visual pathways.power point presentation

Education

1. Visual pathways

2. Retinal pathway from retinal cell to visual cortex

3. The retinal pathway is the series of connections by which light falling on the retina is converted into neural signals and transmitted all the way to the primary visual cortex (V1) for perception

4. 1. Retinal Level ( Phototransduction ) Photoreceptor cells (Rods & Cones) Light is absorbed by photopigments (rhodopsin in rods, opsins in cones). This causes hyperpolarization of the photoreceptor. Instead of firing an action potential, they release less glutamate onto the next cells. Bipolar cells Receive input from photoreceptors. Two main types: ON-center (depolarize when light increases) and OFF-center (depolarize when light decreases). Act as the first stage of contrast detection.

5. Ganglion cells Axons of ganglion cells form the optic nerve . They generate action potentials (the first true spikes in the visual system). Specialized ganglion cells detect motion, edges, and color contrasts. ⚡ Flow in the retina: Photoreceptor → Bipolar cell → Ganglion cell → Optic nerve

6. 2. Optic Nerve and Optic Chiasm Ganglion cell axons converge at the optic disc to form the optic nerve . At the optic chiasm : Nasal retinal fibers (carrying temporal visual field information) cross to the opposite side. Temporal retinal fibers (carrying nasal visual field information) remain uncrossed . 👉 Result: Each optic tract carries information from the contralateral visual field .

7. 3. Optic Tract to Lateral Geniculate Nucleus (LGN) After the chiasm, fibers continue as the optic tract . Most terminate in the lateral geniculate nucleus (LGN) of the thalamus . LGN has six layers . The LGN acts as a relay station + processor , refining contrast, motion, and color information.

8. 4. Optic Radiations Neurons from the LGN project to the primary visual cortex via the optic radiations :

9. 5. Primary Visual Cortex (V1, Striate Cortex, Area 17) Located around the calcarine sulcus in the occipital lobe. Maintains retinotopic mapping (neighboring points in the retina project to neighboring points in V1). Functions: Detects edges, orientation, motion, color, depth . Integrates signals from both eyes for binocular vision .

10. Summary Pathway: Photoreceptor → Bipolar cell → Ganglion cell → Optic nerve → Optic chiasm → Optic tract → LGN (thalamus) → Optic radiations (Meyer’s loop + parietal fibers) → Primary visual cortex (V1).

11. S pecific functions of all five types of retinal cell

12. R etina there are five major types of neurons, each with a distinct role in processing visual information before it even leaves the eye

13. 1. Photoreceptor Cells (Rods and Cones) Rods Extremely sensitive to light → enable night vision (scotopic vision) . Do not mediate color vision (monochromatic). Provide poor spatial resolution (low acuity). Cones Function in bright light → day vision ( photopic vision) . Responsible for color vision (3 subtypes: S, M, L cones for blue, green, red). Provide high visual acuity and fine detail.

14. 2.Bipolar Cells Act as the first-order interneurons between photoreceptors and ganglion cells. Two main types: ON-bipolar cells → depolarize when light increases (reduced glutamate). OFF-bipolar cells → depolarize when light decreases (increased glutamate). Help encode contrast and light-dark differences

15. 3. Ganglion Cells Output neurons of the retina → their axons form the optic nerve . Generate the first true action potentials in the visual pathway. Types: M-cells (magnocellular) : motion detection, gross form. P-cells (parvocellular) : fine detail, color. K-cells ( koniocellular ) : color (blue-yellow) processing. Integrate visual signals and transmit them to the lateral geniculate nucleus (LGN) .

16. 4. Horizontal Cells Located in the outer plexiform layer . Interconnect photoreceptors and bipolar cells laterally. Provide lateral inhibition , which sharpens contrast and enhances edge detection . Adjust retinal sensitivity under different lighting conditions (light adaptation).

17. Amacrine Cells Located in the inner plexiform layer . Interconnect bipolar and ganglion cells laterally. Have diverse subtypes → some use GABA, glycine, dopamine, acetylcholine . Functions: Modulate ganglion cell response (temporal aspects of vision). Important for motion detection and direction selectivity . Influence adaptation to light and circadian rhythms

18. Retinal Cell Type Main Function Photoreceptors (Rods/Cones) Light detection, color vision, acuity Bipolar Cells Relay + contrast (ON/OFF pathways) Ganglion Cells Output → optic nerve, action potentials, motion/color/form coding Horizontal Cells Lateral inhibition, edge detection, contrast enhancement Amacrine Cells Motion detection, timing, modulate ganglion output Summary Table

19. M ain neurotransmitters released by different retinal neurons :

20. Photoreceptors (Rods & Cones) Neurotransmitter: Glutamate Released continuously in darkness (tonic release). Light → hyperpolarization → reduced glutamate release . Glutamate can be excitatory or inhibitory depending on the receptor type on the bipolar cell

21. Bipolar Cells Neurotransmitter: Glutamate Relay signals from photoreceptors to ganglion cells. ON- and OFF-bipolar cells differ in their postsynaptic receptors, not in the transmitter

22. Ganglion Cells Neurotransmitter: Glutamate Their axons form the optic nerve . Generate the first action potentials in the visual system.

23. Horizontal Cells Neurotransmitter: GABA (gamma-aminobutyric acid) Provide lateral inhibition between photoreceptors and bipolar cells. Enhance contrast and edge detection .

24. Amacrine Cells Neurotransmitters: Multiple, depending on subtype GABA and Glycine (most common → inhibitory). Some release Dopamine → modulates light adaptation and circadian rhythms. Others may use Acetylcholine, Serotonin, or Neuropeptides for modulation. Functions in motion detection, timing, and shaping ganglion cell responses .