VS131 Visual Neuroscience

Neural Retina

Auditory system: a ‘classical’ sensory system.Visual system: a ‘quantum’ sensory system.

-> Both our eyes and ears operate near the physical limits. This does not require high gain in the auditory system, but does in the visual system. The price of the high visual gain is low speed. Vision is slower than hearing! (fortunately light is faster than sound!).

-> Still a matter of debate how we compensate for delays in the visual system, we need to ‘lead’ our visual system like a skeet-shooter leads a target.

Question: is the retina more sensitive to hypoxic damage in the light or in the dark?

Televisions, video monitors and digital cameras use regular arrays of red, green, and blue phospors/sensors to record or reproduce an image.

But not the retina!Blue cones are sparse, mostly red and green, but they are located in patches not in regular repeating arrays.

For form vision the red and green cones appear to be used almost exclusively (and perhaps interchangeably – similar wavelength sensitivity).

The blue cones are rare and sparsely localized – absent in the fovea! – used mostly only for color vision.

Synaptic Contacts of Mammalian Photoreceptors:

Pedicles: terminals of cones. Foot-shaped.

Spherules: terminals of rods. Ball-shaped.

-> In the mammalian system, any given bipolar cell contacts either rods or cones but not both.

Triad: dendrite of a cone bipolar cell invaginates the cone pedicle and is flanked by two other invaginating processes from horizontal cells.

Invaginating Bipolar Cell: bipolar cell part of a triad.

Flat Bipolar Cells: contact cone pedicels without invaginating.

Midget Bipolar Cells: in primates, both invaginating and flat, can be postsynaptic to a single cone in the central retina, provide exclusive bipolar input to a single midget ganglion cell.

Synaptic Contacts of Mammalian Photoreceptors (cont):

Rod bipolar cells of all mammals are strictly invaginating, form triads with rod spherules.

Bipolar cells do not generate action potentials: all graded potentials.

-> Light depolarizes invaginating bipolar cells, thus increasing their neurotransmitte release. On bipolar cell.

-> Light hyperpolarizes flat bipolar cells, thus decreasing their neurotransmitter release. Off bipolar cell.

Invaginating = On

Flat = Off

-> All photoreceptors are hyperpolarized by light and decrease their transmitter (glutamate) release in light.

-> So the on- and off- classes of bipolars are created by different kinds of glutamate receptors.

Sign-inverting synapse: cone/rod and on-bipolar cell. Cone hyperpolarizes and bipolar cell depolarizes. Transmitter affects ion channels indirectly through a second messenger (cGMP) system.

Sign-conserving synapse: cone only (never rod) and off-bipolar cell. Transmitter affects ion channels directly.

This splitting up into on- and off- pathways will continue throughout much of the visual system.

RECEPTIVE FIELD (of a neuron) (sound effects)

The region of the retina (or the visual field: ‘visual space’) where a stimulus must be placed for the neuron to be affected. For all cells other than the photoreceptors, the receptive field (“RF”) is a funnction of its functional connections to the photoreceptors.

Receptive fields of bipolar cells:

On-bipolar cells are depolarized by a spot of light in the center, but hyperpolarized by light in the surround. This is an on-center, off-surround receptive field.

Off-bipolar cells have the opposite effects, are said to have off-center, on-surround receptive fields.

Effects thought to be due to horizontal cells. Lateral inhibition.

Effects of center and surround tend to cancel with uniform illumination, though typically some imbalance.

Synaptic terminals of bipolar cells make characteristic dyad junctions in the inner plexiform layer. One contact is usually an amacrine cell, the other may be either an amacrine or a ganglion cell. Can have a reciprocal synapse between amacrine and bipolar. Feedback circuit?

BT: bipolar terminal

A: amacrine cell

G: ganglion cell

Large open arrow: reciprocal synapse from amacrine cell to bipolar cell terminal.

Cartoon of Dyad Junction

Amacrine cells: an enormous variety of types. Function(s) still not very well understood. An area of intensive research.

-> some amacrine cells can generate action potentials.

-> many different neurotransmitters

Interplexiform cells: similar in some ways to amacrine cells, but also send processes into the outer plexiform layer where they contact horizontal cells. Not clear what the point of this is.

Ganglion Cells: the final total output of the retina!

-> The action potentials of ganglion cells are the ONLY thing that make it to the rest of the brain. Retinal activity is only important for visual perception to the extent that it affects the pattern of firing of action potentials for ganglion cells.

-> Transmission is one-way. The brain does not send signals to the retina. (About the only part of the visual system that is a one-way street).

-> Usually have center-surround receptive fields, both on- and off-types.

Neurotransmitters and Neuromodulators in the retina:

Rapid signaling of information in the retina is conveyed by classical small-molecule neurotransmitters (like glutamate+ and GABA-) and also by electrical synapses.

Slower processes are mediated by peptides and also by dopamine. We don’t really know what is being regulated by these slower mechanisms, so you won’t have to memorize them (yet!).

GABA Gabara Gamera

Light and dark adaptation: the human visual system can operate over an incredibly large dynamic range. The difference in light energy between a dark and stormy night and a sunny day at the beach can be over 10 billion to 1.

-> Iris/pupil size changes

-> Photoreceptor adaptation (slower). Non-linear effects of bleached photopigment?

-> “Network” adaptation (faster). Switching from rods to cones (AII amacine cells)

Scotopic vision: dark-adapted vision

Photopic vision: light-adapted vision

Purkinje shift: shift of visual sensitivity towards longer wavelengths with light adaptation, because the balance of cones are responsive to longer wavelengths than are rods.

Electro-retinogram: voltage difference between the cornea and a reference location in response to a flash of light.

a-wave: Persists after occlusion of central retinal artery, acts as if generated by the photorecpetors themselves.

b-wave: Eliminated after occlusion of central retinal artery, due to bipolar cell activity but mediated through Müller cells – changes in extracellular K+.

c-wave: due to pigment epithelium as a result of changes in extracellular K+ due to illumination of photoreceptors.

d-wave: due to off-bipolar cells. So no d-wave in a rod-dominated retina (why not?).

Different categories of Horizontal cells:

H1 -> Terminals around cell body contact (mainly red and green) cones, contacts around axon ends contact rods. The two ends do not communicate electrically (length constants!).

H2 -> Processes around cell body contact cones, processes around axon ends contact mainly blue cones.

Remember the length constants!

Remember the length constants!

Midget Ganglion cells: both on- and off-types. About 80% of the ganlgion cells in primates. Connect to midget bipolar cells. Four basic types:

Red-ON

Red-OFF

Green-ON

Green-OFF

-> Remember, blue cones not really used for detailed pattern vision, therefore blue does not (as far as we can tell) need a direct line to the brain.

-> In central retina midget bipolars get their input from just one cone. A little more diffuse in the periphery but not by much.

-> There are lots of different types of ganglion cells, for example ‘bistratified’ (connections in both ‘on’ and ‘off’ layers). We are not sure of the functional role of many of these types of ganglion cells….

-> Directly photosensitive melatonopsin-containing ganglion cells! (more later on)

-> SMALL RELATIVE NUMBERS OF NEURONS DOES NOT MEAN THAT THEIR FUNCTIONS ARE NOT IMPORTANT!!!!

Technical Jargon:

‘P-Cell’ (parvo) ~= midget ganglion cell

‘M-Cell’ (magno) ~= parasol ganglion cell