Motor control

Question 1

What are the pyramidal tracts? (examples, pathway, function)

Answer 1

• 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

Question 2

What are the extrapyramidal tracts? (examples, pathway, function)

Answer 2

• 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

Question 3

Where is the primary motor cortex?

Answer 3

located in precentral gyrus, anterior to central sulcus

Question 4

What is the premotor area?

Answer 4

• located anterior to primary motor cortex
• involved in planning movements
• regulates externally cued movements e.g. reaching for an object

Question 5

What is the supplementary motor area?

Answer 5

• 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

Question 6

What is the difference between the anterior and lateral corticospinal tracts?

Answer 6

• lateral corticospinal tract: crossed over fibres (85-90%), responsible for limb muscles
• anterior corticospinal tract: uncrossed fibres (10-15%), innervate axial/trunk muscles

Question 7

What is the function of the vestibulospinal tract?

Answer 7

• stabilise head during body movements or head movements
• coordinate head movements with eye movements
• mediate postural adjustments

Question 8

What is the function of the reticulospinal tract?

Answer 8

• originates from medulla and pons

- postural stability and changes in muscle tone

Question 9

What is the function of the tectospinal tract?

Answer 9

• arises from superior colliculus of midbrain

- orientation of head and neck during eye movements

Question 10

What is the function of the rubrospinal tract?

Answer 10

• from red nucleus in midbrain

- mainly taken over by corticospinal tract in humans

Question 11

What are the negative signs of upper motor neuron lesions?

Answer 11

loss of voluntary motor function

• paresis: graded weakness of movements
• paralysis (plegia): complete loss of voluntary muscle activity

Question 12

What are the positive signs of upper motor neuron lesions?

Answer 12

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

Question 13

What is apraxia?

Answer 13

• 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)

Question 14

What are the signs of a lower motor neuron lesion?

Answer 14

• 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

Question 15

What is motor neuron disease (MND)/ amyotrophic lateral sclerosis (ALS)?

Answer 15

• 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

Question 16

What are the upper motor neuron signs of motor neuron disease (MND)?

Answer 16

• spasticity (increased tone of limbs and tongue)
• brisk limbs and jaw reflexes
• babinski’s sign
• loss of dexterity
• dysarthria (difficulty speaking)
• dysphagia (difficulty swallowing)

Question 17

What are the lower motor neuron signs of MND?

Answer 17

• muscle weakness
• muscle wasting
• tongue fasciculations and wasting
• nasal speech
• dysphagia

Question 18

What are the functions of the basal ganglia?

Answer 18

• 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

Question 19

What is Parkinson’s disease?

Answer 19

• 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’

Question 20

What is Huntington’s disease?

Answer 20

• 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

Question 21

What is ballism?

Answer 21

• sudden uncontrolled flinging of limbs
• usually from stroke affecting subthalamic nucleus
• symptoms occur contra laterally

Question 22

Where is the cerebellum located?

Answer 22

• in posterior cranial fossa

- separated from cerebrum above by tentorium cerebelli

Question 23

What is the function of the cerebellum?

Answer 23

• coordinates and predicts movements

- enables fluid movement

Question 24

What are the functions of the vestibulocerebellum?

Answer 24

• 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)

Question 25

What are the functions of the spinocerebellum?

Answer 25

- 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)

Question 26

What are the functions of the cerebrocerebellum?

Answer 26

- coordination of skilled movements - cognitive function, attention, processing of language - emotional control - damage affects mainly arms/skilled coordinated movements (tremor) and speech

Question 27

What are the main signs of cerebellar dysfunction apparent only on movement?

Answer 27

- 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

Question 28

What are alpha motor neurons?

Answer 28

- '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

Question 29

What is a motor unit?

Answer 29

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

Question 30

What are the 3 main types of motor unit?

Answer 30

1. slow (S, type 1) 2. fast, resistant (FR, type IIA) 3. fast, fatiguable (FF, type IIB)

Question 31

What are the characteristics of the nerve fibres of slow motor units?

Answer 31

- small diameter cell bodies - small dendritic trees - thinnest axons - slowest conduction velocity

Question 32

What are the characteristics of the nerve fibres of FR and FF motor units?

Answer 32

- large cell bodies - large dendritic trees - thicker axons - faster conduction velocity

Question 33

Which type of motor unit generates the most force?

Answer 33

fast fatiguable

Question 34

What is recruitment in terms of regulation of muscle force?

Answer 34

- 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

Question 35

What is rate coding in terms of regulation of muscle force?

Answer 35

- motor unit can fire at a range of frequencies--> slow units fire at lower frequency - faster firing rate--> greater force generated

Question 36

What are neurotrophic factors?

Answer 36

- type of growth factor that prevents neuronal death | - promote growth of neurons after injury

Question 37

What is the result of cross innervation where a muscle that is slow has a faster nerve implanted into it? What does this show?

Answer 37

- 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

Question 38

What are the most common situations in which motor units change types?

Answer 38

- 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

Question 39

What is the Jendrassik manoeuvre?

Answer 39

- 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

Question 40

How does the brain influence reflex pathways under normal circumstances?

Answer 40

- higher centres of CNS exert a resting level of inhibition that dampens down reflexes - surgical decerebration reveals this--> exaggerated reflexes

Question 41

What are gamma motor neurons?

Answer 41

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

Question 42

What is hyperreflexia?

Answer 42

- overactive reflexes - due to loss of descending supra spinal inhibition - characteristic of upper motor neuron lesions

Question 43

What is clonus?

Answer 43

- involuntary and rhythmic muscle contractions (hyperreflexia) - due loss of descending inhibition - characteristic of upper motor neuron lesions e.g. stroke

Question 44

What is Babinski sign?

Answer 44

- 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

Question 45

What is hypo-reflexia?

Answer 45

- below normal or absent reflexes | - associated w/ lower motor neuron lesions