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1. The effect of electrical stimulation of the brain stem on reflex transmission has been investigated in decerebrate cats after partial transection of the spinal cord.
2. Brain stem stimuli that do not evoke inhibitory post-synaptic potentials (IPSPs) in motoneurones or primary afferent depolarization may still effectively depress the excitatory and inhibitory synaptic actions evoked from the flexor reflex afferents (FRA) and from Ib afferents. There is no effect on post-synaptic potentials from Ia afferents or on Renshaw IPSPs. The depression is not associated with any measurable change in conductance over the motoneuronal membrane.
3. There is also inhibition from the brain stem of transmission from the FRA (but not from Ia and Ib afferents) to primary afferent terminals and to ascending spinal pathways.
4. It is concluded that this inhibition from the brain stem is exerted at an interneuronal level in spinal reflex paths.
5. The inhibitory action is evoked from the region of Magoun's inhibitory centres in the brain stem and is mediated by axons with a conduction velocity of at least 20 m/sec. The axons are distributed in the dorsal part of the lateral funicle.
6. The pathway mediating the inhibition from the brain stem is named the dorsal reticulospinal system. Its possible role in maintaining the decerebrate control of reflexes is discussed and related to the problem of a selective control of some paths from a primary afferent system.
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