Motor Control - Role of the Motor Cortex and Spinal Reflexes

Question 1

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

Answer 1

Alpha motoneurones.

Question 2

What are central pattern generators?

Answer 2

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 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

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

Where is this structure located?

Answer 4

• 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 5

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

Where are these structures located?

Answer 5

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 6

List the lateral descending pathways.

Answer 6

Lateral descending pathways:

1 - Corticospinal tract.

2 - Rubrospinal tract.

*The corticobulbar isn’t listed here because it doesn’t enter the spinal cord.

Question 7

List the ventromedial descending pathways.

Answer 7

Ventromedial descending pathways:

1 - Tectospinal tract.

2 - Vestibulospinal tract.

3 - Pontine reticulospinal tract.

4 - Medullary reticulospinal tract.

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 to 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

Which pathways constitute the pyramidal tract?

Which pathways constitute the extrapyramidal pathways?

Answer 13

• The lateral descending pathways (but not the rubrospinal tract) constitute the pyramidal tract (so basically just the corticospinal and corticobulbar tracts).
• The ventromedial descending pathways, together with the rubrospinal tract, constitute the extrapyramidal tract.

Question 14

List the structures with which the ventromedial pathways synapse.

Answer 14

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 15

List 2 functions of the pontine reticulospinal tract.

Answer 15

1 - It enhances antigravity reflexes of the spinal cord.

2 - It facilitates the leg extensors to maintain standing posture.

Question 16

List 2 functions of the medullary reticulospinal tract.

Answer 16

1 - It frees antigravity muscles from reflex control.

2 - It facilitates voluntary override.

Question 17

What is the function of the vestibulospinal tract?

Answer 17

It maintains head, neck and leg position by relaying gravitational sensory information from the vestibular labyrinth of the inner ear and stretch receptors in axial muscles to other muscles of the body.

Question 18

What is the function of the tectospinal tract?

Answer 18

It orientates the head and eyes to visual and auditory stimuli by relating visual sensory information from the retina and visual cortex.

Question 19

How many sensorimotor areas are there?

Answer 19

7.

Question 20

List the sensorimotor areas of the cortex that constitute the motor cortex.

Answer 20

Sensorimotor areas that constitute the motor cortex include:

1 - Area 4, including the primary motor cortex (M1).

2 - Area 6, including the supplementary and premotor areas.

Question 21

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 21

• Area 4 is involved in fine motor control of distal muscles.
• Area 4 has a relatively low threshold for excitation.

Question 22

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 22

• The frontomedial area is the supplementary motor area and has a role in:

1 - Planning.

2 - Initiation.

3 - Bimanual coordination of movement.

• The lateral area is the premotor area and has a role in:

1 - Posture and balance through control of axial muscles.

2 - Control of movement sequencing.

3 - Initiation of movement.

• Area 6 has a relatively high threshold for excitation.

Question 23

What are Betz cells?

Answer 23

Upper motoneurones with cell bodies originating in area 5 of the sensorimotor cortex that constitute the upper motoneurones of the corticospinal tract.

Question 24

List 2 effects of stimulation of individual upper motoneurones.

Answer 24

1 - Movement of a small group of muscles in a particular direction.

2 - Movement of a small group of muscles with a particular force.

*The group of muscles affected is in accordance to the somatotopic organisation.

Question 25

List the effects of damage to upper motoneurones.

Is this repairable?

Answer 25

1 - Initial muscle weakness affecting the contralateral side.

2 - Eventual spasticity as the spinal cord takes over, and can only perform simple tasks such as reflexes.

• Recovery is possible because the primary motor cortex circuitry shows adaptive alterations.

Question 26

Define spasticity.

List 2 characteristics of spasticity.

Answer 26

• Resistance to passive movement.

1 - Hypertonia.

2 - Hyperreflexia.