The Reticular System: The Archtypal Motor System Horace Magoun



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Motor 3


  • The Reticular System: The Archtypal Motor System

  • Horace Magoun

  • Reticular activating system

  • Pontine RF: Stimulation facilitates spinal reflexes

  • Medullary RF: Stimulation inhibits spinal reflexes

  • Pontine RF -> medial reticulospinal tract

  • Medullary RF -> lateral reticulospinal tract

  • Contact with interneurons which do alot of local processing

  • Possibility for spinal hierarchy?

  • Feldman, Frog scratch experiments

  • Spinal Frog experiment, surface scratch.

  • Evidence for coordinated movement

  • Problem of spatial trajectories

  • Vestibulo-Spinal Reflexes

  • Magnus and ‘Central’ Reflexes

  • Vestibulocollic relexes

  • Keep head upright as body sways

  • body pitches forward: head tilts up

  • body pulled left, head tilts right

  • Vestbulospinal reflexes

  • Prepare for landing during falls

  • Control limb position

  • Figure 39-5 (K&S): Another example of reflexes switching sign

  • Site of this ‘reflex’ are the vestibular nuclei.

  • Generated by otolith organs

  • Proprioceptive info from neck in vestib n.

  • LIMBS: Otoliths -> lateral vestib n. -> lateral vestibulo-spinal tract

  • semi circ -> medial vestib -> medial vestibulo-spinal tract

  • Vestibulo-spinal reflexes are augmented by the Reticulo-spinal system as seen in cats:

  • Gahery expt

  • Normal Cat lifts forepaw to reach for object:

  • Shift in body weight to other limbs

  • Stimulate Cortex to get same (similar) reach, same adjustment in weigh

  • Inactivate RF and the weight shift occurs much later and in response to muscle feedback, suggesting that the redistribution observed is based upon efference copy.

  • Grillner and Locomotion

  • Locomotion in deafferented cats: Brown in the ‘teens

  • Thorasic Cut generates stepping to treadmill

  • Changes in Gait by Changes in speed

  • L-Dopa initiates stepping

  • Brainstem locomotor region, stim intensity controls gait

  • evidence of hierarchical control

  • NMDA receptors control oscillators

  • Concept that Gait involves phase change for different legs

  • At walk hind legs are at half-center

  • At gallop hind legs are in phase

  • Nashner and Postural Reflexes

  • The basic Nashner sliding or rotating platform approach to postural reflexes

  • System relies on 3 types of sensory inputs

  • vestibular - latency > 100msec

  • visual - latency > 100msec

  • proporioceptive - muscle - latency 70-100 msec

  • Example: Platform sway vs platform rotation

  • Same ankle stimulus exactly, stretching of same muscle

  • Different vestibular response

  • On sway and first rotation gastronemus contracts to straighten out body

  • On later rotations gastronemus relaxes, another example of relex ‘switching’

  • Example: Handle Pulls with shoulder braced and unbraced

  • With shoulder braced: unexpected handle pull and the arm stretch reflex defends arm position.

  • With Shoulder unbraced, contraction of the gastronemus is at same exact time as arm (biceps) contraction.

  • Decerebration Experiments

  • Sherrington & Bard

  • The strategy of the decerebration approach

  • limitations

  • will it recover?

  • Does absence of function localize function?

  • Bulbospinal’ (upper pontine cut)

  • Note meaning of ‘bulbe’

  • All spinal reflexes are preserved but . . .

  • Decerebrate rigidity is observed

  • increase in extensor excitability

  • decrease in flexor excitability

  • **Concept of Anti-gravity Muscles**

  • Animal Cannot:

  • right itself, (though it has some vestibular nucleus remaining)

  • sit/stand dynamically

  • run/walk (but note grillner experiments

  • Anal/defication reflex preserved

  • Bard: Cat on side, pinch tail: front but not back limbs “right”

  • What does that mean - especially reflex hyper excitability:

  • Modulation of reflex level

  • cuts of afferent fibers from muscle indicate that this is a spinal reflex in hyperexcitable state!!!! Note that Sherrington did this

  • General Principle: The brainstem ‘inhibits’ spinal reflexes.

  • Mesencephalic Cut

  • After 10 days to recover, stops looking like bulbospinal cut

  • Bard:

  • righting

  • walking

  • climbing

  • running

  • lordosis w/ estrogen

  • Decorticate Animals

  • Look mostly normal -

  • Learned motor responses are lost

  • Weiss ? He based some of his conclusions about plasticity on this.

  • Revaluation of the Cut experiments today

  • Limited but useful

  • Harvey Grill and motivated behavior

  • If you see it after the cut that’s explicit but if you dont, well . . .

  • Role of the Brainstem

  • Walking from Grillner

  • Inhibiting reflexes from bulbo-cut animals

  • High brainstem

  • Vestibulo-spinal reflexes

  • complete patterns of most behavior

  • No motivated or well regulated behavior governed here

  • Little or no learning happens here

Essay: Compare and contrast the Weiss view of the hierarchical organization of the nervous system and simple reflexology. Start by describing how you could build a complete set of stretch reflexes for a ball joint (p229). What extra-relexive mechanisms would this call for? How might you test Weiss’ contention that interactions below the ‘S’-level are fixed in most vertebrates?

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