Study Guide for PSB2000 Exam 3 Vision Audition Mechanical Senses and Chemical Senses It is always advisable to know the key terms introduced in each section However the study hints listed here by chapter will also help you focus on other important concepts for the exam Chapter 6 Study Hints How far you see depends on how far light travels We only see rays that strike the retina perpendicularly Information reaches your nervous system then you encode it You store this information in terms of responses by neurons in these ways which neurons respond their amount of response and the timing of their responses What is the law of specific nerve energies One aspect of coding is which neurons are active Impulses in one neuron indicate light whereas impulses in another neuron indicate sound In 1838 Johannes Muller described this insight as the law of specific nerve energies statement that each nerve always conveys the same kind of information to the brain Ex if you rub your eyes you may see spots or flashes of light even in a totally dark room You applied mechanical pressure which excited visual receptors in your eyes Anything that excites those receptors is perceived as light Know the anatomy of the eye What are the functions of the labeled parts in the image included here Pupil light enters the eye thought an opening in the center of the iris called the pupil Lens the light is focused by the lens adjustable Cornea light is also focused by the cornea not adjustable Retina the light is projected onto the retina the rear surface of the eye which is lined with visual receptors o Rods are abundant in the periphery of the human retina respond to faint light but are not useful in o daylight because bright light bleaches them Cones which are abundant in and near the fovea are less active in dim light more useful in bright light and essential for color vision o Because of the distribution of rods and cones you have good color vision in the fovea but not in the periphery o Rods outnumber cones by about 20 1 cones provide about 90 of the brains input o Both rods and cones contain photopigments chemicals that release energy when struck by light Iris colored part of the eye that controls light levels inside the eye adjusts the size of the pupil as light enters and leaves Ciliary muscle controls the contraction of the lens which controls the size of the pupil allowing for certain amounts of light in Fovea a tiny specialized area for acute detailed vision Each receptor in the fovea connects to a single bipolar cell which in turn connects to a single ganglion cell which has an axon to the brain The ganglion cells in the fovea of humans and other primates are called midget ganglion cells because each is small and responds to just a single cone As a result each cone in the fovea has a direct line to the brain which registers the exact location of input Macula Blind spot the point at which the optic nerve leaves through the back of the eye which is also where the blood vessels enter and leave is the blind spot because it has no receptors Optic nerve the ganglion cell axons form the optic nerve which exits through the back of the eye In vertebrae retina however messages go from receptors at the back of the eye to bipolar cells located closer to the center of the eye The bipolar cells send their messages to ganglion cells located still closer to the center of the eye The ganglion cell s axon join together and travel back to the brain Additional cells called amacrine cells get information from bipolar cells and send it to other bipolar cells other amacrine cells and ganglion cells For the picture to the right above When light enters through the pupil and strikes the retinal neurons in what order Remember that it passes through the layers of cells pictured above and acts at the RECEPTORS they do the receiving on the far outside first In other words know the retinal circuitry pictured above and how photons stimulate action potentials From the bottom of the picture o Optic nerve o Ganglion cells red o Amacrine cells lower purple o Bipolar cells green o Horizontal cells upper purple o Receptors rods and cones orange o Inner segment bottom orange o Outer segment upper purple Photons Stimulate action potential through photoreceptor cells that absorb light stimuli and trigger the cells membrane potential o Rods and cones are photoreceptors o Photons are basically light stimuli and the smallest amount will start an action potential the eyes are constantly firing receptors even in total darkness light hyperpolarizes receptors bipolar cells are disinhibited and therefore stimulate the ganglion cells by releasing an excitatory neurotransmitter action potential down the optic nerve We talked a lot about rods and cones and their difference in foveal and peripheral vision table 6 1 Also understand the Acuity Sensitivity Tradeoff Retina the light is projected onto the retina the rear surface of the eye which is lined with visual receptors o Rods are abundant in the periphery of the human retina respond to faint light but are not useful in o daylight because bright light bleaches them Cones which are abundant in and near the fovea are less active in dim light more useful in bright light and essential for color vision o Because of the distribution of rods and cones you have good color vision in the fovea but not in the periphery o Rods outnumber cones by about 20 1 cones provide about 90 of the brains input o Remember the midget ganglion cells in the fovea all cones each receptor has its own line to the brain In the periphery mostly rods each receptor shares a line with tens or hundreds of others o Both rods and cones contain photopigments chemicals that release energy when struck by light Photopigments consists of 11 cis retinal bound to proteins called opsins which modify the photopigments sensitivity to different wavelengths of light o Acuity Sensitivity Tradeoff the idea that fovea has good acuity while rods have good motion The fovea is responsible for focusing on objects Rods bad detail vision but good sensitivity While you don t need to know 11 cis retinal or all trans retinal you should know that there are photopigments comprised of retinal and opsins in rods and cones that respond to different wavelengths The exact structure of opsin molecule determines maximal sensitivity to wavelengths of light Long wavelength red light medium wavelength green light short wavelength blue light Shortest visible wavelengths are
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