eye physiology from guyton and halls physiology part 5

BY Muhammad Ramzan Ul Rehman Nistar ken 1

BY Muhammad Ramzan Ul Rehman Nistar ken

2

PHYSIOLOGY OF EYE


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 FUNCTIONSDIFFERENTLY 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


EXCITATION OF SOME BIPOLAR CELLS ANDINHIBITION OF OTHERS—THE DEPOLARIZINGAND HYPERPOLARIZING BIPOLAR CELLS


EXCITATION OF SOME BIPOLAR CELLS ANDINHIBITION OF OTHERS—THE DEPOLARIZINGAND HYPERPOLARIZING BIPOLAR CELLS

There are two possible explanations for this difference. 1. 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.

2. 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 CELLSEach retina has 100 million rods and 3 million cones and 1.6 million ganglion cells.

60 rods and 2 cones for each ganglion cellAt 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 Figure 50-11; Guyton & Hall

X cells ?W cells ?


EXCITATION OF GANGLION CELLSspontaneously 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 THEGANGLION 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 theVisual Cortex

In primary cortex Blobs receive lateral signals from adjacent columns respond to color vision

In secondary cortex color blobs Decipher higher meaning of color


ANALYSIS OF THE VISUAL IMAGEThe 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.

BY Muhammad Ramzan Ul Rehman Nistar ken

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By Muhammad Ramzan Ul Rehman

eye physiology from guyton and halls physiology part 5

Education

rod cells

y cells

interplexiform cells

bipolar cells todendrites

directlyfrom bipolar

synapsewith ganglion

theretina amacrine cells

darkconditions x cells