The Reticular System: The Archtypal Motor System Horace Magoun
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The Reticular System: The Archtypal Motor System
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
Magnus and ‘Central’ Reflexes
Keep head upright as body sways
body pitches forward: head tilts up
body pulled left, head tilts right
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:
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.
Sherrington & Bard
The strategy of the decerebration approach
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**
right itself, (though it has some vestibular nucleus remaining)
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.
After 10 days to recover, stops looking like bulbospinal cut
lordosis w/ estrogen
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
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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