Descending Motor Systems

Question 1

What are the three types of movements? Which ones are stereotypical and repetitive?

Answer 1

1. Reflexes: stereotypical, involuntary. Do not require descending control.
2. Rhythmic movement (walking, breathing): stereotypical, repetitive. Require descending control. Subcortical descending control is sufficient but can be subjected to cortical descending control
3. Directed movements: complex (writing,reaching). Not stereotypical or repetitive. Require continuous voluntary descending control

Question 2

What are the three control systems of the motor system?

Answer 2

1. Cerebral cortex: planning, initiation, execution; olfactory and visual inputs
2. Brainstem: initiation, preparation, execution; auditory and vestibular inputs
3. Spinal cord: integration, execution; cutaneous and proprioceptive inputs

Question 3

what are the two tuning systems?

Answer 3

1. Basal ganglia: selection of motor task, reward-based learning
2. Cerebellum: timing of motor program sequence, error-based learning

Question 4

what are the two types of descending pathways based on origin?

Answer 4

1. (cortex, direct to spinal cord) Corticospinal and corticobulbar
2. (brainstem, indirect via brainstem to spinal cord, SUBCORTICAL DESCENDING PATHWAY)
   red nucleus:rubrospinal
   tectum or colliculi: tectospinal
   vestibular nuclei: vestibulospinal
   medial reticular formation: reticulospinal

Question 5

What are the two descending pathways based on their terminal territory in the spinal cord? What is their function

Answer 5

1. Dorsolateral (control of distal musculature): lrubrospinal
2. Ventromedial (control of axial and proximal musculature): tectospinal, reticulospinal, vestibulospinal

Question 6

What is the function of the corticospinal and corticobulbar pathways? Describe path. (not in the lecture)

Answer 6

Volunary control of movement

Corticospinal: M1 > cerebral peduncle > pontine fibers > pyramid > decussation (most) > (mostly) lateral corticospinal tract

Corticobulbar: M1 > cerebral peduncle > pontine fibers > cranial nerve nuclei

Question 7

what are four strategies to view spinal cord connectivity? (not in the lecture)

Answer 7

1. Transynaptic viral tracers
2. Calcium recording
3. Photostimulation
4. Optogenetics

Question 8

what will a bilateral lesion to the pyramids do to a monkey doing intricate hand work? (not in the lecture)

Answer 8

Lesion affects lateral corticospinal tract so monkey will not be able to do voluntary control of fine movement.

Question 9

example exam questions

Answer 9

1. Describe three characteristics of the rubrospinal pathway and explain how they provide insights into the function of this pathway?
2. Explain why, on the basis of the lesion method alone, it is difficult to infer the function of one descending pathway?

Question 10

the meaning of parallel processing

Answer 10

parallel processing between cerebral cortex and spinal cord;

overlap provides safety and allows for at least partial recovery of function after injury

Question 11

role of rubrospinal pathway?

Answer 11

rubrospinal neuron receive inputs from the cerebral cortex and the deep cerebellar nuclei;
control mostly opposite side of the body;
projects throughout the spinal cord but most intense in lower cervical and upper thoracic segments;
role in skilled limb movements;
role in sensory guidance of skilled limb movements

Question 12

role of tectospinal pathway?

Answer 12

receive inputs from visual and auditory/somatosensory inputs;
purely crossed;
project to upper cervical segments which contains neck motoneurons;
control of head movements

Question 13

role of vestibulospinal pathway?

Answer 13

receive inputs from vestibular organs and cerebellum;
lateral vestibular nucleus > lateral vestibulospinal pathway;
medial vestibular nucleus > medial vestibulospinal pathway;
maintain and stabilize (posture and balance) the body and the head in an upright position

Question 14

role of reticulospinal pathway?

Answer 14

receive inputs from motor cortical areas, cerebellum, superior colliculus, vestibular nuclei and spinal cord;
postural preparation and coordinates the different part of the body during movement;
involved in reaching, grasping, locomotion

Question 15

modulatory actions

Answer 15

5-HT, NA, dopamine, histamine

1. slow acting
   non-synaptic actions via volume transmission;
   synaptic actions via G-protein activation
2. facilitate motor activity
   alter general excitability of spinal neurons;
   readiness to respond

Question 16

different anatomical states of axons at the injury site

Answer 16

spared
demyelinated
cut

myelin debris: releases chemicals that inhibit axon
glial scar

Question 17

strategies for promoting the regenerative potential of descending axons

Answer 17

1. promoting of regrowth of cut axons by:
bypassing the scar with cellular bridge;
preventing inhibition by myelin debris;
promoting axonal regeneration with nerve growth factors
2. promoting remyelination by:
stem cells
3. promoting axonal sprouting by:
neurotrophic factors;
training (activation of residual axons)