Anatomy & Physiology

Anatomy & Physiology Study Guide The ganglion cells that monitor rods are called M cells . M cells provide information regarding the general form of an object, motion, and shadows in dim lighting. Because so much convergence occurs, the activation of an M cell indicates that light has arrived in a general area rather than at a specific location. This kind of retinal processing within ganglion receptive fields improves the detection of the edges of objects within the visual field. Cones typically show very little convergence; in the fovea, the ratio of cones to ganglion cells is 1:1. The ganglion cells that monitor cones, called P cells , are smaller and more numerous than M cells. P cells are active in bright light, and they provide information about edges, fine detail, and color. Because little convergence occurs, the activation of a P cell means that light has arrived at one specific location. As a result, cones provide more precise information about a visual image than do rods. Central Processing of Visual Information Axons from the entire population of ganglion cells converge on the optic disc, penetrate the wall of the eye, and proceed toward the diencephalon as the optic nerve (II). The two optic nerves, one from each eye, reach the diencephalon at the optic chiasm. From there, nearly half of the fibers proceed toward the lateral geniculate nucleus of the same side of the brain, whereas the other half crosses overreaching the lateral geniculate nucleus on the opposite side. Visual information journeys to the occipital cortex of the cerebral hemisphere on that side, from each lateral geniculate nucleus. Many centers in the brain stem receive visual information, either from the lateral geniculate nuclei or through collaterals from the optic tracts. Collaterals that bypass the lateral geniculates synapse in the superior colliculi or the hypothalamus. The superior colliculi of the mesencephalon issue motor commands that control unconscious eye, head, or neck movements in response to visual stimuli. The Field of Vision The perception of a visual image reflects the integration of information that arrives at the visual cortex of the occipital lobes. Each eye receives a slightly different visual image because the foveas are 5–7.5 cm apart, and the nose and eye socket block the view of the opposite side. Depth perception , an interpretation of the three-dimensional relationships among objects in view, is obtained by comparing the relative positions of objects within the images received by the two eyes. 16.8 Hearing and Balance The special senses of equilibrium and hearing are provided by the inner ear, a receptor complex located in the petrous part of the temporal bone of the skull. By monitoring gravity, rotation, and linear acceleration, equilibrium sensations yield information about the position of the head in space. Hearing helps us to detect and interpret sound waves. The basic receptor mechanism for both senses is the same. The receptors provide information from sensations of these mechanoreceptors, called hair cells . The complex structure of the inner ear and the different arrangement of accessory structures enable hair cells to respond to different stimuli and thus provide the input for both senses. Anatomy of the Ear The ear is separated into three anatomical regions: the external ear, the middle ear, and the inner ear. Achieve Page 196 of 368 ©2018