Motor I- Corticospinal System

Question 1

what are voluntary movements?

Answer 1

motor pathways descend from cerebral cortex: corticospinal and corticonuclear

Question 2

what are supraspinal reflexes?

Answer 2

motor pathways descend from brainstem: extrapyramidal pathways

Question 3

where is the prefrontal cortex located?

Answer 3

most rostral part of the frontal lobe; > half of the frontal lobe

Question 4

where is the supplementary motor cortex (SMC) located?

Answer 4

located just rostral to the precentral sulcus, but mostly on medial aspect of the hemispheres (superior frontal gyrus)

Question 5

where is the premotor cortex (PMC) located?

Answer 5

located just rostral to precentral sulcus; only on lateral surface of hemispheres (posterior portion of middle and inferior frontal gyri)

Question 6

where is the primary motor cortex located?

Answer 6

located in the precentral gyrus, which is the most caudal part of the frontal lobe, just rostral to the central sulcus

Question 7

what is the function of the supplementary motor cortex?

Answer 7

contributes to learning sequence of movements and bilateral coordination; stores skilled (know-how) memory; contributes to mental rehearsal of movements

Question 8

what is the function of the premotor cortex?

Answer 8

integrates sensory information into motor plans (sensorimotor transformation); anticipates voluntary movement, hence coordinates on-going movements

Question 9

what is apraxia?

Answer 9

lesion of premotor and supplementary motor cortices; it is the inability to execute a voluntary motor movement despite being able to demonstrate normal muscle function- it also includes inability to imitate a movement

Question 10

what is the function of the primary motor cortex?

Answer 10

execution of movements/motor output; it is somatotopically organised as the motor homunculus

Question 11

what is the cortical origin of the corticospinal pathways?

Answer 11

40% from the primary motor cortex, 40% from the SMC and PMC, 20% from the primary sensory cortex (post central) of the parietal lobe

Question 12

what does a lesion in the primary motor cortex lead to?

Answer 12

paresis (muscle weakness)

Question 13

what does a lesion in the premotor cortices (SMC + PMC) lead to?

Answer 13

apraxia (lack of skilled movements)

Question 14

what does a lesion in the primary sensory cortex (post central gyrus) of the parietal lobe lead to?

Answer 14

leads to degeneration (disturbance) of motor actions

Question 15

describe the pathway of the corticospinal tract.

Answer 15

if majority of the fibres come from the precentral gyrus they go through the posterior limb of the internal capsule

• > if they are corticonuclear fibres they will be the very anterior of the posterior limb or near the genu. they will go to the different motor nuclei
• > corticospinal will be posterior in posterior limb. they will continue to descend through the crus cerebi (mid portion). they get split up a bit as they go through the pons and then descend through the medulla to the pyramid of medulla
• > 90% of the fibres will decussate and cross and descend in the lateral funiculus of spinal cord (lateral corticospinal tract). 10% remain ipsilateral and will descend in the anterior column= anterior corticospinal tract

Question 16

describe the pathway of the lateral corticospinal tract, what does it innervate and what is its function?

Answer 16

pathway:
- most fibres originate from the motor cortex and terminate in the ventral horn of the spinal cord
- 90% of the corticospinal tract neurons
- terminates at the cervical/thoracic and lumbosacral levels to innervate upper and lower limbs
innervates:
- limbs (distal more than proximal) therefore, it targets a motoneurons that feed into the brachial and lumbosacral plexuses
functions:
- primarily concerened w/ precise movements involving distal parts of the limbs (mostly hand)

Question 17

describe the pathway of the anterior corticospinal tract and what does it innervate?

Answer 17

pathway:
- 10% of the corticospinal tract
- does not decussate at the pyramidal decussation
- decussates at different spinal cord levels
innervates:
- innervates mainly axial (neck and trunk) muscles, therefore it synapses with a motoneurons that are located at spinal levels C1-C4 and T2-T12

Question 18

what does lesions in the upper motoneurons cause?

Answer 18

interrupts the descending influences on the LMN (low motoneurons)

Question 19

what does lesions in the lower motoneurons cause?

Answer 19

interrupts the motor input to muscles

Question 20

what are some diseases affecting the motor system?

Answer 20

• poliomyelitis (neurons of the anterior horn are specifically affect by polio virus)
• motor neuron diseases (ALS- fatal, progressive neurodegenerative disease, degeneration of upper and lower motoneurons includes upper and lower motoneurons, leads to muscles weakness and atrophy)