REQUERIMIENTOS DE LOS CLIENTES

S3.

lifricult to deter*:«tae with aufficie&t accuracy# uaa aethod of rinding is to balance the bridge circuit outside the cell# whereupon penetration yields a bridge Imbalance which is true membrane potential (see Methods)# This is not practicable

in the present situation because extracellular is hard to define operationally and tip resistance often alters during probing# i* second method is to unbalance the bridge visually by reference to the space/time^-constant function generated by the current

squure-pulse# fhis is sometimes possible (e#g# Fig# l,d) but could not be used in Fig# 17a# b because the rise-tise was

inside the switching—artefact time of the bridge# a third method has to be used and is simply that if the bridge goes 10 MOhm

out of balance during a flash# cell input resistance must have been at least this high initially# The coupling ratio is then estimated as described in Methods# equation 3*% ee the ratio of

the maximum bridge imbalance in tho cell being polarised to the pulse height in the other cell# This gives an upper limit which is about u#lv7 (9#4 : 1) in the dark for the two cells in Fig# 17 (a#b)# and coupling would be even poorer than this if tho cells did not © iort-oirouit with this intensity of illumination# Using double-barrel microelectrodes# ~>choles (unpublished) found that in some penetrations single cells do not short-circuit with brighi flashes# but it is possible that changes in electrode tip—coupling resistance could account for this result (»*ardell and Tomlta# 196< Method©)# If two separate electrodes are usod this difficulty

36#

may be avoided# and in a few inncances both electrodes have penetrated one cell in the course of the present work# To the extent that the few teats made are representative# the residual resistance at the peaK of the transient phase has always been small or unmeasurable (Fig# l?e>#

it ie not Intuitively obvious whether the same coupling effects would have boon found# were it possible to selectively depolarise one cell with a light stimulus instead of with

currents through one microelectroie# In the latter case the resting potential ion species would presumably be passively redistributed# moving between cells towards the extracellular space down the easiest resistance pathways# out in the former Case# it seems most likely thot depolarization would result from

^ ojaduotance change to different ion species in a membrane patch Vthe rhabdoaere> ,ihich possibly has restricted access to the extracellular space# so that the effects of couplizuy might depend on tao geometry of tho resistance pathways involved in current f1 For the simple steady-state equivalent circuit of two adjacent coupled cells in Fig# 18a (after ioewenstoin 1966) it is obvious

that a light-evoked voltage V a peering at electrode will

produce exactly the same effect at as would an applied voltage provided resistance r^^ does not change appreciably# Thio is

perhaps too simple a representation# since the active and inactiv lyembranes have been lumped# and no account is taken of the

possibility of oonfined extracellular spaces# This is ronedied in Fig# 18b where rosist;^nces and V2 cro affoctod by light#

777-

e

c

r.

-rtr 7fr

Pim. 18 (m) Simplified scheme showing a pair of retinuia cells penetrated by electrodes 1 and 2. If light I produces a small change in resistance r$# the voltage V so produced will have the same effect at electrode 2 as would the same voltage (V*)

applied through electrode 1. The two mean# of testing intercel coupling should therefore produce the same result. (b) is a more realistic scheme# where light-sensitive resisters r% end rg are isolated from the extracellular space by a resistance r^^.

k three-cell **onniatidium*' is used for simplicity. r^# r and r, are# respectively# the resistances coupling the cells# the

intercellular shunt# and the lumped passive membrane resistance. For explanatory purposes# this circuit has been rearranged using 2 star-delta transformations# in (c - e)* the new resistors

r%# r,# ry# are produced by combination of other resistors. OSpolarisâtion by microelectrode currents and by lights now

87.

la and re resents tho iuiaped résistance between the extracellular space arounl the rhabdom and the outside bathing fluid# i?his circuit is rearranged in Fig. 14c$ from which it oa be clearly seen that coupling effects wouli be the sa^i© with both lights ana currents only if wore aegligibio# By roarranging Fig# 18c in Fig# 14d# it is clear that aoy^ativ^

potential shifts could be seen at _<c. with a light stimulus if _0] wore increased to a sufficiently large value; but a further re- arraag ment (Fig. 18e) makes clear that only positive (or sero) potentials could appear at ^2 when tho nicroelectrod© polarizod the cell I since points À and Y would both be positive# /ortuaatc the coupling reported b^ the microolectrole depolarization tost wouli be tho sa*ae as with a light depolarization tost (if r.__ex were small), or stronger# Interaction of voltages during "visual tests should therefore bo less if anything than predicted with microolectrode tests# If isolated results such as Iig# 1/e are representative, r^^ must be negligible, 00 that both the micro- electrode- and "natural" tests would give identical results#

it is difficult to fini Che coupling ratio using "natural" tests, since cells overlap so auc in all their characteriai,ics; the different extremes of colour sensitivity overlap considerably (Bennett at al 1 )G7)$ tbe receptive fields are identical, and no position of polarization plane exists

wnich will not stimulate all cells to some degree# nowaver, at very low ligîit flux levels the random ihoton-arrivals will be

88#

indôperidonti and seldom coinedionoe in different cells, so that the iniividual responses should also be independent# fhis affords a means of testing int ■rcoanoctione between cells using light- evaked responses, where cell meabrane resistance (r^^^2 Fig# IBb) is relatively unaffected# in practice the individual "bwuaps" are not visible much above recording noise level, but in Fig# 17c it is possible to see that c incidences are rare to the extent that noise level permits discrimination# The

largest potential la one cell (arrow, Fig# 1/c) gave no derivatlv in the other coll above noise, so that the coupling ratio must be poorer than about l%v#2)# At higher light intensities, the polarized-light responses (Fig# 16c) appear quite independent— there is no suggestion of asymmetry for ins tance,-4>ut since the actual sensitivity of the a alysers themselves is not independent known, it ca not be said with certainty that the responses are not partly reduced by Interaction of unlike cells# I'his seems unlikely since coupling will got weaker with strong illumination because resistors r^^ and T2 (Fig# 19) will be reduced# Coupling has in fact been snown to get weaker with molerate light

intensities la tho Limulus omaaatidlum (see below), but this caano be accurately tested with the present results#

wimilar out less complote results have been obtained on ton other airs of cells, excluding those cases where the angular response function wore similar but slightly displaced from ono another. It is just possible that if the rhabd^meres become

10TT" L I 1- -3 ,!l!. j!! 360 liiUÜiÜii 360

H

■hi

+fUll