L09 - Motor Control: Role of the Motor Cortex & Spinal Reflexes

Question 1

What are central pattern generators?

Answer 1

Central pattern generators are networks of neurones in the spinal cord which can generate specific learned movements such as chewing, walking etc without initiation from higher centres

*NB input from the brain can still superimpose on this activity

Question 2

Which fibres connect the ventral horn of the spinal cord to muscle fibres in the periphery?

Answer 2

⍺ motoneurones

Question 3

How are motoneurones of the descending pathways arranged in the spinal cord?

Answer 3

Motoneurones of the descending pathway are located in the ventral horn

• Ventromedial motoneurones in the ventral horn are the involuntary component responsible for the innervation of axial muscles and proximal muscles of the limbs
• Lateral motoneurones are the voluntary component responsible for the innervation of distal muscles (hands, feet and digits)

Question 4

List the lateral descending pathways.

Answer 4

Lateral descending pathways:

1 - Corticospinal tract

2 - Rubrospinal tract

Question 5

List the ventromedial descending pathways.

Answer 5

Ventromedial descending pathways:

1 - Tectospinal tract

2 - Vestibulospinal tract

3 - Pontine reticulospinal tract

4 - Medullary reticulospinal tract

Question 6

Give an example of a structure involved in the modulation of the lateral descending pathways.

Where is this structure located?

Answer 6

• The red nucleus is involved in the modulation of the lateral descending pathways
• The red nucleus is located in the medial tegmentum, dorsal to the substantia nigra

Question 7

List 3 structures involved in the modulation of ventromedial descending pathways.

Where are these structures located?

Answer 7

Structures involved in the modulation of ventromedial descending pathways:

1 - The superior colliculus (paired structures on the posterior midbrain)

2 - Vestibular nuclei (in the brainstem - nucleus of CN VIII)

3 - Reticular formation (in tegmentum surrounding aqueduct)

Question 8

Which tract is the main tract used for voluntary movement?

Answer 8

The corticospinal tract

Question 9

What would be expected from a lesion of the lateral pathways?

Answer 9

Slowed voluntary movements but unaffected posture (as posture is controlled by ventromedial pathways)

Question 10

How is loss of control of muscle contraction due to corticospinal tract damage compensated for?

Answer 10

The rubrospinal tract is able to compensate almost entirely for the loss of corticospinal tract function, with the exception of fine digit control

Question 11

Where do lateral descending pathways originate?

Where do ventromedial pathways originate?

Answer 11

• Lateral descending pathways originate in the motor cortex

- Ventromedial descending pathways originate in the brainstem nuclei

Question 12

Are the lateral descending pathways ipsilateral or contralateral?

Are the ventromedial descending pathways ipsilateral or contralateral?

Answer 12

• Lateral descending pathways are all contralateral

- Some ventromedial descending pathways are contralateral, whereas some are ipsilateral

Question 13

With which structures do the ventromedial pathways synapse?

Answer 13

1 - The pontine reticulospinal and medullary reticulospinal tracts synapse with the nuclei of the reticular formation

2 - The vestibulospinal and tectospinal tracts synapse with the superior colliculus and vestibular nuclei

Question 14

What are the functions of the pontine reticulospinal tract?

Answer 14

1 - It enhances anti-gravity reflexes of the spinal cord

2 - It facilitates the leg extensors to maintain standing posture

Question 15

What are the functions of the medullary reticulospinal tract?

Answer 15

1 - It frees anti-gravity muscles from reflex control

2 - It facilitates voluntary override

Question 16

What are the functions of the vestibulospinal tract?

Answer 16

1 - Relays gravitational sensory info from vestibular labyrinth in inner ear & stretch receptors in axial muscles, maintaining head & neck position with respect to gravity

2 - Helps maintain upright, balanced position by facilitating extensors in leg

Question 17

What is the function of the tectospinal tract?

Answer 17

Superior colliculus receives visual input from retina & visual cortex and the tract orientates head & eye movement to visual & auditory stimuli

Question 18

What are the sensorimotor areas of the cortex that constitute the motor cortex?

Answer 18

• Primary motor cortex
• Premotor cortex
• Supplementary motor cortex

Question 19

What is the function of area 4 of the motor cortex (primary motor cortex)?

Does area 4 have a high or low threshold for excitation?

Answer 19

• Control of distal musculature (fine motor control)

- Lowest stimulus threshold

Question 20

What is the function of area 6 of the motor cortex (supplementary and premotor areas)?

Does area 6 have a high or low threshold for excitation?

Answer 20

• Premotor cortex – control of proximal musculature (posture, balance), movement sequencing, & prep for movement & initiation
• Supplementary motor area – planning & initiation, & bimanual coordination (crosses to other side)

Question 21

What are Betz cells?

Answer 21

Primary motor cortical output neurones (upper motor neurones) with cell bodies in cortical layer V that send their axons down spinal cord via the corticospinal tract

Question 22

List the effects of damage to upper motoneurones.

Is this repairable?

Answer 22

• Initial muscle weakness
• Eventual spasticity (↑ resistance to passive movement)
• ↑ muscle tone (hypertonia)
• ↑ reflex responses (hyperreflexia)
• Affects contralat side to damage
• Recovery possible -> primary motor cortex circuitry shows adaptive alterations