Special Senses: Vision, Hearing, Taste, and Smell

Vision

• Learning Outcomes:

  • Describe the anatomical structures involved in vision, including the accessory structures and the layers of the eye.

  • Detail the process of image formation on the retina, including the role of refraction and accommodation.

  • Explain the function of photopigments in signal transduction by photoreceptors and how light energy is converted into electrical signals.

  • Trace the visual pathway from the retina to the visual cortex, highlighting key structures and their functions.

Structure of the Eye

• Key Structures:

  • Ciliary body: Controls the shape of the lens for accommodation.

  • Lens: Focuses light onto the retina.

  • Retina: Contains photoreceptors for vision.

  • Cornea: Transparent outer layer that refracts light.

  • Pupil: Opening in the iris that controls the amount of light entering the eye.

  • Iris: Colored part of the eye that regulates pupil size.

  • Sclera (covered by conjunctiva): Protective outer layer of the eye.

  • Muscles: Control eye movement.

  • Optic Nerve (CN II): Transmits visual information to the brain.

Layers and Chambers of the Eye

• Fibrous Tunic:

  • Sclera:

    • Allows attachment of extrinsic eye muscles.

    • Provides protection and shape to the eyeball.

    • Covered externally by conjunctiva (protective mucous membrane).

  • Cornea:

    • Covers the iris and pupil.

    • Primary refractive surface of the eye, bending incoming light to focus on the retina.

• Vascular Tunic (Uvea):

  • Iris:

    • Surrounds the pupil (black hole).

    • Contains muscles to control pupil size, regulating light entry.

  • Ciliary Body:

    • Includes muscles that change the shape of the lens for focusing.

    • Produces aqueous humor, which nourishes the lens and cornea.

  • Choroid:

    • Contains capillaries for nutrient supply to the retina.

    • Absorbs stray light to prevent reflection within the eye.

• Retina:

  • Sensory layer; inner layer of the eye wall.

  • Converts light into electrical signals.

  • Consists of pigmented and neural layers.

• Vitreous Chamber:

  • Contains the vitreous humor, a clear, gel-like substance that supports the shape of the eye and transmits light to the retina.

• Lens:

  • Helps focus images on the retina by changing shape through accommodation.

Pupils

• Pupillary Constriction (Parasympathetic):

  • Circular muscles of the iris contract in bright light, reducing pupil size.

  • Protects the retina from excessive light and improves image sharpness.

• Pupillary Dilation (Sympathetic):

  • Radial muscles of the iris contract in dim light, increasing pupil size.

  • Allows more light to enter the eye, enhancing vision in low-light conditions.

Retina

• Layers:

  • Pigmented Layer:

    • Absorbs stray light and supports photoreceptor cells.

  • Photoreceptor Cell Layer:

    • Contains rods and cones, which transduce light into electrical signals.

    • Rods: Function well in low light for black and white vision.

    • Cones: Responsible for color vision and function best in bright light.

  • Neural Layer:

    • Horizontal Cells: Integrate and regulate signals from photoreceptors.

    • Bipolar Cells: Transmit signals from photoreceptors to ganglion cells.

    • Amacrine Cells: Modulate signals between bipolar and ganglion cells.

    • Ganglion Cells:

      • Receive signals from bipolar and amacrine cells.

      • Optic nerve axons: Transmit signals to the brain.

• Direction of nerve impulses through the retina: from photoreceptors to ganglion cells.

Blind Spot and Most Detailed Spot

• Optic Disc: the location where the optic nerve and retinal blood vessels enter/exit the eye, creating a blind spot due to the absence of photoreceptors.

• Macula: area of the retina responsible for central, high-resolution vision; contains a high concentration of cones.

• Fovea: the central focal point in the macula, containing the highest concentration of cones and providing the most detailed vision.

Photoreceptors and Photopigments

• Rods:

  • Function well in low light (night vision).

  • Contain rhodopsin, a photopigment sensitive to dim light.

  • Do not see color (black and white vision).

  • Low detail vision.

• Cones:

  • Color vision (three different types: red, green, and blue).

  • Require bright light to function.

  • Responsible for high-acuity vision.

Image Formation

• Light travels in a straight line.

• Light bends (refracts) as it passes through the cornea.

• Light bends again at the lens.

• The lens focuses the image on the retina.

• The image is upside down and reversed (inverted) on the retina.

Accommodation

• The lens changes shape to focus the image on the retina.

• Ciliary muscles contract or relax to change the lens' shape.

• This changes the focal point of the image.

• Refractive errors can occur if the eye cannot accommodate properly, such as myopia (nearsightedness) or hyperopia (farsightedness).

Signal Transduction

• Light is absorbed by photopigments (rhodopsin in rods, cone pigments in cones).

• Photopigments change shape (isomerization).

• This initiates a cascade of events leading to graded potentials.

• Changes in the rate of neurotransmitter release (glutamate).

• Light hyperpolarizes photoreceptors, which stops the release of neurotransmitters.

Dark Current

• In Darkness:

  • CGMP-gated Na^+ channels are open.

  • Inflow of Na^+ (dark current).

  • Membrane potential is -30 mV (photoreceptor is depolarized).

  • Glutamate is released at synaptic terminals, inhibiting bipolar cells.

• In Light:

  • Isomerization of retinal activates an enzyme that breaks down CGMP.

  • CGMP-gated Na^+ channels close. Inflow of Na^+ slows.

  • Hyperpolarizing receptor potential (photoreceptor is hyperpolarized).

  • Glutamate