Motor Control

Question 1

What are the 2 broad principles of motor control?

Answer 1

1. Hierarchical organisation - higher order areas = higher order tasks, lower order areas = lower order tasks
2. Functional segregation - motor system divided into different areas that control different aspects

Question 2

How is voluntary and automatic movement brought about?

What is the difference between upper and lower motor neurons?

Answer 2

Interaction between the 2 principles of control

Upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body

Question 3

What are the 2 major descending tracts from the brain?

Where do they pass through and what are their functions?

Answer 3

1. Pyramidal - pass through the pyramids in the medulla, output neurons in the motor cortex to spinal cord / cranial nerve nuclei in the brainstem. Voluntary movement of body and face.
2. Extrapyramidal - do not pass through the pyramids of the medulla, begin in the brainstem and project to the spinal cord, where they synapse. They project to muscles for involuntary actions e.g. balance, posture

Question 4

Where is the primary motor cortex located and what is its function?

Answer 4

Located in the pre-central gyrus

Controls fine and discrete movements, via the spinal cord

Question 5

Where is the pre-motor cortex located and what is its function?

Answer 5

Located just anterior to the primary motor cortex

Planning of movements - regulated externally cued movements e.g. reaching out to grab something

Question 6

Where is the supplementary motor area located and what is its function?

Answer 6

Located anterior and medial to primary motor cortex
Becomes active prior to voluntary movements occurring
Complex movements that are internally driven e.g. speech

Question 7

What are the names of the 2 pyramidal tracts?

Answer 7

Corticospinal and corticobulbar

Question 8

What is the corticospinal tract?

Where it is located, what is its structure and what does it innervate?

Answer 8

Upper motor neurons in the primary motor cortex, cerebral peduncles in the midbrain, and medulla located further down
Vast (90%) crossing over of the upper motor neurons to the other side of the body in the medulla
These then synapse onto lower motor neurons in the spinal cord

Crossed over neurons make up lateral corticospinal tract = limb muscles
Non-crossed over neurons make up anterior corticospinal tract = trunk muscles
This tract contains upper and lower motor neurons

Question 9

What is meant by somatotopic control?

Answer 9

Certain region of the brain corresponds to controlling a certain region of the body

Question 10

What is the corticobulbar tract?

What is its function? What is its structure and what does it innervate?

Answer 10

Provides voluntary movements of the face

Primary motor cortex = upper motor neurons
These synapse with brainstem nuclei onto the lower motor neurons of the cranial nerves that go out to control movements of the face e.g. eyes, jaw

i.e. trigeminal nulei = muscles of mastication, oculomotor and abducens and trochlear nuclei = eyes, facial nuclei = facial expression, hypoglossal nuclei = tongue

This tract contains upper and lower motor neurons

Question 11

What are the 4 extrapyramidal tracts and what are their functions?

Answer 11

Vestibulospinal = originates in lateral vestibular nucleus (Deiters nucleus), projects to ipsilateral motor neurons in spinal cord. Stabilising head, moving eyes within head to keep vision fixed on the retina, coordinates head and eye movements, mediates postural adjustment

Reticulospinal = most primitive descending tract - from medulla (medial reticulospinal) and pons (lateral reticulospinal); changed in muscle tone associated with voluntary movement, postural stability

Tectospinal = originates from superior colliculus in the midbrain and projects to cervical spinal cord. Orientation of head and neck during eye movements

Rubrospinal = from red nucleus in the midbrain to lateral cervical spinal cord. Seen active in primates but in humans mainly taken over by the corticospinal tract (does come into play when there are lesions in the CNS, leads to posture and balance disorders)

Question 12

What occurs if there is a lesion in the upper motor neuron?

HINT: negative and positive symptoms, what is the difference

Answer 12

Both signs = bad: negative = loss of activity, positive = extra unwanted activity

Negative signs: Loss of voluntary motor function e.g. paresis (graded weakness of movements) or paralysis (complete loss of voluntary muscle activity)

Positive signs: Increased abnormal motor function due to loss of inhibitory descending input, spasticity = increased muscle tone, hyper-reflexia (exaggerated reflexes), babinski’s sign

Question 13

What is apraxia?

Answer 13

Disorder of a skilled movement - consequence of a lesion e.g. stroke, dementia

Dysfunction in the supplementary motor area

Patients are not paralysed, but lose the ability to perform skilled movements

Question 14

What occurs if there is a lesion in the lower motor neurons?

Answer 14

Weakness
Hypotonia (reduced muscle tone)
Hyporeflexia (reduced reflexes)
Muscle atrophy (muscle wasting)
Fasciculations: damaged motor units produce spontaneous action potentials, results in a visible (to the naked eye) twitch
Fibrillations: spontaneous twitching of individual muscle fibres, not visible to the naked eye, seen during EMG testing

Question 15

What is Motor Neuron Disease?

AKA Amyotrophic Lateral Sclerosis [ALS]

Answer 15

Affects upper and lower motor neurons

Progressive neuro-degeneration of the motor system - results in death, no cure

Issues with voluntary contraction of the tongue, upper and lower limbs etc.

Issues with respiratiory musces e.g. intercostal muscles

Question 16

What are the upper motor neuron signs and lower motor neuron signs in MND?

Answer 16

Upper motor neuron signs: 
Spasticity (increased tone of limbs and tongue)
Brisk limbs and jaw reflexes 
Babinski’s sign
Loss of dexterity
Dysarthria (difficulty speaking)
Dysphagia (difficulty swallowing)

Lower motor neuron signs: 
Muscle atrophy
Weakness in the muscles
Twitching (fasciculations) and wasting of tongue muscles 
Dysphagia (difficulty swallowing)
Nasal speech

Question 17

What is the basal ganglia and what makes up the basal ganglia?

Answer 17

A group of subcortical nuclei responsible primarily for motor control

Globus pallidus (external and internal)
Striatum - made up of the caudate and putamen nuclei
Substantia nigra (midbrain)

Question 18

What is the function of the basal ganglia?

Answer 18

Decision to move (intention)

Elaboration associated movements (e.g. swinging arms when walking, facial expressions to match emotions)

Moderating and coordinating movement (suppressing unwanted movements)

Performing movements in order

Question 19

What are some disorders associated with the basal ganglia?

Answer 19

e.g. Parkinson’s and Huntington’s disease and Ballism

Parkinsons = associated with substantia nigra (midbrain)

Question 20

What is the order of the basal ganglia structures: caudate nucleus, putamen, external globus pallidus and thalamus in a coronal section from anterior to posterior?

Answer 20

Caudate nucleus, putamen, external globus pallidus and thalamus

Question 21

What is the basal ganglia circuitry?

Answer 21

Cortex communicates with the striatum

Striatum then communicates with the substantia nigra, internal globus pallidus and external globus pallitus

Question 22

What is Parkinson’s disease?

How does it present clinically?

Answer 22

Degeneration of the dopaminergic neurons that originate from the sunstantia nigra and project to the striatum

Bradykinesia - affects ability making small dextrous movements

Hypomimic face - expressionless face

Akinesia - difficulty producing intention of movement

Rigidity - increased muscle tone causing resistance to externally imposed joint movements

Tremor at rest - 4-7 cycles per second (slow tremor)

Question 23

What is Huntington’s Disease?

How does it present clinically?

Answer 23

Degeneration of GABAergic neurons (inhibitory neurons) in the striatum, caudate and the putamen

Choreic movements = rapid, jerky, involuntary movements
Speech impairment
Difficulty swallowing
Unsteady gait
Later stages = cognitive decline and dementia

Question 24

What is Ballism?

Answer 24

Associated with a stroke affecting the subthalamic nucleus

Usually one sided symptoms (contra-lateral)

Uncontrolled flinging of the extremities - rotatory movement of the limbs

Question 25

What is the cerebellum? Where is it located and what is its function?

Answer 25

Located in the posterior cranial fossa

Separated from the cerebrum above by the tentorium cerebelli

Co-ordinator and predictor of movement - compares what is going out of the CNS (signals on the way down to the muscles of the body) to what is returning from sensory receptors from the PNS (leads to fluid movement)

Question 26

What connects the two cerebellar lobes?

What is the purpose of the cerebral peduncles?

Where do the corticospinal tracts cross over?

Answer 26

Pons

Cerebral peduncles take the large motor tracts (from the erebrum) down to the pyramids (brainstem)

In the medulla 90% of the corticospinal fibres cross over

Question 27

What is the function of the vestibulocerebellum?

What happens if there is damage to this area?

Answer 27

At the posterior cerebellum
Regulates gait, posture and equilibrium
Co-ordinates movement of the head and movement of the eyes

Lesions = similar to symptoms of vestibular disease = poor co-ordination when walking (gait ataxia), tendency to fall even when patient is sitting still with eyes open

Question 28

What is the function of the spinocerebellum?

What happens if there is damage to this area?

Answer 28

Responsible for co-ordinating speech and limb movements, adjustment of muscle tone

Damage (can occur with chronic alcoholism ) = affects mainly legs, gait (wide based)

Question 29

What is the function of the cerebrocerebellum?

What happens if there is damage to this area?

Answer 29

Lateral part of the cerebellum
Co-ordinated skilled movements, cognitive control (attention, language processing, emotional control)

Damage = affects skilled and co-ordinated movements and speech (e.g. tremor)

Question 30

What are the main signs of cerebellum disorders?

Answer 30

Reveals itself when a person tries to make movements:

Ataxia - disturbances in posture and gait, legs wider apart, patients may appear drunk

Dysmetria - unable to predict the amount of force required for target directed movement e.g. move the hand to pick up an object

Intention tremor - tremor when trying to reach for something e.g. nose-finger tracking test (touch clinician’s finger, then touch your nose - repeat this)

Dysdiadochokinesia - inability to do rapidly alternating mvoements (e.g. rapid pronation and supination of the hands)

Scanning speech - staccato = lack of co-ordination of the speech muscles