Motor Pathways: Cortical Motor Function

Question 1

What is volition?

Answer 1

Motor systems produce movements that are adaptive and accomplish a certain goal

Question 2

Describe the hierarchical organisation of motor control.

Answer 2

Association Cortex
Motor Cortex
Brainstem
Spinal Cord

Question 3

What are postural adjustments and unconscious processing?

Answer 3

Postural Adjustments – the motor system has to compensate for changes in the body’s centre of mass during movements
Unconscious Processing – many of the postural adjustments occur without our awareness

Question 4

What are the three parts of the motor cortex?

Answer 4

Primary Motor Cortex
Premotor Cortex
Supplementary Motor Area

Question 5

What makes up the association cortex?

Answer 5

Frontal Cortex
Parietal Cortex
NOTE: this is not exactly part of the motor pathway but it influences the planning and execution of movements

Question 6

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

Answer 6

Corticospinal Tract – starts in the cortex and exits and innervates the muscles in the arms and legs
Corticobulbar Tract – starts in the cortex then exits and innervates the muscles in the face

Question 7

Describe the side loops of this descending pathway and their role.

Answer 7

The descending pathway also has two side loops that go to the cerebellum and basal ganglia
The cerebellum and basal ganglia check the motor information before ittravels to the muscles and has its effect

Question 8

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

Answer 8

Frontal (anterior to the central sulcus)
The primary motor cortex is on the precentral gyrus
The premotor cortex and the supplementary motor area are anterior to the primary motor area

Question 9

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

Answer 9

Primary Motor Cortex = Broadmann’s Area 4

Premotor + Supplementary Motor Area = Broadmann’s Area 6

Question 10

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

Answer 10

Betz Cells (pyramidal cells)

Question 11

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

Answer 11

They are found in the 5th layer of grey matter

The corticospinal tracts originate from here

Question 12

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

Answer 12

They travel through the brainstem to the pyramidal decussation in themedulla where 90% of the axons cross the midline.
These axons continue down the spinal cord and synapse with a lower motor neurone and exit into a peripheral nerve to the reach the skeletal muscle.
The pathway of the corticobulbar tract is somewhat similar – upper motor neurones go down into the brainstem and synapse with a lower motor neurone and they exit to the muscles of the face

Question 13

What are the two types of descending pathway?

Answer 13

Lateral and Medial

Question 14

Which tracts fall into each of these types?

Answer 14

Lateral 
 Lateral corticospinal tract
 Rubrospinal tract
Medial  
 Anterior corticospinal tract
 Reticulospinal tract
 Vestibulospinal tract
 Tectospinal tract

Question 15

What is the function of each of these types of pathway?

Answer 15

Lateral  
 Control of proximal and distal musculature
 Voluntary movements or arms and legs
Medial 
 Control of axial muscles 
 Balance and posture

Question 16

Describe the structure and function of the lateral corticospinal tract.

Answer 16

The lateral corticospinal tract originates in the primary motor cortex from the Betz cells.
Their axons pass down through the brainstem and decussate at the pyramidal decussation in the medulla.
It then passes down the spinal cord and synapses with a lower motor neurone.
It goes onto control mainly the distal musculature.
NOTE: 90% of axons from the primary motor cortex decussate at the medulla (these are the lateral corticospinal tract axons). The 10% that don’t decussate form the anterior corticospinal tract.

Question 17

Where does the rubrospinal tract originate?

Answer 17

Red nucleus of the midbrain

Question 18

What is the function of the rubrospinal tract?

Answer 18

It is an alternative pathway that allows voluntary motor commands to be sent down the spinal cord meaning that the body can compensate for a lesion in the primary motor cortex.
It also has a role in movement velocity.

Question 19

Describe the structure and function of the vestibulospinal tract.

Answer 19

The lateral vestibulospinal tract originates at the lateral vestibular nucleus.
The medial vestibulospinal tract originates at the medial vestibular nucleus.
They mediate postural adjustments and head and eye movements

Question 20

Describe the structure and function of the reticulospinal tract.

Answer 20

It originates in the reticular formation in the brainstem then goes down the spinal cord to innervate muscle.  
It is involved in complex actions: 
 Orienting 
 Stretching 
 Maintaining a complex posture

Question 21

Describe the structure and function of the anterior corticospinal tract.

Answer 21

The anterior corticospinal tract is made up of the upper motor neurone axons coming from the primary motor cortex that do not decussate at the pyramidal decussation.
These fibres cross the midline at the level of the spinal cord It controls proximal musculature.

Question 22

What diagram depicts the somatotopic organisation of the primary motor cortex?

Answer 22

Penfield’s Motor Homunculus

Question 23

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

Answer 23

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

Question 24

What is the function of the premotor cortex?

Answer 24

Plans movements and assembles movements into coordinated actions
NOTE: premotor cortex is anterior to the primary motor cortex

Question 25

What is the function of the supplementary motor area?

Answer 25

Planning complex internally driven voluntary movements e.g. speech
It also becomes active when you are thinking about movement before movement (e.g. rehearsing a dance)

Question 26

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

Answer 26

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 27

Describe the features of upper motor neurone lesions.

Answer 27

Initially you get loss of function of the motor neurones leading to:
 Paresis = graded weakness of movement
 Paralysis = complete loss of muscle activity
After a few weeks, the loss of descending inhibitory pathways leads to increased abnormal motor activity such as:
 Spasticity (increased muscle tone)
 Hyperreflexia (exaggerated reflexes)
 Clonus (abnormal oscillatory muscle contraction)
 Babinski’s Sign
 NO muscle atrophy

Question 28

What is Babinski’s Sign?

Answer 28

You stroke the plantar surface of the foot and in a normal subject you will see flexion of the toes (they curl downwards)
In the case of upper motor neurone lesions, the patient will show an EXTENSOR PLANTAR RESPONSE where their toes fan out and their big toe lifts up.

Question 29

Why is muscle atrophy not seen in upper motor neurone lesions?

Answer 29

The lower motor neurones are still in tact and they have a role inproviding nutrients to the muscle.
There will still be partial atrophy due to muscle disuse.

Question 30

Define apraxia.

Answer 30

A disorder of skilled movement not caused by weakness, abnormal tone or posture or movement disorders (tremors or chorea).
It is caused by the loss of information on how to perform skilled tasks rather than loss of motor command to the muscles.

Question 31

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

Answer 31

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

Question 32

What are the two most common causes of apraxia?

Answer 32

Stroke and Dementia

Question 33

Describe the features of lower motor neurone lesions.

Answer 33

It is generally the opposite of upper motor neurone lesions.
 Hypotonia
 Hyporeflexia
 Weakness
 Muscle Atrophy– the metabolic trophic support to the muscles is lost
 FASCICULATIONS– damages motor units produce spontaneous action potentials, resulting in a visible twitch
 Fibrillations – twitch of individual muscle fibres (aren’t visible to the naked eye but are picked up on EMG)

Question 34

What is motor neurone disease?

Answer 34

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 35

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

Answer 35

Amyotrophic Lateral Sclerosis (ALS)

Question 36

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

Answer 36

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

Question 37

List some signs of ALS.

Answer 37

 Increased muscle tone (spasticity in the limbs and tongue)
 Brisk limb and jaw reflexes (hyperreflexia)
 BABINSKI’s SIGN
 Loss of dexterity
 Dyarthria – difficulty speaking
 Dysphagia – difficulty swallowing

Question 38

Which lower motor neurone controls the tongue?

Answer 38

Hypoglossal Nerve (CN XII)

Question 39

What might you see in the tongue of an MND patient?

Answer 39

Fasciculations and spasticity