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The particular set of olfactory neurons that respond to a given odor might serve as a “fingerprint” the brain can use to identify the odorInternal chemoreceptors detect pH and other characteristics Sensory receptors within the body detect a variety of a chemical characteristics of the blood or fluids derived from the blood, including cerebrospinal fluidIncluded among these receptors are the peripheral chemoreceptors of the aortic andcarotid bodiesAre sensitive primarily to plasma pHCentral chemoreceptors in the medulla oblongata of the brainAre sensitive to the pH of cerebrospinal fluidWhen the breathing rate is too low, the concentration of plasma carbon dioxide increases, producing more carbonic acid and causing a fall in the blood pHThe carbon dioxide can also enter the cerebrospinal fluid and lower the pH, thereby stimulating the central chemoreceptorsThis stimulation indirectly affects the respiratory control center of the brainstem, which increases the breathing rateThe aortic bodies can also respond to a lowering of blood oxygen concentrationsThis effect is normally not significant unless a person goes to a high altitude where the partial pressure of oxygen is lower
44.5VisionThe ability to perceive objects at a distance is important to most animalsPredators locate their prey, and prey avoid their predators, based on the three long-distance sense of hearing, smell, and visionVision can act most distantlyWith a naked eye, humans can see stars thousands of light years away A single photon is sufficient to stimulate a cell of the retina to send an action potentialVision senses light and light changes at a distanceVision begins with the capture of light energy by photoreceptorsBecause light travels in a straight light and arrives virtually instantaneously regardlessof distance, visual information can be used to determine both the direction and the distance of an object Other stimuli, which spread out as they travel and move more slowly, provide much less precise informationStructure of the vertebrate eyeScleraThe white of the eye Formed of tough connective tissueCorneaTransparent front part of the eye that covers the iris, pupil, and anterior chamberLight enters the eye through the cornea which begins to focus the lightFocusing occurs because light is refracted or bent when it travels into a medium of different densityIrisColored portion of the eyeContraction of the iris muscles in bright light decreases the size of its opening, the pupilPupilBack portion of the eyeLight passes through the pupil to the lens Lens
Transparent structure that completes the focusing of the light onto the retina at the back of the eyeAttached by the suspensory ligament to the ciliary musclesThe shape is influenced by the amount of tension in the suspensory ligamentSurrounds the lens and attaches it to the circular ciliary muscle When the ciliary muscle contracts, it puts slack in the suspensory ligament, and the lens becomes more rounded and bends light more strongly oThis rounding is required for close vision