The photoreceptor cell of animal vision is by far the most sophisticated cell of the visual system and probably of the entire neural system. It has unique features associated with its physical environment, morphology, cytology and internal Activas (plural). Section 10.10.7 discusses many of these features. Others are discussed throughout PART C and C of the text in relation to other phenomena.
The following figure consists of three distinct frames. Frame A shows the photoreceptor cell within its physical environment and details some of its morphological features. Frame B provides the topological equivalent of the topographic representation in Frame A. Frame C redraws Frame B in a more conventional electrical circuit representation for purposes of discussion.
Frame A stresses the fact that the Outer Segment is not an integral part of the photoreceptor cell. It also shows that the electrolytic environment surrounding the cell is divided into two compartments by an insulating membrane, the Outer Limiting Membrane. This fact plays an important role in understanding the laboratory data related to this cell. The extended, U-shaped dark box on the left also hints at the unique electrical characteristics of the dendritic structure of the photoreceptor cell.
To provide adequate and efficient contact to the individual disks of the Outer Segment, the dendrites of the photoreceptor cell are formed into long microtubules that are positioned in furrows along the perimeter of the Outer Segment. By making the microtubules very small in diameter, the lemma and the reticulum of the dendrites are forced to be very thin and in close proximity. This situation results in at least three unique properties for the resulting structure. First, the combination of the lemma,the reticulum wall, and the fluid between them forms a continuous Activa over most of the length of the microtubule. Second, the extreme thinness of the lemma of the dendrite causes it to exhibit an unusual electrical characteristic known as avalanche breakdown. Third, the closeness of the microtubules to the individual disks of the Outer Segment contributes to the efficient transfer of quantum mechanical energy between them.
Frame B shows the overall electrical circuit resulting from the above unique features combined with the remainder of the electrical elements of the cell. The sawtooth line represents the relatively high resistance of the surrounding electrolytic medium. In this case, the medium is divided into the inter-photoreceptor matrix (IPM) and the inter-neural matrix (INM). The input stimulus to the cell is received from the disk stack as a quantum mechanical impulse (an exciton) that creates a free electron within the base connection of the Activa of one of the microtubules. There are typically nine microtubules associated with the dendritic structure of a given cell. Each of these microtubules contains a continuous Activa.
The presence of a free electron in the base region of an Activa causes a large number of electrons to flow from the emitter region (indicated by the arrowhead) to the collector of the Activa and into the IPM. This current constitutes the initial signal current of the visual system. It causes a voltage to be formed across the impedance (2) shared with the Activa shown on the right in the frame.
The left portion of frame C represents the sum of all of the microtubules of the dendritic structure. The current is also summed to provide one current through the impedances (1) and (2). It is this total current passing through the resistive impedance of the IPM that is measured in the laboratory as the generator potential in the IPM of a retina.
The right portion of frame C represents a second Activa in the photoreceptor cell. This Activa is analogous to the Activa in other neural cells. It is formed within the cell at the junction of the dendritic and axonal conduits.
The number of electrons flowing in the emitter to collector circuit of the left hand Activa is enhanced by the extreme thinness of the lemma forming the collector electrode. The voltage gradient across this lemma formed by the associated electrostenolytic process on its surface is extremely high under small signal conditions. This extremely high electrical gradient causes an avalanche effect, similar to a chain reaction in nuclear physics, to occur. As a result, the average current gain of this circuit at low current levels can be as high as 3500:1. As the intrinsic current due to the initial electrons in the base region increases, the electrostenolytic supply is unable to meet the demand for current and the voltage across the collector (lemma) drops. This fall in voltage quenches the avalanche effect and the average current gain is reduced significantly. It is this feature that gives this element of the photoreceptor cell the name Adaptation Amplifier. The electrical signal gain of the adaptation amplifier is inversely proportional to the signal stimulus over a very wide range of about 3500:1.
When drawn as in frame C, the circuit of the photoreceptor cell is recognized as that of a differential pair of active devices. This type of circuit has the property that a signal current will pass through the right hand Activa that is the mirror image of the signal current passing through the left hand Activa. Because of the structural differences between the type AT Activa on the left and the type AD Activa on the right (see the text or the end-to-end circuit diagram), the voltages relating to these currents may be quite different.
The voltage across the load impedance (4) is quite different from the voltage across the other load (1) because of the nature of the impedance. Load impedance (4) is a pure diode. and the signal current is large. As a result, the voltage across the load is proportional to the logarithm of the current through the left hand Activa and the IPM. This output voltage represents the generator potential measured at the pedicel of the photoreceptor cell. This generator potential is structurally different from the generator potential measured in the IPM.
It is the combination of the extreme variable gain associated with the adaptation amplifier of the photoreceptor cell and the fact that its output voltage is the logarithm of the current resulting from the adaptation amplifier that has caused such difficulty in understanding the operation of the cell under large signal conditions.
