I3IO 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



same specific volley if paired with an earlier unspecific 

 conditioning volley. 



Other individual cortical neurons can be fired In 

 unspecific afferents after the recruiting wave has 

 reached sufficient amplitude. These cells fire repeti- 

 tively on the ascending limb of the recruiting wave 

 and continue to fire during each wave until tlie waning 

 phase of the response when they cease firing. Such 

 units may fire with a delay of 20 to 30 msec, if the re- 

 cruiting wave occurs with this latency. 



The initial waves of the recruiting response always 

 occur before unit firing, suggesting that the waves are 

 primary, and serve to initiate neuronal firing; the 

 waves do not result from neuronal discharge. This is 

 shown more clearly in a deeply anesthetized animal; 

 no unit firing can be found with microelectrodes at 

 any depth in the cortex, yet the recruiting response 

 appears sometimes of even greater amplitude. The 

 recruiting waves are therefore considered analogous 

 to 's\naptic' or 'dendritic' potential waves, not 

 depending upon the firing of the larger cortical cells.i 

 There is a growing bod\ of evidence in favor of the 

 conception that recruiting waves of the cortex of the 

 unspecific type fall into the class of responses or activity 

 called 'dendritic' (9, 13, 14, 16, i?, '^o, 67, 68, 83). 

 This implies that they do not depend upon the actual 

 all-or-nonc firing of cortical cells and is consistent 

 with the variable relationship found between cell fir- 

 ing and these waves in microelectrode studies. Also 

 they can be recorded when the firing of cells has been 

 suppressed by anesthesia. -Recruiting' implies the suc- 

 cessixe addition of increasing numbers of units to the 

 response. If these waves represent summation of 

 successive increments of depolarization upon a den- 

 dritic network which possesses a long time constant 

 and no refractory period, the addition of an increas- 



' The increasing amplitude of the recruiting wave has also 

 been shown to cause an increased number of cortical cells to 

 fire, but the principal effect appears to be on the number of 

 repetitive discharges in a given cell. Certain other cells, which 

 are firing spontaneously before a recruiting response, may be- 

 come grouped so that they give rise to brief bursts by the re- 

 cruiting waves, even though this may represent a decrease in 

 their total firing rate. This illustrates a mechanism of timing or 

 'gating' of cortical cell firing by the unspecific system which may 

 have important functional implications. Verzeano el al. (85) 

 have come to a different conclusion with regard to barbiturate 

 spindle bursts recorded with microelectrodes in unspecific tha- 

 lamic nuclei. They found a close relationship between spikes 

 and waves and believed that the waves of the spindle bursts in 

 the thalamus might represent positive after -potentials. \ lack 

 of consistent relationship was found, however, between unit 

 spikes and convulsive waves induced by pentylenetetrazol 

 (Metrazol) or picrotoxin. 



ing number of units would not be necessary to explain 

 the potential changes recorded, as pointed out by 

 Clare & Bishop (18, 19). Furthermore, the frequency 

 specificity of this response in close relationship with 

 the spontaneous electrical rhythms indicates dri\ing 

 of an 'oscillating' or rhythmic system (84). The fre- 

 quency of stimulation must approach the periodicity 

 of the 'oscillating' system to be efTective. It has been 

 suggested that these are the properties of dendritic 

 networks in central grey matter (9, 83). There re- 

 mains the possibility that small interneurons of the 

 Golgi II type may play a role in cortical activit\- 

 which is classified as 'dendritic' (46). 



Anatomical studies provide some exidence in fa\or 

 of these conceptions. The description of unspecific 

 afferent fibers to the cerebral cortex by Lorente de No 

 {50) stimulated Morison & Dempsey to postulate 

 the existence of the unspecific thalamocortical pro- 

 jection system, even though no connection had heen 

 shown between intralaminar nuclei of the thalamus 

 and such afferent fibers in the cortex. Chang has 

 suggested that dendritic responses are mediated by 

 axodendritic synapses as contrasted with the axo- 

 somatic synapses which would presumably charac- 

 terize most specific afferent terminals. Such a classifi- 

 cation of synaptic terminals, suggested by Ramon y 

 Cajal (70), may be of far reaching physiological 

 significance due to the special properties of dendrites 

 summarized by Bishop (9). It has yet to be shown, 

 however, that the projection fibers to the cortex from 

 the thalamic reticular system are actually of the 

 unspecific axodendritic type. Evidence from the work 

 of Nauta & Whitlock (62) would suggest that such 

 terminals may originate in either specific or unspecific 

 thalamic nuclei. 



TH.'VL.AMIC DISTRIBUTION OF I'NSPECIFIC 

 PROJECTION SYSTEM 



The thalamic distribution of the unspecific pro- 

 jection system was originally identified with the cells 

 and fibers found within the internal medullary lamina 

 of the thalamus, including a caudal expansion into 

 the n. centrum medianum. The intralaminar cells do 

 not show retrograde degeneration following neo- 

 cortical lesions in the same manner as do the cells of 

 specific thalamic nuclei so that they have long been 

 considered not to have cortical projection fibers.- 



- Degenerative changes of a minor character have been shown 

 bv Nashold el al. (.60). 



