Motor system: upper motor neuron circuits

Question 1

What are the two main structures grouping upper motor neurons? What are they responsible for?

Answer 1

• Motor and premotor areas of the frontal lobe -> planning, initiating and directing movements
• Brainstem -> postural control

Question 2

In the spinal cord, how is organized white matter from motor cortex? From brainstem? Why does this organization make sense?

Answer 2

• Laterally -> skillful voluntary movements

- Medially -> posture and balance

Question 3

Are the upper motor neurons from the cerebral cortex mostly crossing the midline? From brainstem?

Answer 3

• Yes 90% -> mostly contralateral

- No 50% -> bilateral

Question 4

Cortical motor areas (primary motor cortex and premotor cortex) receive inputs mainly from which structures?

Answer 4

• Basal ganglia
• Cerebellum
• Parietal lobe

Question 5

Where is located the primary motor cortex?

Answer 5

Precentral gyrus

Question 6

Why can we say that the primary sensory cortex requires a low threshold to elicit movements?

Answer 6

Because of its almost direct connections to some alpha motor neurons (also attest for dexterity).

Question 7

Where are found Betz cells?

Answer 7

Only primary motor cortex.

Question 8

What are the Betz cells?

Answer 8

• Neurons with largest soma in CNS

- Project directly from primary sensory cortex to alpha motor neurons in spinal cord

Question 9

Why are the descending motor cortical axons not passing into the thalamus?

Answer 9

To maximize the efficiency of the motor descending pathways from the cortex.

Question 10

What are the two descending cortical tracts? Where are they terminating?

Answer 10

• Corticobulbar tract → terminate in brainstem

- Corticospinal tract → terminate in spinal cord

Question 11

The corticospinal tract splits further into two tracts. What are they? What is their % distribution? What are their respective roles?

Answer 11

Lateral corticospinal tract (90%) ->synapse in lateral ventral horn of spinal cord (serving distal muscles)
Ventral corticospinal tract (10%) ->serves proximal and axial muscles

Question 12

The collaterals of the corticobulbar tract innervating multiple nuclei of the brainstem are unilateral or bilateral?

Answer 12

Bilateral.

Question 13

What type of organization do we found in the primary motor cortex situated in the precentral gyrus?

Answer 13

A topographic map of movement.

Question 14

What does account for the idea that movements are encoded, instead of specific muscle contractions?

Answer 14

Intracortical microstimulation combined with recordings of muscle showed that small currents can excite several muscles.

Question 15

How is the direction of movement encoded?

Answer 15

In a “population vector” which represents the integration of a population of individual upper motor neuron’s preferred directions.

Question 16

True or false: premotor cortex is also projecting to primary motor cortex.

Answer 16

True (indirect influence on motor behaviour).

Question 17

What are the two divisions of premotor cortex? How are they activated? What are their respective neurons encoding?

Answer 17

Lateral division (closed-loop):

• Motor outputs initiated by a sensory input
• Neurons encode intention to move

Medial division (open-loop):

• Motor outputs initiated by no external stimuli
• Neurons encode selection and initiation of movements

Question 18

Where are found the mirror motor neurons? How do they work? What are they suggesting?

Answer 18

They are found in the lateral division of the premotor cortex (closed-loop).
Mirror motor neurons respond upon observed (or known) familiar movements executed by another individual.
The mirror motor neurons suggest that the premotor cortex plays a role in encoding observed actions.

Question 19

What are 3 (4) descending motor pathways from the brainstem? What each of them is responsible for?

Answer 19

1. Vestibular nuclei -> mediating reflexes responding to a disturbance and reflexes responsible for balance and posture
2. Reticular formation ->responsible for feedforward adjustments
3. Superior colliculus ->responsible for control of axial muscles in neck
   (4. ) Red nucleus -> responsible for arm and hand movements in primates

Question 20

What are the two tracts descending from the vestibular nuclei? Are they innervating bilaterally or unilaterally? What are their respective roles? What are they responding to?

Answer 20

Medial vestibulospinal tract:

• Bilateral
• Mediating feedback or responding to a disturbance
• Respond to vestibular semicircular canals

Lateral vestibulospinal tract:

• Unilateral (ipsilateral)
• Involved in proximal muscles responses for balance and upright posture
• Respond to vestibular otolith organs

Question 21

True or false: feedforward adjustments programmed by the reticular formation follow the primary movements encoded by the cortex.

Answer 21

False: precede the primary movements encoded by the cortex.

Question 22

How are the neurons descending from the reticular formation reaching the spinal cord? What part of the spinal cord are they innervating?

Answer 22

Descend through reticulospinal tract to innervate medial ventral horn.

Question 23

Why does the superior colliculus input to the reticulospinal tract?

Answer 23

To control axial muscles in the neck and because it has functions in head orientation.

Question 24

Where are the projections of the red nucleus terminating?

Answer 24

Lateral regions of ventral horn and intermediate zone (make sense with function bc. they are responsible for arm and hand movements in primates).