The Visual System

1. The Visual System General plan for visual system material: • How the visual input is received and transduced at the retina by photoreceptors (rods and cones) • How that input is modified by processing in the retina, concept of receptive fields • How that input is projected to the central nervous system (to the lateral geniculate nucleus (LGN) and cortex), the anatomy and processing within that pathway

2. The Retina Images focused by the lens go through the liquid within the eyeball and fall on the retina. In most of the retina, layers of neurons lie over the photoreceptors, and light must go through them. Fig 15-1

3. The Retina At the fovea, the overlying cells are displaced, so the visual image is clearest there. There are more photoreceptors at the fovea.

4. The Retina – classes of neurons • Light goes through other layers of neurons before being transduced at the rod and cone photoreceptors • Photoreceptors send the light information to the bipolar cells, which send it to the ganglion cells, and the information exits the retina to the brain • Horizontal cells and amacrine cells modulate the information Fig 15-2

5. Photoreceptors of the Retina • Rods and cones Fig 15-3

6. Photoreceptors of the Retina Rods are highly sensitive to light, and thus are good for dim light or night vision. They are able to capture more light, but they do not respond well to moving stimuli because their response time is slow. Cones are not as sensitive to dim light, and are able to respond quickly, and therefore transduce moving stimuli. They are used more in daytime vision, and also are able to detect color because three classes of cones express three different photopigments. Cones are more concentrated in the fovea of the retina. • Rods and cones are specialized

7. Photoreceptors of the Retina The outer segments contain stacks of membranes, where the light-absorbing molecules are found Fig 15-7

8. Light response • In darkness, the rod membrane potential is relatively depolarized. Light causes a hyperpolarization by reducing a Na permeability; the current is through a cGMP-dependent cation channel. • Light causes a hyperpolarization

9. Light response • Light causes closure of a cation-permeable channel, via a process using cGMP as a second messenger. If more cGMP-dependent cation channels are closed, the membrane potential is more hyperpolarized. • Light causes a hyperpolarization

10. Light transduction • Rhodopsin (from rods) is a combination of retinal, the light-absorbing molecule, and a large protein called opsin. The opsins are closely related in structure to the G-protein-coupled receptors. • Light is detected by retinal Fig 15-9

11. Light transduction • When retinal absorbs light, it undergoes a conformational change. This change moves the position of the opsin molecule, which activates a G protein (transducin) and begins a second messenger pathway. • Light changes retinal conformation Fig 15-8

12. Phototransduction - cGMP changes • In the dark, phosphodiesterase activity is quite low, thus levels of cGMP are high; the cGMP-dependent cation channels are gated open. When light activates rhodopsin, the levels of cGMP are reduced by the phosphodiesterase, and the channels are closed. • Transducin activates a phosphodiesterase

13. Phototransduction - dark current • In the rod, the cGMP channels are located in the outer segment, and a K channel is present in the inner segment. In the dark, when the cGMP concentrations are high, current flows in through the cGMP-gated channels and out through the K channels. • In the dark, Vm is approximately -40 mV

14. Phototransduction - dark current With light stimulation, the cGMP channels close. Pathway: light rhodopsin rhodopsin transducin transducin phosphodiesterase phosphodiesterase cGMP cGMP cGMP-gated channel • Light hyperpolarizes the photoreceptor

15. Phototransduction - dark current With light stimulation, the cGMP channels are closed. Pathway: light rhodopsin rhodopsin transducin transducin phosphodiesterase phosphodiesterase cGMP cGMP cGMP-gated channel • Light hyperpolarizes the photoreceptor Fig 15-11

16. Phototransduction - dark current One rhodopsin molecule absorbs one photon  500 transducin molecules are activated  500 phosphodiesterase molecules are activated  105 cyclic GMP molecules are hydrolyzed  250 cation channels close  106-107 Na+ ions per second are prevented from entering the cell for a period of ~1 second   rod cell membrane is hyperpolarized by 1 mV • AMPLIFICATION!

17. Retinal Processing • Horizontal cells are post-synaptic to many photoreceptors, and in the dark, receive a large glutamate stimulus; thus they are depolarized in the dark. Light causes hyperpolarization. Via electrical synapses (gap junctions) they also then hyperpolarize their neighboring cells. • Horizontal cells mediate lateral interactions Fig 15-13

18. Processing in the retina • Bipolar cells are a direct pathway from the photoreceptors to ganglion cells. In the dark, cones are depolarized and release glutamate, which has opposite effects on two types of bipolar cells that mediate the parallel direct pathways to ganglion cells. When a cone is stimulated by light, it reduces transmitter release. • Cones send information in parallel pathways Excitatory Inhibitory

19. Retinal Processing • In the dark, cones release glutamate onto off-center bipolar cells and open a ionotropic cation channel. This depolarizes the bipolar cells until a light stimulus. The bipolar cell then hyperpolarizes, and reduces the firing of the off-center ganglion cell that is next in the sequence. • Off-type bipolar cells hyperpolarize with light Off-type

20. Retinal Processing • The on-center bipolar cells are inhibited by glutamate (by two different mechanisms). With light stimulation, the glutamate concentration from the cone is decreased and the bipolar cells are depolarized. This increases the firing of the on-center ganglion cell next in line. • On-type bipolar cells depolarize with light On-type

21. Retinal Processing • At the photoreceptor-horizontal cell synapse, in the dark, glutamate will depolarize horizontal cells, and horizontal neuron GABA will hyperpolarize cones. With light, the horizontal cell is hyperpolarized and less GABA is released. • Light stimulus at one point of the horizontal cell receptive field causes a depolarization in other photoreceptors in the receptive field. • Horizontal cells influence a wide distribution of cones. - Fig 15-17 +

22. Retinal Processing • Off-center bipolar cells respond to a spot of light by hyperpolarizing, because of reduced cone glutamate. Horizontal cells hyperpolarize with light, reducing their inhibitory transmitter output onto cones; with less hyperpolarization, the cones secrete more transmitter and bipolar cell is depolarized. With the inverted influence of the horizontal cell, the bipolar cell response is a combination of the two. • Synaptic interactions increase the receptive field of bipolar cells Fig 15-16