5. Motor systems

Question 1

how do upper and lower motor neurones connect?

Answer 1

upper motor neurones descend through cord or brainstem and synapse onto lower motor neurones

Question 2

where are the cell bodies of lower motor neurones found?

Answer 2

in the ventral horn and in cranial nerve motor nuclei

Question 3

how are lower motor neurones activated?

Answer 3

by incoming pulses from sensory neurones that communicate with muscle spindles

Question 4

why do some spinal reflexes as a baby disappear?

Answer 4

due to the maturation of descending upper motor neurone pathway

Question 5

what signs can be seen in the muscle supplied by the lower motor neurone when that neurone is damaged?

Answer 5

weakness as no longer innervated
areflexia as no longer innervated
wasting due to loss of tropic support to the muscle from lower motor neurone across neuromuscular junction
hypotonia due to loss of muscle activation
fasciculation due to upregulation of nAChR’s to try compensate for loss of innervation

Question 6

where are upper motor neurones found?

Answer 6

in the primary motor cortex

Question 7

how do upper neurones communicate with lower motor neurones?

Answer 7

synapse onto LMN’s directly in ventral horn or cranial nerve motor nuclei

Question 8

what is the net effect of upper motor neurones on lower motor neurones?

Answer 8

inhibition

Question 9

how do upper motor neurones’ descend from the motor cortex?

Answer 9

through corona radiata, internal capsule, cerebral peduncle in midbrain, pons, medullary pyramids, lateral corticospinal tract, ventral horn and then synapse (either directly or via inhibitory interneurones on LMN’s)

Question 10

how is the facial motor nucleus different?

Answer 10

1. facial motor nucleus is a special case as it splits into two halves, one supplies the superior face and one the inferior face
2. the part of the facial motor nucleus that supplies the upper hlaf of the face receives upper motor neurones from both hemispheres wheras lower only recieves a contralateral ipper motor neurone input
3. hence upper motor neurone lesions involving the face will spare the forehead

Question 11

what signs are shown when the upper motor neurones are damaged?

Answer 11

weakness (loss of direct excitary inputs onto lmns from umns)
hypertonia due to loss of descending inhibition as inhibition is the net effect of umns on lmns
hyperreflexia due to overactive reflex arc
extensor plantar reflexes due to loss of descending modulation of spinal reflexes

Question 12

what is spinal shock?

Answer 12

following an upper motor neurone lesion. initially, flaccid paralysis with a areflexia but then tone increases and reflexes become exaggerated (mechanism not known)

Question 13

what is the internal capsule?

Answer 13

white matter pathway connecting the cerebral hemispheres with the rest of the CNS

Question 14

what is superior to the internal capsule?

Answer 14

corona radiata

Question 15

what is inferior to the internal capsule?

Answer 15

cerebral peduncle of the midbrain

Question 16

what does the internal capsule contain?

Answer 16

descending axons of upper motor neurones and ascending axons of third order sensory neurones

Question 17

what are the 3 anatomical divisions of the internal capsule?

Answer 17

anterior limb -contains axons connecting motor cortex with cerebellum
genu - is a bend which contains axons of upper motor neurones supplying face
posterior limb
posterior limb - contains axons of upper motor neurones supplying limb, trunk and lower (in that order ant to posteriorly) and third order sensory axons connecting thalamus to post central gyrus (face, arm, trunk, leg from front to back)

Question 18

what does the lateral corticospinal tract supply?

Answer 18

distal muscles

Question 19

why is lateral CST most important?

Answer 19

at decussation of pyramids, most axons decussate to form the lateral CST, those that remain ipsilateral and descend int he ventral funiculus as the ventral CST (they then decussate at level of target LMN)

Question 20

what does the ventral corticospinal tract supply?

Answer 20

proximal and trunk muscles