A06 Topography of the Major Motor Systems

Question 1

Where are the cell bodies of lower motoneurones?

Answer 1

• In the ventral grey horn of the spinal cord.

or

• In the motor nuclei of cranial nerves.

Question 2

List 9 components of the brain responsible for the execution of properly coordinated movements of muscle.

Answer 2

1 - Cerebral cortex.

2 - Corpus striatum.

3 - Thalamus.

4 - Red nucleus.

5 - Substantia nigra.

6 - Inferior olivary nucleus.

7 - Cerebellum.

8 - Vestibular nuclei.

9 - The reticular formation.

Question 3

List the components of the corpus striatum.

Answer 3

The corpus striatum is composed of:

1 - Caudate nucleus.

2 - Lentiform nucleus, which contains:

• Putamen.
• Globus pallidus.

Question 4

From which areas of the cerebral cortex do the corticospinal and corticobulbar tracts arise?

Which type of pathways are contained within these tracts?

Where do the remainder of the fibres forming the corticospinal and corticobulbar tracts arise?

Answer 4

• The corticospinal and corticobulbar tracts arise from the sensorimotor cortex around the central sulcus (aspects of both the sensory and motor cortices).
• These tracts carry descending pathways.
• The remainder of the fibres originate in the:

1 - Premotor area.

2 - Supplementary motor area.

3 - Cingulate motor area.

Question 5

Where is the premotor area?

What is the function of neurones originating here?

Answer 5

• Anterior to the primary motor cortex in the precentral gyrus.
• Neurones originating here do not initiate impulses to voluntary muscles, but act as suppressors, preventing them from over-discharging in responding reflexively to sensory stimuli.

Question 6

What are the medullary pyramids?

What is the functional implication of distinguishing pyramidal tracts from extrapyramidal tracts?

What marks the lower limit of the medullary pyramids?

Answer 6

• Medullary pyramids are paired white matter structures of the medulla oblongata that contain motor fibers of the corticospinal and corticobulbar tracts.
• Pyramidal tracts (corticospinal and corticobulbar) are involved in controlling voluntary movements, whereas extrapyramidal tracts (rubro, vestibulo, reticulo and tectospinal) are involved in controlling involuntary movements.
• The lower limit of the pyramids is marked by the decussation of fibres of the tracts, and is where the medulla oblongata becomes the spinal cord.

Question 7

Which muscles are innervated by the corticospinal and corticobulbar tracts?

Answer 7

• The corticospinal tract innervates skeletal muscles of the limbs and trunk.
• The corticobulbar tract innervates muscles of the head and neck.

Question 8

What type of neurones are the lower motoneurones of the corticobulbar tract?

Answer 8

Cranial nerves.

Question 9

Give an example of a function of the corticospinal and corticobulbar tracts other than initiation of movement.

Answer 9

The tracts also modulate movement:

• Some descending fibres terminate in the thalamus and brainstem.
• They then send information to the sensory centres which is used to monitor and modify ongoing movement.

Question 10

List 2 roles of the cerebellum in control of movement.

Answer 10

1 - It is involved in learned and therefore repeated movements.

2 - Coordination of afferent and efferent information to provide balance and enable smooth movements.

Question 11

What type of sensory information is received by the cerebellum?

Answer 11

The cerebellum receives proprioceptive information from muscle spindles and golgi tendon organs.

Question 12

What are Renshaw cells?

What is their function?

Answer 12

• Renshaw cells are cells in the spinal cord that act as inhibitory interneurones between afferent fibres and motoneurones in stretch and deep tendon reflexes.
• They act as a feedback mechanism to prevent overactivity.

Question 13

List 4 descending tracts other than the corticospinal and corticobulbar tracts.

What type of descending tracts are these?

Answer 13

1 - Vestibulospinal tracts.

2 - Reticulospinal tracts.

3 - Rubrospinal tracts.

4 - Tectospinal tracts.

• These are subcortical (and extrapyramidal) descending tracts.

Question 14

What is the function of gamma motoneurones?

Answer 14

• They are the neurones with which the descending pathways influence the sensory receptors that respond to muscle stretch.
• They are therefore involved in the modulation of sensitivity to muscle stretch.

Question 15

What type of reflex are the stretch and deep tendon reflexes?

Answer 15

Stretch and deep tendon reflexes are spinal reflexes.

Question 16

Briefly describe the route taken by axons of the upper motoneurones of the lateral corticospinal tract.

How does the route differ for ventral corticospinal fibres?

Answer 16

• The lateral corticospinal tracts leave the cortex and enter the internal capsule.
• They then traverse the midbrain in the crus cerebri of the cerebral peduncles.
• They then pass through the basilar pons, where most of the fibres decussate to form the pyramids on the ventral surface of the medulla oblongata.
• The fibres that don’t cross in the pyramidal decussation form the ventral corticospinal tract.

Question 17

List the segments of the internal capsule.

Which tracts run through these segments?

Answer 17

1 - Anterior limb (frontopontine).

2 - Genu (corticobulbar).

3 - Posterior limb (corticospinal).

Question 18

If the genu of the internal capsule is damaged, which tracts, and ultimately which muscles, will be affected?

Answer 18

• Lesions of the genu affect fibres of the corticobulbar tract.
• It therefore affects muscles of the head and neck.

Question 19

Summarise the origin, function and decussation of the corticospinal tract.

Answer 19

Corticospinal tract:

• Its origin is the pyramidal (Betz) cells in the sensorimotor areas around the central gyrus.
• Its function is initiation of movement in the limbs (lateral corticospinal tract) and trunk muscles (ventral corticospinal tract).
• The lateral corticospinal tract decussates at the medulla.
• The ventral corticospinal tract decussates at the level of the second order neurone in the spinal cord.

Question 20

Summarise the origin, function and decussation of the reticulospinal tract.

Answer 20

Reticulospinal tract:

• Its origin is in the pons (pontine division) and the medulla (medullary division)
• Its function is in exciting (pontine) and inhibiting (medullary) the antigravity muscles to control locomotion or posture.
• It either descends ipsilaterally (pontine division) or decussates partially (descends bilaterally) at the level of the medulla (medullary division).

Question 21

Summarise the origin, function and decussation of the tectospinal tract.

Answer 21

Tectospinal tract:

• Its origin is the superior colliculus of the midbrain.
• Its function is coordination of head and eye movements.
• It decussates at the dorsal tegmental decussation of the midbrain.

Question 22

Summarise the origin, function and decussation of the rubrospinal tract.

Which fibres supply the origin of the rubrospinal tract with afferent fibres?

Answer 22

Rubrospinal tract:

• Its origin is the red nucleus of the tegmentum of the midbrain.
• The efferents of the cerebellum and motor cortex supply the red nucleus with afferent fibres (these inputs are integrated to produce an output).
• Its function is tonal innervation of flexors and to take over the function of the corticospinal tract if the corticospinal tract fails.
• It decussates at the level of origin in the midbrain.

Question 23

Summarise the origin, function and decussation of the vestibulospinal tract.

Answer 23

Vestibulospinal tract:

• Its origin is the medial vestibular nucleus in the medulla oblongata and and lateral vestibular nucleus in the pons.
• The vestibular nuclei receive input from the vestibular nerve and the fastigial nucleus of the cerebellum.
• Its function is control of balance by modulation of trunk muscles and muscles of the limbs.
• Neither tract decussates. They remain ipsilateral.

Question 24

Summarise the origin, function and decussation of the corticobulbar tract.

Answer 24

Corticobulbar tract:

• Its origin is the motor cortex of the precentral gyrus.
• Its function is initiation of movement of muscles in the head and neck.
• Some fibres decussate and some do not so that there is bilateral innervation of the cranial nerves from corticobulbar fibres on both sides.

Question 25

List 4 movements associated with damage to the basal nuclei.

Briefly describe each movement.

Answer 25

1 - Chorea (abnormal quick movements of the hands and feet).

2 - Hemiballism (violent movements of ipsilateral limbs).

3 - Athetosis (writhing movements of the hands).

4 - Dystonia (sustained / repetitive muscle contractions).

Question 26

Which component of the CNS is damaged in parkinson’s disease?

Answer 26

The substantia nigra is damaged in Parkinson’s disease.

Question 27

Describe the course of the corticospinal tract.

Answer 27

• Upper motoneurones originate from sensorimotor areas around the precentral gyrus, and are known as pyramidal (Betz) cells.
• Upper motoneurones leave the cerebrum through the corona radiata and internal capsule.
• They then enter the crus cerebri of the cerebral peduncles of the midbrain.
• They then pass through the ventral portion of the pons to reach the medulla oblongata, where they form two prominent columns known as pyramids.
• In the medulla, the majority of fibres decussate and enter the contralateral lateral corticospinal tract, and the rest remain ipsilateral and enter the ventral corticospinal tract.
• The upper motoneurones of the lateral corticospinal tract descend and synapse with lower motoneurones innervating muscles of the limbs.
• The upper motoneurones of the ventral corticospinal tract descend, decussate at the level of synapse, and synapse with lower motoneurones innervating muscles of the trunk.

Question 28

Describe the course of the rubrospinal tract.

Answer 28

• Upper motoneurones originate from the red nucleus of the tegmentum of the midbrain, and receive afferent input from the cerebellum.
• The upper motoneurones course ventromedially and decussate in the central tegmental decussation of the midbrain.
• They then descend to the spinal cord, where they synapse with lower motoneurones.

Question 29

Which tract is damaged with a positive Babinski reflex?

Answer 29

The corticospinal tract is damaged with a positive babinski reflex.