Table 1.

Anatomic comparison of upper motor neuron and lower motor neuron, and their representations in the motor homunculi

Anatomical feature	Upper motor neuron	Lower motor neuron
Gross anatomy	Primary motor cortex (Brodmann area 4 or M1)	Cranial motor nuclei and spinal anterior horns
Cyto-architecture	Betz cells, which are layered in layer V, are relatively uniformly distributed along the motor cortex—this is not directly represented in girth of somatic regions of the homunculus	Alpha motor neurons, which are stacked in columns in brainstem and Rexed lamina IX of spinal cord, have relative densities of 3:3:1:5 in hypoglossal nucleus, cervical, thoracic, and lumbar anterior horns—this is represented as girths of somatic regions
Somatotopic organization (head to toes)	Lateral to medial along cortex to organize into motor tracts	Rostral to caudal to organize into cranial nerves and motor roots (myotomes)
Anatomic dimensions	Span along motor gyrus from Sylvian fissure to cingulate gyrus is 12 cm per hemisphere and this is represented in the homunculus by overall height (arm length is estimated to be one-third and subtracted from overall height.) Note disproportionate length of face and arms.	Height of motor column from pons to sacral cord is 45 cm (brainstem 4.5–6; cervical cord, 10–13 cm; thoracic cord, 20–25 cm; and lumbosacral cord, 5–8 cm) and this is represented in the homunculus by overall height (arm length is subtracted from overall height.) Note disproportionate length of trunk.

For further information, see the Supplementary material.