Chandan Singh | Motor system

motor system

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notes from Neuroscience, 5th edition + Intro to neurobiology course at UVA

16 lower

  • sensory in dorsal spinal cord, motor in ventral
    • farther out neurons control farther out body parts (medial=trunk, lateral=arms,legs)
  • one motor neuron (MN) innervates multiple fibers
    • the more fibers/neuron, the less precise
    • MN pool - group of MNs=motor units
  • muscle tone = all your muscles are a little on, kind of like turning on the car engine and when you want to, you can move forward
    • more firing = more contraction
  • MN types
    1. fast fatiguable - white muscle
    2. fast fatigue-resistant
    3. slow - red muscles, make atp
      • muscles are innervated by a proportion of these MNs
  • reflex
    • whenever you get positive signal on one side, also get negative on other
    • flexor - curl in (bicep)
    • extensor - extend (tricep) 1. proprioceptors (+) - measure length - more you stretch, more firing of alpha MN to contract
    • intrafusal muscle=spindle - stretches the proprioceptor so that it can measure even when muscle is already stretched
      • $\gamma$ motor neuron - adjusts intrafusal muscles until they are just right
        • keeps muscles tight so you know how much muscle is streteched
        • if alpha fires a lot, gamma will increase as well
          • high gamma allows for fast responsiveness - brainstem modulators (serotonin) also do this
      • opposes muscle stretch to keep it fixed
      • spindle -> activates muscles -> contracts -> turns off
      • sensory neurons / gamma MNs innervate muscle spindle
    • homonymous MNs go into same muscle, antagonistic muscle pushes other way 2. golgi tendon (-) measures pressure not stretch
    • safety switch
    • inhibits homonymous neuron so you don’t rip muscle off
  • ALS = Lou Gehrig’s disease
    • MNs are degenerating - reflexes don’t work
    • progressive loss of $\alpha$ MNs
    • last neuron to go is superior rectus muscle -> people use eyes to talk with tracker
  • CPG = central pattern generator
    • ex. step on pin, lift up leg
    • walking works even if you cut cat’s spinal cord
    • collection of interneurons

17 upper

  • cAMP is used by GPCR
  • lift and hold circuit
    1. ctx->lateral white matter->lateral ventral horn->limb muscles
      • lateral white matter - most sensitive to injury
    2. brainstem->medial white matter->medial horn->trunk
      • medial white matter -> goes into trunk
  • bulbarspinal tracts
    1. lateral and medial vestibulospinal tracts - feedback
      • automated system - not much thinking
      • posture - reflex
      • too slow for learning surfing
    2. reticular - feedforward = anticipate things before they happen
      • command / control system for trunk muscles (posture)
      • feedforward - not a reflex, lean back before opening drawer
      • caudal pontine - feeds into spinal cord
    3. colliculospinal tract
      • has superior colliculus - eye muscles, neck-looking
      • see ch. 20 - reflex
  • corticular bulbar tract (premotor->primary motor->brainstem)
    • motor cortexes - this info is descending
    • can override reticular reflexes in reticular formation
    • premotor cortex (P2) - contains all actions you can do
      • has mirror neurons that fire ahead of primary neurons
        • fire if you think about it or if you do it
    • primary motor cortex (P1)
      • layer 1 ascending
      • layer 4 input
      • layer 5 - Betz cells - behave like 6 (output)
      • layer 6 - descending output
      • has map like S1 does
        • Jacksonian march get seizure that goes from feet to face (usually one side)
          • epileptic seizure - neurons fire too much and fire neurons near them
            • insular - flashes of moods
            • pyriform - flashes of smells
    • Betz cells - if they fire, you will do something
      • dictate a goal, not single neuron to fire
      • axons to ventral horn of spinal cord
  • lesions
    1. upper
      • spasticity - unorganized leg motions
      • increased tone - tight muscles
      • hyperactive deep reflexes
      • ex. babinski’s sign
      • curl foot down a lot because you don’t know how much to curl
      • curling foot down = normal plantar
      • more serotonin can cause this
    2. lower
      • hypoactive deep reflexes
      • decreased tone
      • severe muscle atrophy
  • pathways
    • Betz cell
      • 90% cross midline in brainstem - control limbs
      • 10% don’t cross - trunk muscles

18 basal ganglia (choose what you want to do)

  • “who you are”
  • outputs
    1. brainstem
    2. motor cortex
  • 4 loops (last 2 aren’t really covered)
    • motor loops
      1. body movement loop
        • SnC -> S (CP) -> (-) Gp -> (-) VA/VL -> motor cortex
      2. oculomotor loop
        • cortex -> caudate -> substantia nigra pars reticulata -> superior colliculus
    • non-motor loops
      1. prefrontal loop - daydreaming (higher-order function)
        • spiny neurons corresponding to a silly idea (alien coming after you) filtered out because not fired enough
        • schizophrenia - can’t filter that out
      2. limbic loop - mood
        • has nucleus accumbens
        • can make mood better with dopamine
  • substantia nigra
    1. pars compacta - dopaminergic neurons (input to striatum)
      • more dopamine = more strength between cortical pyramidal neurons and spiny neurons (turns up the gain)
      • dopamine helps activate a spiny neuron
      • may be the ones that learn (positive outcome is saved, will result in more dopamine later)
      • Parkinson’s - specific loss of dopaminergic neurons
      • dopaminergic neurons form melanin = dark color
      • when you get down to 20% what you were born with
      • know what they need to do - don’t have enough dopamine to act
      • treat with L Dopa -> something like dopamine -> take out globus pallidus - cocaine, amphetamine - too much dopamine - Huntington’s - death of specific class of spiny neurons
      • have uncontrolled actions - Tourette’s - too much dopamine
      • also alcohol - MPPP (synthetic heroin)
      • MPTP looks like dopamine but turns into MPP and kills dopaminergic neurons
      • treated with L Dopa to reactivate spiny neurons
    2. pars reticulata
      • doesn’t have dopamine (output from striatum)
      1. striatum contains spiny neurons
    3. caudate (for vision) - output to globus pallidus and substantia nigra (pars reticulata)
    4. putamen - output only to globus pallidus
      • each spiny neuron gets input from ~1000 cortical pyramidal cells
      1. globus pallidus
        • each spiny neuron connects to one globus pallidus neuron
        • deja vu - spiny neuron you haven’t fired in a while
      2. VA/VL (thalamus)
        • all motor actions must go through here before cortex
        • has series of commands of all actions you can do
        • has parallel set of betz cells that will illicit those actions
        • VA/VL is always firing, globus pallidus inhibits it (tonic connection)

19 cerebellum (fine tuning all your motion)

  • redundant system - cortex could do all of this, but would be slow
  • repeated circuit - interesting for neuroscientists
  • all info comes in, gets processed and goes back out
    • cerebellum gets motor efferant copy
    • all structures on your brain that do processing send out efferent
    • cerebellum sends efferant copy back to itself with time delay (through inferior olive) 1. cerebrocerebellum
    • deals with premotor cortex (mostly motor cortex)
      1. spinocerebellum = clarke’s nucleus, knows stretch of every muscle, many proprioceptors go straight into here
    • motor cortex
    • has a map of muscles 3. vestibular cerebellum - vestibular->cerebellum->vestibular
    • vestibular system leans you back but if wind blows, have to adjust to that
  • input
    • pontine nuclei (from cortex)
    • vestibular nuclei (balance)
    • cuneate nucleus (somatosensory from spinal upper body)
    • clarke (proprio from spinal lower body)
  • processing
    • cerebellar deep nuclei
  • output
    • deep cerebellar nuclei
      • go to superior colliculus, reticular formation
    • VA/VL (thalamus) - back to cortex
    • red nucleus
  • circuit 1 - fine-tuning
  • circuit 2 - detects differences, adjusts
    • cerebellum -> red nucleus (is an efferant copy) -> inferior olive -> cerebellum
    • compare new copy to old copy
  • cells
    • purkinje cells - huge number of dendrite branches - dead planar allows good imaging
      • GABAergic
    • (input) mossy fibers -(+)> granule cells (send parallel fibers) -(+)> purkinje cell -(-)> deep cerebellar nuclei (output)
      1. mossy->granule->parallel fibers connect to ~100,000 parallel fibers
      2. climbing fiber - comes from inferior olive and goes back to purkinje cell (this is the efferent copy) = training signal
    • loops
      • deep excitatory loop (climbing/mossy) -(+)-> deep cerebellar nuclei
      • cortical inhibitory loop (climbing/granule) -(+)-> purkinje
        • the negative is from purkinje to deep cerebellar nuclei
  • alcohol
    • can create gaps = folia
    • long-term use causes degeneration = ataxia (lack of coordination)

20 eye movements/integration

  • Broca’s view - look at people with problems
  • Ramon y Cajal - look at circuits
  • 5 kinds of eye movements
    1. saccades
      • use cortex, superior colliculus (visual info -> LGN -> cortex, 10% goes to brainstem)
      • constantly moving eyes around (fovea)
      • ~scan at 30 Hz
      • 5 Hz=200 ms for cortex to process so pause eyes (get 5-6 images)
      • there is a little bit of drift - can’t control this - humans are better than other animals at seeing things that aren’t moving
    2. VOR - vestibular ocular reflex - keeps eyes still
      • use vestibular system, occurs in comatose
      • fast
      • works better if you move your head fast
    3. optokinetic system - tracks with eyes
      • ex. stick head out window of car and track objects as they go by
      • slower than VOR (takes 200 ms)
      • works better if slower
      • reflex
      • in cortex (textbooks) but probs brainstem (new)
    4. smooth pursuit - can track things moving very fast
      • suppress saccades and track smoothly
      • only in higher apes
      • area MT is highest area of motion coding and goes up and comes down multiple ways
      • high speed processing isn’t understood
      • could be retina processing
    5. vergence - crossing your eyes
      • suppresses conjugate eye movements
      • we can control this
      • only humans - bring objects up very close
      • reading uses this
  • eye muscles
    • rectus
      • vertical
        • superior
        • inferior
        • use complicated vertical gaze center
          • last to degenerate in ALS
          • locked-in syndrome - can only move eyes vertically
          • controls oculomotor nucleus
      • lateral
        • medial
        • lateral (controlled by abducens)
        • use horizontal gaze center=PPRF which talk to abducens -MLF connects abducents to opposite medial lateral rectus muscle
      • oblique - more circular motions
        • superior (controlled by trochlear nucleus)
        • inferior
    • everything else controlled by oculomotor nucleus
  • superior colliculus has visual map
    • controls saccades, connects to gaze centers
    • takes input from basal ganglia (oculomotor loop)
    • also gets audio input from inferior colliculus (hear someone behind you and turn)
    • gets strokes
    • redundant with frontal eye field in secondary motor cortex
      • connects to superior colliculus, gaze center, and comes back
      • if you lose one of these, the other will replace it
      • if you lose both, can’t saccade to that side

21 visceral (how you control organs, stress levels, etc.)

  • parasympathetic works against sympathetic
  • divisions
    1. sympathetic - fight-or-flight (adrenaline)
      • functions
      • neurons to smooth muscle
      • pupils dilate
      • increases heart rate
      • turn off digestive system
      • 2 things with no parasympathetic counterpart
        • increase BP
        • sweat glands - location
      • neurons in spinal cord lateral horn
        • send out neurons to sympathetic trunk (along the spinal cord)
        • all outgoing connections are adrenergic - beta-adrenergic drugs block adrenaline
      • beta agonist - activates adrenaline receptors (do this before EKG)
    2. parasympathetic - relaxing (ACh)
      • location
      1. brainstem
      2. edinger westphal nucleus - pupil-constriction
      3. salivatory nucleus
      4. vagus nucleus - digestive system, sexual function
      5. nucleus ambiguous - heart
      6. nucleus of the solitary tract - all input/output goes through this 1. rostral part (front) - taste neurons 2. caudal part (back) contains all sensory information of viscera (ex. BP, heart rate, sexual
      7. sacral spinal cord (bottom) - gut/bladder/genitals
        • not parallel to sympathetic – poor design - may cause stress-associated diseases
          • hard to make drugs with ACh
    3. enteric nervous system - in your gut
      • takes input through vagus nerve from vagus nucleus
      • also has sensory neurons and sends afferents back to brainstem
  • pathway
    • insular cortex - what you care about
    • amygdala - contains emotional memories
    • hypothalamus - controls a lot
      • mostly peptinergin neurons
      • aging, digestion, mood, straight to bloodstream & CNS
      • releases hormones
      • ex. leptin - stops you eating when you eat calories
    • reticular formation - feedforward, prepares digestion before we eat
  • three examples
    1. heart rate
      • starts at nucleus ambiguous
      • also takes input from chemoreceptors (ex. pH)
      • SA node at heart generates heartbeat - balances Ach and adrenaline
      • sympathetic sends info from thoracic spinal cord - heart sends back baroreceptor afferents
    2. bladder function
    3. parasympathetic in sacral lateral horn make you pee (contracts bladder)
    4. turn off sympathetic NS
    5. open sphincter muscle (voluntary)
      • can also control this via skeletal nervous system
      • circuit
        • amygdala (can’t pee when nervous)
        • pontine micturation center -> parasympathetic preganglionic neurons -> parasympathetic ganglionic neurons
        • inhibitory local circuit neurons -> somatic MNs
    6. sexual function
      • Viagra turns on parasympathetic NS
      • also gives temporal color blindness - sympathetic involved in ejaculation
      • temporal correlation (“Point and Shoot”)