Neuroscience  ·  chapter 4 of 8  ·  9 min read

Motor system

notes from Purves et al. "Neuroscience", 5th edition (chapter numbers in headers) + Intro to neurobiology course at UVA

About this chapter

The motor system is built in layers, and the chapter follows Purves chapters 16 to 21 from the bottom up. Lower motor neurons in the spinal cord drive muscles and run reflexes on their own. Upper motor neurons in cortex and brain stem send commands down to them, and the upper vs. lower lesion table tells the two apart.

Two loops then shape those commands without sending anything to the spinal cord directly: the basal ganglia pick which action to release, and the cerebellum compares copies of the command with what happened and fine-tunes it. The last two sections apply the same parts to the eyes (five kinds of eye movement and their gaze centers) and to the organs (sympathetic vs. parasympathetic control).

Chapter outline: 6 sections

1. 16 lowerAlpha motor neurons in the ventral horn contract muscle; gamma motor neurons keep the muscle spindle taut so it can keep measuring stretch.
2. 17 upper3 figuresTwo descending systems.
3. 18 basal ganglia (choose what you want to do)1 interactive3 figuresThe key fact is a double negative.
4. 19 cerebellum (fine tuning all your motion)7 figuresThe cerebellum gets a copy of every motor command (from cortex via the pontine nuclei) and of the body's state (from the spinal cord and vestibular nuclei).
5. 20 eye movements/integration2 figuresFive eye movement systems, from fast reflexes that need no cortex (VOR) to voluntary ones (smooth pursuit, vergence).
6. 21 visceral (how you control organs, stress levels, etc.)4 figuresThe autonomic system has two opposing divisions: sympathetic (fight or flight, adrenergic) from the lateral horn of the spinal cord, and parasympathetic (rest, ACh) from the brain stem and sacral cord.

16 lower

Alpha motor neurons in the ventral horn contract muscle; gamma motor neurons keep the muscle spindle taut so it can keep measuring stretch. Spindles excite the stretched muscle and Golgi tendon organs inhibit it before it tears.

• 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

Two descending systems. Brain stem tracts handle posture, by feedback (vestibulospinal) and by feedforward anticipation (reticulospinal). Motor cortex sends goals through Betz cells, most of whose axons cross the midline to control the limbs.

• signs and symptoms of upper and lower motor neuron lesions (table 17.1)

  upper motor neuron syndrome	lower motor neuron syndrome
  weakness	weakness or paralysis
  spasticity	decreased superficial reflexes
  increased tone	hypoactive deep reflexes
  hyperactive deep reflexes	decreased tone
  clonus	fasciculations and fibrillations
  Babinski’s sign	severe muscle atrophy
  loss of fine voluntary movements

• 

  Fig. 114 Figure 1. The three brain stem tracts to the spinal cord: vestibulospinal (posture, by feedback), reticulospinal (posture, by feedforward), and colliculospinal from the superior colliculus (turning the head to look).

• 

  Fig. 115 Figure 2. Two ways of keeping posture. Feedforward: the central command adjusts posture through the reticular formation at the same time as it moves the limb. Feedback: the vestibular system corrects instability that wasn’t anticipated.

• 

  Fig. 116 Figure 3. The corticospinal and corticobulbar tracts from cortex down through midbrain, pons and medulla. At the pyramidal decussation about 90% of the axons cross to form the lateral tract (limbs); the 10% that stay ventral control posture.

• 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)

The key fact is a double negative. The globus pallidus fires all the time and inhibits the VA/VL thalamus, so no action goes through. Cortex excites the striatum, the striatum inhibits the pallidus, and that releases VA/VL. Dopamine from the substantia nigra turns up the gain of the cortex-to-striatum step.

Interactive 1. A toy rate model of the body movement loop. With normal dopamine, a strong cortical command shuts the pallidus off and VA/VL crosses threshold. Lower dopamine toward 20%, the level the note gives for Parkinson's: the same command no longer gets through, which matches "know what they need to do, don't have enough dopamine to act."

• 

  Fig. 117 Figure 4. Where the motor parts of the basal ganglia sit (caudate, putamen, globus pallidus, subthalamic nucleus, substantia nigra) and the circuit as signs: cortex and dopamine excite the striatum, the striatum inhibits the pallidus, the pallidus inhibits VA/VL, and VA/VL excites the premotor commands.

• 

  Fig. 118 Figure 5. The two outputs of the striatum. Both pass through two inhibitory steps: via the globus pallidus to VA/VL and frontal cortex, and via substantia nigra pars reticulata to the superior colliculus (eye movements).

• 

  Fig. 119 Figure 6. The body movement loop drawn on the brain: cortex to putamen and caudate, putamen to globus pallidus, pallidus to VA/VL and back to cortex, with dopamine from the substantia nigra feeding the striatum.

• “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 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

    4. 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

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

2. globus pallidus

• each spiny neuron connects to one globus pallidus neuron

• deja vu - spiny neuron you haven’t fired in a while

3. 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)

The cerebellum gets a copy of every motor command (from cortex via the pontine nuclei) and of the body’s state (from the spinal cord and vestibular nuclei). Purkinje cells are its only output and are inhibitory; climbing fibers from the inferior olive act as the training signal.

• 

  Fig. 120 Figure 7. Inputs to the cerebellum: cortex reaches the cerebrocerebellum through the pontine nuclei; Clarke’s, cuneate and vestibular nuclei feed the spino- and vestibulocerebellum.

• 

  Fig. 121 Figure 8. Outputs of the cerebellum: the cerebrocerebellum projects to VA/VL and back to cortex; the other divisions project to the red nucleus, inferior olive and the vestibular nuclei.

• 

  Fig. 122 Figure 9. The cerebellar circuit. Mossy fibers excite granule cells, whose parallel fibers excite Purkinje cells; climbing fibers from the inferior olive contact single Purkinje cells. Purkinje cells inhibit the deep nuclei, which also get direct excitation, so there is a deep excitatory loop and a cortical inhibitory loop.

• 

  Fig. 123 Figure 10. Each cerebellar division has its own deep nucleus and target: cerebrocerebellum to dentate to premotor cortex (planning), spinocerebellum to interposed and fastigial nuclei to motor cortex and brain stem (execution), vestibulocerebellum to vestibular nuclei (balance and eye reflexes).

• 

  Fig. 124 Figure 11. Inputs again as a block diagram: frontal and parietal cortex reach the cerebellum through the pontine nuclei, crossing the midline; the inferior olive, spinal cord and vestibular nucleus enter on the same side.

• 

  Fig. 125 Figure 12. The path back to cortex: deep nuclei cross the midline to the VL thalamus, which projects to primary motor and premotor cortex.

• 

  Fig. 126 Figure 13. The path to the brain stem: the fastigial nucleus projects to the superior colliculus and reticular formation, and cerebellar cortex projects straight to the vestibular nuclei, all ending on lower motor neurons in the medial ventral horn.

• 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

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

Five eye movement systems, from fast reflexes that need no cortex (VOR) to voluntary ones (smooth pursuit, vergence). The superior colliculus and frontal eye field both drive saccades, and losing only one of them can be compensated.

• 

  Fig. 127 Figure 14. The six muscles of each eye and the direction each one pulls: four recti (up, down, in, out) and two obliques that rotate the eye.

• 

  Fig. 128 Figure 15. Areas that control saccades and pursuit in the monkey brain: frontal eye field, lateral intraparietal area and MT in cortex; caudate and substantia nigra pars reticulata (the oculomotor loop); superior colliculus, pontine and reticular nuclei, and the cerebellum and vestibular nuclei.

• 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.)

The autonomic system has two opposing divisions: sympathetic (fight or flight, adrenergic) from the lateral horn of the spinal cord, and parasympathetic (rest, ACh) from the brain stem and sacral cord. The nucleus of the solitary tract is the hub where visceral input comes in.

• 

  Fig. 129 Figure 16. Sympathetic (left) and parasympathetic (right) divisions and the organs each one reaches, with the effect on each: e.g. sympathetic dilates the pupil and speeds the heart, parasympathetic constricts the pupil and slows the heart.

• 

  Fig. 130 Figure 17. The nucleus of the solitary tract takes visceral sensory input and drives both local reflexes (through preganglionic neurons) and, through the forebrain, hormonal and behavioral responses.

• A central autonomic network for the control of visceral motor function

  

  Fig. 131 Figure 18. The central autonomic network: insula, medial prefrontal cortex, amygdala and hypothalamus sit above the brain stem reticular formation and the nucleus of the solitary tract, which connect to the preganglionic neurons and end organs.

• 

  Fig. 132 Figure 19. Where autonomic output leaves the CNS: parasympathetic from the brain stem (dorsal nucleus of the vagus) to ganglia near the organs, sympathetic from each spinal level to the chain of ganglia beside the cord.

• 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

       1. edinger westphal nucleus - pupil-constriction

       2. salivatory nucleus

       3. vagus nucleus - digestive system, sexual function

       4. nucleus ambiguous - heart

       5. 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

    2. 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

  3. sexual function

     • Viagra turns on parasympathetic NS

       • also gives temporal color blindness

     • sympathetic involved in ejaculation

       • temporal correlation (“Point and Shoot”)

Key ideas

• Spinal circuits run reflexes on their own: muscle spindles excite a stretched muscle and Golgi tendon organs inhibit it before it tears.

• Upper motor neuron lesions give spasticity and hyperactive reflexes; lower motor neuron lesions give weak reflexes, low tone and atrophy.

• The basal ganglia work by disinhibition: the striatum inhibits the pallidus, which releases the VA/VL thalamus, and dopamine turns up the gain.

• The cerebellum compares copies of motor commands with what happened, with climbing fibers from the inferior olive as the training signal.

• Eye movements range from reflexes that need no cortex (VOR) to voluntary ones (smooth pursuit, vergence), and the superior colliculus and frontal eye field back each other up for saccades.

• Sympathetic and parasympathetic divisions oppose each other, and the nucleus of the solitary tract is where visceral input comes in.