Motor Cortical Control

Question 1

Broad principles of motor control

Answer 1

Hierarchical organisation

Functional segregation

Question 2

Motor system hierarchy

Answer 2

Primary motor cortex
Basal ganglia and cerebellum
Thalamus
Brainstem
Spinal cord

Question 3

Major descending tracts

Answer 3

Pyramidal tracts - pass through thepyramidsof the medulla - Motor cortex to spinal cord or cranial nerve nuclei in brainstem - Voluntary movements of body and face

Extrapyramidal tracts - Do not pass through thepyramidsof the medulla - Brainstem nuclei to spinal cord - involuntary (automatic) movements for balance, posture and locomotion

Question 4

Pyramidal tracts

Answer 4

Corticospinal

Corticobulbar

Question 5

Extrapyramidal tracts

Answer 5

Vestibulospinal
Tectospinal
Reticulospinal
Rubrospinal

Question 6

Primary motor cortex location

Answer 6

Located in precentral gyrus, anterior to the central sulcus

Question 7

Premotor area location

Answer 7

Located anterior to primary motor cortex

Question 8

Supplementary motor area location

Answer 8

Located anterior and medial to primary motor cortex

Question 9

Corticospinal tract

Answer 9

From upper motor neurons down cerebral peduncle to midbrain and pyramids in medulla
Decussate
85-90% crossed fibres - Limb muscles - Lateral corticospinal tract
10-15% uncrossed fibres - Trunk muscles - Anterior corticospinal tract

Question 10

Homunculus and somatotopy

Answer 10

Motor homunculus

Somatotopic representation

Question 11

Corticobulbar tract

Answer 11

Basal ganglia

Principal motor pathway for voluntary movements of the face (and neck)

Question 12

Vestibulospinal

Answer 12

Stabilise head during body movements, or as head moves
Coordinate head movements with eye movements
Mediate postural adjustments

Question 13

Reticulospinal

Answer 13

Most primitive descending tract - from medulla and pons
Changes in muscles tone associated with voluntary movement
Postural stability

Question 14

Tectospinal

Answer 14

From superior colliculus of midbrain

Orientation of the head and neck during eye movements

Question 15

Rubrospinal

Answer 15

From red nucleus of midbrain
In humans mainly taken over by corticospinal tract
Innervate lower motor neurons of flexors of the upper limb

Question 16

Upper motor neuron lesion

Answer 16

Loss of voluntary motor function
Paresis: graded weakness of movements
Paralysis (plegia): complete loss of voluntary muscle activity

Increased abnormal motor function due to loss of inhibitory descending inputs
Spasticity: increased muscle tone
Hyper-reflexia: exaggerated reflexes
Clonus: abnormal oscillatory muscle contraction
Babinski’s sign

Question 17

Apraxia

Answer 17

A disorder of skilled movement. Patients are not paretic but have lost information about how to perform skilled movements

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

Question 18

Lower motor neuron lesion

Answer 18

Weakness
Hypotonia (reduced muscle tone)
Hyporeflexia (reduced reflexes)
Muscle atrophy
Fasciculations: damaged motor units produce spontaneous action potentials, resulting in a visible twitch
Fibrillations: spontaneous twitching of individual muscle fibres; recorded during needle electromyography examination

Question 19

Motor neuron disease (MND)

Answer 19

Progressive neurodegenerative disorder of the motor system

Also known as Amyotrophic Lateral Sclerosis (ALS)

Question 20

Caudate nucleus

Answer 20

Decision to move

Question 21

Lentiform nucleus (putamen + external globus pallidus) – together caudate and putamen are known as the striatum
Nucleus accumbens
Subthalamic nuclei

Answer 21

Elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)

Question 22

Substantia nigra (midbrain)
Ventral pallidum, claustrum, nucleus basalis (of Meynert)

Answer 22

Moderating and coordinating movement (suppressing unwanted movements)

Performing movements in order

Question 23

Parkinson’s disease

Answer 23

Degeneration of the dopaminergic neurons that originate in the substantia nigra and project to the striatum

Question 24

Parkinson’s disease symptoms

Answer 24

Bradykinesia - slowness of (small) movements (doing up buttons, handling a knife)
Hypomimic face - expressionless, mask-like (absence of movements that normally animate the face)
Akinesia - difficulty in the initiation of movements because cannot initiate movements internally
Rigidity - muscle tone increase, causing resistance to externally imposed joint movements
Tremor at rest - 4-7 Hz, starts in one hand (“pill-rolling tremor”); with time spreads to other parts of the body

Question 25

Huntington’s disease

Answer 25

Degeneration of GABAergic neurons in the striatum, caudate and then putamen

Question 26

Huntington’s disease symptoms

Answer 26

Choreic movements (chorea - dance) rapid jerky involuntary movements of the body; hands and face affected first; then legs and rest of body Speech impairment Difficulty swallowing Unsteady gait Later stages, cognitive decline and dementia

Question 27

Ballism

Answer 27

Usually from stroke affecting the subthalamic nucleus. Sudden uncontrolled flinging of the extremities Symptoms occur contralaterally.

Question 28

Cerebellum

Answer 28

Located in posterior cranial fossa Separated from cerebrum above by tentorium cerebelli Coordinator and predictor of movement

Question 29

Vestibulocerebellum

Answer 29

Regulation of gait, posture and equilibrium Coordination of head movements with eye movements Damage (tumour) causes syndrome similar to vestibular disease leading to gait ataxia and tendency to fall (even when patient sitting and eyes open)

Question 30

Spinocerebellum

Answer 30

Coordination of speech Adjustment of muscle tone Coordination of limb movements Damage (degeneration and atrophy associated with chronic alcoholism) affects mainly legs, causes abnormal gait and stance (wide-based)

Question 31

Cerebrocerebellum

Answer 31

Coordination of skilled movements Cognitive function, attention, processing of language Emotional control Damage affects mainly arms/skilled coordinated movements (tremor) and speech

Question 32

Main signs of cerebellar dysfunction

Answer 32

Ataxia General impairments in movement coordination and accuracy. Disturbances of posture or gait: wide-based, staggering (“drunken”) gait Dysmetria Inappropriate force and distance for target-directed movements (knocking over a cup rather than grabbing it) Intention tremor Increasingly oscillatory trajectory of a limb in a target-directed movement (nose-finger tracking) Dysdiadochokinesia Inability to perform rapidly alternating movements (rapidly pronating and supinating hands and forearms) Scanning speech Staccato, due to impaired coordination of speech muscles

Question 33

Alpha motor neuron

Answer 33

The lower motor neurons of the brainstem and the spinal cord Innervate the extrafusal muscle fibres of the skeletal muscles Motor neuron pool contains all alpha motor neurons innervating a single muscle

Question 34

Motor unit

Answer 34

a single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force.

Question 35

Types of motor unit

Answer 35

Slow (S, type I) - slowest conduction velocity Fast, fatigue resistant (FR, type IIA) - faster conduction velocity Fast, fatiguable (FF, type IIB) - faster conduction velocity

Question 36

Regulation of muscle force

Answer 36

Governed by the “size principle”. Smaller units are recruited first. As more force is required, more units are recruited. This allows fine control (e.g. when writing), under which low force levels are required. A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency. As the firing rate increases, the force produced by the unit increases. Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

Question 37

Neurotrophic factors

Answer 37

Prevent neuronal death Promote growth of neurons after injury Motor unit and fibre characteristics are dependent on the nerve which innervates them. The motor neuron has some effect on the properties of the muscle fibres it innervates.

Question 38

Plasticity of motor units/muscle fibres

Answer 38

Type IIB to IIA most common following training Type I to II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle.

Question 39

Reflex

Answer 39

An automatic response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness. An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.

Question 40

Jendrassik manoeuvre

Answer 40

Try clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped. The reflex becomes larger

Question 41

Descending (supraspinal) control of reflexes

Answer 41

Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex. Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.

Question 42

Descending (supraspinal) control of reflexes process

Answer 42

1. Activating alpha motor neurons 2. Activating inhibitory interneurons 3. Activating propriospinal neurons 4. Activating gamma motor neurons 5. Activating terminals of afferent fibres

Question 43

Hyper-reflexia

Answer 43

Overactive reflexes Loss of descending inhibition Associated with upper motor neuron lesions 

Question 44

Clonus

Answer 44

Involuntary and rhythmic muscle contractions Loss of descending inhibition Associated with upper motor neuron lesions 

Question 45

Babinski sign

Answer 45

When sole stimulated with blunt instrument the big toe: Curls downwards - normal Curls upwards – abnormal in adults. This is a positive Babinski sign. Associated with upper motor neuron lesions  Toe curls upwards in infants – this is normal.

Question 46

Hypo-reflexia

Answer 46

Below normal or absent reflexes | Associated with lower motor neuron diseases