5. UMN And LMN Flashcards
Where are lower motor neurones cell bodies found?
Ventral horn (anywhere down spinal cord) and in frail nerve motor nuclei
What are lower motor neurones usually activated by?
Incoming impulses from sensory neurones that communicate with muscle spindles, but can also be inhibited
Why do primitive spinal reflexes in baby’s disappear?
Disappear as baby grows due to maturation of descending upper motor neurone pathways
What signs can be seen in muscles when lower motor neurones are damaged?
Weakness Areflexia Wasting Hypotonia Fasciculation
Where are upper motor neurones found?
Primary motor cortex which then synapse onto LMN directly (or indirectly) in the ventral horn or cranial nerve motor nuclei
What is the net effect of UMN on LMN?
Inhibition
Where do the UMN axons descend from the motor cortex through?
Corona radiata Internal capsule Cerebral peduncle in midbrain Pons Medullary pyramids Decussation of the pyramids Lateral corticospinal tract Ventral horn Synapse on LMN
What do UMNs that supply facial structures leave the pathway in brainstem to form and what does it innervate?
Corticobulbar tract, innervates LMNs in cranial nerve motor nuclei
How is the facial nerve motor nucleus different to other cranial nerve nuclei?
It is split in 2 halves
One supplies the superior face and one the inferior face
Part that supplies upper half of face receives UMNs from both hemispheres
Part that supplies lower face only receives a contralateral UMN input
Therefore UMN lesions involving face will spare forehead
What are the signs when UMNs are damaged?
Weakness (due to loss of direct excitatory inputs onto LMNs)
Hypertonia (due to loss of descending inhibition)
Hyperreflexia
Extensor plantar reflexes (due to loss of descending modulation of spinal reflexes)
What is spinal shock?
Phenomenon that occurs in the days immediately following a UMN lesion
Initially there is a flaccid paralysis with areflexia but then tone increases and reflexes become exaggerated
