Special Senses: The Eyes

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

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)

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

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

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

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

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

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

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

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

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

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

Feast Your Eyes!

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)

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

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!

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

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

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

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

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!

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

Thursday: the Ear!

• And after that: prep for lecture exam #2!