see also:

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

• muscle weakness due to pathology affecting the upper motor neurons which characteristically results in hyperreflexia, spasticity, clonus and muscle weakness
• in a patient with decreased conscious state, roving eye movements suggest either:
  • bilateral lesions above the brainstem, or,
  • encephalopathy, or,
  • other causes of coma
• the presence of horizontal myoclonus of the eyes (“eyes shaking”) with clonus of legs > arms suggests serotonin syndrome

• UMN “pyramidal” 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)
• the activity of these circuits is strongly modulated particularly by the basal ganglia and substantia nigra
  • striatum
    • the striatum receives diverse inputs from the cortex, midbrain, hippocampus, and other regions, which it uses to coordinate planning, movement, and decision-making, as well as processing reward
    • the medium spiny neurons in the striatum are inhibitory neurons and respond to dopamine
      • most belong to either:
        • the direct pathway, which helps to promote movement and express dopamine receptor 1 (D1)
        • the indirect pathway, which suppresses unwanted movements and express dopamine receptor 2 (D2).
      • there appear to be 9 types of medium spiny neurons in the striatum including:¹⁾
        • D1 outliers: show high expression of genes involved in addiction and substance use disorder, especially genes related to opioid response (the mu opioid receptor (OPRM1) )
        • D2 outliers: show high expression of genes that respond to antidepressants
      • role in Huntington's disease:
        • dorsal populations of medium spiny neurons express higher levels of the genes MSH2 and MSH3, which play a role in increasing the number of CAG repeats found in the huntington gene and a rare population of medium spiny neurons that forms island-like structures in the ventral striatum were more resistant to the accumulation of CAG repeats
• the term supranuclear refers to parts of the CNS above the cranial nerve motor nuclei in the brainstem
  • if the actual cranial nerve motor nuclei are intact, reflex movements often still work
    • for example, in eye disorders, reflex movements like the vestibular-ocular reflex (doll's head maneuver) can bypass a damaged supranuclear area
  • bilateral supranuclear lesions affecting eye movements:
    • often impair voluntary control, such as the ability to intentionally look up, down, or side-to-side (saccades), even though involuntary reflex tracking might remain partially preserved
  • Progressive Supranuclear Palsy (PSP) is a neurodegenerative disorder causing balance issues, rigidity, and vertical gaze palsy

• 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
• decorticate posturing
  • occurs due to severe cerebral cortical dysfunction
  • results in arms are bent inward at the elbows and held tightly against the chest with wrists and fists clenched, while the legs are stiffly extended and rotated inward
• decerebrate posturing
  • occurs due to severe brainstem dysfunction below the red nucleus
  • results in all four limbs are rigidly straightened and extended out, with the arms pronated (palms facing away), toes pointed downward, and the head and neck arched backward

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