UPPER MOTOR NEURON AND LOWER MOTOR NEURON
Neurons of the motor system are divided into upper motor neurons and lower motor neurons, depending upon their location and termination.
Upper Motor Neuron
Upper
motor neurons are the neurons in higher centers of brain, which control the
lower motor neurons.
Upper
motor neurons are of three types:
1. Motor
neurons in cerebral cortex. Fibers of these
neurons
form corticospinal (pyramidal) and
corticobulbar
tracts.
2. Neurons
in basal ganglia and brainstem nuclei
3. Neurons
in cerebellum.
Motor
neurons in cerebral cortex, which give origin to pyramidal tracts belong to the
pyramidal system and the remaining motor neurons belong to extrapyramidal system.
Some
controversy exists in including the neurons of extrapyramidal system under the
category of upper
motor
neurons. However, considering in terms of the definition, neurons other than
lower motor neurons are to be named as upper motor neurons.
Lower Motor Neuron
Lower
motor neurons are the anterior gray horn cells in spinal cord and motor neurons
of cranial nerve nuclei, situated in brainstem, which innervate the muscles directly.
Thus, the
lower motor neurons constitute ‘final common pathway’ of motor system. Lower
motor neurons
are under
the influence of upper motor neurons.
Effects of Motor
Neuron Lesions
Effects of
lower motor neuron lesion are the loss of muscle tone and
flaccid
paralysis. Effects of upper motor neuron lesion depends upon the type of neuron
involved. Effects of upper motor neuron lesion are:
1. Lesion
in pyramidal system causes hypertonia and spastic paralysis. Spastic paralysis
involves only
one group
of muscles, particularly the extensor muscles.
2. Lesion
in basal ganglia produces hypertonia and rigidity
involving both flexor and extensor muscles
3. Lesion
in cerebellum causes hypotonia, muscular weakness and incoordination of
movements.
Paralysis
Paralysis
is defined as the complete loss of strength and functions of muscle group or a
limb.
Causes
for paralysis
Common
causes for paralysis are trauma, tumor, stroke, cerebral palsy (condition
caused by brain injury
immediately
after birth), multiple sclerosis and neurodegenerative diseases.
Types of
paralysis
Paralysis
of muscles in the body depends upon type and location of motor neurons affected
by the lesion.
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