2.6 - Motor control and movement disorders Flashcards
What is hierarchical organisation?
- high order areas of hierarchy are involved in more complex tasks (programme and decide on movement, coordinate muscle activity)
- lower level areas of hierarchy perform lower level tasks (execution of movement)
What is functional segregation?
Motor system organised in a number of different areas that control different aspects of movement
What is the role of the motor cortex?
Receives information from other cortical areas and sends commands to the thalamus and brainstem
What is the role of the cerebellum and basal ganglia?
Adjust commands received from other parts of the motor control system
What is the role of the brainstem?
Passes commands from cortex to spinal cord
What are the two types of major descending tracts?
- pyramidal tracts
- extrapyramidal tracts
What are the two pyramidal tracts?
- corticospinal
- corticobulbar
Why are the pyramidal tracts called pyramidal?
They pass through the pyramids of the medulla
Where do the nerves of the pyramidal tracts go from and to?
From motor cortex to spinal cord (corticospinal) or cranial nerve nuclei in brainstem (corticobulbar)
What do the nerves of the pyramidal tracts control?
Voluntary movements of body and face
What are the four extrapyramidal tracts?
- vestibulospinal
- tectospinal
- reticulospinal
- rubrospinal
Why are the extrapyramidal tracts called extrapyramidal?
Do not pass through the pyramids of the medulla
Where do the nerves of the extrapyramidal tracts go from and to?
- UMN in cortex
- LMN in brainstem nuclei to spinal cord
What do the extrapyramidal tracts control?
Involuntary (automatic) movements for balance, posture and locomotion
What does the vestibulospinal tract do?
- stabilise head during body movements or as head moves
- coordinate head movements with eye movements
- mediate postural adjustments
What does the tectospinal tract do and where is it from?
- from superior colliculus of midbrain
- orientation of head and neck during eye movements
What does the reticulospinal tract do and where is it from?
- most primitive descending tract - from medulla and pons
- changes in muscle tone associated with voluntary movement
- postural stability
What does the rubrospinal tract do and where is it from?
- from red nucleus of midbrain
- in humans mainly taken over by corticospinal tract
- innervate lower motor neurons of flexors of the upper limb
Where is the primary motor cortex located?
In precentral gyrus, anterior to central sulcus
What is the primary motor cortex responsible for?
- controls fine, discrete, precise voluntary movements
- provides descending signals to execute movements
Where is the premotor area located?
Located anterior to primary motor cortex
What is the premotor area responsible for?
- involved in planning movements
- regulates externally cued movements e.g. seeing an apple and reaching out for it
Where is the supplementary motor area located?
Anterior and medial to primary motor cortex
What is the supplementary motor area responsible for?
- involved in planning complex movements (i.e. internally cued, speech)
- becomes active prior to voluntary movement
Where do the neurons decussate in the corticospinal tract?
- 85-90% of fibres decussate and cross over in the medulla, forming the lateral corticospinal tract
- innervate limb muscles (contralateral)
- 10-15% of fibres do not decussate and are uncrossed forming the anterior corticospinal tract
- innervate trunk muscles (ipsilateral)
Where are the upper and lower motor neurons of the corticobulbar tract?
- UMN in motor cortex
- LMN in relevant cranial nerve nuclei (oculomotor, trochlear, trigeminal motor, abducens, facial, hypoglossal)
In the corticobulbar tract, what do motor nerves from the oculomotor and trochlear nucleus control?
Eye movements
In the corticobulbar tract, what do motor nerves from the trigeminal motor nucleus control?
Muscles of the jaw
In the corticobulbar tract, what do motor nerves from the abducens nucleus control?
Eye movements
In the corticobulbar tract, what do motor nerves from the facial nucleus control?
Muscles of the face
In the corticobulbar tract, what do motor nerves from the hypoglossal nucleus control?
Tongue
What are negative signs of an upper motor neuron lesion? (3)
- loss of voluntary motor function
- paresis - graded weakness of movements
- paralysis (plegia) - complete loss of voluntary muscle activity
What are the positive signs of an upper motor neuron lesion? (5)
- 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
What is apraxia (UMN lesion)?
- a disorder of skilled movement
- patients are not paretic but have lost information about how to perform skilled movements
- due to a lesion of inferior parietal lobe or frontal lobe (premotor cortex, supplementary motor area)
What are the two most common causes of apraxia?
Stroke and dementia (although any disease affecting the areas can cause it)
What are the symptoms of a lower motor neuron lesion? (6)
- weakness
- hypotonia (reduced muscle tone)
- hyporeflexia (reduced reflexes)
- muscle atrophy
- fasciculations
- fibrillations
What are fasciculations?
Damaged motor units produce spontaneous action potentials, resulting in a visible twitch
What are fibrillations?
Spontaneous twitching of individual muscle fibres, recorded during needle electromyography examination
What is a summary of the power, tone and reflexes of an UMN vs LMN lesion?
- UMN - reduced power, increased tone, increased reflexes (think upper = up for tone and reflexes)
- LMN - reduced power, reduced tone, reduced reflexes (think lower = down for tone and reflexes)
What is motor neuron disease (MND)?
- progressive neurodegenerative disorder of the motor system
- spectrum of disorders
- both UMN and LMN lost
What is MND also known as?
Amyotrophic lateral sclerosis (ALS)
What are the upper motor neuron signs of MND? (6)
- spasticity (increased tone of limbs and tongue)
- brisk limbs and jaw reflexes
- Babinski’s sign
- loss of dexterity
- dysarthria (difficulty speaking)
- dysphagia (difficulty swallowing)
What are the lower motor neuron signs of MND? (5)
- weakness
- muscle wasting
- tongue fasciculations and wasting
- nasal speech
- dysphagia
What are the parts to the basal ganglia?
- caudate nucleus
- lentiform nucleus (putamen + external globus pallidus) - together caudate and putamen are known as striatum
- nucleus accumbens
- subthalamic nuclei
- substantia nigra (midbrain)
- ventral pallidum, claustrum, nucleus basalis (of Meynert)
What are the functions of the basal ganglia? (4)
- decision to move - subthalamic
- elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)
- moderating and coordinating movement (suppressing unwanted movements) - lentiform
- performing movements in order - caudate + nucleus accumbens
What should we be aware of in relation to the basal ganglia circuitry?
Some diseases (e.g. Parkinson’s, Huntington’s, ballism) target specific parts of the basal ganglia
What is Parkinson’s disease?
Degeneration of the dopaminergic neurons that originate in the substantia nigra and project to the striatum
What are the symptoms of Parkinson’s? (5)
- bradykinesia - slowness of (small) movements (doing up buttons, handling knife)
- hypomimic face - expressionless, mask-like (absence of movements that normally animate the face)
- akinesia - difficulty in initiation of movements because cannot initiate movements internally
- rigidity - muscle tone increase, causing resistance to externally imposed joint movements
- tremor at rest - 4-7Hz, starts in one hand (‘pill-rolling tremor’); with time spreads to other parts
What is Huntington’s disease?
Degeneration of GABAergic neurons in the striatum - caudate and then putamen
What is the aetiology of Huntington’s disease?
- genetic neurodegenerative disorder
- chromosome 4 has a CAG repeat
- autosomal dominant
What are the symptoms of Huntington’s disease? (5)
- choreic movements - rapid jerky involuntary movements of hands and face first, then legs, then rest of body
- speech impairment
- dysphagia
- unsteady gait
- later stages - cognitive decline and dementia
What is ballism?
- usually from stroke affecting subthalamic nucleus
- sudden uncontrolled flinging of the extremities (one-sided)
- symptoms occur contralaterally
Where is the cerebellum?
- located in posterior cranial fossa
- separated from cerebrum above by tentorium cerebelli
What is the overall function of the cerebellum?
Coordinator and predictor of movement
What are the three main regions of the cerebellum?
- vestibulocerebellum
- spinocerebellum
- cerebrocerebellum
What does the vestibulocerebellum do?
- regulation of gait, posture and equilibrium
- coordination of head movements with eye movements
What does a lesion to the vestibulocerebellum cause?
Damage (tumour) causes syndrome similar to vestibular disease leading to gait ataxia (unsteady, staggering gait) and tendency to fall (even when patient sitting and eyes open)
What does the spinocerebellum do? (3)
- coordination of speech
- coordination of limb movements
- adjustment of muscle tone
What does damage to the spinocerebellum cause?
Damage (degeneration and atrophy associated with chronic alcoholism) affects mainly legs, causes abnormal gait and stance (wide-based)
What does the cerebrocerebellum do? (5)
- coordination of skilled movements
- cognitive function
- attention
- processing of language
- emotional control
What does damage to the cerebrocerebellum cause?
Damage mainly affects arms/skilled coordinated movements (tremor) and speech
When are the signs of cerebellar dysfunction apparent?
Only on movement
What are the main signs of cerebellar dysfunction? (5)
- ataxia - general impairments in movement coordination and accuracy; disturbances of posture/gait - wide-based, staggering gait
- dysmetria - inappropriate force and distance for target-directed movements (knocking over cup instead of 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
What is a mnemonic for the signs of cerebellar dysfunction?
DANISH
- dysdiadochokinesia + dysmetria
- ataxia (Romberg’s)
- nystagmus (draw H)
- intention tremor (touch nose then pen)
- slurred, staccato speech
- hypotonia / heel-shin test
What are alpha motor neurons?
- lower motor neurons of the brainstem and spinal cord
- innervate the extrafusal muscle fibres of the skeletal muscles
- their activation causes muscle contraction
Where are the alpha motor neurons located?
- anterior horn of grey matter of spinal cord
- also located in brainstem
What is the motor neuron pool?
Contains all alpha motor neurons innervating a single muscle
What is a motor unit?
- a single (alpha) motor neuron together with all the muscle fibres it innervates
- one muscle fibre cannot be innervated by >1 neuron
- stimulation of one motor unit = contraction of all muscle fibres
What are the three types of motor units?
- slow (S, type I)
- fast, fatigue resistant (FR, type IIA)
- fast, fatiguable (FF, type IIB)
How are muscle fibre types distributed through muscle?
Randomly distributed - different muscles have different proportions of slow and fast twitch muscles (e.g. back = more slow)
By which two ways does the brain regulate muscle force?
- recruitment (slow –> FR –> FF)
- rate coding (already recruited motor units increase firing rates)
How does recruitment work?
- motor units not randomly recruited - order
- governed by ‘size principle’ - smaller (slow) recruited first
- more force required = more units recruited
- allows fine control (low force needed e.g. writing)
How does rate coding work?
- a motor unit can fire at a range of frequencies - slow units fire at a lower frequency
- as firing rate increases, force produced by unit increases
- summation occurs when units fire at frequency too fast to allow muscle to relax between arriving APs
What are neurotrophic factors?
- type of growth factor
- prevent neuronal death
- promote neuronal growth after injury
What are motor unit and fibre characteristics dependent on?
- nerve which innervates them
- if a fast and slow muscle are cross innervated, they switch = motor neuron has some effect on properties of muscle fibre it innervates
What is the plasticity of motor units/muscle fibres?
- fibre types can change properties under different conditions
- type IIB–>IIA common following training
- type I–>II possible in severe deconditioning/SC injury/microgravity
- ageing associated with loss of type I and II fibres - preferentially type II = larger proportion type I (slower contraction times)
What is a reflex?
- 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 without reaching the level of consciousness
- involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli
- magnitude and timing determined respectively by intensity and onset of stimulus
How do reflexes differ from voluntary movements?
Once reflexes are released, they cannot be stopped
What are reflexes thought of as being?
Automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors
What is the Jendrassik manoeuvre and why does it happen?
- if you clench your teeth, make a fist or pull against locked fingers when your patellar tendon is tapped, the reflex becomes larger
- because descending inhibition from the brain is removed
What is the descending (supraspinal) control of reflexes?
- higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
- inhibitory control dominates in normal conditions
- decerebration reveals the excitatory control from supraspinal areas
What can cause rigidity and spasticity?
Can result from brain damage giving over-active or tonic stretch reflex
What are the pathways that produce descending spinal reflexes? (5)
- activating alpha motor neurons
- activating inhibitory interneurons
- activating propriospinal neurons
- activating gamma motor neurons
- activating terminals of afferent fibres
What is hyper-reflexia?
- overactive reflexes
- loss of descending inhibition
- associated with UMN lesion
What is clonus (hyper-reflexia)?
- sign of UMN dysfunction
- involuntary and rhythmic muscle contractions
- loss of descending inhibition
What is Babinski’s sign (hyper-reflexia)?
- when sole stimulated with blunt instrument, big toe usually curls downwards
- curls upward - abnormal in adults = positive Babinski sign
- associated with UMN lesions of corticospinal tract
- note - toe curls upwards in infants - normal
What is hypo-reflexia?
- below normal or absent reflexes
- associated with LMN diseases
