7.7. motor system¶

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

7.7.1. 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

  1. 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

7.7.2. 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

    1. 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

    1. 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

    1. 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

    1. 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

7.7.3. 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

      1. oculomotor loop

      • cortex -> caudate -> substantia nigra pars reticulata -> superior colliculus

    • non-motor loops 3. 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

      1. 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

    1. pars reticulata

    • doesn’t have dopamine (output from striatum)

  1. striatum contains spiny neurons

  2. caudate (for vision) - output to globus pallidus and substantia nigra (pars reticulata)

  3. 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

  1. 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)

7.7.4. 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)

  2. spinocerebellum = clarke’s nucleus, knows stretch of every muscle, many proprioceptors go straight into here

  • motor cortex

  • has a map of muscles

  1. 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)

7.7.5. 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

    1. VOR - vestibular ocular reflex - keeps eyes still

    • use vestibular system, occurs in comatose

    • fast

    • works better if you move your head fast

    1. 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)

    1. 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

    1. 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

7.7.6. 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)

    1. 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

    1. 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

    1. bladder function

    2. parasympathetic in sacral lateral horn make you pee (contracts bladder)

    3. turn off sympathetic NS

    4. 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

    1. sexual function

    • Viagra turns on parasympathetic NS

      • also gives temporal color blindness

    • sympathetic involved in ejaculation

      • temporal correlation (“Point and Shoot”)