Chapter 7F - Visual System
Visual pathways are interesting and clinically important. The retina is a complex structure that consists of three main cellular layers, with interspersed layers of connections containing other cells that modify the signals. The deepest layer contains the rods and cones, which themselves have an inner and outer segment. The receptive element of the rods and cones are contained on lamella of membranes in the outer segment, meaning that light has to pass through all retinal layers to produce its physiologic effects. Rods are very sensitive to small number of photons of light meaning they are used in low-light situations but do not convey color information. There are three types of cones representing the three primary colors, distinguished by their photoreceptive pigment. The rods and cones are constantly depolarizing and releasing neurotranmistter (called the dark current). Stimulation by light closes the ion channels that produce the dark current and results in hyperpolarization with a decrease in neurotransmitter release onto bipolar cells. Bipolar cells do not generate action potentials. However they do release transmitter onto ganglion cells, which are the first site of action potential generation in the visual system. Ganglion cells receive input from many bipolar cells, some of which are excitatory and some inhibitory. Uniform lighting of the receptive field of a bipolar cell will produce no response. Some ganglion cells respond to a pinpoint of light illuminating the center of the receptive field ("on-center") and some are inhibited by light falling on the center ("off-center"). This makes the ganglion cell signals very good at discriminating edges and points.
The optic nerve consists of the myelinated processes of ganglion cells (figure 21), which leave the retina at the optic disc (physiologic blind spot because there are no photoreceptors). The nerve leaves the orbit through the optic canal to reach the optic chiasm located just superior to the pituitary gland. The ophthalmic artery has a branch that enters the optic nerve (central retinal artery) and supplies the inner part of the retina. In the optic chiasm, the nerve fibers from the nasal side of the retina cross to the contralateral side. These crossing fibers convey signals from the lateral (temporal) visual fields. As the retinal axons progress posteriorly from the optic chiasm, those from the nasal part of the retina that have just crossed the midline join those from the opposite temporal part of the retina in the optic tract. Note that the nasal part of the retina of one eye and the temporal part of the retina of the other eye detect light from an overlapping part of the visual field. Therefore, the left optic tract contains all of the signals from the right side of the visual world. All portions of the visual system posterior to the optic chiasm convey information from overlapping (homonomous) portions of the visual world.
The optic tract terminates in the lateral geniculate nucleus of the thalamus. This nucleus has 6 layers, each one receiving input from one or the other eye. The optic radiations are a broad band of fibers that arise from neurons in the lateral geniculate and terminate in the primary visual cortex located in the medial occipital lobe adjacent to the calcarine sulcus. Information from the lower contralateral visual world passes through optic radiations that traverse the white matter of the parietal lobe to terminate on the gyrus above the calcarine sulcus (the cuneus) and that from the upper visual world passes through the temporal lobe (Meyer's loop) to reach the gyrus inferior to the calcarine sulcus (the lingula). Central vision is represented most posteriorly, right at the occipital pole of the cerebral cortex.
Geniculo-cortical connections terminate in adjacent, vertically oriented stripes (therefore the name "striate cortex" for visual cortex). These stripes represent input from overlapping areas of each eye (ocular dominance columns). Each column of cells receives input from one eye and the column adjacent to it receives input from portions of the retina of the other eye that detect light from the same portion of the world. It is at this level that inputs from the two eyes begin to be integrated.
The visual cortex does not function as a camera conveying point-to-point information. Instead, neurons are arranged to be responsive to progressively more complex functions (lines, edges, or particular orientations or directions of movement of edges). The visual association areas surrounding the primary visual cortex are responsible for synthesizing the information into recognizable and useful representation of the world. There are certain specific areas of critical importance, such as the posterior part of the middle temporal gyrus (MT) that is necessary for processing information about movement and part of the inferior temporal gyrus that is necessary for facial recognition.