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Understanding Vision and Light Transduction

Mar 16, 2025

Lecture Notes: Vision and Light Transduction

Key Concepts

  • Light Transduction: The process by which light energy is converted into electrical signals in the retina.
  • Photoreceptors: Site of light transduction in the retina, consisting of rods and cones.

Photopigments

  • Role: Integral proteins that absorb light and initiate the transduction process.
  • Structure: Composed of opsin (a glycoprotein) and retinal (a derivative of vitamin A).
  • Types:
    • Rods: Contain rhodopsin.
    • Cones: Contain different opsins for blue, green, and red light.

Transduction Process

  1. Absorption of Light:
    • Photopigments change shape upon absorbing light.
    • Retinal changes from cis-retinal (bent) to trans-retinal (straight).
  2. Isomerization:
    • Change from cis to trans triggers chemical reactions leading to receptor potential.
    • Trans-retinal separates from opsin (bleaching).
  3. Regeneration:
    • Enzyme retinal isomerase converts trans-retinal back to cis-retinal.
    • Cis-retinal binds again with opsin, reforming the photopigment.

Receptor Potential and Action Potential

  • Dark Conditions:
    • Retinal in cis form allows sodium influx, depolarizing photoreceptor.
    • Calcium influx causes release of glutamate (inhibitory neurotransmitter).
    • Inhibition prevents action potentials in bipolar cells and ganglion cells.
  • Light Conditions:
    • Retinal in trans form breaks down cyclic GMP, closing sodium channels.
    • No calcium influx, no glutamate release.
    • Depolarization of bipolar cells and ganglion cells leads to action potential.

Neural Pathway

  • Optic Nerve:
    • Exits the eye and passes through the optic chiasm.
    • Axons either cross over or remain on the same side to become optic tracts.
  • Optic Tracts:
    • Most terminate in the thalamus, synapse with neurons projecting to the occipital lobe.
    • Involved in visual perception.
  • Non-Thalamic Pathways:
    • Synapse in the superior colliculi of the midbrain, controlling reflexive visual responses.

Summary

  • Phototransduction Cycle: Starts with light absorption and concludes with neural signals sent to the brain for visual processing.
  • Critical Steps: Involves changes in retinal structure, ion channel activity, neurotransmitter release, and neural signaling leading to vision.

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