L11 Sensory System

The Eye and Vision

Learning Outcomes

  • Understand the structure of the eye and its main compartments.
  • Know the cellular structure of the retina.
  • Explain phototransduction: the conversion of light into electrical signals by photoreceptors.
  • Discuss the transmission of electrical activity through the retina.
  • Understand the pathways from the eye to the brain.
  • Reference: Neuroscience: Exploring the Brain. 4th Ed., Chapter 9.

Special Features of the Eye

  • Produces clean, crisp images.
  • Adjusts to light levels and focus.
  • Tracks moving objects.
  • Self-cleaning mechanisms (tears, blinking).
  • The retina is part of the brain (CNS tissue) and converts light to electricity.
  • Enables in-depth view of the world.

Structure of the Eye

  • Anterior Segment: Contains the cornea, anterior chamber, iris, posterior chamber, ciliary body, lens.
  • Posterior Segment: Contains the sclera, choroid, retina, optic nerve, vitreous body.
  • Key Structures: Cornea, iris, pupil, lens, retina, optic nerve.

Compartments of the Eye

  • Vitreous Humor: Fills the posterior segment.
  • Aqueous Humor: Nourishes the cornea, secreted into the posterior chamber, flows to the anterior chamber, and is reabsorbed.
  • Glaucoma: Increased pressure due to reduced removal of aqueous humor.
    • Angle-closure: Iris adheres to the cornea, blocking reabsorption.
    • Open-angle: Dysfunction of the trabecular meshwork/Schlemm's canal reduces outflow.
    • Treatment: Surgery, beta-blockers (reduce aqueous humor production), prostaglandin analogs (increase blood drainage).

The Retina

  • Converts light into neural activity.
  • Direct Pathway: Photoreceptors → Bipolar cells → Retinal Ganglion Cells (fire Action Potentials) → Optic Nerve → Brain.
  • Indirect Pathway: Horizontal and Amacrine cells modulate signaling.
  • Retinal pigment epithelium supports photoreceptor function.

Retinal Structure

  • Cell Layers: Light passes through ganglion cells, amacrine cells, bipolar cells, horizontal cells, to reach photoreceptors.
  • Optic Nerve: Axons of retinal ganglion cells form the optic nerve, which exits the eye at the optic disc.
  • Only retinal ganglion cells fire action potentials due to the long distance electrical signal needs to travel.
  • Photoreceptors, bipolar, horizontal, and amacrine cells change electric potentials but do not generate APs due to insufficient voltage-gated channels.

Visual Pathway

  • Light → Photoreceptor → Bipolar cells → Retinal ganglion cells → Optic nerve → Lateral geniculate nucleus → Visual cortex.

Blinding Diseases

  • Primary Open Angle Glaucoma: Affects 80 million people worldwide.
  • Corneal Opacity: Resulting from infection, affects 30 million people worldwide.
  • AMD (Age-related Macular Degeneration): Affects 200 million people worldwide.
  • Neurodegenerative Diseases: Retina is part of the brain; treatments are still under development.

Photoreceptors

  • Only light-sensitive cells in the retina: Rods and Cones.
  • Structure: Outer segment (contains membranous disks with photopigments), inner segment, synaptic terminal.
  • Membranous disks contain light-sensitive photopigments that alter photoreceptor membrane potential.

Opsins

  • G-protein-coupled receptors that become light-sensitive via a chromophore (retinal).
  • Upon light absorption, they change conformation, activating a G protein and initiating a signaling cascade.
  • Cone opsins: Responsible for photopic vision (color vision).
    • Activation depends on the amino acid sequence and varies with the wavelength of light absorbed.
  • Rhodopsin: Present in rod photoreceptor cells; responsible for scotopic vision (black and white vision).

Rods vs. Cones

  • Rods: More disks, contain rhodopsin, more sensitive to light (night vision).
  • Cones: Fewer disks, three types of opsins (red, green, blue), responsible for color vision (day vision), require higher light levels.

Regional Organization of the Retina

  • Blind Spot: Where the optic nerve leaves the eye.
  • Fovea: Contains only cones, high visual acuity.
  • Macula: Central retina, more cones.
  • Peripheral Retina: High sensitivity to light, mostly rods, high convergence (many rods to one ganglion cell) leading to low visual acuity.

Fovea

  • Pit inside the Macula, contains all cones.
  • Low sensitivity (less pigment/cone).
  • 1 cone : 1 ganglion cell, requires substantial light signal to be picked up by RGC.
  • High acuity due to:
    • Different light levels on different cones allowing fine detail.
    • Lateral displacement of cells other than photoreceptors.

Phototransduction

  • Conversion of light into electrical signals.

Phototransduction in Rods (Dark)

  • cyclicGMP (cGMP)-gated cation channels are open.
  • Na+Na^+ influx, causing depolarization (dark current). Na+Na^+ influx > K+K^+ efflux.
  • Resting membrane potential (Em) is approximately −30mV-30 mV.
  • Depolarization leads to glutamate release at the synapse with bipolar cells.
  • Rhodopsin (inactive) = Retinal (chromophore) + Opsin (G protein-coupled receptor).

Phototransduction in Rods (Light)

  • Retinal absorbs light, activating opsin.
  • GTP binds to G-protein (transducin), activating it.
  • G-protein activates phosphodiesterase (PDE).
  • PDE breaks down cyclicGMP (cGMP).
  • cGMP-gated channels close, stopping Na+Na^+ influx.
  • Rod hyperpolarizes, reduces glutamate release.

Transmission Through Retina

  • Direct Pathway: Photoreceptor → Bipolar cell → Ganglion cell → Visual cortex.
  • Photoreceptors always hyperpolarize in response to light.
  • Electrical response of bipolar and ganglion cells depends on cell type.
    • ON cells: Depolarize in response to light.
    • OFF cells: Hyperpolarize in response to light.

ON and OFF Bipolar/Ganglion Cells

  • ON cells: Depolarized by light.
    • Dark: Glutamate released from photoreceptor causes hyperpolarization (inhibitory synapse).
    • Light: Reduced glutamate removes hyperpolarization, allowing depolarization.
  • OFF cells: Hyperpolarized by light.
    • Dark: Glutamate from photoreceptor causes depolarization (excitatory synapse).
    • Light: Reduced glutamate causes hyperpolarization.

Retinal Processing – Direct Pathway

  • Photoreceptors: Light → Reduced glutamate release.
  • Bipolar Cells:
    • ON: Reduced glutamate → Depolarized (glutamate hyperpolarizes cell).
    • OFF: Reduced glutamate → Hyperpolarized (depolarized in dark by glutamate).
  • Ganglion Cells:
    • ON: Depolarized → Action Potentials (APs).
    • OFF: Hyperpolarized (APs in dark!).

Retinal Processing – Indirect Pathway

  • Each Bipolar or Ganglion cell has a Receptive Field
    • Region of retina that influences that cell
      • Central part – involve direct connections from photoreceptor (e.g. PR->BC->GC)
      • Surround part - indirect connections only via Horizontal/Amacrine cells to reach GC
  • Light in Receptive field Surround: ® Opposite electrical responses in BC / GC cells (compared to response via direct pathway)
    • Because of the influence of Horizontal/Amacrine cells
    • Important for contrast at image borders (enhancing the image that we see)
    • Central part has greater influence than Surround part

Importance of Receptive Field

  • OFF Ganglion cell
    • Hyperpolarised by light – via direct pathway (Centre)
    • Depolarised by light – indirect pathway (Surround)

Optic Tracts

  • Convey information relating to a visual hemifield.
  • Visual Hemifields: Binocular visual field, Right visual hemifield, Left visual hemifield.
  • Pathways: Right optic nerve, Left optic nerve, Optic chiasm, Right optic tract, Left optic tract.
  • N = Nasal, T = Temporal

Visual Field Deficits

  • Transection of left optic nerve: Loss of vision in the left eye.
  • Transection of left optic tract: Loss of right visual hemifield.
  • Transection of optic chiasm: Bitemporal hemianopia (loss of peripheral vision).

References

  • Neuroscience: Exploring the Brain 4th Ed. Chapters 9 and 10.
  • Materials on canvas (YouTube Videos).