1. Ion exchange and membrane transport are essential for generating and transmitting electric impulses in retinal tissue. Small molecules and ions use transporters and channels to move across cell membranes via diffusion, active transport, and cotransport. 2. In the retina, photoreceptors generate receptor potentials in response to light that are transmitted to bipolar, horizontal, amacrine and ganglion cells. Bipolar cells transmit either excitatory or inhibitory signals. Ganglion cells transmit signals to the brain via action potentials along the optic nerve. 3. Parallel processing in the retina involves ON and OFF bipolar and ganglion cell pathways that enhance contrast sensitivity and allow for detection of light and dark areas.

1. Concept : Ion exchange, Electric
impulse generation and
transmission in retinal tissue
Kalpana Bhandari
Master of clinical optometry 2nd batch
Tilganga institute of ophthalmology

2. Presentation layout
• Ion exchange.
• Overview of membrane transport.
• Action potential.
• Electric impulse generation.
• Transmission of visual impulse.
• Physiological activities in retinal cells.
• References.

3. Ion Exchange
Terminology
• Small molecules, such as gases,
lipids, and lipid-soluble molecules,
can diffuse directly through the
membranes of the endothelial
cells of the capillary wall.
• Glucose, amino acids, and ions—
including sodium, potassium,
calcium, and chloride—use
transporters to move through
specific channels in the
membrane by facilitated
diffusion.

4. • Glucose, ions, and larger molecules may also leave the
blood through intercellular clefts. Larger molecules can
pass through the pores of fenestrated capillaries, and
even large plasma proteins can pass through the great
gaps in the sinusoids. Some large proteins in blood
plasma can move into and out of the endothelial cells
packaged within vesicles by endocytosis and exocytosis.
• Water moves by osmosis.

5. • Active transport is the movement of
molecules across a cell membrane from a
region of lower concentration to a region of
higher concentration—against the
concentration gradient. Active transport
requires cellular energy to achieve this
movement.
• Cotransporters are a subcategory of
membrane transport proteins that couple the
favorable movement of one molecule with its
concentration gradient and unfavorable
movement of another molecule against its
concentration gradient.

6. Overview of Membrane Transport

7. Membrane Mechanisms of RPE
• The retinal pigment epithelium (RPE) interacts closely with
photoreceptors, an activity that is essential to maintain
excitability of photoreceptors.
• The RPE helps to control the environment of the sub-retinal
space, supplies nutrients and retinal to the photoreceptors,
phagocytoses shed photoreceptor outer segments in a
renewal process, and secretes a variety of growth factors,
helping to maintain the structural integrity of the retina.

8. RPE functions that involve the movements of
ions across the cell membranes
• The RPE transports ions and metabolic end products
from the retinal to the choroidal (vascular) side.
• Ion transport serves to control the ion composition in
the subretinal space, which is essential for the
maintenance of photoreceptor excitability and also
drives water transport across the RPE.

9. • Stimulation of photoreceptors by light decreases the
potassium concentration in the subretinal space. To
compensate, the RPE releases potassium ions through
the apical membrane into the subretinal space.
• Changing from light to dark increases the potassium
concentration in the subretinal space, which is
compensated by the absorption of potassium ions.
• Both the capability for epithelial transport and fast
instantaneous compensation can be monitored by ERG or
EOG measurements.

10. Action potential
• The transmission of signals in the nervous system
occurs in the form of electrical impulses.
• These electrical impulses are generated on the
membrane of the nerve cells.
• Different types of ion channels are involved in the
transmission of electrical impulses through nerve cells.

11. • Typically, the sodium ion concentration outside the nerve cell
membrane is high while the concentration of the potassium
ions inside the nerve cell membrane is high. The potential at
this stage is known as the resting membrane potential.
• Depolarization and hyperpolarization are two variations of
the resting membrane potential.
• The main difference between depolarization and
hyperpolarization is that depolarization refers to a decrease in
the resting membrane potential whereas hyperpolarization
refers to an increase in the resting membrane potential.

12. Steps of action potential

13. Electric impulse generation

14. Electric impulse generation
• Normally the inner segment of the photoreceptor
continually pumps Na+ from inside to outside,
thereby creating a negative potential on the inside
of entire cell.
• However, the Na+ channels present in the cell
membrane of the outer segment of the
photoreceptor are kept open by the cyclic GMP, in
the dark. So, Na+ from the extracellular fluid,
flows inside the outer segment i.e. in dark.
• As a result cell membrane in the outer segment is
hypopolarized with respect to inner segment i.e.
current flows from inner to outer segment.
• Current also flows to the synaptic ending of the
photo receptor, this is called dark current.

16. • The photoreceptors, both rods and cones, release
neurotransmitter during the dark, because under dark
conditions, the membrane of the sensory neuron is in a
depolarized state. Cyclic GMP–gated channels are open to
sodium influx in the dark state.
• In darkness, small transient voltage changes (“bumps”) can be
recorded across photoreceptor membranes. These are
presumably caused by thermal activation of molecules in the
excitatory cascade.
• These changes are “smoothed” at the receptor-bipolar
synapse and receptor-receptor junctions, thus permitting the
unambiguous detection of very weak signals.
• Maintaining the dark current requires high activity in the ATP-
requiring Na+/K+ pump and helps to explain why in
mitochondrial diseases, the retina may be disproportionately
affected.

17. • On light exposure, the rhodopsin molecules undergo their
conformational change, and a resulting phototransduction
cascade closes the membrane channels, sodium is kept out,
and the membrane of the whole cell goes into a hyperpolarized
state for as long as the light is present.
• Excitation of photo receptor causes increased negativity of the
membrane potential (hyperpolarization), rather than
decreased negativity (depolarization).
• Normally, the electronegativity inside the rod membrane is
about-50 mv (millivolts) and after excitation hyperpolarizes to
about -60 to -70mv.*
*Fusao Kawai, Masayuki Horiguchi, Hiromitsu Suzuk et.al. May 2001 .Japan. Na+ action
potential in human photoreceptors

18. • The hyperpolarizing response of a cone has a small
area over which it responds that is not much bigger
than the diameter of the cone.
• This space over which the cone gives its response is
known as its receptive field.

20. Transmission of visual impulse

21. Transmission of visual impulse
• The receptor potential generated in the photo
receptor is transmitted by electronic conduction (i.e.
direct flow of electric current, not action potential)
to the other cell of the retina i.e. horizontal cells,
bipolar cells, amacrine cells and ganglion cells.
However, the ganglion cells transmit the visual signal
by the means of the action potential.

22. Neurotransmitters in the retina
Glutamine; an excitatory transmitter, is release by rods
and cones at their synapses with bipolar and horizontal
cells.
Amacrine cells produce five different types of inhibitory
transmitters. They include: gamma aminobutyric acid
(GABA), glycine, dopamine, acetylcholine and indolamine.
Cholinesterase has been found in the processes of muller,
horizontal, amacrine and ganglion cells.
Carbonic anhydrase has been isolated from cones and
RPE but not rods.

23. Physiological activities in the retinal cells

24. Horizontal cells
• It transmit signals horizontally
in the outer plexiform layer
from rods and cones to the
bipolar cells.
• Their main function is to
enhance visual contrast by
causing lateral inhibition.
Fig A: phenomenon of lateral inhibition in the surround receptive plexiform layer.
The central photoreceptor has been stimulated with light and the inner portion of
the cell membrane has become more negative. The signal is transmitted to the
bipolar cell and also to the horizontal cells. This horizontal transmission results in
inhibition of the photoreceptor-bipolar cell synapse of the neighbouring
photoreceptor element. The stimulated bipolar cell may be hyperpolarized or
depolarized.

25. Bipolar cells
• These are neurons of the first order of visual pathway.
• Their dendrites are stimulated by the light induced
hyperpolarization of the photoreceptors.
• Some bipolar cells depolarize while others
hyperpolarize when the photoreceptors are excited
i.e two different types of bipolar cells provide
opposing excitatory and inhibitory signals in the visual
pathway.
• The bipolar cell receives either excitatory input and
thus responds like the photoreceptor, horizontal cell
and has a hyperpolarizing response (due to iGluRs) or
gets an inhibitory input (due to mGluRs) and gives a
depolarizing response to light.

26. • Different bipolar cell types selectively express different types
of receptors for glutamate, allowing each bipolar type to
respond to photoreceptor input in a different way.
• Retinal bipolar cells are of two types: ON-center and OFF-
center.
• In centre depolarizing (also called ON-center bipolar cells) the
light striking the centre of receptive field activate and the light
striking the surround inhibits. The reverse occurs in the centre
hyperpolarizing (also called off-centre bipolar cells).
• Electron microscopy shows that bipolar cell dendrites make
different types of contact with the cone or rod synaptic
region, either beneath the synaptic ribbon or at more distant
basal contacts.

27. Amacrine cells
• They receive information at the synapse of the bipolar
cell axon with ganglion cell dendrites and use these
information for temporal processing at the other end
of the bipolar cell.
Uses:
• Some amacrine cells are part of the direct pathway for
rod vision i.e. the impulse travels from rod to bipolar
cells to amacrine cells to ganglion cells.
• Other type of amacrine cells are direction sensitive and
respond to movement of a spot across the retina in a
specific direction.
• Thus, amacrine cells help in temporal summation and
in the initial analysis of the visual signals before they
leave the retina

28. Amacrine cells play an important role in transmitting information
from rod photoreceptors to ganglion cells. The amacrine cells collect
messages from many rod-connected bipolar cells, allowing the
perception of very dim light. The amacrine cells feed information
directly to OFF ganglion cells. They also co-opt the ON cone bipolar-
to-ganglion cell architecture by means of gap junctions.

29. Ganglion cells
• The electric respond of bipolar cells after modification by the
amacrine cell is transmitted to the ganglion cells which inturns
transmit their signals by means of action potential to the brain.
• The ganglion cells are of two types in terms of their centre
response: “on – centre” cells that increase their discharge and “
off – centre” cells that decrease their discharge upon
illumination of the centre of their receptive fields.
• The number of ganglion cell in the centre of the fovea (about
3,5000) is equal to the number of cones; this account for the
high degree of visual acuity in the centre retina in comparison
with poorer acuity peripherally.

30. • Peripheral retina has much more sensitivity to weak
light than the central retina. This is because rods are
300 times more sensitive to light as compared to
cones and also as many as 200 rods converge on the
same optic nerve fibre in the peripheral retina.

31. Retinal parallel processing
• Parallel bipolar channels transmit inputs to ganglion cells.
• The parallel sets of visual channels for ON (detecting light
areas on dark backgrounds) and OFF (detecting dark areas on
light backgrounds) qualities of an image are fundamental to
our seeing.
• For example, we read black letters against a white
background using the OFF channels that start in the retina.
• Such parallel processing enhances the contrast sensitivity of
the eye.

32. Retinal parallel processing contd…
• Mammalian rod bipolar cells depolarize in response to
light that is absorbed within the center of their receptive
field.
• Cone bipolar cells are of two types: those that depolarize
(ON center) and those that hyperpolarize (OFF center) in
response to central illumination, so called because they
provide excitatory or inhibitory inputs to amacrine or
ganglion cells in response to light.
• The ON and OFF bipolar cells make synaptic contact with
amacrine and ganglion cells in different parts of the IPL.

33. Fig: retinal parallel processing

35. • Axons of retinal ganglion cells converge to form a
optic nerve fibre and carries total output of retina to
the brain via optic nerve, optic chaisma, optic tract,
LGB, optic radiation and finally into visual cortex
where the image is perceived.

37. References
• Principle and practice of electrophysiology of vision 2nd
edition.
• https://webvision.med.utah.edu/wp
content/uploads/2011/01/2003-01Kolb.pdf. How the retina
works.
• Anatomy and physiology of vision A.K. khurana.
• Internet