18.E) The term umami is derived from the Japanese word for savory or delicious and is often described as similar to the taste of meat. Glutamate is the chemical believed to elicit the umami taste sensation.
19.B) Visual information from the primary visual cortex (Brodmann’s area 17) is relayed to Brodmann’s area 18, and then into other areas of the cerebral cortex for further processing. Analysis of 3-D position, gross form, and motion of objects occurs in the posterior midtemporal area and occipitoparietal cortex. Analysis of visual detail and color occurs in the inferior ventral and medial regions of the occipital and temporal cortex.
20.C) Melanin is the dark pigment that occurs in great abundance in the pigment layer of the retina. The pigment layer functions to limit light scattering inside the globe of the eye and increase contrast and visual acuity.
21.D) Rhodopsin is the light-sensitive chemical in rods. Scotopsin and all-trans retinal are the breakdown products of rhodopsin, which has absorbed light energy. The all-trans retinal is converted into 11-cis retinal, which can recombine with scotopsin to form rhodopsin.
22.A) One form of vitamin A is all-trans retinol, which is converted through two different pathways into 11-cis-retinal, which then combines with scotopsin to form rhodopsin. Vitamin A is stored in large quantities in the liver. However, many months of a diet deficient in vitamin A can lead to night blindness, because rhodopsin is crucial for rod function.
23.E) In presbyopia, each eye remains focused permanently at an almost constant distance. The eyes can no longer accommodate for both near and far vision. Hyperopia and myopia refer to farsightedness and nearsightedness, respectively. Emmetropia is normal vision. Amblyopia has several causes that result in either an absence or loss of binocular vision.
24.C) The ampulla and saccule are part of the vestibular apparatus, not the cochlear apparatus. The cochlea has three main compartments, with fluid movement occurring in the scala vestibuli and scala media in response to sound vibrations. The organ of Corti is contained within the scala media.
25.C) All taste fibers synapse in the solitary nucleus and send second-order neurons to the ventral posteromedial nucleus of the thalamus. Third order neurons project to the lower tip of the postcentral gyrus in the parietal cortex.
26.E) The simple cells of the primary visual cortex detect orientation of lines and borders, whereas the complex cells detect lines oriented in the same direction but are not position-specific. That is, the line can be displaced moderate distances laterally or vertically, and the same few neurons will be stimulated as long as the line is the same direction.
27.A) The malleus is connected to the tympanic membrane, the incus articulates with the malleus and stapes, and the stapes is connected to the oval window.
28.B) In low light conditions the level of cyclic GMP is high. Cyclic GMP-dependent sodium channels in the outer portions of the rods and cones allow sodium ions to pass from the extracellular space to the intracellular space of the photoreceptor. This results in a membrane potential that is somewhat lower than the resting membrane potential of a typical neuron. The movement of the sodium ions and resulting electrical potential change as a result of this enhanced permeability is known as the dark current.
29.C) Taste impulses from the anterior two thirds of the tongue pass first into the lingual nerve, then through the chorda tympani into the facial nerve, and finally to the solitary nucleus. Taste sensations from the posterior third of the tongue are transmitted through the glossopharyngeal nerve to the solitary nucleus. Taste signals from the pharyngeal region are transmitted via the vagus nerve.
30.A) The receptor cells for the smell sensation are bipolar nerve cells derived originally from the central nervous system itself.
31.E) Although there are three to four times as many outer hair cells as inner hair cells, about 90% of the auditory nerve fibers are stimulated by the inner hair cells.
32.E) Light rays from a distant object do not require as much refraction (bending) as do light rays from an object close at hand. Therefore, the curvature of the lens required to focus these rays on the retina is less.
33.C) In the dark state, cGMP helps maintain the open state of the sodium channels in the outer membrane of the rod. Hydrolysis of cGMP by light causes these sodium channels to close. Less sodium is able to enter the rod outer segment, thus hyperpolarizing the rod.
34.E) During photoreception the active compound metarhodopsin is formed. This in turn activates a g-protein called transducin. The transducin activates a c-GMP phosphodiesterase which destroys c-GMP. C-GMP dependent sodium channels close and the influx of sodium ions into the outer segment of the photoreceptors decreases.
35.A) Astigmatism most often results when there is too great a curvature of the cornea in one of its planes. Because the curvature of the astigmatic lens along one plane is less than the curvature along the other plane, light rays striking the two planes will be bent to different degrees. Thus, light rays passing through an astigmatic lens do not come to a common focal point.
36.C) The scala media is bordered by the basilar membrane and Reissner’s membrane, and contains a tectorial membrane. The apical border of hair cells has stereocilia that are embedded in the tectorial membrane.
37.B) The abducens nerve innervates the lateral rectus muscle. The trochlear nerve innervates the superior oblique muscle. The oculomotor nerve innervates the medial rectus, inferior oblique, superior rectus, and inferior rectus muscles.
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