Special Senses: The Eyes

1. Special Senses: The Eyes

2. By the end of this class you should understand: • The general structure of a sensory neuron and the types found in the body • The properties of light as it relates to vision • The major parts of the eye and their roles in focusing light • The different types of photoreceptors in the eye and their functions

3. The Six Senses • Classically, humans are thought to have five senses • Reality is we have many! • Vision • Hearing • Taste • Smell • Touch (actually many kinds of senses) • Balance (vestibular sense)

4. Sensory Neuron • All sensory neurons, or receptors, have some type of molecule that causes them to receive signals from the environment • These signals create action potentials (depolarizations of membrane) • The axon sends this action potential to the spinal cord and ultimately the brain

5. Types of Receptors • Mechanoreceptor • Responds to mechanical stress such as pressure or stretching • Thermoreceptor • Reponds to high or low temperatures • Nociceptor • Pain receptor that signals damage to cells • Chemoreceptor • Responds to chemical stimulus • Photoreceptor • Reponds to light

6. Mechanoreceptors • Mechanoreceptors generate our sense of touch • They also are responsible for hearing and balance (more on that next class) • Typically, when something pushes on the cell it opens mechanically gated ion channels • When ions move into the cell it depolarizes the membrane and creates an action potential

7. Thermoreceptors • Thermoreceptors are embedded in our skin and organs • Relate information about heat and cold • Only function within a certain range and can be killed by extreme temperatures • Frostbite and burns often begin with numbness until pain receptors kick in

8. Nociceptor • Nociceptors detect imbalances in tissues and send action potentials as a result • Combination mechanoreceptor and chemoreceptor • Potassium is one stimulus that they respond to • Potassium is supposed to be inside cells, so a large amount of potassium is often caused by cell lysis • Responsible for itching and pain

9. Chemoreceptor • Chemoreceptors send action potentials in response to having chemicals bind to the cell membrane • Responsible for senses of smell and taste • Taste: only five flavors (sweet, sour, bitter, salt, savory) • Over 1000 smell receptor types • Much of “taste” is smell, which is why food tastes bland when you have a cold

10. Photoreceptors • Photoreceptors respond to light by sending action potentials • Found only in the eye • Produce the sense of sight • The eye’s function is to focus light onto these photoreceptors so they can send action potentials to the brain • The human eye has three types of cone and one type of rod, all different kinds of photoreceptors

11. Properties of Light • Light is made up of particles called photons that are so small and move so fast they also behave as waves • The more energy a photon has, the faster its frequency • The perceived color of a particle of light depends on what frequency it has

12. Wavelengths of Light • Only particles of light with certain energies are visible light • These are the frequencies that activate our photoreceptors • Higher-energy particles such as UV light and X-rays pass through without stimulating our photoreceptors • Lower-energy particles such as infrared, microwaves and radio waves don’t have enough energy to stimulate our photoreceptors

13. The Structure of the Eyeball • The eye has one function: to focus light on the retina which is a tissue filled with photoreceptors • The light is allowed in through a small hole called the pupil and is focused (bent) by the lens • The lens can change its thickness to change the focus to be closer or farther away • All the other parts of the eye are protection and support for these active parts

14. Feast Your Eyes!

15. Outer Protection of Eye • Sclera • Also known as the “white” of the eye • Fibrous connective tissue that envelops the eye • Conjunctiva • A thin transparent membrane around the outside of the sclera • Cornea • The portion of the conjunctiva in front of the pupil • Bends light (Lasik surgery changes the shape of the cornea)

16. Inner Structure of Eye • Pupil • The hole though which light enters • Iris • The colored part of the eye • Changes size to allow more or less light in • Humors (liquids) • Aqueous humor is between the pupil and lens • Vitreous humor fills main eyeball and keeps it round and taut

17. Focusing of Light • Light is focused by the lens and the cornea • The eye’s shape is vital for this focusing to work • Anyone who has tried wearing the wrong glasses prescription can tell you so!

18. Retina • The retina lines the back of the eyeball • Filled with rods and cones • The very center of the retina is called the macula and is filled primarily with cones • The rest of the retina is filled primarily with rods

19. Blind Spot • The optic disk on the retina is where the axons from all the interneurons of the photoreceptors meet and become the optic nerve • This produces a blind spot that our visual cortex (in the occipital lobe) fills in

20. Rods and Cones • Rods are sensitive to many different wavelengths of light • Since action potentials are all-or-none, rods do not distinguish between different colors of light and produce only grayscale vision • Most humans have three types of cones (red, green and blue) • They require much more light to function than rods but produce color vision

21. Two Rods Converged • Rods also have a property called convergence • Many rods are attached to the same interneuron • When any of those rods fire, the interneuron fires • Produces a fuzzy picture • Cones do not have convergence • They produce clear images but require a lot of light • This is why they are concentrated in the macula

22. Activation of Photoreceptors • Rods and cones all have different versions of the same molecule, rhodopsin • Rhodopsin is a protein with a pigment called retinal contained inside • Retinal is made from vitamin A • Eat your carrots!

23. Retinal • Different retinal structures respond to different frequencies of light but all of them change shape when struck by the right photon • The change in shape causes the rhodopsin to alter the behavior of sodium channels • This ultimately creates action potentials in the interneurons of the eye which go to the brain

24. Thursday: the Ear! • And after that: prep for lecture exam #2!