PAGE 2

OPTIC NERVE

 

It is SPECIAL SOMATIC AFFERENT. Soma meaning body part refers to the eye since it carries visual stimulus from the eye, afferent means it is sensory, named special because it carries the special sensation of vision.

 

It forms as an out pouching of the diencephalon to form an optic vesicle. The optic vesicle gets detached from the diencephalon and a lens invaginates the optic vesicle anteriorly to form the optic cup. The inner wall of the cup develops into the retina while the outer wall develops into the choroid.

 

OPTIC NERVE IS NOT A PEREPHERAL NERVE.

 

·       Myelinated by oligodendrocytes which cause myelination in the CNS, and are affected by multiple sclerosis, a demyelination disease of the CNS.

·       The peripheral nerves can regenerate while nerves of CNS can’t. The optic nerve can’t regenerate either.

·       Out pouching of the diencephalon.

·       The peripheral nerves are surrounded by epineurium (collagen covering) while the optic nerve is surrounded by the three meninges of the brain.

 

VISUAL PATHWAY:

 

FIG 1.

Each retina has a temporal part, towards the temporal lobes on lateral side and a nasal part, towards the nose on medial side and each visual field has a left A part and right B part.

On the right visual cortex, vision forms from the left sides (A part) of both right and left visual fields (follow the red color) and vice versa (follow the blue color). For example with your right eye you see a dog (RIGHT VISUAL FIELD) while with your left eye you see a cat (LEFT VISUAL FIELD). The right visual field has a RIGHT A AND LEFT B SIDE.

The left visual field has a RIGHT A AND LEFT B SIDE.

 

                                       

  

On the right visual cortex, images of both A sides will form, while on the left visual cortex images of both B sides will form. Furthermore A (left) of cat will be sensed by nasal hemi retina of left eye, while B (right) of cat will be sensed by temporal hemi retina of left eye. Same is true for the right side of the eye.

 

Image in the form of light on hitting the retina from the inner side penetrates the various layers of retina to be received by the dendrites of rod and cone cells. The stimulus produce graded (local) potentials inside rod and cone cells {that allow only few Na channels to open. This causes little Na influx that is not enough to reach threshold potential (-70 to -55 mV) and generate an action potential}. This graded potential is transmitted to the bipolar cells and then the ganglionic cells via neurotransmitter glutamine at both synapses.

 

The bundles of axons of the ganglionic cells form the optic nerve. The right optic nerve consists of all the fibers from the right eye and all the vision seen by the right eye. Same is true for the left eye.

 

Fibers from both eye’s temporal hemi retina DONOT CROSS at the optic chiasma and move in their side’s optic tract while fibers from both eye’s nasal hemi retina CROSS and move in the opposite optic tracts. THE RIGHT OPTIC TRACT WILL HAVE FIBERS FROM THE LEFT SIDES OF BOTH RIGHT AND LEFT VISUAL FIELDS WHILE THE LEFT OPTIC TRACT WILL HAVE FIBERS FROM THE RIGHT SIDES OF BOTH RIGHT AND LEFT VISUAL FIELD.

Note: the optic nerve leaves the orbital cavity (cavity of cranium containing the eye) through the optic canal (a small opening). After coming out of the optic canal it joins the optic nerve from the other eye to form the optic chiasma.

 

90% fibers from optic tracts terminate at their side’s lateral geniculate nucleus while 10% go to the midbrain.