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It is possible, however, that other mechanisms may be involved in its mediation. There is some indication that the cortical effect of reticular stimulation may be mediated by this diffuse thalamic projection system, for synchronized activity within it is similarly prevented by reticular excitation, and direct high frequency stimulation of this system, within the thalamus, reproduces the reticular response.
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The cortical recruiting response induced by low frequency stimulation of the diffuse thalamic projection system is reduced or abolished by reticular stimulation. The convulsive spikes produced by local strychnine and those of a fit following supramaximal cortical excitation, are not decreased by stimulating the reticular formation. Outside the sensory receiving area, secondary responses themselves may be reduced or prevented. Consequent sensory after-discharge is abolished, however, as is pyramidal tract discharge and jerky movements referable to it. In the chloralosane preparation, the secondary cortical response evoked by a sensory volley is generally unaffected by reticular stimulation. With barbiturate anesthesia, the reticular response is difficult to elicit or is abolished.
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In full chloralosane anesthesia, high voltage slow waves are blocked but no increase in lower amplitude, fast activity occurs. With synchrony, in spontaneous drowsiness or light chloralosane anesthesia, the effect of reticular stimulation is strikingly like Berger's alpha wave blockade, or any arousal reaction. In the "encephale isolé", reticular stimulation has no additional effect upon the fully activated EEG. Some background synchrony of electrocortical activity is requisite for manifestation of the response. The excitable substrate possesses a low threshold and responds best to high frequencies of stimulation. The bulbar effect is due to ascending impulses relayed through these more cephalic structures. This response can elicited by stimulating the medical bulbar reticular formation, pontile and midbrain tegmentum, and dorsal hypothalamus and subthalamus. The alteration is a generalized one but is most pronounced in the ipsilateral hemisphere and, sometimes, in its anterior part. Stimulation of the reticular formation of the brain stem evokes changes in the EEG, consisting of abolition of synchronized discharge and introduction of low voltage fast activity in its place, which are not mediated by any of the known ascending or descending paths that traverse the brain stem.