Suzanne de Bruijn

16 Chapter 1.1 cells (3 rd layer). Finally, the larger axons of the ganglion cells form the optic nerve that will transmit the stimulus to the rest of the central nervous system. The horizontal neuron cells and the amacrine neuron cells enable the lateral interactions between the photoreceptor-bipolar cells and the bipolar-ganglion cells, respectively. The retinal cell types contribute in different ways to visual function, such as allowing a direction selective response. 5,7 There are two types of photoreceptors in the neural retina: the rod and cone photoreceptors ( Figure 1B ). 8 Both types are composed of an outer segment, inner segment, cell body and a synaptic region. The outer segment consists of membranous discs thatcontainlight-sensitivephotopigments, andthis iswherethephototransduction processes take place. The inner segments house the metabolic machinery of the cell (the mitochondria, the endoplasmic reticulum, the Golgi complex, the lysosomes, and the ribosomes) that is responsible for protein synthesis and energy production. The outer and inner segment are connected by the connecting cilium. This structure plays a pivotal role in protein trafficking between both segments. 9 Lastly, the cell body contains the nucleus of the photoreceptor cells, and the synaptic region contains the synaptic terminals through which the signals are propagated to the bipolar or horizontal cells. The two photoreceptor cell types can be distinguished based on shape, type of bounded photopigment, and the distribution across the retina. Each cell type is specialized for a different aspect of vision. Rod photoreceptors allow light detection at extremely low luminance, but only with a low spatial resolution. Cone photoreceptors on the other hand, have a high spatial resolution but lack sensitivity for vision in low light conditions. Whereas one light photon is enough to initiate a stimulus in a rod photoreceptor, 100 light photons are required to activate a cone photoreceptor. 5 Additionally, rods only mediate achromatic vision and cone photoreceptors are responsible for color vision. Cone photoreceptors can be divided in three types, that all contain a different type of photopigment that is able to absorb light with a different wavelength. They are referred to as short (S, “blue”), medium (M, “green”) and long (L, “red”) wavelength cones. Cones are highly concentrated at the macula, although present throughout the retina. 7 The macula is located near the center of the retina and contains yellow pigment, and it supports high visual acuity. This acuity is highest at the center of the macula: the fovea, which can be observed as a small pit in the retina. Within the retina, there are 6 million cones and 120 million rod photoreceptors of which the latter are predominating in the mid-peripheral retina. 8 The region of the optic disc does not contain any photoreceptors and is insensitive to light stimuli. It is referred to as the blind spot. 5