntary muscle
contraction and relaxation), fibrillation (rapid, irregular unsynchronized contraction), hypotonia (low
muscle tone, less strength), hyporeflexia( reflexs are decreased), decreased strength. These
sympthoms only act on innervted muscles with the damaged nerve.
Links:
https://en.wikipedia.org/wiki/Lower_motor_neuron
https://en.wikipedia.org/wiki/Upper_motor_neuron
https://en.wikipedia.org/wiki/Lower_motor_neuron_lesion
https://en.wikipedia.org/wiki/Upper_motor_neuron_lesion
http://www.slideshare.net/SMS_2015/upper-and-lower-motor-neuron-lesions-by-drifra
https://www.youtube.com/watch?v=APuiZCxDnTA
https://www.youtube.com/watch?v=Ma4i6nH3qMQ
Book: Neuroscience Exploring the brain – 4th edition