Task 1 - Voluntary Motor Control Flashcards
CNS
Cerebrum
Cerebellum: movement control center
Brain Stem: vital functions + relay station from cerebrum to spinal cord
Spinal cord: communication between brain & skin, joints, muscles
-> dorsal root: afferent, ventral root: efferent
PNS
Somatic PNS: all spinal nerves innervating skin, joints muscles under voluntary motor control
- motor axons
- sensory axons
- ANS/Visceral PNS: neurons innervating internal organs, blood vessels, glands
Motor System levels
High: association areas of neocortex & basal ganglia
-> Strategy
Middle: motor cortex & cerebellum
-> tactics
Low: brain stem & spinal cord
-> execution
all relies on sensory information except for ballistic movements
Ballistic movements
Brief all-or-none high speed movements
-> don’t rely/ are not influenced by sensory feedback
High level motor systems
Association areas of neocortex & basal ganglia
- > strategy: fiture out goal of movement and best strategy
- sensory info from cortex about spatial information and alternatives are filtered through basal ganglia
Middle level momtor systems
Motor cortex & cerebellum
- > tactics: sequences of muscle contractions to smoothly and accurately achieve goal
- basd on memory of sensory info fom past movements
- translation of action goals into movement instructions for lower level
Low level motor systems
Brain stem & spinal cord
- > Execution: activate neurons generating the goal directed movements & necessary adjustments of posture
- motor neurons + interneurons
- sensory feedback used to maintain posture, muscle lenght + tension before & after each voluntary movement
Posterior Parietal Cortex
Directs behavior by providing spatial information,
Area 5: input from primary somatosesnroy cortical areas
Area 7: input from higher order visual areas
-largely connected to PFC
-> damage produces deficits in perception and memory of spatial relationships (Apraxia, Contralateral negleect)
Apraxia
- loss of skilled action
- Difficulty of performing movements when asked about it out of context, can readily perform movement in natural situations
- > results from parietal cortex lesions
Contralateral Neglect
inability of a person to process and perceive stimuli on the opposite side of the brain damage even though they can be subconsiously perceived
Prefrontal Cortex
Makes decisions about what actions to take & their likely outcome
- > evaluation of external stimuli perceived by parietal cortex
- > Posterior parietal + PFC together encode what actions are desired and send axon to area 6
Planning (secondary motor areas)
Supplementary Motor Area (SMA)
Premotor Area (PMA)
-convert info about what action is desired into how the action will be carried out
-> programming specific patterns of movment after receiving instructions from PFC,
-output to primary motor cortex
-each has own somatotopic map
-> lesions lead to apraxia
Supplementary Motor Area (SMA)
- sends axons directly to innervate distal motor units
- interconnected with M1/area 4
- strong connections with PFC
- fires about a second before action
Premotor Area (PMA)
Primarily connects with reticulospinal neurons innervating proximal motor neurons
Ideomotor Apraxia
still have rough sense of desirec action but problems with proper execution
Ideational Apraxia
Don’t know how to perform actions anymore
Ready-set-go principle
Ready: depends on parietal & PFC activity
Set: depends on SMA & PMA
Go: depends on M1
Initiation Primary Motor Cortex
M1
- only fires when doing, not when imagining (like secondary areas)
- precentral gyrus (motor strip)
- strongest connections with motor neurons
- input: cortical areas (6), cerebellum through thalamus
- output: to spinal cord
- rostral neurons: terminate on spinal interneurons
- caudal neurons: terminate on alpha neurons
Hemiplegia
Loss of voluntary movement on contralateral side due to lesion to primary motor cortex
Motor Action Coding
Cells encode force & direction of movement by population coding
- > each cell has tuning curve + activity of cell represents direction vector
- > direction of movement depends on average of direction vectors of cells: population vector
- the larger the population the finer the possible control
Spinal Tracts
Corticospinal tract Rubrospinal tract Vestibulospinal trac Tectospinal tract Pontine & medullary Reticulospinal tracts
Lateral Pathways
Corticospinal trct
Rubrospinal tract
-> voluntary movement of distal muscles (direct cortical control
Ventral Pathways
Vestibulospinal tract
Tectospinal tract
Pontine & Medullary Reticulospinal tracts
-> control, maintenance, of posture & locomotion
-> proximal muscles (brainstem control)
Corticospinal tract
-longest & largest in CNS
-direct
-2/3 originate in motor cortex (areas 4,6), 1/3 in somatosensory areas
Trajectory:
cortex -> internal capsule -> cerebral peduncle -> pons -> medulla -> forms medullary pyramid (pyramidal tract) where it decussates at junction to spinal cord -> runs down lateral column of spinal cord
-> terminates in dorsolateral region of ventral horns & intermediate grey matter
-> controlls distal muscles
Rubrospinal Tract
- indirect
- originates in red nucleus of midbrain
- decussates in pons and joins corticospinal tract
- no major role
- compensates when corticospinal tract is lesioned
Vestibulospinal tract
keeps head balanced and turns in response to stimuli
- originate in vestibular nuclei of medulla
- bilateral tract: controls neck & back muscles -> head movement
- ipsilateral tract: runs down lumbar spinal cord to maintain posture (up to motor neurons of legs)
Tectospinal tract
- originates in superior colliculus (input from retina)
- colliculus creates map of environment: stimulation of one point on map leads to orienting response
Pontine & Medullary Reticulospinal tracts
Origin: reticular formation
- pontine: enhances antigravity reflexes of spinal cord (standing posture, maintains muscle length)
- medullary: frees antigravity muscles from reflex control (opposite of pontine)
Effector
Part of body that can move as result of muscle contraction e.g. finger, neck,
Alpha motor neurons
Innervate muscle fibers & provide physical basis for translating nerve signals into mechanical action
-spinal cord -> ventral root -> muscles fibers -> ACh -> contraction
Spinal interneurons
integrate sensory feedback with motor commands resulting in voluntary movement
- input from descending motor fibers
- output to motor neurons
Brain Machine Interface (BMS)
Microelectrode implanted, spike patterns analyzed to see how they encode movement
- > decoded activity used to move e.g. a cursor on a screen
- > video game can be played only using neural signals from M1, bypassing usual pathway involving muscles
- works with cells in M1, PMA, SMA, parietal cortex
Amyotrophic Lateral Sclerosis (ALS)
Muscle weakness & atrophy
- > degenration of large alpha motor neurons & neurons innervaing them
- cognitive functions and intelligence unchanged
- possible causes: genetic mutation affecting enzyme superoxide dismutate (normally reduced toxic superoxide radicals; Excitotoxicity: elevated levels of glutamate lead to overstimulation (cell death)
- Treatment: Roluzole: blocks glutamate release, can only slow progress of disease
Duchenne Muscular Dystrophy
Progressive weakness & deterioration of muscle in boys (passed on from mothers)
-defective region on X chromosome encoding dystrophin contributing to cytoskeleton under muscles -> lack mRNA
Treatment: gene therapy; replacing defective gene by sending virus or transplanting stem cells
Myasthenia Gravis
Weakness & fatigability of voluntary muscles (typically facial expression)
- severity fluctuates a lot
- fatal when respiration compromised
- causes: autoimmune disease: system generates antibodies against own receptors; less effective ACh release at neuromuscular junction
- Treatment: drugs reinhibiting reuptake (leads to desensitization
- suppression of immune system (drugs, removal of thymus gland)
Premotor - primary motor area order
Info influencing movement must first be processed by association and higher order sensory cortices
- > then info must be communicated to non-primary motor areas
- > premotor areas coordinate output lvel of M1 & spinal cord (primary motor areas)
