Hierarchy of Motor Control

Question 1

What is the motor system?

Answer 1

The means by which we interact with our environment

Putting voluntary desires into action

Question 2

What are lower motor neurones?

What will lesions cause?

Answer 2

Spinal and cranial nerves that directly innervate muscles

Flaccid paralysis and muscle atrophy

Question 3

What are upper motor neurones?

What will lesions cause?

Answer 3

Neurons in the brain and spinal cord that change a-motorneuron activity

Spasticity
Some paralysis (although may be transient)

Question 4

What is the difference between lesions in many posture regulating pathways compared with corticospinal lesions?

Answer 4

Spastic paralysis

Weakness (paresis), rather than paralysis

Question 5

What tracts make up pyramidal tracts?

Answer 5

Made up of both corticospinal and corticobulbar tracts

Question 6

What are corticobulbar tracts?

Answer 6

Two neuron white matter motor tract carrying fibres to the cranial nerves

Question 7

What are corticospinal tracts?

Answer 7

Conducts signals from the brain to the spinal cord.

Made up of lateral corticospinal tract and anterior corticospinal tracts

Question 8

How do the lateral and anterior corticospinal tracts differ in function?

Answer 8

Lateral corticospinal tract controls the fine movements of limbs of the ipsilateral side

Anterior corticospinal tract controls more central axial and girdle muscles

Question 9

What happens when corticobulbar tract nuclei are damaged?

Answer 9

Paresis

Question 10

What is paresis?

Answer 10

Slight paralysis (weakness of voluntary movement)

Question 11

What is direct control of muscles?

Answer 11

a-motorneurones in the ventral horn of the spinal cord

Question 12

Where do segmental reflexes occur?

Answer 12

Spinal cord level

Question 13

Segmental reflexes are subject to further control.

What does this, what are these reflexes called and what do they do?

Answer 13

Brainstem nuclei

Higher order reflexes

Control of posture and balance

Question 14

What is the vestibulospinal tract?

Answer 14

The vestibulospinal tract is a neural tract in the central nervous system.
It is a component of the extrapyramidal system.
The vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head.
The nuclei relay motor commands through the vestibulospinal tract.
The function of these motors commands are to alter muscle tone, extend, and change the position of the limbs and head to supporting posture and maintaining balance

Question 15

What muscle groups are involved in the control of balance and posture?
What is their input?

Answer 15

Primarily those of the trunk and limbs.

Descending input via a number of tracts of which the vestibulospinal and reticulospinal tracts are very important

In turn the brainstem nuclei are controlled by higher brain centres

Question 16

What are the higher brain centres involved with motor control?

Answer 16

Cerebral cortex (motor cortex and association areas, premotor and supplementary motor areas)

Basal ganglia

Cerebellum

Question 17

How do the higher brain centres depend on each other for motor control?

Answer 17

Functionally interdependent but control different aspects of the voluntary movement process

Question 18

As well as receiving descending input via brainstem, there is also a direct cortical connection to the spinal cord.
What is this called?

Answer 18

Corticospinal tract (pyramidal tract)
Motor cortex to spine

Question 19

What do we make movements in response to?

Answer 19

Visual, olefactory, auditory, emotional and intellectual cues

Question 20

Sensory input is vital for the motor system.

How and why is it vital?

Answer 20

It comes in at all levels:

• The spinal cord in the form or proprioceptors, touch etc
• The brainstem in the form of the vestibular system
• At the level of the cortex

We make movements based on all sensory input: Visual, olfactory, auditory etc

Damage to sensory inputs (spinal level) causes paralysis as if the motoneurons themselves had been damaged

Question 21

Loss of all proprioception can basically paralyse someone as there is no feedback for movements as to where limb position is.
How can you overcome this?

Answer 21

You can train your brain to use your visual system to provide feedback on where the parts of the body are.

Absolute requirement for the lights to be on.

Question 22

What is the most dominant sensory system we have?

Answer 22

Visual system

Question 23

What is the simplest segmental reflex?

Answer 23

The stretch reflex

Question 24

Describe the stretch reflex

Answer 24

Stimulus: blow to tendon stretches muscle

Receptor: Muscle spindle stretches and fires

Afferent path: Action potential passes through sensory neuron

Integrating centre: Sensory neurone synapses in spinal cord.

Efferent path 1:
Somatic motor neurone -> quadriceps muscle -> quadricep contracts, swinging leg

Efferent path 2:
Interneuron inhabiting somatic motor neuron -> hamstring muscle -> hamstring stays relaxed allowing extension of leg (reciprocal inhibition)

Question 25

What muscles are the stretch reflex present?

Answer 25

All skeletal muscles in the body

Question 26

How can the stretch reflex be influenced by higher centres?

Answer 26

1. Directly onto the a-motorneurone
   If inhibit powerfully by descending voluntary input can override the stretch reflex.
2. By acting on interneurones, influencing the balance between agonist and antagonist
3. By acting on y-motoneurones to change the sensitivity of the spindles (exclusively controlled by brain -> not involved in stretch reflex)

Question 27

How does the corticospinal tract generally operate?

How does this process work?

Answer 27

a-y coactivation

The a motoneurons are activated to make the desired movement and y-motoneurones are activated at the same time to maintain sensitivity of the spindles to muscle length changes = important information needed by the brain

Question 28

What crucial cord segments are you testing when you test for each of the reflexes?

Answer 28

Biceps jerk = C6
Triceps jerk = C7
Patellar tendon = L4
Achilles tendon reflex = S1

Question 29

What is the significance of testing for reflexes?

Answer 29

Helps to detect level of spinal cord damage. Ability to evoke reflexes above but not below level of damage.

Impaired reflexes above but not below level of damage e.g. in diabetes, patients may lose their ankle jerk reflexes

(Patients must be distracted to avoid voluntary influencing of responses

Question 30

What is diabetic neuropathy?

Answer 30

At least 50% of patients

Nerve starts to die back from muscle to cell body.
Can monitor the progress using stretch reflex tests

Question 31

What is a flexor reflex?

Answer 31

Withdrawal reflex + crossed extension

Withdrawal of ipsilateral limb, shift of balance to contralateral limb to maintain balance

Basic spinal cord circuitry has been hijacked and refined in voluntary movements such as walking and running, i.e. input does not have to be PAIN

Question 32

When may flexor reflexes be evoked by mild cutaneous stimulation?

Answer 32

In certain sensitive parts of the body (sole of the foot)

Question 33

Stroking the lateral aspect of the sole of the foot should illicit what in healthy individuals?
What does this response depend on?

Answer 33

Plantar flexion, mainly downward curling of the big toe

Subtleties of balance between flexor and extensor reflexes.
Only works properly when CS tract input is intact and functioning normally

Question 34

If damage or disruption of the corticospinal tract occurs what clinical sign will you see when you stroke the lateral aspect of the sole of the foot?

Where else may you see this?

Answer 34

Extension of the big toe rather than flexion
Toes fan up and out.
This is called the Babinski sign

As well as in pathology, seen in children

Question 35

What occurs after transection of the spine?

Answer 35

Sensory and ANS effects

Loss of sensation and bowel
Bladder and sexual control

There is also a period of spinal shock when reflexes cannot be evoked. Lasts 2-6 weeks in man

Question 36

What is spinal shock and what occurs after it?

Answer 36

Spinal shock is an effect occurring after spinal transection where there is a period of 2-6 weeks where reflexes cannot be evoked

Cause is uncertain, clearly loss of supra spinal excitatory influences.
Gradual return of reflexes, not stereotyped.

Eventually reflexes may become exaggerated to the point that powerful withdrawal reflexes of the whole limb can be evoked by light touch.

Extensor reflexes may also be so exaggerated that body weight can be briefly supported = a positive supporting reaction

Question 37

What may be a cause of hyperactivity of reflexes?

Answer 37

Collateral sprouting which increases the effectiveness of segmental input as well as increased sensitivity to remaining excitatory neurotransmitters

Question 38

Explain collateral sprouting

Answer 38

Spinal cord is transected

Central neurones die back from a-motoneuron vacating an area of postsynaptic membrane

Segmental inputs send collaterals to occupy the vacant post synaptic membrane

Increased effectiveness of segmental input + decreased higher centre input -> exaggerated reflex response

Question 39

What makes up most of the human cerebral cortex?

Answer 39

Association cortex which is involved with complex processing.

Question 40

What is brodman’s area 4?

Answer 40

Precentral gyrus = main motor area in the cerebral cortex.

It lies right next to the primary somatosensory area with which it has very strong connections.

Question 41

What do cortical cells give rise to?

Answer 41

Corticospinal projections to a-y motoneurons

Question 42

What percentage of corticospinal tract neurones make direct rapid contact?
What happens to the other axons?

Answer 42

10%

Other axons end on interneurones

Question 43

Where do collaterals from the corticospinal tract go?

Answer 43

Pass to many other brain areas

Provide control mechanisms and sensory system with update of commands being sent to the motoneurons

Question 44

Describe the major crossing over of the corticospinal tract in the brainstem

Answer 44

Decussation of corticospinal tract fibres in medulla (80% of axons)

This creates a major crossed lateral tract and a minor uncrossed ventral (anterior) tract

Question 45

What is a motor homunculus and what is its significance?

Answer 45

The largest part of the body, the trunk, has a very small area of cortex devoted to it.
The hands and face, particularly the mouth and tongue have considerable cortical representation.

The hands and face are involved with movements which require high level of precision control e.g. manual dexterity, facial expression and speech
By contrast, the muscles of the trunk and limbs are mainly used for gross movements and postural adjustment, much of their control is by brainstem nuclei.

Motor cortex is involved in fine precision voluntary movements. Therefore damage to the MC/CS -> CLUMSINESS and primarily distal involvement

Question 46

Is the motor cortex involved in the initiation of movement?

Answer 46

NO.
When you measure brain activity using EEG before a voluntary movement (“thinking about it) -> there is an increase in electrical activity in all cortical areas and some subcortical ones.

This is the readiness potential.

(motor cortex just gets the muscles to move after you have thought about it)

Question 47

How long does the readiness potential last?

Answer 47

Its duration depends on the complexity of the motor task

e.g. 2 sec for speech.
This is why its easy to say stuff you don’t mean (think before you say)

About 50ms before the movement actually occurs, there is an extra burst of electrical activity in the area of the motor cortex which controls the muscles needed for the desired movement = signals being sent down the CS tract to activate the correct motoneurones

Question 48

Describe the association cortex

Answer 48

The association cortex is subdivided into a number of areas, some of which are assigned specific motor functions.

Include the PREMOTOR CORTEX which is responsible for identifying goals and choosing a course of action and the SUPPLEMENTARY MOTOR AREA, which is involved in programming voluntary movements.

Question 49

What is the mental rehearsal of complex motor tasks (e.g. playing a piano piece) associated with?

Answer 49

Increased electrical activity and increased blood flow in the supplementary motor area

Question 50

Why is the association cortex so complex?

Answer 50

There are very strong portico-cortical projections throughout the cerebral cortex

Movements are made in response to multiple complex cues.
There are very strong connections with the frontal cortex, meaning, memory intellect and the posterior parietal cortex.

Question 51

How does the brain cope with all the sensory information it receives?

Answer 51

Although the brain craves information, a lot of the time there is simply too much information to deal with, so it filters out the unimportant stuff.

Very important processing for attention goes on in the parietal cortex.

The POSTERIOR PARIETAL CORTEX seems to be essential for the processing of sensory stimuli leading to purposeful movements

Question 52

What do lesions of the posterior parietal cortex cause?

Answer 52

Neglect Syndrome

An inability to respond to stimuli on the contralateral side of the body
Patients may deny that the contralateral arm or leg is theirs when brought passively into the field of view.