Anatomy and Physiology Answer Key

3 a horizontal cells are interconnecting neurons that

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Unformatted text preview: g information about light level. 2. (a) The incoming light is refracted (primarily by the cornea) and the amount entering the eye is regulated by constriction of the pupil. The degree of refraction is adjusted through accommodation (changes to the shape of the lens) so that a sharp image is formed on the retina. (b) Accommodation is achieved by the action of the ciliary muscles pulling on the elastic lens and changing its shape. Note: When the ciliary muscle contracts there is decreased pressure on the suspensory ligament and the lens becomes more convex (to focus on near objects). When the ciliary muscle relaxes there is increased tension on the suspensory ligament and the lens is pulled into a thinner shape (to focus on distant objects). 4. The blind spot is where all the nerve fibers leave the eye as the optic nerve so there are no photoreceptor cells there (hence "blind"). In contrast, the fovea has the greatest density of photoreceptor cells and hence it is the point of most acute vision. 5. Each rod cell synapses with a number of bipolar cells and this gives poor acuity but high sensitivity. Each cone cell synapses with only one bipolar cell and this gives high acuity but poor sensitivity. 6. (a) A photochemical pigment molecule contained in the membranes of the photoreceptor cells that undergoes a structural change when exposed to light (and is therefore light-sensitive). (b) Rhodopsin in rods and iodopsin in cones. 7. Light falling on the retina causes structural changes in the photopigments in the rods and cones. These changes lead to the development of graded electrical signals (hyperpolarizations) which spread from the rods and cones, via the bipolar neurons, to the ganglion cells. The ganglion cells respond by depolarization and transmit action potentials to the brain. 3. (a) The pupil is the hole through which light enters the eye. By constricting (in bright light), the pupil can narrow the diameter of this entry point and prevent light rays entering from the periphery....
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