404 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



slowK' adapting 'skin pressure' units arc driven by- 

 light mechanical stimulation of the skin surface. This 

 correlation is probaljly not perfect, for some units 

 excited by movement of specialized hairs have been 

 observed which adapted slowly (68), and some rapidly 

 adapting units have been noted whose peripheral re- 

 ceptive fields were located in hairless parts of the skin. 

 Neurons responding steadily or with only an onset 

 transient to mechanical stimulation of the skin have 

 been observed at the level of the ventrobasal thalamic 

 complex and in the postcentral homologue (181; and 

 Mountcastle, V. B. & J. E. Rose, unpublished 

 obser\ations). Records of the action potentials 

 of a postcentral cortical neuron responding stead- 

 ily to a steady cutaneous stimulus are shown in 

 figure 6. Results such as these indicate that neurons 

 located at the various levels of the system reflect rather 

 faithfully the discharge properties of either the periph- 

 eral receptors themselves or those of the first order 

 neurons. At each level of the svstem the fast and slowlv 



adapting neurons are intermingled within a single 

 topographical pattern. 



PERiPHER.AL RECEPTIVE FIELDS. Quantitative measure- 

 ments of the cutaneous fields of distribution of single 

 fibers have been very few. By recording unitary action 

 potentials from fibers within the dorsal columns, 

 Yamamoto et al. (280) found the peripheral fields to 

 vary in size from a maximum of 2 to 3 cm'- on the 

 trunk, to a few square millimeters at the distal ends 

 of the limbs. Working with single fibers in cutaneous 

 ner\es, Maruhashi et al. (168) have in general found 

 similar results, though they emphasize that a) many 

 large afferents may have truly 'spot-like' receptive 

 fields, and 6) that smaller (3 to 5 n) slowly adapting 

 afferents may have wide receptive fields in the range 

 of 1 4 to 40 cm'-. 



Only scattered data are available concerning the 

 size of the fields which project upon neurons of the 

 dorsal column nuclei and of the ventrobasal complex. 



rMPULSES PER SECOND 



CORTICAL NEURON RESPONDING TO STEADY 

 PRESSURE OF SKIN OF FOREARM 



FIG. 6. .\ction potentials of a single cell in the postcentral gyrus of the monkey, Macacus rhesus. The 

 neuron is driven by steady pressure applied to the cutaneous receptive field located on the volar 

 surface of the contralateral forearm. Onset and release of pressure indicated by solid bar under the 

 graph, which shows the number of impulses per second plotted at 200 msec, intervals. Experimental 

 conditions described by Mountcastle el at. (183). [From Mountcastle, 'V. B. & T. P. .S Powell, 

 manuscript in preparation.] 



