Motor control Flashcards
What are the pyramidal tracts? (examples, pathway, function)
- corticospinal and corticobulbar tracts
- cell bodies of upper motor neurons in motor cortex
- -> pass through the medullary pyramids on ventral surface of brainstem
- -> lower motor neurons in anterior horn of spinal cord OR cranial nerve nuclei (LMNs of corticobulbar) in brainstem
- voluntary movements of body and face
What are the extrapyramidal tracts? (examples, pathway, function)
- vestibulospinal, tectospinal, reticulospinal, rubrospinal
- do not start in cerebral cortex
- upper motor neurons in nuclei in brainstem–> axons travel down spinal cord
- -> many synapse with interneurons to indirectly modulate movement
- coordinate involuntary movements for balance, posture and locomotion
Where is the primary motor cortex?
located in precentral gyrus, anterior to central sulcus
What is the premotor area?
- located anterior to primary motor cortex
- involved in planning movements
- regulates externally cued movements e.g. reaching for an object
What is the supplementary motor area?
- located anterior to primary motor cortex and medial to premotor area
- involved in planning complex movements e.g. internally cued speech
- becomes active prior to voluntary movements
What is the difference between the anterior and lateral corticospinal tracts?
- lateral corticospinal tract: crossed over fibres (85-90%), responsible for limb muscles
- anterior corticospinal tract: uncrossed fibres (10-15%), innervate axial/trunk muscles
What is the function of the vestibulospinal tract?
- stabilise head during body movements or head movements
- coordinate head movements with eye movements
- mediate postural adjustments
What is the function of the reticulospinal tract?
- originates from medulla and pons
- postural stability and changes in muscle tone
What is the function of the tectospinal tract?
- arises from superior colliculus of midbrain
- orientation of head and neck during eye movements
What is the function of the rubrospinal tract?
- from red nucleus in midbrain
- mainly taken over by corticospinal tract in humans
What are the negative signs of upper motor neuron lesions?
loss of voluntary motor function
- paresis: graded weakness of movements
- paralysis (plegia): complete loss of voluntary muscle activity
What are the positive signs of upper motor neuron lesions?
increased abnormal motor function due to loss of inhibitory descending inputs
- spasticity: increased muscle tone (stiff muscles)
- hyperreflexia: exaggerated reflexes
- clonus: abnormal oscillatory muscle contraction
Babinski’s sign
What is apraxia?
- disorder of skilled movement
- not paretic, but lost information about how to perform skilled movements
- stroke and dementia most common causes
- lesion of inferior parietal lobe, or frontal lobe (premotor cortex, supplementary motor area/SMA)
What are the signs of a lower motor neuron lesion?
- weakness
- hypotonia* (reduced muscle tone)
- hyporeflexia* (reduced reflexes- size and briskness)
- muscle atrophy
- fasciculation: damaged motor units produce spontaneous action potentials–> visible twitch
- fibrillations: smaller spontaneous twitching of individual muscle fibres recorded during needle electromyography (invisible to naked eye)
*opposite to upper motor neuron lesion
What is motor neuron disease (MND)/ amyotrophic lateral sclerosis (ALS)?
- progressive neurodegenerative disorder of motor system
- affects upper motor neurons, lower motor neurons in brainstem (issues w/ voluntary contraction of tongue)
- issues w/ intercostal muscles and voluntary contraction of upper and lower limb muscles
- no cure
- eventually results in death due to lack of respiratory muscle function
What are the upper motor neuron signs of motor neuron disease (MND)?
- 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?
- muscle weakness
- muscle wasting
- tongue fasciculations and wasting
- nasal speech
- dysphagia
What are the functions of the basal ganglia?
- decision to move
- elaborating associated movements e.g. arms swinging when walking, changing facial expression to match emotions
- moderating and coordinating movements (and suppressing unwanted movements)
- performing movements in order
What is Parkinson’s disease?
- degeneration of dopaminergic neurons that originate in the substantial nigra and project to the striatum
- bradykinesia: slow small movements e.g. doing up buttons
- hypomimic face: expressionless
- akinesia: difficult in initiation of movements, as cannot generate movements internally
- rigidity: inc. muscle tone, causing resistance to externally imposed joint movements
- tremor at rest (4-7Hz): starts in one hand but spreads to other parts of body- ‘pill rolling tremor’
What is Huntington’s disease?
- degeneration of GABAergic neurons in the striatum, caudate and then putamen
- choreic (dance-like) movements: rapid, jerky involuntary movements of body–> hands and face first, then legs an rest of body
- speech impairment
- dysphagia
- unsteady gait
- later stages, cognitive decline and dementia
What is ballism?
- sudden uncontrolled flinging of limbs
- usually from stroke affecting subthalamic nucleus
- symptoms occur contra laterally
Where is the cerebellum located?
- in posterior cranial fossa
- separated from cerebrum above by tentorium cerebelli
What is the function of the cerebellum?
- coordinates and predicts movements
- enables fluid movement
What are the functions of the vestibulocerebellum?
- regulation of gait, posture and equilibrium
- coordination of head movements w/ eye movements
- damage (tumour) causes syndrome similar to vestibular disease–> gait ataxia and tendency to fall (even when sitting)
What are the functions of the spinocerebellum?
- coordination of speech
- adjustment of muscle tone
- coordination of limb movements
- damage (degeneration and atrophy w/chronic alcoholism) affects legs, causing abnormal gait and stance (wide)
What are the functions of the cerebrocerebellum?
- coordination of skilled movements
- cognitive function, attention, processing of language
- emotional control
- damage affects mainly arms/skilled coordinated movements (tremor) and speech
What are the main signs of cerebellar dysfunction apparent only on movement?
- ataxia: general impairments in movement coordination and accuracy, disturbances of posture or gait–> wide based, staggering gait
- dysmetria: inappropriate force and distance for target-directed movements e.g. knocking over cup rather than grabbing it
- intention tremor:during target-directed movements (nose-finger tracking)
- dysdiadochokinesia: inability to perform rapidly alternating movements e.g. rapidly pronating and supinating hands/forearms
- scanning speech: staccato, due to impaired coordination of speech muscles
What are alpha motor neurons?
- ‘ventral/anterior horn cells’
- the lower motor neurons of the brainstem and spinal cord
- occupy ventral horn of grey matter of spinal cord
- innervate the extrafusal muscle fibres
- when activated, cause contraction of muscle fibres
- motor neuron pool= all alpha motor neurons innervating a single muscle
What is a motor unit?
a single motor neuron and all the muscle fibres it innervates- stimulation of 1 motor unit causes contraction of all muscle fibres in that unit
*Innervation ratio relates to function of muscle
What are the 3 main types of motor unit?
- slow (S, type 1)
- fast, resistant (FR, type IIA)
- fast, fatiguable (FF, type IIB)
What are the characteristics of the nerve fibres of slow motor units?
- small diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
What are the characteristics of the nerve fibres of FR and FF motor units?
- large cell bodies
- large dendritic trees
- thicker axons
- faster conduction velocity
Which type of motor unit generates the most force?
fast fatiguable
What is recruitment in terms of regulation of muscle force?
- if the CNS determines that more force is required, it can recruit a bigger number of motor neurons and therefore muscle fibres
- ‘size principle’–> smaller units recruited first (usually slow twitch)
- more force required–> more units recruited
- allows fine control e.g. when writing
What is rate coding in terms of regulation of muscle force?
- motor unit can fire at a range of frequencies–> slow units fire at lower frequency
- faster firing rate–> greater force generated
What are neurotrophic factors?
- type of growth factor that prevents neuronal death
- promote growth of neurons after injury
What is the result of cross innervation where a muscle that is slow has a faster nerve implanted into it? What does this show?
- starts to take on characteristic of a fast muscle
- the motor neuron has some effect on the properties of the muscle fibres it innervates–> there can be some plasticity within motor unit types
What are the most common situations in which motor units change types?
- from IIB to IIA (fast fatiguable–> fatigue resistant) most common following training
- type I–>II possible with severe reconditioning or spinal cord injury (e.g. microgravity in space results in shift from slow to fast)
- loss of both type I and II fibres in elderly, but preferential loss of type II–> so larger proportion of type I fibres in aged muscle, leading to slower contraction times
What is the Jendrassik manoeuvre?
- when a reflex becomes larger if you clench your teeth, make a fist, or pull against locked fingers at the same time
- reduces inhibition that CNS usually exerts over reflexes
How does the brain influence reflex pathways under normal circumstances?
- higher centres of CNS exert a resting level of inhibition that dampens down reflexes
- surgical decerebration reveals this–> exaggerated reflexes
What are gamma motor neurons?
lower motor neurons that don’t contribute to contraction of a muscle, but alter the sensitivity of the sensory organ in skeletal muscle/ intrafusal muscle fibres
What is hyperreflexia?
- overactive reflexes
- due to loss of descending supra spinal inhibition
- characteristic of upper motor neuron lesions
What is clonus?
- involuntary and rhythmic muscle contractions (hyperreflexia)
- due loss of descending inhibition
- characteristic of upper motor neuron lesions e.g. stroke
What is Babinski sign?
- when sole of foot stroked w/ blunt instrument e.g. spoon, toes curl downwards normally (esp. big toe)
- if big toe curls upwards–> ABNORMAL in adults- +ve Babinski sign
- associated w/ upper motor neuron lesion
N.B. normal in infants
What is hypo-reflexia?
- below normal or absent reflexes
- associated w/ lower motor neuron lesions
