Special senses the eyes
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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

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Special Senses: The Eyes

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Special senses the eyes

Special Senses: The Eyes

By the end of this class you should understand

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

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

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

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 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 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



  • 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



  • 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 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

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

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 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

Feast Your Eyes!

Outer protection of eye

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

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

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!



  • 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

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 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

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

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!



  • 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

Thursday: the Ear!

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

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