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 Thermoreceptor Nociceptor
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 Iris Humors (liquids)
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. Wednesday: the Ear!
And after that: prep for lecture exam #2!