Processing by the Central Nervous System

Abstract

Light absorption by photopigment in a retinal photoreceptor (rod or cone) causes the cell membrane to hyperpolarise. This potential change is then transmitted electronically to synapses on both bipolar and horizontal cells. In turn, the bipolar cells transmit a graded electrical signal to the ganglion and amacrine cells with which they are in contact. In the ganglion cells action potentials are generated and these pass along the optic nerve and tract to the lateral geniculate nucleus and the visual cortex (Fig. 11.1). In the retina lateral interactions are provided by the horizontal and amacrine cells. Generally, the number of fibres in the optic nerve approximate the number of retinal ganglion cells. In primates and birds this number is high while in various amphibians and fishes it is relatively small. The visual pathway from retina to cortex consists essentially of six types of neurones or nerve cells; of which three are in the retina (photoreceptors, bipolar cells, ganglion cells), one in the lateral geniculate nucleus, and two in the cortex (complex and simple cortical cells) (Fig. 11.1). Nothing very mysterious is occurring, but it is nevertheless difficult to fully comprehend the parts and how they are integrated into a whole.