EVS Week 6 - Anatomy of retina & visual pathway

what is the neural portion of the retina?

• photoreceptors

• bipolar cells

• retinal ganglion cells

what do the neural cells of the retina do?

• conduct signals from the photoreceptors to the optic nerve

what is the non-neural portion of the retina?

retinal pigment epithelium (RPE)

where do rods dominate in the retina?

• further from the fovea

• at the peripheral retina

where do cones dominate in the retina?

• at the fovea

what creates a blind spot?

• there are no retinal components in the optic nerve head, so this creates a blindspot

where is the retina thickest and thinnest?

• thickest at the optic nerve head (>0.5mm thick)

• thinnest at the ora serrata (~0.1mm)

where is the fovea thinnest?

• at the foveola — this is the very centre of the fovea

what do the axons of the RGCs form?

• the axons of the RGCs form the optic nerve

when are the axons of RGCs unmyelinated and myelinated?

• the axons are unmyelinated when travelling across the retina

• the axons become myelinated after they pass through the lamina cribrosa

what cells are considered first order neurons in the retina, and why?

• bipolar cells

• these are first order neurons because signals pass through the bipolar cells first before getting to the RGCs

what cells are considered second order neurons, and why?

• retinal ganglion cells

• RGCs transmit the signals from the bipolar cells to the brain via their axons, forming the optic nerve

what cells are considered interneurons, and why?

• horizontal cells and amacrine cells

• these cells modulate transmission between photoreceptors and bipolar cells

• they also modulate transmission between bipolar cells and RGCs

which retinal layer is closest to the choroid and sclera?

• layer 1 — the RPE is closest to the choroid and sclera

which retinal layer is the closest to the vitreous?

• layer 10 — the internal limiting membrane is closest to the vitreous

what is the outer plexiform layer (layer 5) composed of?

• it is composed of neuronal synapses between dendrites from the inner nuclear layer (bipolar cells) and the outer nuclear layer (inner segments/cell bodies of photoreceptors

• contains photoreceptor branches connecting with dendrites of bipolar and horizontal cells

what is the inner nuclear layer (layer 6) composed of?

• cell nuclei of bipolar cells, horizontal cells, amacrine cells, and Muller glial cells

what is the outer nuclear layer (layer 4) composed of?

• it contains the cells bodies of photoreceptors

what is the function of the RPE (layer 1)?

• absorbs stray light — ensures that light does not scatter across the retina

• transports nutrients from choroid to retina

• forms the blood-retina-barrier

• phagocytoses photoreceptor outer segments

what is the photoreceptor layer (layer 2) composed of?

• contains rods and cones

what is the external limiting membrane (layer 3)?

• it is a layer that separates the inner segments of photoreceptors from the outer nuclear layer

• it is a network of junctions formed by Muller cells

what are muller cells?

• it is a type of glial cell that supports the retina

what is the function of the inner plexiform layer (layer 7)?

• it is a synaptic layer where bipolar cells connect to RGCs

• amacrine cells modulate this interaction

• contain branches of the bipolar and some amacrine cells connecting with the RGC dendrites

what is the ganglion cell layer (layer 8) composed of?

• cell bodies of RGCs — the axons of RGCs form the optic nerve

what is the nerve fibre layer (layer 9) composed of?

• contains axons of ganglion cells which head towards the optic disc to form the optic nerve

what is the function of the internal limiting membrane (layer 10)?

• it is formed by the footplates of Muller cells

• serves as the innermost boundary of the retina

• adjacent to the vitreous humor

• internal limiting membrane has a structural function for blood vessels but not at the fovea

what is the fovea centralis in the macula?

• it is a dip/depression in the retinal area that is in line with the visual axis

• this depression is formed because the neurons and capillaries of the inner layers of the retina are displaced/pushed peripherally

how does the fovea centralis ensure highest VA?

• has a high concentration of cones — for colour and detailed vision

• 5 degrees of vision

• inner retinal layers are pushed peripherally; allowing light to strike the photoreceptors without scattering

• it is surrounded by the foveal avascular zone — no blood vessels are present to minimise light scatter and maximise clarity

what is the foveola?

• it is the most central part and occupies 1.2 degrees of the visual field

• only contains cones

what are some features of the layers of the fovea?

• there are no RGCs or bipolar cell bodies

• there are fibres of Henle — stretched out photoreceptor axons that connect with bipolar cells that have been pushed off the retina/fovea

what makes a structure bright/hyper-reflect on an OCT?

• if the structure is dense like the RPE or nerve fibre layer

• if the layer is dark, it will be hyper-reflected on an OCT

where is the retinal nerve fibre layer thickest and thinnest at, and how is this reflected on an OCT?

• thicker towards the nasal side — more hyper-reflected on OCT

• thinner towards the temporal side — not really shown on OCT

where do the nerve fibres in the retina travel?

• the nerve fibres are found all throughout the retina and they all travel towards the optic nerve head

how do the macula/fovea bundle of nerve fibres travel towards the optic nerve head?

• they travel straight across to the optic nerve head

how do the peripheral nerve fibres travel towards the optic nerve head?

• they arc around the macular bundles to get to the optic nerve

what is the lamina cribrosa?

• it a sieve-like structure in the posterior part of the sclera where the optic nerve fibres exit to form the optic nerve

what is the structure and function of the lamina cribrosa?

• structure — made up of collagenous connective tissue, contains small pores (cribriform openings) through which the RGC axons pass

• function — supports + protects the optic nerve fibres as they leave the eye, maintains structural integrity of the ONH

how do the RGC axons travel towards the optic nerve?

• RGC axons travel in bundles towards the optic nerve, e.g. the macula bundles and peripheral bundles travel together

what is the topography of the optic nerve/tract?

• Retina

  • The retina is divided into nasal (medial) and temporal (lateral) hemiretinas.

  • Each hemiretina sees the opposite half of the visual field:

    • Temporal retina sees the nasal (inner) visual field.

    • Nasal retina sees the temporal (outer) visual field.

• Optic Nerve (CN II)

  • Formed by axons of retinal ganglion cells.

  • Each optic nerve carries all visual input from one eye (both temporal and nasal retina).

  • Organized so that macular (central) fibers run centrally, and peripheral fibers run peripherally within the nerve.

• Optic Chiasm

  • Site where fibers from the nasal retina cross to the opposite side.

  • Temporal fibers remain uncrossed.

  • Result: The left visual field (from both eyes) is processed in the right hemisphere, and vice versa.

• Optic Tract

  • Carries visual information from the contralateral visual field:

    • Left optic tract = right visual field (nasal retina of right eye + temporal retina of left eye).

    • Right optic tract = left visual field.

  • Fibers remain organized:

    • Macular fibers remain central and posterior.

    • Superior retinal fibers (inferior visual field) are lateral.

    • Inferior retinal fibers (superior visual field) are medial.

what are the secondary visual pathways?

• optic tract —> hypothalamus

• optic tract —> pretectum

• optic tract —> superior colliculus

what is the role of the optic tract to hypothalamus pathway?

• helps regulate circadian rhythms via light

• our brain is very sensitive to light during the night and around one hour after we wake up

• too much light during this sleep period disturbs sleep patterns

what is the role of the optic tract to pretectum pathway?

• mediates the pupillary light reflex

• uses light intensity to regulate pupil diameter

what is the role of the optic tract to superior colliculus pathway?

• receives input from rod-rich retinal areas (sensitive to movement)

• Function: Controls eye movements, visual attention, and head orientation to visual stimuli.

• Involved in:

  • Saccadic eye movements

  • Visual tracking

  • Reflexive gaze shifts

• Integrates visual, auditory, and somatosensory input for quick responses to environmental changes.

how many cells in the primary cortex are devoted to the central visual field?

• around 80% of cortical cells are devoted to the central 10 degrees

• 25% of visual cortex is involved in central 2.5 degrees

what do the magnocellular layers (layer 1 & 2) in the LGN detect?

• they detect motion, luminance, and low spatial frequency

what do the parvocellular layers (layers 3-6) in the LGN detect?

• they detect fine detail, colour, and high spatial frequency