Upper and lower motor neuron systems

Lower motor neurons:

  • Make direct contact with muscles.
  • Form final common pathway for control of muscle activity. 
  • Comprise all motor cranial and spinal nerves

Upper motor neurons

  • Do not make direct contact with muscles.
  • Control muscle activity indirectly by driving lower motor neurons
  • Comprise descending tracts e.g. corticospinal and vestibulospinal

Effects of lesions

Lower Motor
Upper Motor
  1. Total flaccid paralysis of muscles

  2. Diminished/absent tendon reflexes

  3. Progressive atrophy of muscles
  1. Voluntary Spastic paralysis of muscles.

  2. Exxagerated tendon reflexes

  3. Little or no muscle atrophy

  4. Primitive reflexes e.g. babinski sign




Giant pyramidal neurons of cortical layer 5 of:

  • Primary motor area
  • Premotor area
  • The frontal eye field
  • Somatosensory area

Course , Components and Termination


Cerebrum: Corona radiata, Posterior limb of internal capsule

Mid brain : Middle of crus cerebri

Pons: Basal pons

Medulla: Pyramid, 75-85% pyramidal decussation

Spinal cord: Lateral corticospinal- Lateral funiculus, Ventral corticospinal - ventral funiculus


  1. Corticobulbar
  2. Corticospinal



Anterior horn cells ( ventral horn ) at the following levels:

  1. 55% Cervical
  2. 20% Thoracic
  3. 25% Lumbosacral

Nuclei of:

  1. Oculomotor
  2. Trochlea
  3. Trigeminal
  4. Abducens
  5. Facial
  6. Glossopharyngeal
  7. Vagus
  8. Hypoglossal


  • Control of fine voluntary movements