A physiological and morphological study of the horizontal cell types of the rabbit retina

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Abstract
A perfused, isolated retina‐eyecup preparation of the rabbit was utilized to correlate the physiology and morphology of horizontal cells. Neurons were physiologically characterized by intracellular recording techniques and subsequently stained with intracellular iontophoretically injected horseradish peroxidase for morphological identification. Three types of rabbit horizontal cell recordings have been differentiated, based on variations in response waveform, amplitude‐intensity properties, and area summation characteristics. These three types have been unequivocally associated with the axonless A‐type horizontal cells and the somatic and axon terminal endings (each displaying its own distinct physiology) of B‐type horizontal cells first described in studies using Golgi‐impregnation techniques (Fisher and Boycott, 1974). In addition, the sizes of A‐type horizontal cells were found to be directly related to their retinal eccentricities from the optic disk. However, a unique subclass of A‐type cells has been discovered (elongated or Ae type) which displayed the largest dendritic field of any cells studied here, yet had the smallest eccentricities—within 1.4 mm of the optic disk. Moreover, elongated A‐type cells exhibited long asymmetrical dendritic fields which were oriented parallel with the visual streak. The unique asymmetry and orientation of these cells suggests that they may have orientation‐biased receptive field properties. Physiological evidence for an orientation‐biased horizontal cell is presented in support of this notion.