NEURAL FUNCTIONS OF RETINA Physiology Lecture Slideshow

• Rods and cones
• Horizontal cells
• Bipolar cells
• Amacrine cells
• Ganglion cells
• Interplexiform cells

• Rods and cones– Photoreceptors
• Horizontal Cells—Inhibitory cells
• Bipolar Cells– Depolarising/Hyperpolarising
• Amacrine Cells—30 types
• Ganglion cells—W, X and Y cells

Visual Pathway in Retina

• Cone vision– foveal portion of retina
  – 3 cell pathway
  – Cones → bipolar cells → Ganglion cells
  – New and fast system
• Rod vision– peripheral retina
  – 4 cell pathway
  – Cones → bipolar cells → amacrine cells →
  Ganglion cells
  – New and fast system

Neurotransmitters

• Glutamate
• Amacrine cells secrete 8 types of NT
  – GABA
  – Glycine
  – Dopamine
  – Acetylcholine
  – Indolamine

Signal Transmission in Retina

• Ganglion cells and amacrine cells
  – True action potential
• Amacrine cells, horizontal cells and
  Photoreceptors
  – Electrotonic conduction
  – Graded Potential

Photoreceptors

• Rods and cones
• Transmits signals to outer plexiform layer
• Synapse with bipolar and horizontal cells

Horizontal cells

• Transmits signals horizontally to bipolar cells
• In the outer plexiform layer
• Transmits inhibitory signals laterally
• Plays role in lateral connectivity along with
  amacrine cells
• Synapse with dendrites of bipolar cells and
  cell bodies of rods and cones
• Obeys phenomena of lateral inhibition

• Lateral inhibition– transmission of visual
  patterns with proper visual contrast
• Light strikes eye– central area excited
• Outer zone–inhibited by lateral inhibition via
  horizontal cells
• amacrine cells also contribute in lateral
  inhibition
• Enhancement of visual contrast

Bipolar Cells

• 2 types
  – Hyperpolarizing cells
  – Depolarizing cells
• 1
  st order neuron in visual pathway
• Cells respond to glutamate released by rods
  and cones in their specific manner

• Bipolar (Depolarizing) cells takes excitation
  from rods and cones
• Bipolar (Hyperpolarizing) cells takes signals
  from horizontal cells
• Half of the cells transmit positive signals and
  rest half sends negative signals
• Contributes in lateral inhibition and provides
  contrast border in the image

Amacrine Cells

• 30 types
• Functions

1. Conduction Pathway (Rod vision)
2. Offset image
3. Respond to change in illumination
4. Directional sensitivity
5. Visual image analysis

Ganglion cells

• 1.6 million
• 60 rods and 2 cones converge on ganglion cell
  – Central fovea
  – Peripheral retina
• 3 types
  – W Cells
  – X Cells
  – Y Cells

W cells

• 40 per cent of all the ganglion cells
• Small, diameter less than 10 micrometers
• Transmit signals in optic nerve fibers at slow velocity
  of 8m/sec
• Ganglion cells receive most of excitation from rods
  transmitted via bipolar and amacrine cells
• Have broad fields in the peripheral retina because
  dendrites of ganglion cells spread widely in the inner
  plexiform layer, receiving signals from broad areas
• W cells are sensitive for detecting directional
  movement in the field of vision
• Important for crude rod vision in dark

X Cells

• Most numerous of the ganglion cells
• 55 per cent of the total
• Medium diameter, 10-15 micrometers
• Transmit signals in their optic nerve fibers at 14 m/sec
• Small fields because their dendrites do not spread
  widely in the retina
• Signals represent discrete retinal locations
• Fine details of visual image are transmitted
• Every X cell receives input from at least one cone
• X cell responsible for all color vision

Y cells

• Largest and least numerous (5%) , up to 35
  micrometers in diameter
• Transmit their signals at 50 m/sec or faster
• Broad dendritic fields—from widespread
  retinal areas
• Y ganglion cells also respond to rapid changes
  in the visual image

P and M cells

• 20 types of retinal ganglion cells
• Magnocellular(M) and Parvocellular(P) cells
• P cells
  – Beta cells project in to parvocellular cell layer of
  lateral geniculate nucleus of thalamus
• M cells
  – Project in to magnocellular cell layer of lateral
  geniculate nucleus of thalamus
  – Relays information from optic tract to visual cortex

Ganglion cells

P Cells

• Smaller receptive field
• Conduct impulses
  slowly
• Sustained color stimuli
• Sensitive to color
  stimulus and fine details
• Not sensitive to black
  and white

M Cells

• Larger receptive field
• Conduct impulses
  rapidly
• Rapidly changing color
  stimuli
• Not sensitive to color
  stimulus
• Sensitive to black and
  white

On-off Response

Rapid impulses for a fraction of a second when a light is first turned on and decreasing rapidity in the next fraction of a second.

On-off Response

Ganglion cell showing lateral to the spot of light, cell is markedly inhibited Light is turned on—lateral inhibition

On-off Response

• Ganglion cells are excited by changes in light intensity
• Light is turned off, opposite effects occur–“on- off” and “off-on” responses
• Phenomena occur depolarizing and hyperpolarizing bipolar cells
• Temporary responses are also contributed by the amacrine cells