CENTRAL CONTROL OF RECEPTORS AND SENSORY TRANSMISSION SYSTEMS 



745 



suppression reappears if the stimulus is reapplied 

 peripherally to the point of division of the bundle. 

 Rasmussen's efferent fibers are therefore evidently 

 capable of suppressing activity in auditory afferents 

 either at or near the point of impulse origin within the 

 cochlea. 



Optic j\erve Activity 



Granit, by stimulating the midbrain tegmentum, 

 induced a lasting augmentation of the frequency of 

 firing of individual ganglion cells in the retina, 

 whether the latter were spontaneously active or were 

 made active by test flash illumination (29). Occasion- 

 ally, from the same general region, inhibition is 

 elicited. Both the facilitatory and inhibitory effects 

 appear to be conveyed by fine efferent fibers described 

 by Ramon \ Cajal and others (e.g. 65, vol. II, fig. 

 211, p. 366). Dodt, by stimulating the optic tract in 

 rabbits, elicited small, late-appearing retinal spikes 

 which are unlike antidromic spikes; these he inter- 

 preted as due to impulses conveyed along the cen- 

 trifugal fibers to the retina (15)- The exact central 

 origin of such centrifugal fibers is not yet determined. 

 These efferent effects upon retinal activity are most 

 readily initiated by stimulation of the reticular forma- 

 tion of the midbrain and are reliably obtained only in 

 animals lacking central anesthesia (29, 39). 



Olfactory Bull) Activity 



The anterior commissure contains cflerent fibers, 

 described by Ramon y Cajal and others (e.g. 65, vol. 

 II, p. 664, figs. 423-425; 66, p. 12), which apparently 

 arise in basal rhinencephalic areas and pass out to the 

 olfactory bulb. These and similar fibers coming from 

 the opposite bulb are believed to terminate on granule 

 cells and in the periventricular and external plexiform 

 layers. In this location they have access to the synaptic 

 junction between receptor-cell terminals and bulbar 

 neurons. Kerr & Hagbarth (46) studied the effects of 

 exciting this centrifugal system upon the electrical 

 activity of the olfactory bulb, both in the resting state 

 and following olfactory stimulation. Excitation of the 

 anterior commissure, the prepyriform cortex, the 

 cortical amygdaloid nucleus and the olfactory tubercle 

 induces a diminution of olfactory-bulb activity. Ef- 

 ferent fibers apparently exercise a tonic inhibitory 

 influence upon the olfactory bulb since the addition 

 of central anesthesia or a surgical division of the an- 

 terior commissure is followed by an augmentation of 

 olfactory-bulb activity. 



CONTROL OF CENTRAL SENSOR"!' REL.AYS 



Spinal Ascending Relays 



Magoun observed in 1950 (58) that the "study of 

 descending influences of the reticular formation has 

 so far been preoccupied entirely with the pronounced 

 effects exerted upon the discharge of spinal motor 

 neurons. It would be of considerable interest to 

 know whether or not these generalized reticulospinal 

 influences are capable also of altering the transmission 

 of afferent impulses within the cord." The effect of 

 centrifugal influences upon the synaptic relay of im- 

 pulses from dorsal root fibers to second order ascend- 

 ing neurons was first tested by Hagbarth & Kerr in 

 ■954 (sO- Using cats immobilized with curare and 

 lacking central anesthesia, they applied test shocks to 

 individual lumbosacral dorsal roots and analyzed the 

 effects of intervening (conditioning) excitation applied 

 elsewhere in the central nervous .system. They found 

 that stimulation in either the inhibitory or facilitatory 

 zones of the reticular formation diminishes or abol- 

 ishes responses being conveyed within both the ventral 

 and lateral funiculi of the spinal cord. The relayed 

 response in the dorsal columns is also affected although 

 the primary dorsal column spike, representing con- 

 duction along primary afferent fibers, is unaltered. 

 Stimulation of a number of other parts of the central 

 nervous system, the sensorimotor cortex, the second 

 somatic sensory area, the anterior part of the cingulate 

 gyrus and the anterior vermis of the cerebellum, has 

 similar but less pronounced effects. An example of 

 this is shown in figure 3. 



When central anesthetics are administered, there is 

 an augmentation in amplitude of the relayed re- 

 sponse as compared to preanesthetic levels (fig. 4). 

 Additionally, if the spinal cord is divided in the cer- 

 vical region in animals without central anesthesia, a 

 similar 'release' appears, resulting in an increase of 

 amplitude in the second order neuron responses to a 

 standard dorsal-root volley (31). Evidently in anes- 

 thetized animals the high amplitude of .sensory-e\oked 

 responses recorded within the classical sensory path- 

 wa\s is due to the anesthetic having interrupted a 

 tonic descending inhibitory influence. 



Dorsal Column and Other Bulbar Relays 



Excitation of the brain-stem reticular formation 

 induces a prolonged depression of transmission 

 through the dorsal column relay nuclei (39, 71). A 

 moderately intense i-sec. stimulation causes a rapid 



