“On Old Olympus’ Towering Tops, A Finn And German Vend Some Hops.” This was the mnemonic taught to pre-med and medical students to help memorize the cranial nerves. O.O.O.T.T.A.F.A.G.V. S.H. (i) Olfactory. (ii) Optic. (iii) Occulomotor. (iv) Trochlear. (v) Trigeminal. (vi) Abducens. (vii) Facial. (viii) Auditory (now called “vestibulocochlear”). (ix) Glossopharyngeal. (x) Vagus. (xi) Spinal Accessory (now called “cranial accessory” or just “accessory”). (xii) Hypoglossal.
And there it is. Cranial Nerve Number II. Optic Nerve. Note that this is the first time I’ve used the word “nerve”, because this is the first time we are dealing with a bundle of *axons* which project from a sensory organ to the brain.
The eye consists of several structural details of cornea, lens, iris, retina, etc. which can be found on many on-line sources and Biology textbooks. However, the *neural* details important to our discussion the Brain are in the retina. The photoreceptors of the retina come in two varieties – black and white (rods) and color (cones). However, the most fascinating processing of the visual system takes place in the “retinal ganglion” neurons. These neurons receive connections from dozens of photoreceptor neurons and organize their inputs into a “field” of vision that responds to light in its center, and dark in its surrounding area (see Figure 1, right). These neurons operate by receiving an excitatory connection from photoreceptor neurons in a particular location of the retina, and inhibitory connections from the photoreceptor neurons surrounding it.
|Figure 1: Receptive Fields|
What this means is that the neurons in the retinal are best tuned to detecting spots of light very similar to pixels in a computer image or on an LCD screen. The optic nerve is actually made of the axons from the retinal ganglion neurons, and not the photoreceptor neurons. They enter the base of the brain and travel to one of the specialty regions of the thalamus. If you recall the earlier description, the thalamus is a relay gateway for sensory information entering the brain, and vision is no exception. At the “Lateral Geniculate Nucleus” of the thalamus, the axons of the retinal neurons are joined to form receptive fields that resemble bars of light – or light-dark edges of objects in the visual field. This neurons then project to the V1 primary visual cortex, and form the orientation-specific and ocular dominance columns discussed yesterday.
In parallel pathways, the color sensitive photoreceptors form similar “center-surround” structures, but this time they are paired by color. Red and green sensitive neurons are paired to produce the red-green fields shown in Figure 1. Blue and yellow are likewise paired. Interestingly, there are no true “yellow” sensitive photoreceptors, and the yellow fields shown in the figure result from the fact that green-sensitive photoreceptors differentially respond to blue-green vs. yellow-green light.
In case you were wondering, red-green color-blindness results from deficiencies of the retinal cones, and not the ganglion or LGN neurons. The “after-image” effect you “see” when you stare at a color picture, then look at a white piece of paper is a result “rebound” when the inhibition is released on the surrounding color fields of the retina and LGN neurons.
|Figure 2: Visual Field and Optic Chiasm|
Most, but not all, of the sensory and motor neurons from the body actually connect to the *opposite* side of the brain. That crossover usually takes place in the spinal cord about midway between the brain and the location where the nerve enters the spinal cord. For the cranial nerves, not all of the connections cross over. The optic nerve is very strange in that half of the connection cross, and the other half do not. The distinction is in what part of the visual field is represented. Figure 2, left shows how this works. The lens of the eye reverses and inverts the image, so that objects that appear in our vision on the right, end up on the left half of the retina. Each eye receives input from left and right visual areas, but they are combined so that the images from the right visual field go to the left LGN and visual cortex, and images from the left visual field go to the right LGN and visual cortex. Once in the visual cortex, projections from the two eyes, but for the same visual field, form the ocular dominance columns.
The “X” crossing is actually called the “decussation” or “optic chiasm” and certain brainstem or pituitary tumors or strokes can be diagnosed by their effects on the visual field caused by pressure on the optic nerve and chiasm. Visual information goes to additional locations to aid in visual tracking, localization and papillary reflexes. A novel form of stroke, involving the thalamus and the optic nerve projections, results in agnosia, for visual neglect, in which the person is unable to acknowledge any vision in one half of the visual field. Both eyes function normally, and the person will *track* a moving object anywhere in sight, but if shown a picture in the neglected area, they cannot identify it, and in fact will deny that there is anything to see (however, if the picture is a sexy, horrific or embarrassing one, they will show emotional reactions, still without recognizing that the picture is there).
So, the eyes really do “have it.” They detect light, color, motion – and even do a large part of the visual processing before the signals even get to the brain. Is it true that the retina preserves the last image seen before death? No. The retina is not like film or a video camera. Vision consists of electrical signals from neurons that detect light by chemical means, however those chemicals break down pretty rapidly when neurons are deprived of oxygen, and the electrical signal is actually a variation in the frequency of action potentials. Once those processes stop, there is not image, no picture, no information. But even without the old myth, the visual system is a fascinating place to hang out.
Next up, we will move on to the intersection of the parietal and temporal lobes for the auditory system. After that we will return to the association cortices to discuss how the brain puts multiple signals together to recognize the outside world.