The Sight – Transmission

The optic nerve carries those impulses up to the thalamus and then on to the visual cortex.

Once the light pass through the optic nerve, it becomes electrical signals.
The brain is symmetrically divided into two hemispheres.

          we can clearly distinguish the two hemispheres of the brain.

Optic nerve leaves both eyes at the location of the optical discs, and continue his path through the fat tissue behind the eyes at the eye sockets, then enter the brain through holes at the base of the skull. The optic nerves of both eyes come together to form the optic chiasm, and then the optic tract. Any signal from the left visual field reaches the right primary visual cortex and vice versa.

in the brain, visual information passes through several channels. The most important is the retino-geniculo-striated pathway. 90% of the axons of ganglion cells are part of this pathway. Most axons of the optical tract innervate the lateral geniculate body in the dorsal part of the thalamus. Other neurons carry the information to visual centres elsewhere in the cortex where the visual information is further processed and integrated.

Structure of the LGB:

The LGB is a kind of gateway to the visual cortex, and therefore to conscious visual perception.

• six layers that fold around the optical tract.

• three layers receive signals from one eye, and the other three layers receive signals from the other eye.

• The majority of LGB neurons that project into the primary visual cortex end in the lower part of layer 4. This layer 4 is thick. It forms characteristic bands. (if we look at the cortex in cross-section)

there are 2 independent visual information processing channels that relay in each LGB:

• A pathway that relays into the upper 4 layers (P / parvocellular) because these layers are made up of neurons formed of SMALL cellular bodies. These layers are sensitive to colour and detail, to stationary objects moving slowly. Indeed, the cones provide a major access to the P layers.

• A path concerning the 2 lower layers (M / Magnocellular), because these layers are made up of BIG cellular bodies. These layers are sensitive to moving objects. Indeed, the sticks provide major access to the M layers.

=> Magnocellular neurons project just above the parvocellular neurons, on the lower part of layer 4 of the striated cortex. These M and P portions in turn project onto different parts of the visual cortex.

---

DID YOU KNOW THAT ?

rattlesnakes  see infrared rays, allowing them to detect warm-blooded prey.

---