see also:

• neurology
• clonus
• weakness of the lower limb(s) / weak in the knees

• muscle weakness due to pathology affecting the upper motor neurons which characteristically results in hyperreflexia, spasticity, clonus and muscle weakness

• UMN “pyraamidal” fibres
  • NB. UMNs do not technically have “nuclei” in the classical sense of clustered lower motor neuron cell bodies, instead, their cell bodies form layers (like layer V pyramidal cells in the cortex) or reside within specific brainstem motor centers. They project their axons downward to synapse on lower motor neuron nuclei in the brainstem or spinal cord
  • cerebral cortex / pyramidal tracts:
    • these control voluntary, fine and skilled movements and pass through the medullary pyramids where the lateral corticospinal tract fibres decussate/cross over and consist of:
      • corticospinal tracts - synapse with spinal nerves
        • within the spinal cord they form:
          • the lateral corticospinal tract
            • 89-90% of all corticospinal fibres
            • decussate in the medullary pyramids
            • controls fine, precise, and skilled voluntary movements of the distal limbs (like fingers and hands)
          • the anterior corticospinal tract
            • do not cross in the brainstem, they descend ipsilaterally and cross over later at the specific spinal cord level where they synapse
            • controls voluntary movements of the axial and proximal muscles (the neck, trunk, and shoulders) to maintain posture
      • corticobulbar tract - synapse with cranial nerves
  • UMN “nuclei” locations
    • primary motor cortex:
      • located in the precentral gyrus (Brodmann area 4) of the frontal lobe
    • premotor cortex and supplementary motor area:
      • located just anterior to the primary motor cortex in the frontal lobe
    • other cortical areas:
      • portions also arise from the parietal lobe and postcentral gyrus
• UMN brainstem / extrapyramidal tracts:
  • these are for involuntary, postural and autonomic controls and completely bypass the medullary pyramids
  • red nucleus:
    • located in the midbrain (giving rise to the rubrospinal tract)
  • vestibular nuclei:
    • located in the pons and medulla (giving rise to vestibulospinal tracts)
  • reticular formation nuclei:
    • located throughout the brainstem (giving rise to reticulospinal tracts)
  • superior colliculus:
    • located in the midbrain (giving rise to the tectospinal tract)

• cerebral palsy
• traumatic brain injury
• motor neurone disease (MND)
• serotonin syndrome
• spinal cord injury
• spinal cord ischaemia
• transverse myelitis
• encephalitis
• auto-immune post-viral leukoencephalopathy (ADEM) - acute, rapidly progressive, mainly in children
• pseudobulbar palsy - involves muscles innervated by the cranial nerves IX, X and XII, may have labile emotions and bilateral upper motor limb signs

• stroke (CVA)
• other focal intracranial pathology causing unilateral weakness eg. tumour, abscess, multiple sclerosis (MS)
• spinal cord injury
• spinal cord ischaemia