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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