organ systems III: visual system part 3- the retina

this lecture covered basic ideas about the structure and function of the retina. the retina is the layer in the back of the eye that contains the photoreceptor cells that transduce light energy into neural signals. the fovea is the area in the back of the eye which focuses light most readily and the macula lutea is the yellow region surrounding the fovea. the retina is made up of several layers: at the base is the retinal epithelial layer, then the photoreceptor cells, then the integrative cells (horizontal, amacrine, bipolar), then the ganglionic cells. this configuration is called "inverted" because the light has to travel through the ganglionic and integrative cells before hitting the photoreceptors. the photoreceptors need to be close to a blood supply, the choroid vessels (see previous lecture), and thus the inverted configuration serves to keep the thick blood supply from blocking the light from hitting the photoreceptors.

rods and cones are the two types of photoreceptors in the retina, and have complementary properties to each other. cones are better suited for bright light sources while rods are better suited for dim light sources. cones have a lower concentration of pigments and thus a lower sensitivity, while rods have a higher concentration and thus a higher sensitivity (making them better suited for detecting dim light sources). cones have a high concentration in the fovea, the area where light focuses best, and thus provide high visual acuity and spatial resolution while rods are diffusely spread peripherally to the fovea and are more involved in movement and have lower spatial resolution. finally, cones have three colored pigments (isomers of iodospin?) while rods have one color pigment (rhodopsin).

the retinal epithelial layer is the layer that supports the photoreceptor in several ways. first, the pigments within the layer absorb light, preventing the bounceback of light from obscuring the image. during the absorption of light by the photoreceptors, pigments create trans-retinol, which is converted back to cis retinol and transported back into the photoreceptors. this process also creates debris, which is phagocytosed by the retinal epithelial layer. finally, the pigments in this layer absorb blue light preferentially, which helps prevent the production of free radicals.

photoreceptor cells create a graded receptor potential via their pigments, which change conformation after light absorption, opening sodium channels and changing membrane potential. the photoreceptor cells then send signals to the layer of integrative cells which includes the horizontal, amacrine, and bipolar cells. bipolar cells receive signals from several photoreceptor cells, whereas amacrine and horizontal cells integrate signals from several photoreceptors. ganglionic cells receive the processed signal, which it sends to the optic nerve. the receptive field of the ganglionic cell, which spans the photoreceptors that it receives signals from, undergoes lateral inhibition via the integrative cell layer, which creates "center-surround" receptive fields. these are either stimulatory in the center and inhibitory in the periphery, or vice versa. the receptive fields are most stimulated when the edge between the center and periphery "subtends" the light and dark edge; these receptive fields, and thus the signals that the ganglionic cells send to the optic nerve, are most receptive to the contrast between dark and light, making it well suited for distinguishes edges and borders. center surround receptive fields are also involved in color perception; ganglionic cells can receive impulses from colored pigments from cones, creating a receptive field which sends a signals based on the wavelengths of incoming light; thus providing the neural information necessary to construct a colored image.

questions
1. what is the retina?
2. where do the receptor cells lie in relation to the retinal cells?
3. what is the fovea?
4. what is the macula lutea?
5. what is the blind spot?
6. where does the retina blood supply come from?
7. what are the retinal layers starting from the retinal epithelium?
8. what do bipolar, horizontal, amacrine and ganglion cells do?
9. what is the physical makeup of the fovea?
10. what are the pigments in rods and cones that respond to the visible light spectrum?
11. how do photoreceptors produce a receptor potential?
12. what do the retinal integrative cells do?

difference between rods and cones...
13. bright light sources vs. dim light sources...
14. sensitivity / concentration of pigment...
15. acuity / concentration in the fovea...
16. direct vs. scattered light...
17. color...

18. why do photoreceptors need to be close to the choroid blood layer?
19. what does it mean that the retina is inverted and what is the purpose of this orientation?
20. why do the ganglionic and bipolar cells not distort the incoming light?
21. what is the retinal pigment epithelium (RPE)?
22. what is the purpose of the RPE?
23. what is retinal detachment?
24. what causes retinal detachment?

25. bipolar cells receive input from...
26. what do horizontal and amacrine cells do?
27. lateral inhibition of horizontal and amacrine cells creates...
28. what do ganglionic cells do?
29. what are the glial cells that support the optic nerve?
30. what is papilledema?
31. what are on-center and off-center ganglionic cells?
32. both off-center and on-center ganglionic cells produce the most stimulation when...
33. what is the implication of the receptive field of ganglionic cells responding most to the light/dark edge?
34. how do ganglionic cells code color?
35. what is the blind spot?

answers
1. the back inner lining of the eyeball.
2. deep to the retinal cells.
3. the central area in the back of the eye where the light most clearly focuses.
4. the yellow area surrounding the fovea.
5. the area in the back of the retina where the optic nerve exits; contains ganglion cells but no receptor cells.
6. internal carotid provides the central artery which supplies the retina.
7. retinal epithelium, photoreceptors, bipolar/horizontal/amacrine cells, ganglion cells.
8. bipolar, horizontal, amacrine cells integrate visual information and ganglion cells send information to brain stem.
9. fovea has the highest concentration of cone receptor cells which receive more incoming light by the outward spreading of the other retinal layers.
10. rhodopsin in rods and iodopsin in cones.
11. the pigments contained within photoreceptors absorb light and change conformation, causing the opening of sodium channels and the production of a receptor potential.
12. generate EPSP's and IPSP's and summate the potentials from neighboring retinal cells to send an integrated signal to the ganglion cell.

13. rods for bright light, cones for dim light.
14. rods have high concentration of pigment and thus sensitivity, cones have low concentrations of pigment and thus low sensitivity.
15. high concentration of rods in the fovea and thus high sensitivity. low concentration of cones in the fovea and thus low sensitivity.
16. rod receptors are best with direct axial light and cones receptors are best with scattered light.
17. rod receptors have 3 pigments which absorb different colors; cones only have one color pigment.

18. because photoreceptor turnover requires a large oxygen supply.
19. the inverted retina refers to the fact that the photoreceptor cells are the last layer that the light hits. this orientation prevents the blood supply to the photoreceptor cells from blocking the light from hitting the photoreceptors.
20. because the bipolar and ganglionic cells have the same refractive index as the vitreous humor.
21. the pigmented layer deep to the photoreceptors.
22. absorbing light that passes through the photoreceptor to prevent the bounceback of light which could potentially cause blurring. absorbing blue light in particular, which is particularly involved in the production of free radicals. regeneration of trans-retinal back into cis-retinal in the retinal cycle. delivery of nutrients such as glucose and retinol to photoreceptor cells. phagocytosis of debris which might accumulate from the photoreceptor's light absorption. (bouncing blue ball regenerates nutrient debris)
23. when the retinal layer mechanically detaches from the RPE.
24. buildup of pressure in the macula region, or buildup of cellular waste material.

25. several receptor cells.
26. integrate signals from several receptor cells.
27. center-surround receptive fields.
28. transmit signals to the optic nerve which then transmits to the lateral geniculate nucleus of the thalamus.
29. oligodendrocytes myelinate the axons and astrocytes surround the cell bodies and dendrites.
30. limited venous return from the retina caused by increased CSF pressure.
31. on-center ganglionic cells produce receptive fields that produce stimulatory signals from light shining in the middle and the opposite effect from light shining in the periphery. off-center ganglionic cells produce the opposite type of receptive field.
32. their receptive fields subtend the light-dark edge.
33. the purpose of this mechanism is for the photoreceptors to be sensitive to contrasts between light and dark; to distinguish fine features.
34. the summation of the excitatory / inhibitory activity of receptive fields of ganglionic cells which receive signals from different colored pigments within cone cells.
35. the spot on the retina where there are no receptor cells, where the optic nerve exits.

organ systems III: somatosensory 2b

this lecture dealt with some of the higher order processing of the somatosensory cortex; namely, the cortex's ability for localization and discrimination of fine textures and resolutions. localization refers to how accurately the somatosensory cortex can distinguish a stimuli from a certain location on the body. the two point discrimination test measures the smallest distance between two simultaneous stimuli which will indeed produce a sensation of two stimuli.

lateral inhibition is introduced as a means of increasing the resolution and sensitivity to stimuli, a mechanism which is used widely in the cortex, thalamus, and dorsal column nuclei. the neuron at the center of a given receptive field essentially produces the strongest signal because of its inhibition of neighboring nuclei, which are also inhibiting neighboring nuclei. this inhibition occurs via inhibitory interneurons, which are stimulated by recurrent branches of the dorsal column nuclei.

the cortex can also regulate lateral inhibition to enhance the selectivity of the sensory input even further; this is seen especially during movement, where the cortical neurons will directly stimulate or inhibit dorsal column nuclei to select for stimuli that is relevant to the movement, allowing for greater discrimination of complex textures.

the next section looked at the somatosensory association cortex, which lies posterior to the primary somatosensory cortex. it is responsible for deciphering the meaning behind tactile input, formation of an egocentric space via the integration of tactile, proprioceptive, auditory, and visual information, and language comprehension via Wernicke's area. lesions within this area can produce such syndromes as astereognosis, which is the inability to identify an object by touch, or neglect syndrome, which is a loss of cognition in half of the sensory field.

finally, two conditions are discussed related to spinal cord injury: syringomyelia is the formation of cysts in spinal grey matter which blocks decussation of anterolateral second order neurons and thus the bilateral loss of pain and temperature sensation. brown-sequard syndrome results from a hemisection of the spinal cord, an injury that slices through the dorsal and anterolateral columns on one side, that results in loss of ipsilateral motor control and discriminative touch sensation, and loss of contralateral pain and temperature sensation.

questions
localization...
1. cortical localization requires...
2. how are receptive fields differentiated in peripheral structures?
3. what is the relationship between localization and receptive field size?
4. how are receptive fields differentiated in the CNS?

lateral inhibition...
5. what is lateral inhibition?
6. describe what happens with stimulation of the center and peripheral region of a receptive field for a given dorsal column nuclei neuron.
7. what are inhibitory interneurons and what are their function in lateral inhibition?
8. what is the net effect of lateral inhibition?

two point discrimination, cortical regulation...
9. what is meant by "two point discrimination"?
10. describe how two point discrimination is related to localization and lateral inhibition.
11. how does syphilis affect the two point discrimination test?
12. how does the cortex regulate lateral inhibition?
13. what is the purpose of cortical regulation of lateral inhibition?
14. why does somatosensory perception depend on movement?

somatosensory association cortex...
15. somatosensory association cortex lies...
16. what is the somatosensory association cortex involved in?
17. what is the region within the somatosensory cortex involved in language comprehension?

pathologies...
18. what is astereognosis?
19. what is neglect syndrome?
20. most neglect is due to...
21. why do lesions to the left parietal lobe not necessarily lead to neglect syndrome?
22. what is syringomyelia?
23. what is brown-sequard syndrome?

answers
1. differentiation of neural input from receptive fields at many different levels of processing including the dorsal column nuclei, thalamus, and somatosensory cortex.
2. by the specific activity of sensory neurons that innervate each RF.
3. the smaller receptive fields have a greater degree of localization.
4. via neural interactions between pathways between different RF's.
5. a neural mechanism employed in the cortex, thalamus, and dorsal column nuclei that enhances the difference between somatosensory receptive fields.
6. stimulation at the center will produce a stimulatory effect on the dorsal column nuclei neuron and stimulation at the periphery of the receptive field will produce an inhibitory effect.
7. inhibitory interneurons are the neurons that are stimulated by the recurrent branches of dorsal column nuclei which inhibit action of neighboring dorsal column nuclei neurons.
8. lateral inhibition enhances the signal received at the central region of activation and decreases the signal away from the center.

9. the ability to distinguish between separate but simultaneous pinpricks to the skin.
10. lateral inhibition refines the localization of sensory stimulation, which is what is being measured in two point discrimination.
11. the dorsal column degeneration interferes with the reception of discriminative touch and thus reduces one's two point discrimination ability.
12. via descending pathways from the primary somatosensory cortex and motor cortex which modulate afferent inputs.
13. to focus on the afferents that are delivering useful information.
14. during movement, motor cortex sends messages to dorsal column nuclei to facilitate sensory stimuli related to the movement, in a way that increases the discrimination of complex textures.

15. posterior to the primary somatosensory cortex.
16. deciphering meaning of tactile stimuli, integration of touch, proprioception, vision, audition (formation of ego-centric space), language comprehension.
17. Wernicke's area.

18. the inability to identify an object by touch.
19. loss of cognition of half of a sensory field.
20. lesions in the right inferior parietal lobe.
21. the functioning right hemisphere has the capacity to compensate for the left's dysfunction but not vice versa.
22. formation of cysts in the center of the spinal grey area that prevents decussation of second order neurons in the anterolateral system, leading to bilateral loss of pain and temperature sensation.
23. a hemisection of spinal cord which cuts through the dorsal and anterolateral columns on one side, leading to loss of ipsilateral discriminative touch and motor control and loss of contralateral pain and temperature.