Motor Pathways

Question 1

state the broad principles of motor control

Answer 1

Functional segregation and Hierarchical organisation

Question 2

describe functional segregation

Answer 2

Motor system organised in a number of different areas that control different aspects of movement

Question 3

describe Hierarchical organisation

Answer 3

high order areas of hierarchy are involved in more complex tasks (programme and decide on movements, coordinate muscle activity)

lower level areas of hierarchy perform lower level tasks (execution of movement)

Question 4

Motor system hierarchy

Answer 4

(association cortex)
1) Motor Cortex: Receives info from other cortical areas and sends commands to brainstem and thalamus
2) Brainstem and Basal ganglia adjust commands fro other parts of motor system
3) Brainstem passes commands from cortex to spinal cord
(see diagram)

Question 5

what is the location of Primary motor cortex

Answer 5

Primary motor cortex (M1):

Location: precentral gyrus, anterior to the central sulcus

Question 6

what is the function of Primary motor cortex

Answer 6

Function: control fine, discrete, precise voluntary movement

Provide descending signals to execute movement

Question 7

how is Somatotopical Organisation shown

Answer 7

This is called Penfield’s Motor Homunculus

In the motor cortex there is a representation of the muscles of different parts of the body

Question 8

how is increased muscle use represented in the cortex

Answer 8

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

The motor homunculus is very distorted because different parts of the body get used more than other parts

This is different in all of us - the cortical representation of the hand in a child is much smaller than that of a pianist

Question 9

what are types of corticospinal tract

Answer 9

anterior and lateral

They are both composed of upper motor neurones

Question 10

function of anterior corticospinal tract

Answer 10

The anterior tract is for axial UMNs

Axial: the muscles of the back and spine
Ie, UMNs supplying information to LMNs which will go on to innervate axial muscles

Question 11

function of lateral corticospinal tract

Answer 11

The lateral tract is for appendicular UMNs

Appendicular: Muscles of the limbs

Question 12

which neurones exit via the ventral root

Answer 12

Somatic and autonomic motor neurones

Question 13

what order of neuornes exist in the corticospinal tract

Answer 13

Upper motor neurone
Cell body in the motor cortex in lamina V, betz cells
Axons travel in the lateral/ ventral corticospinal tracts to synapse with LMN.

Lower motor neurone
Cell body in the ventral horn
Axon travels out of the ventral root

Question 14

describe the general path of upper and lower motor neurones before decussation

Answer 14

UMNs start in the motor cortex:

Path: corona radiata - internal capsule - peduncles at the midbrain

Question 15

describe lateral corticospinal tract path

Answer 15

decussate in medullary pyramids
descend contralaterally in the lateral corticospinal tract
synapse with contralateral LMNs in the ventral horn

Question 16

describe anterior corticospinal tract path

Answer 16

continue to descend ipsilaterally in the anterior corticospinal tract
decussate in the spinal cord at the anterior white commissure
Only then will they synapse with a LMN cell body in the ventral horn

Question 17

what percentage of neurones decussate in medulla and what type of neurones are they

Answer 17

90% (UMNs of lateral corticospinal tract)

10% that don’t cross over are of anterior corticospinal tract

Question 18

which other descending pathway is there and what does it do

Answer 18

Corticobulbar Tract - starts in the cortex, then exits and innervates the muscles in the face

Question 19

describe corticobulbar pathway

Answer 19

arise from the lateral aspect of the primary motor cortex - internal capsule - brainstem
neurones terminate on the motor nuclei of the cranial nerves (hypoglossal nucleus)
Here, they synapse with LMN (hypoglossal nerve), which carry motor signals to muscles of face and neck

Question 20

describe the location of the premotor cortex

Answer 20

frontal lobe anterior to M1

Question 21

describe the function of the premotor cortex

Answer 21

Function: planning of movements
Regulates externally cued movements
e.g. reaching out for an object:
moving a body part relative to another body part (intra-personal space)
movement of the body in the environment (extra-personal space)

Question 22

describe the location of the Supplementary motor area

Answer 22

frontal lobe anterior to M1, medially

Question 23

describe the function of the Supplementary motor area

Answer 23

Function: planning complex movements; programming sequencing of movements
Regulates internally driven movements (e.g. speech)
SMA becomes active when thinking about a movement before executing that movement

Question 24

briefly describe the association cortex and its constituants

Answer 24

Brain areas not strictly motor areas as their activity does not correlate with motor output/act
(Posterior parietal cortex and Prefrontal cortex)

Question 25

Role of posterior parietal cortex

Answer 25

ensures movements are targeted accurately to objects in external space
(part of association cortex)

Question 26

Role of posterior Prefrontal cortex

Answer 26

involved in selection of appropriate movements for a particular course of action
(part of association cortex)

Question 27

describe the timeline for Upper motor neuron lesion

Answer 27

Initially you get loss of function (‘negative signs’)

After a few weeks of having this lesion you will get increased abnormal motor function (‘positive signs’)

This is due to the loss of inhibitory descending inputs

Question 28

what are negative signs in UMN lesion

Answer 28

Paresis: graded weakness of movements

Paralysis (plegia): complete loss of muscle activity

Question 29

what are positive signs in UMN lesion

Answer 29

Spasticity: increased muscle tone
Hyper-reflexia: exaggerated reflexes
Clonus: abnormal oscillatory muscle contraction
Babinski’s sign

Question 30

what is Babinski’s sign

Answer 30

important sign of an upper motor lesion

If you stroke the plantar side of the foot, the toes will flex and the big toe will also flex (in a normal subject) but after upper motor neurone lesions the toes will fan and the big toe will go up

Also called the extensor plantar response

Question 31

what is Apraxia

Answer 31

A disorder in skilled movement NOT caused by weakness, abnormal tone or posture or movement disorders (tremors or chorea)

Patients are NOT paretic (partial motor paralysis) but have lost information about how to perform skilled movements

Question 32

what causes Apraxia

Answer 32

Lesion of inferior parietal lobe, the frontal lobe (premotor cortex, supplementary motor area)

Any disease of these areas can cause apraxia, although stroke and dementia are the most common causes

Question 33

what is a LMN lesion

Answer 33

Affects the second motor neurone (the one that starts in the grey matter of the spinal cord and exits to form peripheral nerves)

Lower motor neurone lesions have the opposite set of signs to upper motor neurone lesions

Question 34

what are signs of LMN lesion

Answer 34

Weakness 
Hypotonia (reduced muscle tone) 
Hyporeflexia (reduced reflexes)  
Muscle Atrophy - the metabolic trophic support lost  
Fasciculations  
Fibrillations

Question 35

what is a fasciculation

Answer 35

damaged motor units produce spontaneous action potentials, resulting in a visible twitch

Question 36

what is a fibrillation

Answer 36

twitch of individual muscle fibres - these aren’t visible to the naked eye but can be recorded if the patients have needle electromyography

Question 37

what is Motor Neuron Disease (MND)

Answer 37

Progressive neurodegenerative disorder of the motor system
Spectrum of disorders

MND can affect only UMNs, only LMNs or both

When MND affects both UMNs AND LMNs it is called Amyotrophic Lateral Sclerosis (ALS)

Question 38

Upper motor neuron signs of MND

Answer 38

Increased muscle tone (spasticity of limbs and tongue)
Brisk limbs and jaw reflexes 
Babinski’s sign
Loss of dexterity
Dysarthria
Dysphagia

Question 39

Lower motor neuron signs of MND

Answer 39

Weakness
Muscle wasting
Tongue fasciculations and wasting
Nasal speech
Dysphagia