Motor Pathways: Cortical Motor Function

Question 1

Describe the hierarchical organisation of motor control.

Answer 1

High order areas: involved in more complex tasks (programme + decide on movements, coordinate muscle activity)
Lower level areas: perform lower level tasks (execution of movement)

Question 2

What are the three parts of the motor cortex?

Answer 2

Primary Motor Cortex
Premotor Cortex
Supplementary Motor Area

Question 3

What are the two parts of the pyramidal (descending) system?

Answer 3

Corticospinal Tract: starts in the cortex + exits to innervate the muscles in the arms + legs
Corticobulbar Tract: starts in the cortex then exits to innervate muscles in the face

Question 4

Describe the side loops of the pyramidal system and their role.

Answer 4

2 side loops that go to the cerebellum + basal ganglia

Cerebellum + basal ganglia adjust the commands received from other parts of the motor system

Question 5

Which lobe are the three parts of the motor cortex found in? Describe their arrangement.

Answer 5

Frontal (anterior to the central sulcus)
Primary motor cortex is on the precentral gyrus
Premotor cortex + Supplementary motor area are anterior to the primary motor area

Question 6

What is another name given to the three parts of the motor cortex?

Answer 6

Primary Motor Cortex = Brodmann’s Area 4

Premotor + Supplementary Motor Area = Brodmann’s Area 6

Question 7

What are the most important cells in the primary motor cortex?

Answer 7

Betz Cells (pyramidal cells)

Question 8

Where are Betz cells located within the grey matter and which tracts originate from here?

Answer 8

5th layer of grey matter

Corticospinal tracts originate from here

Question 9

Describe what happens to the upper motor neurones that come from the primary motor cortex.

Answer 9

Travel through the brainstem to the pyramidal decussation in the medulla where 90% of the axons decussate.
These axons continue down the spinal cord + synapse with a lower motor neurone + exit into a peripheral nerve to the reach the skeletal muscle.
Pathway of the corticobulbar tract is somewhat similar – upper motor neurones go down into the brainstem + synapse with a lower motor neurone, then exit to muscles of the face

Question 10

What are the two types of descending pathway?

Answer 10

Lateral

Anterior

Question 11

Which tracts fall into each of these 2 types of descending pathway?

Answer 11

Lateral: Lateral corticospinal tract + Rubrospinal tract
Anterior: Anterior corticospinal tract, Reticulospinal tract, Vestibulospinal tract + Tectospinal tract

Question 12

What is the function of each of the 2 main descending motor pathways?

Answer 12

Lateral: Control of proximal + distal musculature, Voluntary movements or arms and legs
Anterior: Control of axial muscles, Balance + posture

Question 13

Describe the structure and function of the lateral corticospinal tract.

Answer 13

Originates in the primary motor cortex from the Betz cells.
Their axons pass down through the brainstem + decussate at the pyramidal decussation in the medulla.
It then passes down the spinal cord + synapses with a lower motor neurone.
It goes on to control mainly the distal musculature.

Question 14

Describe the structure and function of the anterior corticospinal tract.

Answer 14

Made up of upper motor neurone axons coming from the primary motor cortex that do not decussate at pyramidal decussation.
These fibres decussate at the level of the spinal cord
Controls proximal musculature.

Question 15

How can the cortical representation of a muscle in the motor cortex change?

Answer 15

The more we use a muscle, the bigger the representation of that muscle in the cortex.

Question 16

What is the function of the supplementary motor area?

Answer 16

Planning complex movements
Programming sequencing of movements
Regulates internally driven movements e.g. speech
Becomes active when thinking about movement before executing movement

Question 17

What are the two parts of the association cortex that are involved in motor control? State their functions.

Answer 17

Posterior Parietal Cortex: ensures movements are targeted accurately to objects in external space
Prefrontal Cortex: involved in the selection of appropriate movements for a particular course of action

Question 18

Describe the features of upper motor neurone lesions.

Answer 18

Loss of function (negative signs):
Paresis = graded weakness of movement
Paralysis = complete loss of muscle activity
Increased abnormal motor activity (positive signs) due to loss of inhibitory descending inputs:
Spasticity (increased muscle tone)
Hyperreflexia (exaggerated reflexes)
Clonus (abnormal oscillatory muscle contraction)
Babinski’s Sign

Question 19

What is Babinski’s Sign?

Answer 19

Stroke the plantar surface of the foot
Normal subject: flexion of toes (curl downwards)
Upper motor neurone lesions: Toes fan out + big toe lifts up.

Question 20

Define apraxia.

Answer 20

A disorder of skilled movement
Patients aren’t paretic but have lost information about how
to perform skilled movements

Question 21

Lesions in which part of the brain tend to cause apraxia?

Answer 21

Inferior parietal lobe  
Frontal lobe (premotor cortex + supplementary motor area)

Question 22

What are the two most common causes of apraxia?

Answer 22

Stroke

Dementia

Question 23

Describe 6 features of lower motor neurone lesions.

Answer 23

Hypotonia
Hyporeflexia
Weakness
Muscle Atrophy: metabolic trophic support to muscles is lost
FASCICULATIONS– damaged motor units produce spontaneous AP’s, resulting in a visible twitch
Fibrillations: spontaneous twitching of individual muscle fibres (seen on needle electromyography)

Question 24

What is motor neurone disease?

Answer 24

A progressive neurodegenerative disorder of the motor system– it is a spectrum of disorders.
MND can affect upper motor neurones, lower motor neurones or both

Question 25

What is the term given for upper AND lower motor neurone disease?

Answer 25

Amyotrophic Lateral Sclerosis (ALS)

Question 26

Describe how the symptoms of ALS change as the disease progresses.

Answer 26

Some patients may present with only upper motor lesion symptoms or only lower motor lesion symptoms but as the disorder progresses, both upper + lower motor neurone signs will be coexistent.

Question 27

Describe 6 upper + 5 lower motor neuron signs of MND

Answer 27

Increased muscle tone (spasticity in the limbs + tongue)
Brisk limb + jaw reflexes (hyperreflexia)
BABINSKI’s SIGN 
Loss of dexterity 
Dysarthria– difficulty speaking 
Dysphagia– difficulty swallowing
Tongue fasciculations + wasting
Nasal speech
Muscle wasting
Weakness
Dysphagia

Question 28

Which lower motor neurone controls the tongue?

Answer 28

Hypoglossal Nerve (CN XII)

Question 29

What is the function of the Primary Motor cortex?

Answer 29

Control fine, discrete, precise voluntary movement

Provide descending signals to execute movement

Question 30

Describe the organisation in the Primary Motor cortex. Thus, what is the consequence of an anterior artery stroke?

Answer 30

Somatotopic
Homunculus is inverted
Anterior Artery stroke more likely to affect lower limb

Question 31

What is the function of the premotor cortex?

Answer 31

Planning of movements

Regulates externally cued movements

Question 32

Where are lower motor neurones found?

Answer 32

Spinal cord

Brainstem

Question 33

Where are upper motor neurones found?

Answer 33

Corticospinal

Corticobulbar

Question 34

What constitutes the pyramidal area?

Answer 34

Corticospinal tracts

Corticobulbar tracts

Question 35

What constitutes the extrapyramidal area?

Answer 35

Basal ganglia

Cerebellum