MOTOR CORTICAL CONTROL Flashcards
What us the hierarchical organisation of motor control
High order areas of hierarchy are involved in more complex tasks (programme and decide on movements, coordination)
Lower level areas of hierarchy perform lower level tasks (execution of movement)
How is the motor system organised?
By functional segregation
Name the descending pyramidal tracts
Corticospinal
Corticobulbar
Voluntary movements of body and face
Name the extra pyramidal tracts
Vestibulospinal
Tectospinal
Reticulospinal
Rubrospinal
Involuntary movements for balance, posture and locomotion
Describe the decussation of the corticospinal tract
85-90% decussate to the lateral corticospinal tract
10-15% remain on same side via anterior corticospinal tract
What does the lateral corticospinal and anterior corticospinal tract innervate?
Lateral - limb muscles
Anterior - trunk muscle
What does the corticobulbar tract do?
Principal motor pathway for voluntary movements of the face and neck
What does the vestibulospinal tract do?
Stabilises head during body movements or as head moves
Coordinate head movements with eye movements
Mediate postural adjustments
What does the reticulospinal tract do?
Changes in muscle tone associated with voluntary movement
Postural stability
Descends from medulla and pons
What does the tectospinal tract do? And where does it descend from?
Orientation of head and neck during eye movements
Descends from superior colliculus of midbrain
What does the rubrospinal tract do?
In humans mainly taken over by corticospinal tract
Innervates lower motor neurons in flexors of upper limb
Descends from red nucleus of midbrain
What does an upper motor neuron lesion cause?
Negative signs:
- Loss of voluntary motor function
- Paresis (graded weakness of movements)
- Paralysis
Positive signs: - Increased abnormal motor function due to loss of inhibitory descending inputs - Spasticity (increased muscle tone) - Hyper-reflexia - Clonus - Babinski's sign
What is apraxia and what could cause it?
Disorder of skilled movement. Not paretic but patients have lost info on how to perform skilled movements
Lesions/diseases of:
- inferior parietal lobe
- frontal lobe (premotor cortex/supplementary motor area)
What are the most common diseases causing apraxia?
Stroke and dementia
What does a lower motor neuron lesion cause?
Weakness Hypotonia Hyporeflexia Muscle atrophy Fasciculations Fibrillations
What are fasciculations?
Damaged motor units which produce spontaneous APs resulting in a visible twitches
What are fibrillations?
Spontaneous twitching of individual muscle fibres recorded during needle electromyography examination
What is motor neuron disease (MND)?
Spectrum of progressive neurodegenerative disorder affecting upper and lower motor neurones
AKA amyotrophic lateral sclerosis (ALS)
What are the upper motor neuron symptoms of MND?
Spasticity Brisk limbs and jaw reflexes Babinski's sign Loss of dexterity Dysarthria Dysphagia
What are the lower motor neuron symptoms of MND
Weakness Muscle wasting Tonge fasciculations and wasting Nasal speech Dysphagia
List the nuclei in the basal ganglia
Caudate nucleus
Putamen (caudate and putamen together called striatum)
Lentiform nucleus
Nucleus accumbens
Subthalamic nuclei
Substantia nigra
Ventral pallidum, claustrum, nucleus basalis
What is the function of the caudate nucleus?
Decision to move
Which nuclei are responsible for elaborating associated movements e.g. swinging arms when walking, changing facial expression to match emotion
Lentiform nucleus (putamen + external globus pallidus)
Nucleus accumbens
What is the function of the substantia nigra?
Moderating and coordinating movement (suppressing unwanted movement)
Which parts of the basal ganglia circuitry are affected in Parkinson’s disease, Huntington’s disease and Ballism?
Parkinson’s disease - between striatum and substantia nigra (rostral)
Huntington’s disease - striatum leading to globus pallidus
Ballism - subthalamic nucleus
What is Parkinson’s disease?
Degeneration of the dopaminergic neurons that originate in the substantia nigra and project to the striatum
List the symptoms of Parkinson’s disease
Bradykinesia - slowness of small movements
Hypomimic face - expressionless, mask-less
Akinesia - difficulty in the initiation of movements because cannot initiate movements internally
Rigidity - muscle tone increase causing resistance to joints
Tremor at rest - 4-7 Hz starting in one hand with time spreading to other parts of body “pill rolling tremor”
What is pathology of Huntington’s disease?
Genetic degeneration of the GABAergic neurons in the striatum, caudate and then putamen
Describe the genetic mutation which causes Huntington’s disease
Chromosome 4
Autosomal dominant
CAG repeat
What are the symptoms of Huntington’s disease?
Choreic movements - jerky movements of hands and face affected first then legs and rest of body
Speech impairment
Dysphagia
Unsteady gait
Cognitive decline and dementia in later stages
What is ballism?
Sudden uncontrolled flinging of the extremities
Usually caused by stroke affecting subthalamic nucleus with symptoms occuring on contralateral side
What is the function of the cerebellum?
Coordinator and predictor of movement
Takes signals on their way down and compares it with sensory signals from receptors on their way up to coordinate movements and make them fluid
Name the 3 parts of the cerebellum and point them out on a diagram
Vestibulocerebellum
Spinocerebellum
Cerebrocerebellum
What is the function of the vestibulocerebellum?
Regulation of gait, posture and equilibrium
Coordination of head movements with eye movements
What is the function of the spinocerebellum?
Coordination of speech
Adjustment of muscle tone
Coordination of limb movements
What is the function of the cerebrocerebellum?
Coordination of skilled movements
Cognitive function, attention, processing of language
Emotion control
What does damage e.g. tumour in vestibulocerebellum cause?
Syndrome similar to vesitbular disease:
- gait, ataxia and tendency to fall
What does damage (e.g. chronic alcoholism causing degeneration and atrophy) in spinocerebellum cause?
Affects mainly legs causing abnormal gait and wide-based stance
What does damage to cerebrocerebellum cause?
Damage affects mainly arms/skilled coordinated movements (tremor) and speech
List the main signs of cerebellar dysfunction
Ataxia Dysmetria Intention tremor Dysdiadockokinesia Scanning speech
What is the difference between extrafusal and intrafusal muscles
Extrafusal - muscle fibres have contractile elements in them
Intrafusal - muscle fibres which contain the sensory organs which respond to stretch and tension. Responsible for conveying the sensory info about the status of a muscle
What is an alpha motor neuron?
The lower motor neurons of the brainstem and spinal cord which innervate the extrafusal muscle fibres of the skeletal muscles.
Which part of the spinal cord do alpha motor neurons occupy?
Ventral horn
What is a motor neuron pool?
All the alpha motor neurons that innervate a single muscle
What are the two mechanisms in which the brain regulates the amount of force that a muscle produces?
Recruitment - Smaller units recruited and derecruited first allowing fine control
Rate coding - The more a motor unit fires the greater the force produced. Slow units fire at a lower frequency.
What causes muscle fibre type IIB - IIA change?
Training e.g. gym
What causes type I to II?
Severe deconditioning:
- Spinal cord injury
- Astronauts due to microgravity
What happens to motor units in ageing?
Loss of type I and II
Preferential loss of type II thus greater proportion of I
What are descending (supraspinal) control of reflexes
Higher centres of CNS exert inhibitory control on stretch reflex
Reflexes can be influenced by specific motions which remove the descending inhibition from the brain causing the reflex to become larger
What is the Jendrassik manoeuvre?
Clenching teeth, making a fist, pulling against locked fingers causes the patellar tendon reflex to be larger
What is decerebration?
Surgery to separate cortex from lower brain stem and spinal cord regions
Shows that excitatory control (inhibition) comes from higher centres of CNS. Without the higher centres only lower centres left causing rigidity and spasticity giving over-active and tonic stretch reflex
List the 5 types neurons which control reflexes
Activating alpha motor neurons Activating inhibitory interneurons Activating propriospinal neurons Activating gamma motor neurons Activating terminals of afferent fibres
What are activating propriospinal neurons
Interneurons which go up and down to activate nearby muscles to help with movement
What are gamma motor neurons
Alter the sensory organs that are housed in intrafusal muscles. Allows the muscle to remain sensitive to stretch when muscle at different lengths
What is clonus?
Form of hyper-reflexia - involuntary and rhythmic muscle contractions
What is Babinski’s sign?
When sole of foot stimulated with blunt instrument the big toe curls upwards when it normally curls downwards
Associated with upper corticospinal tract neuron lesion
Why does Babinski’s sign not work in infants?
Their big toe will curl upwards due to an undeveloped corticospinal tract
Why does hyper-reflexia occur?
Loss of descending inhibition
Upper motor neuron lesion
Why does hypo-reflexia occur?
Lower motor neuron diseases as efferent limb of reflex not functioning
