Neuro 56: U & LMN Flashcards
2 components of the UMN
- lateral corticospinal tract
- bulbospinal tract
* *run together –> so damage will effect both & cause UMN lesion sx!
Lateral corticospinal tract
- provides comands from the cortex to the spinal chord to allow for mvmnt of the limbs
- starts in precentral gyrus and the associated motor areas of the cortex –> 90% cross over when they get to the spinal chord
- supply the LMN in the ventral horn which then cause the contraction
- DIRECT pathway from cortex to LMN
Bulbospinal tract
- INDIRECT pthwy
- arise from a few diff locations in the brainstem that terminate on the MN networks in the spinal chord
- play a role in adjusting muscle tone and posture
- get input from cortex
Why is there increased tendon reflexes with UMN lesions?
-b/c the bulbospinal signals from the brainstem are normally inhibatory to the stretch reflex of the DTR –> so since these are damaged the inhibition is taken away and the DTR will be increased
Location of LMN’s
-ventral horn (lamina IX) of the SC
alpha mn
-innervate extrafusal muscle fibers
gamma mn
- innervate intrafusal muscle fibers
- the CNS controls the sensitivity of muscle spindles by these mns
Motor neurons for axial muscles
-located more medially in the longitudinal column of cell bodies that runs the length of the spinal cord
Motorneurons for limbs
-located more laterally in a discontinuous lateral cell column located in the cervical and lumbosacral enlargements
Spinal reflexes
-depend on local circuts w/in the sc that couple incoming sensory info to outgoing motor info
Origin of corticospinal T cell bodies
- cell bodies are located in the frontal cortex motor areas
- cells in the somatosensory cortex also contribute, but are not used for mvmnt
- the amnt of cortex devoted to a body area is related to the need for fine motor control
Cortical motor areas
- highly interconnected regions invlved in planning and initiation of voluntary mvmnt
- divided into regions based on structural and functional differences:
1. motor cortex
2. pre-motor cortex
3. supplementary motor cortex
Motor cortex (cortical motor area)
- the precentral gyrus will stimulate a single UMN here that will activate several muscles
- shows that UMNs encode PURPOSEFUL mvmnts rather than contraction of individual muscles
- *lesions = weakness/paralysis
Premotor cortex (cortical motor area)
- immediately anterior to the precentral gyrus
- receievs info from the somatosensory and visual cortex
- mediates mvmnts that require sSENSORY GUIDANCE –> such as tactile and visual reaching
- *lesions = apraxia
Supplementary motor area (cortical motor area)
- planning of self-initiates mvmnts
- bimanual coordination (using 2 hands together)
- *lesions = apraxia
