The retina contains different types of neurons that work together to transmit visual signals to the brain. Photoreceptors (rods and cones) detect light and transmit signals via bipolar and horizontal cells. Amacrine cells help analyze the visual signal before it leaves the retina via ganglion cells, whose axons form the optic nerve. Different regions of the retina contain more rods or cones depending on the need for visual acuity or light sensitivity. Ganglion cells come in X, Y, and W types that transmit different aspects of the visual image like fine details, rapid movement, or directional movement.

1. Neural Functions of Retina
Dr. Sara Sadiq
(MBBS, M.Phil)

4. Direction of
light

5. Neurons in Visual Pathway
• Foveal Portion of retina:
• Cones
• Bipolar cells
• Ganglion cells
• Pure rod vision:
• Rods
• Bipolar cells
• Amacrine cells
• Ganglion cells

6. Signal Transmission in the Retina
• Transmission of signals in the retina is by electrotonic conduction.
• Allows graded response proportional to light intensity.
• The only cells that have action potentials are ganglion cells and
amacrine cells  send signals all the way to the brain

7. Horizontal Cells
• Horizontal cells connect laterally between the rods and cones and the
bipolar cells
• Output of horizontal cells is always inhibitory
• Prevents the lateral spread of light excitation on the retina
• Have an excitatory center and an inhibitory surround
• Essential for high visual accuracy in transmitting contrast borders in the
visual image

9. Role of Lateral Inhibition

10. Amacrine Cells
• About 30 different types
• Some involved in the direct pathway from rods to bipolar to amacrine to
ganglion cells
• Some respond to movement of the light signal across the retina
• Amacrine cells are a type of interneuron that help to analyze visual signal
before they leave retina.

11. • Central retina
• About 35000 cones
• No rods
• High degree of visual acuity
• Number of optic nerve fibers are almost exactly equal to cones
• Peripheral retina
• More rods than cones
• Much grater sensitivity to weak light
• Rods are 30-300 times more sensitive to light
• 200 rods converge on a single nerve fiber so signals summate to
give more intense stimulation

12. Types of Ganglion Cells
• W cells (40%)
• Receive most of their excitation from rod cells
• Broader field in retina b/c dendrites of ganglion spread widely
• Sensitive to directional movement in the visual field
• X cells (55%)
• Small receptive field b/c dendrites of ganglion don’t spread widely
• Transmission of fine details of visual image
• Always receives input from at least one cone
• Responsible for color transmission.
• Y cells (5%)
• Large receptive field
• Respond to instantaneous changes in the visual image  rapid movement, rapid
change in light intensity