Microscopic Anatomy of the Retina

The vertebrate retina is a light sensitive part inside the inner layer of the eye. Two of its three types of photoreceptor cells, rods and cones, receive light and transform it into image-forming signals which are transmitted through the optic nerve to the brain. The retina is comparable to the film in a camera.

The third and more recently discovered category of photosensitive cells is probably not involved in image-forming vision. These are a small proportion, about 2% in humans, of the retina's ganglion cells, themselves photosensitive through the photopigment melanopsin, which transmit information about light through the RHT (retinohypothalamic tract) directly to the SCN (suprachiasmatic nucleus) and other brain structures. Signals from these ganglion cells are used to adjust the size of the pupil, entrain the body's circadian rhythms and acutely suppress the pineal hormone melatonin, processes which in fact function in many blind people who do not have functioning rods and cones.

The retina, indeed an extension of the brain, takes light and turns it into chemical energy in turn into nerve impulses sent to the higher regions of the brain via the optic nerve. In reality, the optic nerve is less a nerve than a central tract connecting the retina to the brain.

While rods and cones respond maximally to wavelengths around 555 nanometers (green), the light sensitive ganglion cells respond maximally to about 480nm (blue-violet). There are several different photopigments involved.

Neural signals from the rods and cones undergo complex processing by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. Several important features of visual perception can be traced to the retinal encoding and processing of light.

In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain. Hence, the retina is part of the central nervous system (CNS). It is the only part of the CNS that can be imaged directly.

The unique structure of the blood vessels in the retina has been used for biometric identification.

Picture taken and adapted from Bear, Exploring the Brain in Neuroscience.