BY DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.) PHYSIOLOGY OF EYE BY DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.)

Neural Circuitry of the Retina

Neural Circuitry of the Retina 1. The photoreceptors themselves—the rods and cones—which transmit signals to the outer plexiform layer, where they synapse with bipolar cells and horizontal cells

Neural Circuitry of the Retina 2. The horizontal cells, which transmit signals horizontally in the outer plexiform layer from the rods and cones to bipolar cells

Neural Circuitry of the Retina 3. The bipolar cells, which transmit signals vertically from the rods, cones, and horizontal cells to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells

Neural Circuitry of the Retina 4. The amacrine cells, which transmit signals in two directions, either directly from bipolar cells to ganglion cells or horizontally within the inner plexiform layer from axons of the bipolar cells to dendrites of the ganglion cells or to other amacrine cells

Neural Circuitry of the Retina 5. The ganglion cells, which transmit output signals from the retina through the optic nerve into the brain

The Visual Pathway from the Cones to the Ganglion Cells Functions Differently from the Rod Pathway.

Neurotransmitters Released by Retinal Neurons both the rods and the cones release glutamate at their synapses with the bipolar cells. *** amacrine cells secrete at least eight types of transmitter substances, including gamma-aminobutyric acid, glycine, dopamine, acetylcholine, and indolamine, all of which normally function as inhibitory transmitters. The transmitters of the bipolar, horizontal, and interplexiform cells are unclear, but at least some of the horizontal cells release inhibitory transmitters.

Transmission of Most Signals Occurs in the Retinal Neurons by Electrotonic Conduction, Not by Action Potentials

Lateral Inhibition to Enhance Visual Contrast— Function of the Horizontal Cells

Lateral Inhibition to Enhance Visual Contrast— Function of the Horizontal Cells

Excitation of Some Bipolar Cells and Inhibition of Others—The Depolarizing and Hyperpolarizing Bipolar Cells

There are two possible explanations for this difference. Excitation of Some Bipolar Cells and Inhibition of Others—The Depolarizing and Hyperpolarizing Bipolar Cells There are two possible explanations for this difference. One explanation is that the two bipolar cells are of entirely different types—one responding by depolarizing in response to the glutamate neurotransmitter released by the rods and cones, and the other responding by hyperpolarizing. The other possibility is that one of the bipolar cells receives direct excitation from the rods and cones, whereas the other receives its signal indirectly through a horizontal cell.

Amacrine Cells and Their Functions

Function of Amacrine Cells About 30 different types Some involved in the direct pathway from rods to bipolar to amacrine to ganglion cells Some amacrine cells respond strongly to the onset of the visual signal, some to the extinguishment of the signal Some respond to movement of the light signal across the retina Amacrine cells are a type of interneuron that aid in the beginning of visual signal analysis.

Rods, Cones and Ganglion Cells Each retina has 100 million rods and 3 million cones and 1.6 million ganglion cells. 60 rods and 2 cones for each ganglion cell At the central fovea there are no rods and the ratio of cones to ganglion cells is 1:1. May explain the high degree of visual acuity in the central retina

Three Types of Ganglion Cells W cells (40%) receive most of their excitation from rod cells. Large receptive field sensitive to directional movement in the visual field they are probably important for much of our crude rod vision under dark conditions X cells (55%) small receptive field, discrete retinal locations, may be responsible for the transmission of the visual image itself, always receives input from at least one cone, may be responsible for color transmission. Y cells (5%) large receptive field respond to instantaneous changes in the visual field.

Neural Organization of the Retina Direction of light W cells ? X cells ? Figure 50-11; Guyton & Hall

Excitation of Ganglion Cells spontaneously active with continuous action potentials (basic 5-40 AP per sec) visual signals are superimposed on this background many excited by changes in light intensity respond to contrast borders, this is the way the pattern of the scene is transmitted to the brain

Transmission of Color Signals by the Ganglion Cells

Processing in the Visual Cortex separation of the signals from the two eyes is lost in the primary visual cortex signals from one eye enter every other column, alternating with signals from the other eye allows the cortex to decipher whether the two signals match

Connections in the Visual Cortex In primary cortex Blobs receive lateral signals from adjacent columns respond to color vision Connections in the Visual Cortex In secondary cortex color blobs Decipher higher meaning of color

Analysis of the Visual Image The visual signal in the primary visual cortex is concerned mainly with contrasts in the visual scene. The greater the sharpness of the contrast, the greater the degree of stimulation. Also detects the direction of orientation of each line and border. for each orientation of a line, a specific neuronal cell is stimulated.

Visual Perception is a Creative Process How the brain actually perceives a visual image is not understood well. Visual perception is thought to be mediated by three parallel pathways–the process information on motion, depth and form, and color.