704 



HANDBOOK OF I'HYSKJLOGV 



NEUROPHYSIOLOGY I 



be caused by sources located tangentially within the 

 retina and suggests, on the basis of electrical con- 

 siderations too involved to discuss, that "only the 

 rods and cones contribute substantially to the a-, b- 

 and (/-waves of the electroretinogram, although cells 

 belonging to the inner nuclear layer can produce 

 large electrical changes of similar time course" (30). 

 He himself finds this hypothesis "somewhat surpris- 

 ing." Tomita localizes the same response to the 

 bipolar layer with some contributions from other 

 retinal layers, in particular the inner portion of the 

 receptors [cf. also Noell (114)]. 



Particularly interesting are Brindley's (28) meas- 

 urements of the passive electrical properties of frog 

 retina for radial fields and currents and the method 

 applied to make such measurements possible. The 

 largest step takes place at around 230 fx from the 

 ganglion side and is of the order of 270 fi. This is 

 Brindley's R-membrane which he pro\isionally identi- 

 fies with the external limiting membrane. Its capacity 

 is about 40 /i F. Across this memi^rane also is the largest 

 step or component of the resting potential of the 

 retina. Electron microscopy (134, 135) has led to the 

 view that the external limiting membrane consists 

 of rings or collars around the receptor base (inner 

 segment), a fact difficult to harmonize with the high 

 resistance and capacity if it really can be assumed 

 that there has been no shrinkage in Sjostrand's 

 preparations. 



These papers are thought-provoking and serious 

 attempts to lay bare the considerable difficulties in 

 arriving at evidence for final conclusions and thus 

 form a structure of knowledge upon which further 

 work can be built. There is a large body of informa- 

 tion about the ERG as influenced by alcohol, potas- 

 sium, state of adaptation, etc., which will have to be 

 experimentally applied to microelectrode analysis be- 

 fore a final conclusion can be reached. 



The crucial point in the present position is the 

 identification of the R-membrane, it being highly 

 unlikely that any intracellular potentials in the retina 

 ever have been recorded. The recent findings by 

 McNichol et al. {105a) show that the former cone- 

 potential of Svaetichin actually is obtained below the 

 layer of rods and cones and Tomita (141a) has shown 

 that it can be obtained with electrodes far too big to 

 penetrate individual cells successfully. However, the 

 observations on effects of different wavelengths in fish 

 seem interesting independently of present assumptions 



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FIG. 12. Diagiain illustrating tliree libers in the optic nerve 

 firing spontaneously and their responses to illumination as 

 described in text. [From Granit (73).] 



and also confirm the demonstration of dominators 

 and narrow-banded modulators in this eye (65a). 



NEUR.\L P.^TTERNS 



Work on the spike discharge from the retina started 

 with the classical papers by Adrian & Matthews 

 (3, 4, 5) who used the long optic nerve of the eel 

 Conger for a study of the massed discharge. The dis- 

 covery of different discharge types was a consequence 

 of Hartline's (86) successful attempts to split the 

 frog optic nerve. This he did at the point where it 

 enters the blind spot and is already naturally split 

 into fibers coming from different parts of the retina. 

 The types are shown in the schematic figure 12 in 

 which account also is taken of the microelectrode 

 records from mammals (see 69, 73) in which one 

 often finds more activity between onset and cessation 

 of light than in the frog and less stability of response 

 types. Some fibers (/ in fig. 12) respond to onset of 

 light, others (2) are inhibited by onset of light and 

 instead discharge at 'ofT." The majorit\- of them dis- 

 charge to both onset and cessation of light (j). Re- 

 illumination during the off-discharge inhibits it, as 

 shown by 2h. It should be realized that, since the 

 optic nerve fibers represent highly differentiated con- 

 vergence structures, there are in actual practice al- 

 most as many discharge types as there are optic nerve 

 fibers. Nevertheless the types illustrated show ap- 

 proximately what happens. The inhibition of the 

 ofT-discharge, as stated, coincides with the large 

 negative o-wave on top of the off-effect described 



