Motor Systems Flashcards
what are the 2 major motor systems?
- pyramidal
- extrapyramidal
briefly describe pyramidal and extrapyramidal motor systems
- pyramidal: corticospinal and corticobulbar pathways
- extrapyramidal: cerebellum and basal ganglia
describe motor system hierarchy
describe hierarchical control of movement: reflexes
spinal cord
describe hierarchical control of movement: stereotyped, repetitive movements
- spinal cord
- brain stem
- cerebellum
- dependent on central pattern generators that produce rhythmic output needed for walking, flying, and breathing
describe hierarchical control of movement: goal-directed, voluntary movement
- cortex
- basal ganglia
what is level 1 of the motor system hierarchy?
spinal cord circuits: muscle conraction, reflexes, central pattern generators
describe the pyramidal 2 neuron system
lower and upper motor neurons
- lower motor neurons (LMN) innervate muscle
- upper motor neurons: cortical neurons that innervate LMNs
- reach LMNs through corticospinal and corticobulbar tracts
describe the motor unit
- lower motor neuron and the muscle fibers it innervates
- the building block of movement
describe the neuromuscular junction (lower motor neurons)
- synaptic connection from the nervous system to the muscle
- action potential triggers calcium release
- docking and then release of vesicles containing ACh
describe a reflex arc
- the simplest sensory-motor circuit
- simultaneous control of muscle contraction and relaxation
- the rest of the CNS exists to modify this reflex
describe the stretch reflex
- opposes muscle stretch through contraction
- muscle spindle detects muscle length (static response) and rate of change (dynamic response): these signs are relayed to the spinal cord
- within the cord, motor neurons are excited to activate compensatory contraction
describe gamma fiber activation of muscle spindles
- gamma motor neuron tells us how much stretching is happening
- intra and extrafusal muscle fibers innervated by gamma and alpha motor neurons respectively
describe the motor cortex
2nd neuron
- primary motor cortex
- brodmann area 4
- premotor cortex
- brodmann’s area 6 and 8
- planning and guidance
- projects directly to spinal cord
- giant pyramidal (betz) motor neurons
- largest CNS neurons
- layer 5
- long projection axons
- synapse on alpha-motor neurons
what is somatotopy?
brain maps the motor function of the body
describe the somatotopic organization of the motor cortex
what is the internal capsule?
- between thalamus and basal ganglia
- continuous with the cerebral peduncle
describe descent of the motor tract
describe the structures of the internal capsule
- anterior limb
- frontal functions
- not clinically relevant
- genu
- corticobulbar (face)
- posterior limb
- corticospinal tracts (CST)
- motor deficiencies
is somatotopy retained in the internal capsule?
yes
where does decussation of the corticospinal tract occur?
at the pyramids of the medulla
the motor cortex controls mostly ___ side
contralateral
describe pyramidal decussation
- >90% contralateral
- ~10% ipsilateral
- 8% anterior
- 2% lateral
describe the primary neuron
- decussation is at the level of the medulla
- somatotopy is maintained
what are 2 important features in the sacral region of the spinal cord?
- conus medullaris
- cauda equina
compare upper and lower motor neuron signs
realize that each column is corresponding to signs when either the upper or lower motor neuron is damage/lost
describe the corticobulbar tract
- motor cortex to brainstem cranial nerve nuclei
- muscles of face, head, and neck
- tracts stay ipsilateral, but usually innervate ganglia bilaterally
- travel in the genu of the internal capsule
- do not have major decussation at the pyramids
corticobulbar innervation is mostly bilateral. describe the exceptions
- facial nerve
- hypoglossal nerve
- if one side is blown out, it cannot be compensated for due to the lack of bilaterality
describe the extrapyramidal system
- interact with pyramidal motor systems, but they are distinct
- do not originate in cortex, but respond to cortical over-ride
- includes vestibulospinal, tectospinal, and rubrospinal systems
- also basal ganglia (contralateral control) and cerebellum (ipsilateral control)
describe the vestibulospinal tract
- extrapyramidal
- support posture and balance, control head movements
- relays information from the vestibular nuclei (VIII nerve) to spinal cord
describe the tectospinal tract
- extrapyramidal
- mediates reflex postural movements of the head from visual input
- originates in midbrain tectum (superior colliculus)
- axon fibers decussate in midbrain
- terminates in spinal cord
describe the rubrospinal tract
- extrapyramidal
- cerebellar inputs; perform planned movements (~alternative to corticospinal system)
- originates in red nucleus
- axon fibers decussate in midbrain
what is the clinical definition of basal ganglia?
extrapyramidal motor system
what does the basal ganglia do?
- modulates the initiation, termination, and amplitude of intentional movements
- unilateral disease causes contralateral signs
describe modulatory cortical loops
- basal ganglia afferents
- robust input from almost all parts of the cortex
- basal ganglia efferents
- ventral thalamic relay to cortex
- somatic motor loop - somatomotor control
- occulomotor loop: eye movements
- frontal loop: cognitive functions
- limbic loop: emotional and visceral functions
what are the inputs, outputs, intrinsic, and modulatory characteristics of the basal ganglia?
- input: caudate, putamen
- output: globus pallidus - internal segment; substantia nigra - pars reticulata
- intrinsic: globus pallidus - external segments; subthalamic nucleus
- modulatory: substantia nigra - pars reticulata
describe direct and indirect pathways
- direct pathway - facilitates movement; fewer neurons, so it is faster
- indirect pathway - inhibits movement; more neurons, so it is slower
*red = inhibitory, green = excitatory
describe the general difference between hyperkinesia vs. hypokinesia
- hyperkinesia - excessive movements, often with increasing dimentia; ex. Huntington’s disease/chorea
- hypokinesia - decreased movements; ex. parkinson’s disease
describe huntington’s disease
- decreased striatal output
- decreased inhibition of GPe
- increased inhibition of subthalamic pathways
- disinhibition of thalamic pathways
- hyperkinesis
*D1 and D2 refer to dopamine receptors
what is hemiballism?
- type of hyperkinesia
- lesion of subthalamic nucleus (stroke)
- sudden onset of abnormal movements
describe parkinson’s disease
- decreased dopamanergic output from substantia nigra compacta
- decreased inhibition of GPe (indirect) and GPi (direct)
- increased subthalamic nucleus activity
- increased activity of GPi
- decreased thalamic output
- hypokinesis
bottom line: direct pathway becomes less active, indirect pathway becomes more active
what are 2 therapies for parkinson’s disease?
- levadopa
- ablation or deep brain stimulation
function to re-establish circuit balance
describe cerebellum blood supply
vertebral arteries:
- superior cerebellar a.
- anterior inferior cerebellar a.
- posterior inferior cerebellar a.
what are the afferents and efferents of the cerebellum?
- afferents: climbing fibers (inf. olive) and mossy fibers (come from pons, most input)
- efferents: purkinje cells to cerebellar nuclei (main) or vestibular nuclei
what are the 4 nuclei of the cerebellum?
Fat Guys Eat Donuts
- fastigial nucleus: vestibular
- globose nucleus: muscular tone
- emboliform nucleus: muscle tone
- dentate nucleus: coordination, fine voluntary motor activity)
what are the 3 subcomponents of the cerebellar peduncles?
- superior (SCP): midbrain; efferents to thalamus
- middle (MCP): pons; corticopontocerebellar afferents
- inferior (ICP): midbrain; spinocerebellar afferents and efferents to brainstem
cerebellar peduncles are tracts in and out of the cerebellum
describe the spinocerebellar tract
- proprioceptive info comes in via sensory neurons
- information ascends ipsilaterally through inferior cerebellar peduncle
- axons synapse in cerebellar nuclei and cerebellar cortex
lesion will cause ipsilateral cerebellar signs
describe the cerebellar efferents of the thalamocortical tract
cerebellar efferents
- primary: purkinje cells to dentate nucleus
- secondary: exit to superior CP
- enters middle CP
- END: Cb hemispheres
- note: also sends collaterals to deep cerebellar nuclei
describe the cerebellar afferents of the corticopontocerebellar tract
- primary: corticopontine
- secondary: pontocerebellar (decussate in pons)
- enter middle CP
- END: Cb hemispheres
- note: also sends collaterals to deep cerebellar nuclei
describe corticopontocerebellar tract signs
in general, upper motor signs mostly obscure cerebellar signs
- lesion A: in internal capsule; upper motor neuron signs
- lesion B: in pons; upper motor neuron signs
- lesion C: in MCP; ipsilateral cerebellar signs
what is spinocerebellar ataxia?
- walking, balance, gait problems
- loss of precision and timing of movements
- swallowing, judging distances, spasticity
describe alcoholic cerebellar degeneration
- anterior cerebellar lobe degenerates
- gait, trunk, and lower limb ataxia
what 3 things can lesions of the vermis cause?
- truncal ataxia - loss of axial muscle coordination
- gait ataxia - unsteady gait, tied to anterior lobe syndrome/alcoholism
- abnormal saccades
what 6 things can lesions of the cerebellar hemispheres cause?
- ataxia - loss of coordination of voluntary movements
- hypotonia - decreased muscle tone
- intention tremor - swaying of limb or finger during directed movement
- dysmetria - over or under shooting a target
- dysdiadochokinesia - inability to perform rapidly alternating movements
- speech and cognitive impairments possible
what 2 things can lesions of the flocculonodular lobe cause?
- truncal ataxia - uncoordinated axial muscles result in poor balance
- nystagmus - uncoordinated and unintended eye movements (dancing eyes)
describe pallidotomy
ablation of the globus pallidus internal segment
- usually done in patients with levodopa associated dyskenisis
- bilateral pallidotomy produced severe apathy and often depression
- can also interfere with executive function and language (left side)
- can produce ipsilateral hemiballism
