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An analysis of rod outer segment adaptation based on a simple equivalent circuit

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Abstract

This model of rod outer segment adaptation is based on the hypothesis that transmitter substance released by bleached rhodopsin closes sodium channels in the outer segment plasma membrane, leading to hyperpolarization of the receptor. The outer segment adaptation processes of the model are associated with the transmitter release, the transmitter background concentration and the plasma membrane leakage. Changes in the three model parameters correspond to the three types of outer segment adaptation processes. According to the model the stimulus-response function is in every adaptational state U/U max −I/(I+I H ). The model predicts how each adaptation process affects I H and U max. Specifically, when the number of liberated transmitter molecules per isomerizing quantum decreases, the amplitude U max remains constant but I H increases. A short description of this model has been published in a paper reporting experimental results on background adaptation (HemilÄ, 1977).

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Abbreviations

B :

the fraction of sodium channels closed

c :

the transmitter concentration in the extradiscal space of the ROS

c i :

the transmitter background, the initial transmitter concentration before stimulation

c H :

the transmitter concentration corresponding to B = 1/2

vc 1, C a :

the parameters of the general photoresponse equation (Eq. 5)

c 10, C a0 :

the parameters c 1 and C a in the dark-adapted state

E :

the source voltage (e.m.f.) in the rod inner segment

e :

the fraction of the sodium channels closed temporarily by a strong bleach independently of the transmitter blockade

G 0 :

= Ng

G v :

= G 0 (1-B), the total conductance of the open sodium channels

G L :

the leakage conductance of the ROS plasma membrane

g :

the conductance of one sodium channel

I :

the stimulus light intensity

I H :

the stimulus intensity which elicits a response 1/2U max

I h :

the value of I H in the dark adapted state

I 0 :

the threshold intensity corresponding to the criterion response U c

I 0 :

the absolute threshold intensity (dark adapted state)

I B :

the background light intensity

N :

number of sodium channels in the ROS plasma membrane

Q :

the transmitter coefficient (c = c i + QI)

Q 0 :

the value of Q in the dark adapted state

q :

the transmitter release factor, the average number of transmitter molecules liberated by an isomerizing quantum

R e :

the resistance of the interreceptor space

R t :

the resistance of the inner segment plasma membrane

R c :

the resistance of the cilium (plus a third of the longitudinal cytoplasmic resistance)

R s :

= R e + r i + R c

U :

the receptor potential, the change in voltage across the resistance R e brough about by the stimulus light

U max :

the maximum receptor potential in the prevailing state of adaptation

U m :

the value of U max in the dark adapted state

U c :

the criterion photoresponse (threshold measurements)

δU i :

the hyperpolarization of the rod inner segment plasma membrane

δt :

duration of the flash stimulus

η :

the absorption factor, the fraction of incident quantum flux absorbed by rhodopsin and causing photopigment isomerization

Τ :

the life time of the transmitter molecules

Τ i :

the integrating time in the response to the step of light

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Authors and Affiliations

  1. Department of Zoology, University of Helsinki, Arkadiankatu 7, SF-00100, Helsinki 10, Finland

    Simo Hemilä

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  1. Simo Hemilä
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Hemilä, S. An analysis of rod outer segment adaptation based on a simple equivalent circuit. Biophys. Struct. Mechanism 4, 115–128 (1978). https://doi.org/10.1007/BF00539226

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  • DOI: https://doi.org/10.1007/BF00539226

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