Motor systems Flashcards
Spinal cord anatomy
Neurons/grey matter in middle
- dorsal horn - sensory, integration
- ventral horn - motor - A-alpha (fast!), gamma (slower, fewer)
- interneurons - activate patterns of alpha (excite one muscle, inhibit antagonist)
- smaller caudally
- cervical, lumbar enlargements - sensory and motor neurons for limb
Nerves - dorsal root and ventral root -> spinal nerve -> branches
- includes branches to SNS chain (T1-L2) from lateral horn
Overview of motor systems
Descending
- cortex - planning, initiating, voluntary
- brainstem - basic movements, posture
- basal ganglia -> cortex, (brainstem) - initiation
- cerebellum -> cortex, brainstem - coordination
Spinal cord
- input from cortex -> interneurons (most common, patterns), alpha (rapid, skilled)
- sensory input to interneurons (pain, stretch, tension)
- interneurons - patterns -> excite/inhibit muscle groups via alpha
- alpha/”lower motor” neurons -> all output to motor units
Fiber types
Alpha - A-alpha -> extrafusal (normal) muscle fibers
- 80-120 m/s (big diameter)
Gamma - A-gamma -> intrafusal (spindle) fibers
- 4-24 m/s (smaller)
Sensory
- Spindle (Ia) and golgi tendon (Ib) both A-alpha (fat, 80-120 m/s)
Organization of motor neurons
Motor pool - all neurons to same muscle
- clustered over several spinal segments (-> mult spinal nerves)
- surrounded by interneurons that control
Topography
- distal (hand) - lateral; axial - medial
- flexors - dorsal; extensors - ventral
- tracts mirror - lateral funiculus = limbs, anterior funiculus = axial
Recruitment of motor fibers
Motor unit - all fibers innervated by single neuron
- always activated together
Contraction - increase in firing rate -> recruit additional units
- max firing rate = 20 AP/sec
Muscle fiber types
Type I - red, aerobic (glycogen), small, weak, endurance
Intermediate - white, fast, powerful, resist fatigue
Type II - white, anaerobic (creatinine phos), large, fast, strong
Fiber type is determined by neuron contacting fiber
- ex small alpha neuron -> induces red Type I fibers
- can’t be changed via exercise (can strengthen differentially)
Contraction always begins with Type I -> recruit II for more power
Muscle spindle
Sensory organ - modified skeletal muscle fiber
Mechanical stretch -> annulospiral sensory nerve fiber -> 1A afferent (fastest!)
Innervated by gamma neuron (highly innervated)
- > only ends of fiber contract (nucleus is in middle)
- > new set point after muscle contraction (alpha, gamma together)
Respond to muscle stretch or contraction (-> stretch)
Not enough power to actually contract muscle
Muscle stretch reflex
Annulospiral/muscle spindle stretch (ie hitting tendon with hammer) -> A-alpha fiber ->
Synapses
- > direct to same muscle (quad contracts)
- > interneuron -> inhibits antagonist (hamstring relaxes)
Depends on level of excitability
Function - stabilizes, prevent unintended movement (hitting back of knee, pouring liquid into cup)
Super fast! (all A-alpha fibers)
Gamma loop
Efficient mechanism for slow, routine movements
Activate small group of gamma neurons
- > spindle senses stretch
- > activates spinal reflex
- > contraction of muscle, relaxation of antagonists
Once muscle spindles are set -> resist change from this length
Spasticity
Overactive response of gamma neurons
- due to absence of descending inhibitory input (brainstem)
- > excessive response to stretch
Golgi tendon organ
At junction of fiber and connective tissue
“Inversion myostatic”
Contraction -> tension -> 1B fiber (very fast, slightly slower than spindle)
-> interneurons -> relaxation of muscle, contract antagonist
-> also to cerebellum for coordination
Passive stretch - most absorbed by muscle fibers -> little activity
- active only at limits of compliance (protective)
Spasticity -> “clasp knife” - gives way under stretch
Flexion reflex
aka withdrawal
Pain -> type IV fibers ->
- flexion of limb
- extension of contralateral limb
Reciprocal/bilateral response
Mediated by interneurons (complex, multisynaptic)
Overactive = Babinski (extension of big toe)
Interneuron patterns
Interneurons can activate patterns (vs direct cortical control)
- ex walking
Also reflexes (cough, sneeze, cornea), many voluntary movements
- bilateral and vertical coordination
- both excitatory and inhibitory
- integrate sensory and cortical input
Topography of motor tracts
Mirrors topography of motor neurons
Anterior funiculus (medial) - axial, bilateral, gross movements
Lateral funiculus - limbs, detail, unilateral
Medial -> lateral
- medial vestibulospinal
- anterior/ventral corticospinal
- tectospinal
- pontine reticulospinal
- lateral vestibulospinal (anterior to pontine)
- medullary reticulospinal
- rubrospinal
- corticospinal
Medial motor systems
Tectospinal (adjacent to MLF) Medial vestibulospinal (MLF) Pontine reticulospinal (MLF) Anterior corticospinal - voluntary control of axial (Lateral vestibulospinal) (Medullary/lateral reticulospinal)
Terminate bilaterally in upper torso
-> neck and trunk movements
Most from MLF -> anterior/ventral funiculus
Tectospinal tract
Superior colliculus ->
decussates in midbrain (“dorsal tegmental decussation”) ->
adjacent to MLF -> anterior funiculus
cervical, upper thoracic (bilateral)
Sudden stimuli -> head and neck movements
Medial vestibulospinal tract
Medial vestibular nucleus (vestibular input) ->
medial to MLF -> descends -> anterior funiculus ->
cervical, upper thoracic
Vestibular stimuli, movement -> head and neck tone
Pontine reticulospinal tract
aka medial reticulospinal
Para-median pontine reticular formation (PPRF)
-> MLF -> anterior funiculus -> cervical, upper thoracic
Lateral eye movement -> head turning
Can function as indirect cortico-spinal tract (for gross movements)
Lateral vestibulospinal tract
Lateral vestibular nucleus ->
anterior portion of lateral funiculus (ipsilateral) ->
extensors at all spinal levels
Vestibular righting reflex (tip head to right -> extension on right)
Medullary reticulospinal tract
aka lateral reticulospinal tract
Medulla -> anterior part of lateral funiculus -> all spinal levels
inhibits interneurons -> activates gamma loop
Gross movements, muscle tone
Can be activated by cortico-bulbar tract (fx as indirect cortico-spinal tract for gross movements)
Lateral motor systems
Rubrospinal
Corticospinal
Control limbs, detailed movements
Anterior part of lateral funiculus (not technically lateral)
- medullary reticulospinal
- lateral vestibulospinal
Rubrospinal tract
Red nucleus (midbrain) ->
decussates in midbrain (ventral tegmental decussation) ->
lateral funiculus ->
interneurons
Proximal flexor muscles -> crawling
Can be indirect corticospinal tract
Corticospinal tract
Most important motor tract
Motor cortex (topography, pyramidal/Betz/upper motor neurons)
- > internal capsule (head at genu, arms, legs posterior)
- > cerebral peduncles (small area/percentage of fibers)
- > base of pons -> medullary pyramids
- > pyramidal decussation (90% of fibers cross)
- > lateral funiculus
Most to interneurons (pattern activation)
- some direct to alpha motor (fast, highly skilled, distal limbs)
- few to dorsal horn (sensory transmission, reflexes)
Uncrossed fibers -> anterior funiculus -> gross axial
Isolated lesion -> weakness of distal, not effect on reflexes
Cortical motor areas
Amount of cortex corresponds to complexity of movement
Precedes muscle movement (20 ms upper limb, 30 ms lower)
Motor - homonculus, simple movements (finger tapping)
- neurons are specific for direction of movement (not muscle or motor unit)
Premotor - preparation, rehearsal
Supplementary (SMA) - part of premotor
- deciding between movements, initiating
- lesion -> abulia (difficulty initiating)
Corticobulbar tracts
Cerebral cortex -> brain stem
Multiple functions
- majority pontine -> cerebellum
- motor cranial nerve nuclei (facial, eye)
- indirect corticospinal pathways
(corticotectal -> superior colliculus, corticoreticular -> reticular)
- sensory transmission
- autonomic
Facial control
Motor cortex (lateral) -> lower face = strictly crossed (unilateral "supranuclear" lesion -> contralateral deficit) -> upper face = bilateral (unilateral lesion -> no deficit) Facial nucleus - receives cortical input -> ipsilateral nerve (unilateral lesion -> complete ipsilateral deficit) Unilateral lesion -> lower face (most), tongue/swallow (some), upper face (little), jaw (none)
Anterior cingulate -bilateral, limbic (direct)> emotional movements
- can have hyperactive emotional responses if preserved (hypermimia, “pseudobulbar” affect)
Indirect cortico-spinal tracts
Muscle tone and stabilization
- ex flexing calf before pulling up on lever
Cortico-reticulo-spinal pathway
- premotor/supplementary motor -> medulla -> medullary reticulospinal pathway
- inhibitory (lesion -> overactive reflexes)
