Neurophysiology: Vision- Pt 2
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Questions and Answers

Which type of potentials do photoreceptors, bipolar cells, horizontal cells, and amacrine cells use?

  • Graded potentials (correct)
  • Electrical conduction
  • Action potentials
  • Synaptic potentials
  • What type of conduction do retinal neurons use to conduct their visual signals?

  • Neurotransmitter conduction
  • Electrical conduction (correct)
  • Ion channel conduction
  • Chemical conduction
  • What is the result of the hyperpolarization (due to the sodium-calcium channels closure) during the activation of photoreceptors by light?

  • Depolarization of the membrane
  • Decreased glutamate release (correct)
  • Increased sodium-calcium channel opening
  • Release of neurotransmitters
  • What is the function of glutamate in photoreceptors?

    <p>Inhibitory</p> Signup and view all the answers

    What happens to cGMP in the dark?

    <p>It accumulates and binds to ligand gated sodium-calcium ion channels</p> Signup and view all the answers

    What is the result of the closure of sodium-calcium channels in photoreceptors' light activation?

    <p>Hyperpolarization of the membrane</p> Signup and view all the answers

    What is the function of transducin in photoreceptors?

    <p>It activates PDE6</p> Signup and view all the answers

    What type of cells generate action potentials?

    <p>Ganglionic cells</p> Signup and view all the answers

    Which of the following is NOT a factor that affects visual acuity?

    <p>Population of retinal cells</p> Signup and view all the answers

    What is the purpose of the lens in the eye?

    <p>To converge light rays to a focal point on the retina</p> Signup and view all the answers

    What is the characteristic of images formed on the retina?

    <p>Real, inverted, and smaller than the object</p> Signup and view all the answers

    What is the purpose of the retina in the process of image formation?

    <p>To act as a film, detecting light and transmitting signals to the brain</p> Signup and view all the answers

    What is the difference in the way cone cells and rod cells feed signals to ganglion cells?

    <p>Hundreds of rod cells feed signals to a single ganglion cell, while only a few cone cells feed signals to a single ganglion cell</p> Signup and view all the answers

    What is the characteristic of the central retina?

    <p>It has the highest visual acuity in the retina</p> Signup and view all the answers

    What happens to the light rays emanating from the top and bottom of the object as they approach the eye?

    <p>They converge and cross each other</p> Signup and view all the answers

    What is the function of the optic radiation?

    <p>To transmit visual signals from the thalamus to the occipital lobe</p> Signup and view all the answers

    What is the role of the pretectal nucleus in the pupillary light reflex arc?

    <p>To act as a brainstem center for integration</p> Signup and view all the answers

    What is the function of the cones and rods in the retina?

    <p>To transform light into nerve impulses</p> Signup and view all the answers

    Choose the correct order of fiber termination for visual reflexes.

    <ol> <li>Midbrain 2) Tectospinal tract 3) Cranial nerve nuclei</li> </ol> Signup and view all the answers

    What is the order of light refraction in the eye?

    <p>Air-cornea-aqueous humor-lens-vitreous humor</p> Signup and view all the answers

    What is the function of the optic nerve?

    <p>To transmit visual signals from the eye to the brain</p> Signup and view all the answers

    Which species have the highest percentage of cranial nerve 2 (optic nerve) decussation?

    <p>Birds and lizards</p> Signup and view all the answers

    Which of the following is not an example of an effector organ used for the pupillary light reflex?

    <p>Pretectal nucleus, and the parasympathetic nucleus of the oculomotor nerve (cranial nerve 3) for integration</p> Signup and view all the answers

    Study Notes

    Activation of Photoreceptors by Light

    • Photoreceptors, bipolar cells, horizontal cells, and amacrine cells use graded potentials rather than action potentials.
    • These cells conduct their visual signals along the cell by direct flow of electric currents (electrical conduction).
    • The strength of the hyperpolarizing output signal is directly related to the intensity of the illumination.

    Activation of Photoreceptors by Light: Steps

    • Step 1: Activation of rhodopsin by light leads to activation of a G-protein, transducin.
    • Step 2: The activated transducin splits away and activates PDE6.
    • Step 3: The activated PDE6 hydrolyzes cGMP to 5'-GMP (inactive form), causing the sodium-calcium channel to close.
    • Step 4: The sodium-calcium channel closure leads to hyperpolarization (membrane potential becomes more negative).
    • Step 5: The decrease in glutamate released will excite the bipolar cells.

    Photoreceptors in the Dark

    • In the dark, cGMP phosphodiesterase is inactive (PDE6).
    • cGMP accumulates and binds to ligand-gated sodium-calcium ion channels.
    • The flow of cations into the rods keeps the membrane depolarized.

    Visual Acuity

    • Acuity of visual images reflects the following factors: population of retinal cells, ratio of rods to cone photoreceptor cells, and ratio of photoreceptor cells to ganglion cells.
    • Cone cells provide better acuity than rod cells.
    • Hundreds of rods feed signals via bipolar cells to a single ganglion cell.
    • Only a few cone cells feed signals to a single ganglion cell.

    Image Formation: General Info

    • Images are formed by the eye, which is optically equivalent to a photographic camera.
    • The eye catches the light reflected by objects and guides its passage until the image is formed.
    • The lens works by converging the light rays to a certain focal point on the retina.

    Image Formation: Light Refraction Steps

    • Step 1: Light is refracted off of the interface between the air and the anterior surface of the cornea.
    • Step 2: Light is refracted at the interface between the posterior surface of the cornea and the aqueous humor.
    • Step 3: Light is refracted at the interface between the aqueous humor and the anterior surface of the lens of the eye.
    • Step 4: Light is refracted at the interface between the posterior surface of the lens and the vitreous humor.

    Visual Pathway

    • The images formed on the retina are transformed by cones and rods into nerve impulses.
    • The brain processes and reorients the image.
    • Visual signals are carried by optic nerve fibers that form the optic nerve, optic chiasm, and optic tract.

    Pupillary Light Reflex (PLR)

    • The PLR is a reflex arc composed of receptors for light within the retina, afferent neurons, brainstem centers, and efferent neurons.
    • The PLR is involved in pupillary constriction.

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    Understanding the activation of photoreceptors by light, graded potentials, and electrical conduction in retinal neurons.

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