Motor control

Question 1

What is the main route via which the motor cortex communicates with muscles of the body?

Answer 1

Corticospinal tract

Question 2

What is the main route by which the motor cortex communicates with muscles of the head and neck?

Answer 2

Corticobulbar tract

Question 3

What types of cells project from the motor cortex into the corticospinal tract and from which layer?

Answer 3

Pyramidal cells (layer V)

Question 4

What are the structures of the brain that fibres of the corticospinal tract run through before reaching the spinal cord?

Answer 4

1. Internal capsule (forebrain)
2. Cerebral peduncles (midbrain)
3. Medullary pyramids (ventral medulla, hindbrain)

Question 5

What happens to the fibres of the CST before they enter the spinal cord?

Answer 5

• ~90% cross to contralateral side and descend in lateral corticospinal tract
• ~10% don’t cross and remain on ipsilateral side to descend in the ventral corticospinal tract

Question 6

What is the structure of the corticobulbar tract?

Answer 6

Fibres descend with the fibres of the corticospinal tract, but exit the brainstem at an appropriate level to innervate the motor nuclei of the cranial nerves (V, VII, IX, X, XI, XII).

Question 7

What is the location of M1?

Answer 7

• Frontal lobe
• Anterior to central sulcus
• Brodmann’s area 4

Question 8

What are the location of the secondary motor areas?

Answer 8

• Brodmann’s area 6

Question 9

How can M1 be distinguished from other parts of the frontal lobe?

Answer 9

It contains very few granule cells, so is classified as agranular cortex.

Question 10

What are the features of the upper midbrain?

Answer 10

1. Cerebral peduncles: Contains descending motor fibres from the internal capsule.
2. Substantia nigra
3. Red nucleus
4. Cerebral aqueduct
5. Superior colliculus

Question 11

What is the significance of the pons in terms of behaviour of motor neurones as they pass through?

Answer 11

Many fibres from the cerebral peduncles project into the cerebellum. Consequently, the medullary pyramids are narrower than the cerebral peduncles.

Question 12

What is the nature of connectivity of CST with spinal cord neurones?

Answer 12

• Indirect: CST neurones synapse onto spinal cord interneurones that the synapse onto motor neurones.
• Direct: CST neurones synpase directly onto motor neurones (cortico-motoneuronal connections), especially those controlling intrinsic muscles of hands and feet (may explain manual dexterity).

Question 13

What are the features of the mid-medulla?

Answer 13

1. Medullary pyramids
2. Dorsal column nuclei
3. Trigeminal nuclei
4. Medial lemniscus

Question 14

What are the features of the lower medulla?

Answer 14

1. Medullary pyramids
2. Motor decussation
3. Trigeminal nuclei
4. Dorsal column

Question 15

What are the locations of the ventral and lateral corticospinal tracts?

Answer 15

Question 16

What are the additional descending pathways descending from the brain into spinal cord?

Answer 16

1. Vestibulospinal tract: Arises from the vestibular nuclei and descend in the ventral spinal cord.
2. Reticulospinal fibres: Arises from the reticular formation of the brainstem.
3. Rubrospinal tract: Originates from the red nucleus (receives input form cerebral cortex and cerebellum) of the midbrain and projects mainly into the inferior olivary nucleus, from which cerebellar climbing fibres arise.

Question 17

What are the functions of the vestibulospinal tract?

Answer 17

Fibres control antigravity extensors of the lower limb and proximal muscles (trunk).

Question 18

What are the functions of the reticulospinal tract?

Answer 18

Fibres are involved with controlling stereotyped motor activity (e.g. walking, chewing) and other essential functions (e.g. breathing, heart rate).

Question 19

What are the functions of the rubrospinal tract?

Answer 19

May be involved in motor learning.

Question 20

What are the features of the cerebellum?

Answer 20

1. Vermis: Midline part of the cerebellum, between the 2 cerebellar hemispheres.
2. Cerebellar hemispheres: Lateral to the vermis.
3. Middle cerebellar peduncle: Connects the cerebellum to the pons of brainstem, containing fibres from pons projecting into cerebellum.
4. Flocculus: Semi-detached part of the cerebellum located on the ventral aspects (cerebello-pontine angle). Concerned with vestibular function.
5. Tonsils: Part of the cerebellum overhanging the dorsolateral aspects of the medulla.

Question 21

What are the deep cerebellar nuclei?

Answer 21

1. Dentate nucleus
2. Nucleus interpositus (globose + emboliform)
3. Fastigial nucleus

Question 22

What is special about the output from the flocculus of the cerebellum?

Answer 22

Projects directly into the vestibular nuclei without synapsing in a deep cerebellar nuclei

Question 23

What are the cerebellar peduncles?

Answer 23

1. Superior cerebellar peduncles: Output from deep nuclei of cerebellum to rest of the brain.
2. Middle cerebellar peduncles: Input to cerebellum from pons.
3. Inferior cerebellar peduncles: Input to cerebellum from medulla.

Question 24

What is the pathway associated with the cerebellar inputs from the cortex (via middle cerebellar peduncle)?

Answer 24

Crossed pathway

Question 25

What is the function of inputs into the cerebellum from the cortex?

Answer 25

Fibres originate form sensory/sensory association areas and relay information concerning vision, somatosensation, sound…). Probably allows integration of sensory input with motor output.

Question 26

What is the pathway associated with the cerebellar inputs from the body?

Answer 26

Uncrossed pathway

Question 27

What is the function of inputs into the cerebellum from the body?

Answer 27

Carries sensory information (mainly proprioception) from the body to the cerebellum.

Question 28

What is the pathway associated with the cerebellar inputs from the inferior olive?

Answer 28

Question 29

What is the function of inputs into the cerebellum from the inferior olive?

Answer 29

Important in motor learning and is thought to mediate plasticity in mossy fibre – granule cell – Purkinje cell pathway.

Question 30

What is the pathway associated with the cerebellar outputs?

Answer 30

Deep cerebellar nuclei ↓ Superior cerebellar peduncle ↓ Lower midbrain ↓ (decussation) ↓ Upper midbrain ↓ Red nucleus ↓ Thalamus: - Ventrolateral nucleus → M1 - Ventromedial nucleus → Secondary motor areas

Question 31

What is the nature of cerebellar inputs and outputs into/from the cortex in terms of representation of body?

Answer 31

- Cerebellar hemispheres represent ipsilateral sides of the body - Cortex respresents contralateral sides, so input fibres from the cortex into the cerebellum are crossed. - Outputs from the cerebellum to the cortex are also crossed.

Question 32

What are the layers of the cerebellum?

Answer 32

1. Molecular layer: Low cell density. Composed of mostly granule cell axons, running parallel to each other. Perpendicular to these are the Purkinje cell axons. Also in this layer are basket and stellate cells. 2. Purkinje cell layer: Composed of exactly 1 layer of Purkinje cell bodies. 3. Granular layer: Contains large numbers (~ 50 billion) granule cells and a few golgi cells.

Question 33

What is the structure of the cerebellar network?

Answer 33

1. As the parallel fibres of the granule cells run in the molecular layer, they contact the axons of the Purkinje cells, making contact with 1/3-5 they pass, making total of ~80 synaptic connections with Purkinje cells. 2. Each Purkinje cell makes contact with ~250000 parallel fibres. 3. All fibres synapsing onto granule cells are mossy fibres from the middle/inferior cerebellar peduncles. 4. Granule cells synapse onto Purkinje cell dendrites via excitatory glutaminergic synapses. 5. The Purkinje cells are the only cells that send fibres out of the cerebellar cortex. They synapse with neurones in the deep cerebellar nuclei via inhibitory GABAinergic synapses.

Question 34

What are the upper motor neurones?

Answer 34

- Corticospinal tract - Corticobulbar tract

Question 35

What are the lower motor neurones?

Answer 35

Spinal cord motor neurones

Question 36

What is the functional significance of uncrossed fibres in ventral corticospinal tract?

Answer 36

They do not innervate muscles of ipsilateral limbs (fibres cross to innervate contralateral limb at level of spinal cord)