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 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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