Lab 8 The Special Senses Hearing Vision and Orientation

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Convex Lens, Fig 8 5 Accomodation In order for light from a distant. object to be brought into focus on the retina the ciliary. muscles relax tightening the suspensory ligaments and. stretching out the lens and reducing its refractive power. To view near objects the ciliary muscle contracts thus. slackening the suspensory ligaments and allowing the lens. to recoil into a thicker shape with greater refractive. Concave Lens,Most lenses have a fixed refractive power. Fig 8 3 A comparison of convex and concave lenses and thus an object must be a specific distance. Convex lenses converge light toward a single focal point from the lens in order for light from that object. whereas concave lenses disperse light away from a single to be focused by that lens However the. focal point,refractive power of the lens of the human eye is. adjustable The circumference of the lens is, the thickness of the lens Thicker lenses bend attached to the ciliary muscle of the eye through. light more i e have greater refractive power a series of ligaments called suspensory. thus light that passes through the lens converges ligaments Fig 8 1 By contracting the ciliary. into a single point in a shorter distance i e muscle to different degrees the tension exerted. shorter focal length than for a thinner lens Fig by the suspensory ligaments on the edges of the. 8 4 Concave lenses Fig 8 3 in contrast lens can be altered allowing the lens to be. disperse light passing through them rather than stretched thin or allowed to elastically recoil into. converge that light and thus have a negative a thicker shape Thus as the thickness of the. refractive power based on the degree to which lens is altered its refractive power is altered. they disperse light This alteration in lens thickness can be used to. focus on objects at different distances from the,eye through a process called accomodation Fig.
8 5 To view distant objects from which light,enters the eye at a narrow range of angles the. ciliary muscles are relaxed This causes this ring,Focal Length. of muscle to become thin which in turn pulls,outward increasing the tension on the. suspensory ligaments As these ligaments are, pulled taut they stretch out the lens causing it to. Focal Length assume a thin shape with a lower refractive. power In contract to view close objects the, Fig 8 4 Effect of lens thickness on refractive power in ciliary muscles contract As this ring of muscle.
convex lenses Thicker lenses bend light more thus the. focal length for the lens is less and the refractive power. thickens it releases tension on the suspensory, in diopters is greater ligaments and the lens elastically recoils back to. a thicker shape with a higher refractive power, Far Normal Cornea Frontal View Astigmatism Frontal View. Near Dorsal, Fig 8 7 Astigmatism as illustrated with a mishapen. Fig 8 6 Myopia and hyperopia In myopia they eye has cornea Notice that the cornea on the right is wider. abnormally high refractive power thus whereas near than it is high This means that light in the vertical. objects can be seen in focus since refractive power would plane will be refracted more than light in the horizontal. need to be increased anyway the refractive power of the plane As a result if the lens is adjusted to bring light. eye cannot be lowered enough to focus on distant objects in the vertical plane into focus light in the horizontal. as light passing through the eye reaches its focal point in plane will go out of focus. front of the retina In hyperopia the eye has abnormally. lower refractive power In this case distant objects can. bee seen clearly but the refractive power of the eye inward before it enters the eye are used to. cannot be increased enough to see near objects as the correct for this visual disorder. focal point for light passing through the eye would be Astigmatisms are visual disorders causes by. behind the retina a mishaping of some refractive structure in the. eye such that the focal length for light entering, Many visual disorders are associated with in the eye is not the same at all angles Fig 8 7. abnormal refraction of light due to mishaped as a result the lens cannot be adjusted to bring. structures in the eye Myopia or light at all angles into focus simultaneously. nearsightedness is a condition where an Corrective lenses for astigmatisms have an. individual has trouble seeing distant objects angular alteration in refractive power that. although near objects are seen clearly Fig 8 6 corrects for the angle of mishapening in the eye. Myopia results from having an unusually Fig 8 8,elongated eyeball or an unusually thick cornea.
or lens In effect the eye cannot reduce,refractive power enough to view distant objects. so light from these objects comes into focus at a,point in front of the retina and is back out of. focus by the time it reaches the retina Concave,lenses which spread out light into a wider array. of angles before it enters the eyes are prescribed. to correct this visual disorder,Individuals with hyperopia or. farsightedness have the opposite problem of,those with myopia Having a relatively short.
eyeball shallow cornea or thin lens individuals,with hyperopia cannot focus on nearby objects. but can focus on distant objects because the, eye does not have enough refractive power to Fig 8 8 Optometry prescription form Sphere indicates. bring the image for close objects into focus by the overall refractive power adjustment in diopters The. the time it reaches the retina and instead the cylinder indicates refractive power change for an. astigmatism and the axis indicates the angle of the. focal point is located somewhere behind the eye astigmantism correction From. Corrective convex lenses which bend light http www pearlevision com veex ve page17 html. The ability of the eye to adjust its refractive,power can change with age As a person gets. older the lens becomes less flexible and,attachments for the the suspensory ligaments are. moved forward on the lens As a result the,lense remains in a stretched state even when the.
ciliary muscles constrict This reduces the, ability of the eye to increase its refractive power. to see near objects This form of farsightedness, called presbyopia is nearly ubiquitous among Optic disk. people over the age of 45 years and is part of,the overall aging process. Once light is bent correctly by the refractive, Fig 8 10 Projection of light from laterally oriented. structures of the eye a clearly focused image of objects onto the retina Note that more laterally. objects in the visual field is projected on the postioned objects project images on more medial. retina of the eye The ability to see objects in regions of the retina Also note that images projected. the visual field and determine their spatial onto the optic disk e g the purple circle above cannot. be seen since therre are no photoreceptors at that. relationship to one another depends upon what location on the retina. part of the retina light from a point in the visual. field is projected Light that passes through the, axis extending through the centers of the cornea acuity is very high in the fovea centralis Acuity.
and lens is directed toward a structure in the tends to decrease somewhat for light projected. retina called the macula lutea Figs 8 1 and 8 9 on more peripheral regions of the macula lutea. where most of the cone cells of the retina those where each ganglion cells receives signals from. that permit color vison and high visula acuity multiple cone cells creating a larger receptive. are located At the center of the macula lutea is field for each ganglion cell and even more so. a pit like region called the fovea centralis This for light falling outside of the macula lutea. particular location has an extremely high density where each ganglion cell is stimulated by many. of cone cells each connected with the peripheral individual rod cells those that have high light. nervous system in such a way that each cone cell sensitivity but no color discrimination and low. stimulates its own ganglion cell sensory acuity, neuron This means that the receptive field for Light from objects that are positioned in the. each ganglion cell is very small and thus visual lateral parts of the visual field are projected onto. the media portions of the retina Fig 8 10, Interestingly it is here in the media region of the. retina where the optic nerve joins with the eye, and on the inner surface of the retina a structure. called the optic disk can be seen that demarcates,this connection point Figs 8 9 and 8 10 The. optic disk lacks photreceptor cells so is light,from an object is projected directly on the optic.
disk no image will be perceived This creates a,blind spot within the visual field although the. brain perceives a continous visual field based on,sensory inputs from areas surrounding the blind. If a person directly faces an object light,from that object which would be medially. oriented will tend to be projected somewhat,laterally on the inner surface of the retina Fig. Fig 8 9 A view of the fundus of the eye showing, structures on the internal surface of the retina 8 11 The degree to which the image is.
8 11 Effect of object distance on lateral projection of. images in stereoscopic vision Light from near objects. tends to be projected onto more lateral regions of the. retina than does light from distant objects Thus Fig 8 12 Internal anatomy of the ear Illustration from. differential stimulation of photoreceptors in both eyes http www vestibular org gallery html. simultaneously allows depth perception for objects that. are relatively near to the individual 100,inner ear with enough strength to stimulate the. projected laterally is related to the distance of hair cells the hair cells must be able to respond. the object from the eyes Recall that light from to these vibrations by releasing enough. points on distant objects enter the eye at a very neurotransmitter to sensory neurons to trigger an. narrow range of angles Light from these action potential the action potentials must. objects would be projected more or less towards propagate through the auditory nerve into the. the center of the retina for both eyes However central nervous system and through appropriate. light from points on near objects enter the eye at second order and third order neurons to reach. a broader range of angles so light from those the auditory cortex in the temporal lobe. objects would tend to be projected onto more Hearing impairments can be caused by a. lateral regions fo the retina This lateral number of different conditions but can be. projection onto the retinas of both eyes categorized into two different types The first. stimultaneously coupled with the relative size of conductive deafness results from a condition. the object in relation to other objects appearing where the conduction and amplication of. in the visual field enables depth perception in vibrations between the external environment and. human vision In order for lateral projection of the fluid of the cochlea As a result the. images on the retina to be effective however vibrations that reach the hair cells are not strong. the image must fall into the visual field of both enough to lead to action potential generation in. eyes Thus the stereopsis the viewing of the the sensory neurons of the auditory nerve. same object from two slightly different angles Examples of conditions that cause conductive. simultaneously provided from having two eyes deafness include occlusion of the external. postioned at the front of the head with auditory meatus e g excessive earwax. overlapping visual fields is important component production perforation of the tympanic. of our ability to see in three dimensions membrane abnormal development or damage to. the audittory ossicles or damage to the oval or,round windows Conductive deafness can often. Hearing be corrected through the use of hearing aids that. amplify sound entering the ear so that the, The outer and middle regions of the ear act as a vibrations reaching the inner ear are strong. conduction and amplification system that enough to effectively stimulate the hair cells. collects sound waves in the air and amplifies The other type of deafness sensorineural. these vibrations enough to generate waves of deafness results from damage to either the. fluid pressure in the cochlea of the inner ear sensors hair cells for hearing to the nerve. where the sensory receptors hair cells are pathways that conduct signals from the ear to the. located In order for hearing to take place auditory cortex or to the auditory cortex itself. sound must be effectively conducted into the In this case the basic process of sensation. cannot take place because the sensory pathway The positioning of the ears on the lateral. surfaces of the head enables binaural hearing,where the central nervous system not only. detects vibrations in the surrounding,environment but by comparing characteristics.
of the sound detected by each ear can perceive,the direction from where the sound originated. To illustrate why imaging that you have your,left ear oriented toward a stereo speaker and. your right ear oriented away from the speaker,Sound eminating from the speaker will reach the. left ear a fraction of a second before the right,ear Thus there will be difference in stimulation. Lab 8 The Special Senses Hearing Vision and Orientation Background The special senses vision hearing equilibrium gustation and olfaction differ from the somatesthetic senses in two fundamental ways First the receptors for the special senses are all found within specific locations the head and often within complex organs designed to modify the environmental change in a way that

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