Motor function

Question 1

Describe the role of the motor cortex

Answer 1

The motor cortex controls the descending pathways that activate motor neurones
However, it is not the site of voluntary initiation of movement; the decision to move comes from elsewhere in the brain

Question 2

What are the functions of descending motor pathways?

Answer 2

Control of voluntary movement
Modulate reflexes
Gamma bias

Question 3

Which centres of the brain send descending projections to the spinal cord to inhibit pain transmission? Which neurotransmitters are involved?

Answer 3

PAG and RVM centres

Noradrenaline and Serotonin

Question 4

Axons from which descending tract form the medullary pyramids?

Answer 4

The corticospinal tract

Question 5

Name the two major descending tracts

Answer 5

Corticospinal tract

Rubrospinal tract

Question 6

Which pathways control axial muscles? What movements are these pathways responsible for?

Answer 6

Medial pathways

Posture, balance, coarse control of movement

Question 7

Where do the corticospinal tracts originate?

Answer 7

In the motor cortex, areas 4 and 6

Question 8

Describe the course of the corticospinal tracts

Answer 8

Motor cortex
Internal capsule
Cerebral peduncle
Medullary pyramids
Caudal part of the medulla - this is where they decussate
Lateral corticospinal tract (contralateral side)
Synapse in ventral horn onto interneurones or a-motorneurones
Exit spinal cord via ventral root

Question 9

What forms the ventral corticospinal tract?

Answer 9

Axons from the corticospinal tract that do no decussate in the medulla (about 10%)
- these decussate in the spinal cord at the level at which they terminate

Question 10

What is the role of nerves in the rubrospinal tract?

Answer 10

Carry voluntary motor commands
Excites flexor muscles and inhibits extensor muscles
Can potentially act as an alternative tracts to the corticospinal tract

Question 11

Where does the rubrospinal tract originate? Where does it receive most of its input from?

Answer 11

The rubrospinal tract originates in the red nucleus of the midbrain and receives most of its input from the cerebellum (also receives some input from the motor cortex)

Question 12

Name the three ventromedial motor pathways

Answer 12

Vestibulospinal tracts
Tectospinal tracts
Reticulospinal tracts

Question 13

What is the main role of the ventromedial pathways?

Answer 13

Maintenance of posture and balance

Question 14

What is the role of the tectospinal tract?

Answer 14

Ensures that the eye remains stable as the body moves

this means that most fibres terminate in the upper cervical levels of the spinal cord

Question 15

What are the roles of the vestibulospinal tract?

Answer 15

Stabilise posture and head movements

Primarily controls extension of lower limbs (and upper limbs but to a lesser extent)

Question 16

What is the role of the reticulospinal tract?

Answer 16

Integration of sensory information into motor output:

• control trunk muscles to maintain balance and body position
• primarily controls flexion of the upper limbs (and lower limbs but to a lesser extent)

Question 17

What is the role of gamma motoneurones?

Answer 17

innervate intrafusal muscle fibres which helps to maintain the tautness of muscle spindles to keep them sensitive to changes in muscle length

Question 18

What is the role of alpha motoneurones?

Answer 18

innervate extrafusal muscle fibres, causing muscle contraction

• receive and integrate inputs from various arts of the brain and from sensory receptors
• release acetylcholine at the neuromuscular junction

Question 19

Name a type of cell that makes up the upper motor neurones

Answer 19

Pyramidal cells

Question 20

Describe the function(s) of the vestibular nuclei

Answer 20

Receives sensory input from the vestibulocochlear nerve (Cr N VIII) regarding head position.
Is the origin of the vestibulospinal tract, a descending motor pathway that controls extension of the lower (and upper) limbs.

Question 21

What sort of lesion causes decorticate rigidity? Describe the effects of such a lesion

Answer 21

A lesion in the cortex, above the level of the red nucleus in the midbrain.
Both the rubrospinal and vestibulospinal tracts are still intact, but cortical innervation is disrupted, so the patient tends to have flexed upper limbs and extended lower limbs.

Question 22

What sort of lesion causes a decerebrate rigidity? Describe the effects of such a lesion

Answer 22

A lesion below the level of the red nucleus but above the level of the vestibular nucleus (i.e. between the midbrain and the pons).
The rubrospinal tract is no longer active, so the upper limbs are no longer flexed.
The vestibulospinal tract however, is still intact, so both the upper and lower limbs are extended.

Question 23

What is the difference between the cortex and the cerebrum?

Answer 23

The cerebral cortex is the outer layer of gray matter of the cerebrum