Functional Anatomy: Rods and Cones
The light-sensitive layer of the retina is nonuniform and consists of two types of cells, each serving distinct functions.
Rods are localized predominantly in the periphery. Their numbers are vast—approximately 130 million. The primary function of these cells is to provide scotopic (dim-light) vision. Rods do not distinguish colors, responding exclusively to black-and-white stimuli, yet they possess exceptionally high light sensitivity and form a wide visual field.
Cones, in contrast, are densely packed in the central zone of the retina—the macula lutea, specifically the fovea centralis. They make color vision possible. Due to their specific pattern of synaptic connection with downstream neurons, cones guarantee maximum visual acuity and perception clarity, although higher light intensities are required for their activation.
Biochemistry of Visual Perception
The conversion of light into a nerve impulse is a complex biochemical cascade. It begins when a photoreceptor absorbs a quantum of light.
- Isomerization: A molecule of 11-cis-retinal (the chromophore portion of the pigment) changes its spatial structure and converts into all-trans-retinal.
- Protein Alteration: Conformational changes in opsin lead to the bleaching of rhodopsin and the formation of an active product—metarhodopsin II.
- G-Protein Activation: Metarhodopsin II binds to a specific retinal G-protein—transducin. As a result, transducin is activated, exchanging bound guanosine diphosphate (GDP) for guanosine triphosphate (GTP).
- Enzyme Activation: Activated transducin stimulates phosphodiesterase activity.
- Drop in cGMP Levels: Phosphodiesterase actively hydrolyzes cyclic guanosine monophosphate (cGMP). The concentration of free cGMP in the cytoplasm of the outer segment of the cell drops sharply.
Dark Current and Potential Generation
The electrical activity of photoreceptors is paradoxical: they are active in the dark and "inhibited" by light.
In complete darkness, ion channels in the membrane of the outer segment remain open. Through these channels, $Na^+$ and $Ca^{2+}$ ions continuously flow into the cell. This process is termed the dark current. In this state, the photoreceptor continuously releases the inhibitory neurotransmitter glutamate into the synaptic cleft.
As soon as light triggers the biochemical cascade and cGMP concentration falls, sodium-calcium channels close. The influx of positively charged ions stops, causing hyperpolarization of the cell membrane. This is the receptor potential. As a result of hyperpolarization, glutamate release from the presynaptic terminals decreases, which in turn alters the membrane polarization of bipolar and ganglion cells.
Ganglion Cells and Image Processing
Signals from photoreceptors are transmitted via bipolar cells to ganglion neurons, whose axons form the optic nerve and project to the lateral geniculate nucleus. Ganglion cells act as detectors: they can extract motion, angles, arcs, straight lines, object edges, and colors.
There are two main systems of retinal ganglion cells:
- P-cells (Parvocellular): Small neurons with a slow conduction velocity and tonic response type. They are essential for recognizing fine details and color vision (their activity depends on wavelength).
- M-cells (Magnocellular): Large neurons producing a rapid response to complex stimuli. They do not perceive color, but possess much higher sensitivity to luminance than P-cells.
Additionally, the retina organizes complex receptive fields with center-surround antagonism. For instance, in the red-green system, stimulating the field center with red light excites the neuron, whereas applying green light to the surround inhibits it.