Synsinnet

Synsinnet, or the visual system, is the sensory system responsible for sight. It includes the eye itself and the neural pathways that translate light into meaningful information. The system supports visual acuity, color perception, motion detection, and depth perception, enabling interpretation of the surrounding environment.

Anatomy and physiology: The eye comprises cornea, lens, iris, and sclera, with the retina as the light-sensitive inner layer. Photoreceptors—rods for low light sensitivity and cones for color and detail—detect light and initiate electrical signals. These signals are processed by retinal neurons, including bipolar and ganglion cells, and transmitted via the optic nerve. The optic nerve carries information from the retina to the brain, where it is further processed.

Neural pathways and processing: At the brain, the optic nerves partially cross at the optic chiasm, so information from the left visual field is routed to the right hemisphere and vice versa. Signals reach the lateral geniculate nucleus of the thalamus and then project to the primary visual cortex in the occipital lobe. Beyond early processing, two major streams emerge: the dorsal stream, related to spatial location and motion, and the ventral stream, related to object identification and form.

Development and evolution: The retina develops from neural ectoderm, featuring an orderly layering of cells and the specialized fovea for high-acuity vision. The optic nerve emerges at the optic disc, creating a blind spot. Evolutionarily, vertebrate retinas exhibit a layered structure with varying degrees of photoreceptor specialization to support diverse visual demands.

Clinical relevance: The synsinnet is assessed in ophthalmology and neurology. Common conditions include refractive errors (myopia, hyperopia, astigmatism), cataracts, glaucoma, and retinal diseases such as age-related macular degeneration and retinitis pigmentosa. Diagnostic tools include visual acuity tests, perimetry, ophthalmoscopy, and imaging techniques like optical coherence tomography.