UMN

upper motor neurons

• descending patways of SC composed of axons of UMNs
• cell bodies of UMNs located in cerebral cortex
• axons of UMNs CROSS to contra side and end on a motor neuron in the ventral horn either directly or through an interneuron

LMN

lower motor neurons

• motor neurons in the ventral horn or final common pathway through which UMNs exert their control over motor activity
• UMNs excite or inhibit LMNs
• axons of LMNs leave SC through ventral roots of spinal nerves and innervate voluntary skeletal mms

• corticospinal or pyramidal tracts end on the ventral horn cells
• corticobulbar tracts end on neurons in the motor nuclei of cranial nerves

location of UMN cell bodies

40% are in upper 2/3 of primary cortex of frontal lobe

40% = premotor area of frontal lobe area 6

20% = somesthetic cortex of parietal lobe(3,1,2)

• corticospinal tract descends post limb of internal capsule, crus cerebri or basis pedunculli through the pons and forms the pyramids of the medulla
• caudal end of medulla - 85% of fibers CROSS to contra side in decussation of the pyramids and descend in dorsal part of lateral column as LCST - cervical fibers are medial; sacral fibers are most lateral
• 15% remain UNCROSSED and continue into the VCST