Question

1a) Starting at the level of the external environment, identify the environmental condition and series of steps that would be required to hyperpolarize an ‘on’ bipolar cell. Assume that the photoreceptor is wired directly to the bipolar cell.

1b) How would the environmental condition and/or series of steps in part 1a) be different (if at all) if you had hyperpolarized an ‘off’ bipolar cell? Again, assume that the photoreceptor is wired directly to the bipolar cell.

1c) In question 1b), the stimulus would have been received in the center of an “off centre, on surround” receptive field. What would the response of the ‘off’ bipolar cell be if the same stimulus as question 1b) had been received in the ‘on surround’? Include all steps necessary to explain your answer starting at the glutamate release from the photoreceptor.

Answer 1

The retinal impulses are carried to visual center in the Cerebral cortex by the nervous pathway called the VISUAL PATHWAY.

In binocular vision, the light rays from temporal half of the visual field fall upon the nasal part of the corresponding retina.The rays from the nasal half of the visual field fall upon the temporal part of the retina.

VISUAL RECEPTORS:

Rods and Cones which are present in the retina of the eye, form the visual receptors.Fibers from the visual receptors synapse with dendrites of BIPOLAR CELLS of inner nuclear layer of retina.

FIRST ORDER NEURONS:

First order neurons or the primary neurons and are the bipolar cells in the retina.Axons from the bipolar cells synapse with dendrites of ganglionic cells.

SECOND ORDER NEURONS:

Second order neurons or the secondary neurons are the ganglionic cells in the ganglionic cell layer of retina.The axons of the ganglionic cells form the optic nerve.The optic nerve leaves the eye and terminates in the lateral geniculate body.

THIRD ORDER NEURONS:

The third order neurons are in the lateral geniculate body.Fibers arising from this reach the visual cortex.

CONNECTIONS OF VISUAL RECEPTORS TO OPTIC NERVE:

There are two pathways between the visual receptors and optic nerve:

1.Private pathway

2.Diffuse pathway

PRIVATE PATHWAY:

The individual cones in fovea centralis are connected to separate bipolar cells.Each bipolar cell is connected to separate ganglionic cell namely,THE MIDGET GANGLIONIC CELL.Thus individual cone is connected to individual optic nerve fiber.This private pathway is responsible for visual acuity and intensity discrimination.

DIFFUSE PATHWAY:

A number of rods and cones are connected with a polysynaptic bipolar cell.The bipolar cells are connected to diffused ganglionic cells.So ,there is great overlapping.This type of pathway is present outside the fovea.

COURSE OF VISUAL PATHWAY:

1.OPTIC NERVE:

It is formed by the axons of ganglionic cells.Optic nerve leaves the eye through optic disc.The fibers from the temporal part of the retina are in lateral part of the nerve and carry the impulses from nasal half of the visual field of the same eye.The fibers from the nasal part of the retina are in medial part of the nerve and carry the impulses from temporal half of the visual field of the same eye.

2.OPTIC CHIASMA:

Here, the medial fibers of each optic nerve cross the midline and join the uncrossed lateral fibers of opposite side to form the optic tract.

3.OPTIC TRACT:

After forming optic chiasma , all the fibers run backward and outward towards the cerebral peduncle.While reaching the peduncle,the fibers pass between tuber cinereum and anterior perforated substance.Then the fibers turn around the peduncle to reach the lateral geniculate body.Here , many fibers synapse while some fibers just pass through this and run towards superior colliculus.Fibers from fovea do not enter superior colliculus.Some fibers from fovea of each side pass through optic tract of same side and others through optic tract of opposite side.Due to crossing of medial fibers in optic chiasma , the left optic tract carries impulses from temporal part of left retina and nasal part of right retina ,that is it is responsible for vision in nasal half of left visual field and temporal half of right visual field.The right optic tract contains fibers from nasal half of left retina and temporal half of right retina.It is responsible for vision in temporal half of left visual field and nasal half of right visual field.

4.LATERAL GENICULATE BODY:

The lateral geniculate body forms the subcortical center for visual sensation.Many fibers from optic tract end in lateral geniculate body,which is in thalamus.From here, the geniculocalcarine tract(optic radiation)arises.This tract is the last relay of the visual pathway.

Some of the fibers from optic tract donot synapse in lateral genuculate body but, pass through it and terminate in any of the other two centers namely the SUPERIOR COLLICULUS and SUPRAOPTIC NUCLEUS OF HYPOTHALAMUS.

5.OPTIC RADIATION:

Fibers from lateral geniculate body pass through internal capsule and form optic raiation.This ends in visual cortex,The fibers between lateral geniculate body and visual cortex are also called GENICULOCALCARINE FIBERS.

6.VISUAL CORTEX:

The primary cortical center for vision is calle visual cortex ,that is located on the medial surface of the occipital lobe.It forms the walls and lips of the calcarine fissure in the medial surface of occipital lobe.There is definite localization of retinal projections upon visual cortex.In fact the point to point projection of retina upon visual cortex is well establlished.The peripheral retinal representation occupies the anterior part of visual cortex.The macular representation occupies the posterior part of the visual cortex near the occipital pole.

AREAS OF VISUAL CORTEX:

1.PRIMARY VISUAL AREA----------area 17

2.VISUAL ASSOCIATION AREA---area 18

3.OCCIPITAL EYE FIELD------------area 19

PHOTORECEPTORS:

The retina comprises several layers of which the light sensitive receptors are outmost.The retina possesses 3 layers of neurones, i.e,. from inwards to outwards

1.Ganglionic cells

2.Bipolar cells

3.Light sensitive rods and cones

Beyond the receptor elements lie the pigment epithelium and lamina vitrea.The light traverses the ganglionic cells and bipolar cells before finally reaching the rods and cones.At the posterior pole of the eye lateral to the optic disc fovea centralis is found in which the path of the light rays to the PHOTORECEPTORS is uninterrupted by the ganglion and bipolar cells and their respective fibers..The fovea centralis is occupied solely by cones.It is on this part of the retina that the image of small objects falls when the eye is used for discriminate vision.In fovea centralis each cone cell is connected to one ganglion cell.The ganglion cells and their fibers cannot function themselves as light receptors , for the point of entrance of the optic nerve is quite insensitive to light and constitutes the BLIND SPOT.

The retinal ganglion cells discharge fairly steadily even when they are not subject to any input from the rods and cones.The response depends on where in the visual field ,the light fell.As "ON" response resulted in an increased discharge during the light stimulus and an "OFF" response resulted in a decreased discharge.In one receptive field comprised a central "on" area surrounded by a circular zone which caused "off" responses.In other light falling on the center of the receptive field caused an "off" responses and on the surrounding zone induced "on" discharge.

Two spots of light shown on separate parts of an "on" area induced a bigger "on" response.If one part of an "on" area was stimulated by light simultaneously with a point in the "off" area the ganglion cell response was very weak.the two effects trend to neutralize each other.