NEURO217 Motor system

motor hierarchy (3 main components)

lower motor neurons, upper motor neurons, extrapyramidal system

lower motor neurons (final common pathway)

spinal reflexes (proprioceptors and spinal interneurons)

cranial nerves

skeletomuscular efferents

autonomic efferent

upper motor neurons (descending control)

pyramidal (primary) motor system — planning, initiating, and directing voluntary movements

brainstem systems — basic movements and postural control

extrapyramidal system

basal ganglia

cerebellum

parts of midbrain

motor hierarchy movement path

basal ganglia — gates proper initiation of movement; cerebellum — sensory motor coordination; premotor/parietal/parts of the midbrain — coordinate transforms, planning → upper motor neurons (motor cortex, brainstem centers) → 1. local circuit neurons (sensory inputs → spinal cord and brainstem circuits) — reflex coordination → lower motor neurons → skeletal muscles; or 2. lower motor neurons → skeletal muscles

upper motor neurons; ex: lateral corticospinal pathway (pyramidal, primary motor)

provide descending (central) control over lower motor neurons and spinal reflexes

other (extrapyramidal) parallel pathways are important for other functions

ex. corticobulbar tract for facial movements, vestibulospinal tract for postural adjustments

primary motor cortex (M1, BA 4)

source of origin of corticospinal tracts (70%) — large specialized pyramidal cells in La V (Betz cells)

contralateral movement —> somatotopic mappings

motor seizures — “march” down the body; orderly progression — when seizure spreads, spreads in a somatotopic way

what is coded in M1?

m1 microstimulation elicits particular movements of combinations of muscle groups; neurons will code for a bunch of muscles; lower in the spinal cord is more 1:1, but going higher elicits more complex combinations

neurons fire before movement is made (stimulates spinal cord, when then is the proximal cause of the movement (lower motor neurons))

target brainstorm/spinal cord for complex (multi-joint) movement

what else is coded in M1?

Magnitude and direct of force to be exerted

direction of visually-guided movements determined by population coding

relative discharges in large number of neurons yields direction sensitivity

how can action potential & silencing be seen with neurons

how neurons move and what direction they prefer — neurons all favor different directions, but subject wants to move in a specific direction, so neurons that prefer this direction will start firing, and this collective firing results in movement

upper motor neuron syndromes

somatotopic paresis (weakness/hypotonia), especially for extremities, which partially recovers due to plasticity in motor representations — neurons overlap and some can take over representations

in children, long myelinated fibers are not completely developed; this is tested, normal plantar response will be flexion (toes going down), Babinski sign will be a fanning of the toes bc of damage to descending corticospinal pathways

spasticity/decerebrate rigidity (loss of inhibitory control over vestibulospinal/reticulospinal tracts)

hyperreflexia of sensory-motor reflexes

lateral corticospinal damage = loss of fine motor control = lack movement control, no strong movements

support systems for upper motor neurons

premotor/parietal

parietal cortex transforms sensory inputs into a body-centric space to guide action

parietal cortex, somatosensory system

retinatopic system coordinate transform → somatosensory system → parietal takes in sensory inputs → body-centric space → guides action

damage to parietal cortex can disrupt goal-directed reaching and eye movement (optic ataxia — making clumsy movements based on visual input)

the visual system in not in the somatosensory system, but is tuned into the retinatopic system (where you are looking)

parts and functions of the premotor cortex (BA 6)

30% of corticospinal tracts arise from this region

lateral premotor cortex (externally guided)

medial premotor cortex (internally guided)

neurons respond maximally during movement planning: more respond earlier compared to those in the motor cortex, neurons shut off once motor movement is made

more activation in supplementary motor area (medial premotor cortex) in self-initiated movement versus visually triggered movement

lateral premotor cortex function

initiates externally guided motor movements; based on visual input to make motor response = sensory cue → particular movement

visuomotor learning (e.g. red vs green traffic lights)

control of eye movements (frontal eye fields)

medial premotor cortex (supplementary motor cortex) function

initiates internally-guided motor movements

damage = reduced self-initiated movements (e.g., chosen altering driving paths on a whim — change of plans (like getting coffee instead of going straight home)

readiness potential (scalp EEG)

precedes both intentions and actions

starts out anterior and bilateral (premotor), then shifts posterior and contralateral (motor)

enhanced when attention is focused on motivations to act rather than action itself — recruits medial premotor, dorsolateral, prefrontal, and parietal cortices)

basal ganglia anatomy

collections of subcortical structures at the base of the forebrain

neostriatum (corpus striatum,)

paleostriatum (globus pallidus)

sub thalamus (diencephalon)

substantia nigra (midbrain)

output of basal ganglia system: Globus Pallidus internal (GPi)/ Substantia Nigra pars reticulata

protect to thalamus (VA/VL) and superior colliculus

basal ganglia has to remove tonic inhibition to generate action (mostly internal self-initiated)

one of few brain areas without reciprocal connections to the cortex

output is inhibitory with high resting activity

disengagement of output neurons needed to release goal-directed movements in a direct pathway

parallel pathways in basal ganglia anatomy (car analogy)

direct vs indirect pathways

indirect involves subthalamus, which serves as an emergency brake and engages tonic inhibition

direct involves thalamus, which serves as the accelerator → disinhibition on direct pathway

basal ganglia disorders

hemiballismus

parkinson’s disease

huntington’s disease

hemiballismus

disease of the subthalamus

removes brake of indirect pathway so no tonic inhibition = spontaneous, uncontrolled movements on contralateral side

tonic, phasic

tonic: at rest

phasic: once region becomes activated by cortex, inhibition is temporarily released so movement can be executed

Parkinson’s disease

DA (dopamine) nigrostriatal pathway degenerates (50-80%)

linked to neurotransmitter dysfunction

loss of dopamine input to caudate containment → increased inhibitory outflow to thalamus = removes accelerator and simultaneously engages brakes

motor symptoms of parkinson’s

slow movements, lack of movement, difficulty initiating movements, starting a movement but cannot initiate stop, adjusting force (ballistic movements)

impacts postural muscles (gait, posture) — “shuffling” locomotion

special difficulty with internally-drvien motor plans (medial premotor cortex)

treatments for parkinson’s

behavioral: use external sensory cues to help initiate movement

pharmacological: replace DA, but problem with blood-brain barrier, so use L-DOPA as precursor, but problem is non-specific, so can produce schizophrenic-like symptoms, and can target other things

implanted deep brain stimulators (inhibitory or excitatory: thalamus, Globus Pallidus (internal)/subthalamus — impact of degeneration = more tonic movement → increases inhibitory outflow to thalamus so implanted deep brain stimulation to help

Huntington’s disease

progressive degenerative disorder, late adult onset (40s-50s)

inherited

loss of medium spiny neurons that project to Globus Pallidus external (indirect pathway) — so cannot apply the brakehu

Huntington’s disease motor symptoms

excess movements, coordination across multiple areas and not just contralateral control, difficult to test motor functions

irregular gestures because movement is less goal-oriented

loss of sustained muscular contraction

the cerebellum components

nuclear group (deep nuclei) OUTPUT

codex (anterior lobe (paleocortex), flocculonodular lobe (archicortex), lateral lobes

anterior lobe

spinocerebellum (muscle tone/reflexes)

1. vermis (proximal)

2. paramedian zone (distal)

flocculonodular lobe: vestibulocerebellum (balance)

lateral lobes: cerebrocerebellum (sensorimotor coordination/timing/errors)

the cerebellum

body representation in cerebellum is IPSILATERAL

spinocerebellar and vestibular axons don’t cross

descending projections from cortex cross in peduncles and then cross back

somatotopic maps in spinocerebellum

large fibers interconnecting cerebrum and cerebellum (20x corticospinal tract)

results of damage to the lateral cerebellar cortex (neocortical)

ataxia - clumsy, uncoordinated movements; no error correction (feedback)

hypermetria — rapid pointing movements extend beyond target

problems switching between gestures (irregular complex movements)

poorly timed movements, especially for over practiced ones (e.g., tennis serve)

abnormal reflect conditioning/motor learning

error-correcting role of cerebellum

cerebellum involved in on-line and feedback-based guidance of movement trajectories and movement correction following termination

lateral cerebellar activity, motor skill accuracy

participants track position of target with random jumps (simulates hypermetria)

cerebellum signals the difference between motor commands issued and actual movements produced

can be used to help generate accurate internal models of outcomes of motor processes

cerebellar versus basal ganglia loops

externally-guided versus internally-guided sequences/target actions

termination (correction) vs initiation of movements

interaction of each level of motor processing for different aspects of action (e.g., planning, sequencing, executing force, and timing)

what is motor system divided into

upper and lower motor neurons and modulatory circuits

how does primary cortex work

indexes both the direction and force of movements through population coding

medial vs lateral premotor cortices response

movement initiation: medial is internal cues, lateral is external cues

complex movements and coordination

complex movements require coordination across disparate motor components — transformation of coordinate systems into somatomotor space in parietal cortex, planning and selection in the premotor cortex, movement initiation in the basal ganglia and sensorimotor coordination in the cerebellum

basal ganglia pathways

both direct and indirect processing pathways act antagonistically

cause of parkinson’s disease

degeneration of dopamine in nigrostriatal pathway, other syndromes emerge due to imbalance in processing pathways within the basal ganglia

cerebellum has…

parallel pathways for vestibular, balance, sensorimotor control

error-correction mechanisms, which contribute to smooth trajectories of movements and appropriate stopping at target locations, and motor learning