4o6 



HANDBOOK OF PHYSIOLOGV 



NEUROPHYSIOLOGY 



CM' 



PERIPHERAL RECEPTIVE AREAS 

 PROJECTING UPON CORTICAL NEURONS 

 OF SOMATIC AREA I 

 n = 126 



THORACIC UNITS 

 27.7 ± 5.0 CM» 

 n =21 



XI0I±I3 



X95±I9 



X707ttO 



. •X''9l05r 



X306t05 



- J X'I610I4 



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liLC 



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CM FROM TIP OF FORELIMB 



FIG. 8. Plot relating the size (in square centimeters) of excitatory cutaneous receptive fields for 

 cortical neurons to the distances of the centers of those fields from the tip of the forelimb of the cat. 

 Crossed dots locate means (and standard errors) for these fields when .grouped into classes by 4 cm 

 distances from limb tip. The receptive fields close to the tip of the limb are small in size and usually 

 comparable in area. With the increased distance from the tip both the size of the fields and the 

 variability between them increase greatly. [From Mountcastle (181).] 



group of cells within the dorsal column nuclei. The 

 cells in the center of the group will receive a maximum 

 number of synaptic impingements and will discharge 

 repetitive trains at high frequency, probably with 

 shortest latency. Cells surrounding the center are pre- 

 sumably excited to discharge trains of fewer impulses 

 at longer latency and at lower frequency, while cells 

 at the edges of the discharge zone will discharge 

 single impulses at even longer latencies. That a similar 

 distribution of activity occurs among third order cells 

 of the thalamic relay nucleus and among fourth order 

 cells in the cortical receiving area can be deduced 

 from single unit studies of those regions (13, 157- 

 ■59. 183, 208). 



Marshall et al. (167) have shown for the thalamic and 

 cortical slow waves that the recovery time is greatly 

 prolonged by anesthetic agents. Single unit studies 

 have confirmed and extended these original observa- 

 tions (13, 183; and Mountcastle, V. B. & J. E. Rose, 

 unpublished observations). The recosery time of 

 single neurons in the imanesthetized animal is not yet 

 known, though it probably is much briefer than the 

 recovery time observed in an anesthetized animal. In 

 the anesthetized animal, however, the anesthesia 

 itself is not the only factor affecting the recovery tiine. 

 At the same anesthetic level the unresponsive time of 

 the system shortens as the strength of the initial 

 stimulus decreases. 



RESPONSES TO TWO STIMULI AT DIFFERENT INTERVALS. 



Some information concerning the capacity of the 

 somatic afferent system to relay activity has been oij- 

 tained by measuring the ability of the system to 

 respond to a second peripheral stimulus at various 

 time intervals after the first. Marshall ('66) and 



RESPONSES TO REPETITIVE STIMULI AT DIFFERENT FRE- 

 QUENCIES. It is clear from study of indi\-iclual neurons 

 that the two-stimulus experiment does not at all 

 specifv the capacity of the system to respond when 

 trains of stimuli are applied. Individual neurons at 

 any level show one of two types of behavior. Figure 1 2 



