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A. Two cranial nerves, II and V, convey afferents from the eye to the brain
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The optic nerve, cranial nerve (CrN) II, conveys the afferent axons for two functions, the special sense of vision and pupilloconstriction.
The trigeminal nerve, CrN V, conveys the afferents for general sensation:
Ocular pain
Tearing reflex
Corneal reflex
Proprioception from the extraocular muscles
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B. Two motor systems, peripheral and central, innervate the intra- and extraocular muscles
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Peripheral ocular motor nerves consist of CrNs III, IV, and VI and the carotid sympathetic nerve.
Central ocular motor systems control the peripheral movements. The central systems find, fixate, focus/align on, and follow visual targets. CrNs III, IV, and VI innervate the extraocular muscles for these actions.
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C. Two images, a real retinal image and a mental or visual image, made by the mind
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Start with an arrow as the visual target (Fig. 3-2).
Each retina receives an inverted real or actual image, due to the physical optics of the eye.
Neurophysiologic processing then converts this real retinal image into an abstraction called a visual image.
Projection of the visual image by the mind
As Fig. 3-2 shows, the light rays that form the nasal half of the retinal image come from the □ temporal/□ nasal half of the object viewed.
☑ temporal
By a process of learning, we associate the point of retinal stimulation with the reverse half of space. Hence, if light rays fall on the temporal half of the retina, the mind perceives the object as located in the □ temporal/□ nasal half of space.
☑ nasal
If the image of an object falls on the nasal side of the retina, we would reach for the object in the temporal half of space. Similarly, if the image falls on the upper half of the retina, we would reach for the object in the ____________________ half of space.
lower
Thus the law of projection of the visual image states that the mind projects the visual image derived from one half of the retina to the ____________________ half of space.
opposite or reverse
This particular law exemplifies a general law of sensation: The mind projects afferent impulses to their usual site of origin in all sensory systems. If an electrode stimulated your right auditory nerve, you would experience a sound as if it came from the right side of space. If you bump your ulnar nerve at the elbow, you would feel a shock down your forearm into your little finger, even when no afferent impulses arose from the finger itself. In each case, we say that, when afferent impulses reach the brain, the mind projects or refers them to their usual site of origin.
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D. Two areas of the retina: central and peripheral areas
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The retina consists of a central circular macula and a peripheral zone that concentrically surrounds the macula (Fig. 3-3).
The retina has two types of receptors:
Cones in the macula
Rods in the periphery
The retina provides two fields of vision, central and peripheral. Each field has two functions:
The cone receptors of the macula mediate the two functions of the central field of vision:
Visual acuity
Color vision
Mnemonic of the C's for the function of the Cones: The Cones, located Centrally in the maCula, mediate visual aCuity, and Color vision.
In the periphery of the retina, concentrically surrounding the macula, rod receptors mediate the two functions of the peripheral field of vision:
Night vision
Motion detection
Neuronal layers of the retina
Light initiates afferent impulses for vision and pupilloconstriction by exciting the rod and cone receptors of the retina.
The unipolar rod and cone neurons synapse on the bipolar neurons of the adjacent layer of the retina.
The bipolar neurons synapse on multipolar neurons of the ganglion cell layer. The axons from the multipolar neurons converge on the optic disc in a special pattern, pierce the lamina cribrosa of the sclera, and form the optic nerve that emerges from the back of the eyeball and joins the optic chiasm.
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E. Dual pathways in the optic nerve
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The optic nerve runs from the retina posteriorly through the optic chiasm where it divides into two optic tracts. Each optic tract contains two sets of axons.
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One set of axons comes from the ipsilateral, temporal half of the retina (Fig. 3-4).
A second set of axons comes from the contralateral, nasal half of the retina. These two groups of axons unite the right and left visual half fields from each eye.
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A. Complete blindness, L eye
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B. Complete bitemporal hemianopia
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C. Complete nasal hemianopia, L eye
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D. Complete R homonymous hemianopia
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E. Complete R superior homonymous quadrantanopia
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F. Complete R inferior homonymous quadrantanopia
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G. Complete R homonymous hemianopia
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F. Dual pathways branch from the optic tract
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The optic tract conveys visual and nonvisual axons posteriorly from the chiasm.
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The nonvisual axons of the optic tract go to the pretectum of the midbrain and to the hypothalamus.
The retinopretectal tract synapses in the pretectum and midbrain for pupilloconstriction to light (Fig. 4-30).
The retinohypothalamic tract synapses on the paraventricular nucleus and mediates the diurnal cycle of sleep and wakefulness.
The visual axons in the optic tract, the retinogeniculate tract, synapse on the lateral geniculate body of the thalamus (Fig. 3-4).
The geniculocalcarine tract then synapses on the primary visual (calcarine) cortex (Figs. 3-4 to 3-6).
The association cortex surrounding the calcarine cortex then interprets the significance and meaning of the visual image.
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G. Dual banks of the calcarine cortex
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The primary visual cortex forms dual, upper and lower, banks along the calcarine fissure on the medial surface of the occipital lobe (area 17 of Brodmann; Figs. 2-2, 3-4, and 3-5A).
The macula is represented toward the occipital pole of area 17. The remaining retina is topographically represented forward (Fig. 3-5B).
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Mnemonic for the representation of the macula and upper and lower halves of the retina on the calcarine cortex:
Make a sagittal hemisection of an eyeball and place the hemisection on the occipital pole, as in Fig. 3-5B. The actual macula of the eyeball then rests on the occipital pole, which receives the projection from the macula. The rest of the visual cortex represents the successively more forward sectors of the retina. The upper half of the retina rests on the upper bank of the calcarine cortex, and the lower half rests on the lower bank of the calcarine fissure.
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H. Review of the visual pathway
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Think through the afferent pathway for vision until you know it. First of all, notice in Fig. 3-4 that, when both eyes focus on the arrow, the real images fall on corresponding parts of the retina. Then proceed through the retinal rods and cones; bipolar layer; multipolar layer, optic nerve, optic chiasm; optic tract; geniculate body synapse; and geniculocalcarine tract to the primary visual cortex around the calcarine fissure.