UNSPECIFIC THALAMOCORTICAL RELATIONS 



I3I5 



INTERRELATIONS BETWEEN SPECIFIC AND 

 UNSPECIFIC PROJECTION SYSTEMS 



Interrelation between specific and unspecific pro- 

 jection systems occurs at tliree levels: a) by means of 

 collaterals from the principal sensory pathways, b) in- 

 trathalamic connections from intralaminar to specific 

 thalamic nuclei and () o\erlapping projections in al- 

 most all areas of the cerebral cortex. Corticofugal 

 projections provide a fourth means of interaction be- 

 tween specific and unspecific systems. 



The collateral fibers from sensory pathways (visual, 

 auditory and somatic) converge into the reticular net- 

 work of the centrum medianum and other intra- 

 laminar nuclei in a manner similar to that described 

 for the brain-stem reticular system in Chapter LI I by 

 French in the present work. This is a relatively un- 

 specific activation since little if any clear modality 

 specificity has been demonstrated (27, 29, 53, 55, 78, 

 79). The longer latency, variability and adaptation of 

 evoked potentials in the thalamic reticular system, 

 when compared with responses in sensory relay nuclei, 

 suggests a synaptic system with different properties 

 and possibly also some diff'erence in the type of fibers 

 in sensory pathways which terminate in the thalamic 

 reticular system. 



The anatomical studies of Nauta & VVhitlock (62), 

 using a special silver impregnation method for axons 

 and terminals, has shown that numerous fibers leave 

 the main course from the centrum medianum forward 

 in the thalamus to terminate in various specific tha- 

 lamic nuclei along the way. A recruiting response can 

 also be recorded from lateral thalamic nuclei in re- 

 sponse to stimulation of the centrum medianum. The 

 importance of these intrathalamic connections has yet 

 to be determined. E\en though they are not essential 

 to the cortical distribution of recruiting responses, they 

 may still play a significant role in thalamic integrative 

 mechanisms. 



In cortical sensory receiving areas it may be hard to 

 demonstrate recruiting responses, as discussed aijoxe, 

 due to competition with impulses arriving over spe- 

 cific projection fibers. With paired stimulation, alter- 

 nating shocks to unspecific and specific systems, it is 

 possible to demonstrate definite interaction of recruit- 

 ing waves with the surface-negative component of the 

 specific evoked potential complex in sensory areas 

 (40, 81 ). Less effect can be demonstrated upon the in- 

 itial surface positive phase of the specific comple.\. An 

 example of such interaction in the visual cortex is 

 shown in figure 7. Interaction appears to occur mainly, 

 though not exclusively, after the voiles of sensory im- 



FiG. 7. Oscilloscope records of recruiting and specific evoked 

 potentials from the visual corte-x obtained simultaneously by 

 repetitive stimulation with paired shocks. The initial shock was 

 applied to the nucleus ventralis anterior followed by a test shock 

 to the lateral geniculate body 62 msec, later. Note the reciprocal 

 relationship between the variations in the recruiting wave and 

 the amplitude of the surface-negative component of the specific 

 evoked potential. Timeline intervals, 10 msec. [From Jasper & 

 .■\jmone-Marsan (40).] 



pulses has passed the first cortical synapses and is being 

 propagated into the dendritic network of the cortex. 



However, with microelectrodes, it has been shown 

 that unspecific projections may modify the excita- 

 bility and repetitive discharge of neurons which are 

 fired with short latency in response to specific aff^erent 

 volleys (46). Also rhythmic sensory after-discharge can 

 be blocked completely by rapid stimulation of the 

 thalamic reticular systein. Therefore activity in the un- 

 specific system may exert an effect upon the respon.se 

 of sensory cortex by a) modifying the initial cortical 

 neuronal discharge, b} blocking, facilitating or timing 

 the elaboration of impulses throughout the different 

 layers of the cortex, and <) modifying the prolonged 

 aftereffects of a sensory volley. 



We do not know how these electrophysiological 

 changes may be related to subjective sensation or per- 

 ception, although possii:)le relationships have been 

 suggested (10, 47). 



