746 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



onset and slow decay of depression, affecting impulses 

 being relayed to the internal arcuate fibers. The ini- 

 tial spike of impulses arri\-ing via the dorsal columns 

 is not affected. Not only is the relayed response of the 

 dorsal column nuclei modified, but the background 

 activity of reticular neurons at the same level is af- 

 fected, although with a different time course, by the 

 same conditioning reticular stimulation. On the intro- 

 duction of central anesthetics or the production of a 

 mid-line pontine lesion, there is a notable increase in 

 amplitude of evoked responses pa.ssing through the 

 dorsal-column nuclei, indicating that ordinarily there 



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10 msec. 



FIG. 3. Cerebellar inllucnces on .spinal sensory transmission. 

 Responses are elicited by stimulation of the right dorsal root 

 L7, and recorded from the left ventral column of the spinal 

 cord {upper beam') and left sensory cortex (lower beam)- Inter- 

 current stimulation is applied to the ventral part of the anterior 

 vermis of the cerebellum. Test responses recorded (/) prior to 

 cerebellar stimulation, (.?) during cerebellar stimulation, (j) 

 I sec. and (4) 3 sec. after termination of cerebellar stimulation. 

 Dorsal columns of spinal cord were sectioned at L4. [From 

 Hagbarth & Kerr (31).] 



I I t I I I I 



5 m sec . 



FIG. 4. Release' of tonic descending inhibitory influences by 

 anesthesia and by cord transection. Curarized cats without 

 central anesthesia. Top row: Left ventral column response to 

 feeble L7 dorsal root stimulation (^4) before, and (B) after 

 injection of 45 mg chloralose per kg. BoUom row : Effect of high 

 cord section on left ventral column response. A before, and B 

 1 hr. after transection. In each experiment the stimulus in- 

 tensity and location were kept constant; the dorsal columns 

 had been transected at the L4 level. [From Hagbarth & Kerr 



(30-] 



is a tonic descending inhibitory influence acting upon 

 this relay station. 



Impulses being relayed through the spinal root of 

 the trigeminal nerve in response to test shocks applied 

 to the ophthalmic branch of the trigeminal are also 

 diminished by stimulation of the brain-stem reticular 

 formation (36). Sensory-evoked responses in the adja- 

 cent reticular formation are sometimes depressed for 

 more than a minute even though the trigeminal nu- 

 clear response is only transiently affected. Stimulation 

 of the sensorimotor cortex will also bring about active 

 inhibition of the trigeminal synaptic relay, as appears 

 in figure 5. 



Jouvet & Dcsmedt report that stimulation of the 

 mesencephalic reticular forntation will cause a marked 

 reduction in amplitude of auditor\-e\oked responses 

 recorded from the dorsal cochlear nucleus (44). This 

 occurs even when the electrical responses recorded 

 from the round window in response to the same 

 sensorv stimuli are unaffected. They conclude that 



