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 – these are upper motor neurons that start in layer V of cortex and exits and synapses with lower motor neuron in the spinal cord.

Corticobulbar Tract – these are upper motor neurons that start in the cortex then synapses with lower motor fibre that is part of a cranial nerve

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 it travels to the muscles and has its effect

elevator descends from roof to bottom, but in the middle it has to stop to get checked before it reaches the bottom

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 are the most important cells in the primary motor cortex?

Answer 9

Betz Cells (pyramidal cells located in layer V of motor cortex)

Question 10

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

Answer 10

They are found in the 5th layer of grey matter

The corticospinal tracts originate from here

Question 11

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

Answer 11

They travel through the brainstem to the pyramidal decussation in the medulla where 90% of the axons cross the midline (remaining 10% do not cross and are the anterior corticospinal tract that controls postural muscles)

These axons continue down the spinal cord and synapse with a lower motor neurone which innervates 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 (part of cranial nerve).

Question 12

Describe the structure and function of the lateral corticospinal tract.

Answer 12

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 13

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

Answer 13

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

Question 14

What is the function of the premotor cortex?

Answer 14

Premotor cortex is part of the motor cortex but is non-primary motor cortex.
It plans movements and assembles movements into coordinated actions
NOTE: premotor cortex is anterior to the primary motor cortex

Question 15

What is the function of the supplementary motor area?

Answer 15

this is also a non-primary motor cortex 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 16

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

Answer 16

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 17

Describe the features of upper motor neurone lesions.

Answer 17

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 18

What is Babinski’s Sign?

Answer 18

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 19

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

Answer 19

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 20

Define apraxia.

Answer 20

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 21

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

Answer 21

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

Question 22

What are the two most common causes of apraxia?

Answer 22

Stroke and Dementia

Question 23

Describe the features of lower motor neurone lesions.

Answer 23

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 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 and lower motor neurone signs will be coexistent.

Question 27

List some signs of ALS.

Answer 27

 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 28

Which lower motor neurone controls the tongue?

Answer 28

Hypoglossal Nerve (CN XII)

Question 29

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

Answer 29

Fasciculations and spasticity