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