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:
the presence of horizontal myoclonus of the eyes (“eyes shaking”) with clonus of legs > arms suggests serotonin syndrome
Anatomy
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
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
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