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NS I - Descending Motor Systems
DCMS
8
Anatomy
Graduate
07/08/2009

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Cards

Term

UMN

upper motor neurons

Definition
  • 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
Term

LMN

lower motor neurons

Definition
  • 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
Term
Pyramidal Systems
Definition
  • corticospinal or pyramidal tracts end on the ventral horn cells
  • corticobulbar tracts end on neurons in the motor nuclei of cranial nerves
Term
Extrapyramidal systems
Definition
includes all other centers and tracts that have a significant influence on motor cortex or on LMNs
Term
Corticospinal or Pyramidal Tract
Definition

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)

Term
Course of UMN axons
Definition
  • 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
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