The second cranial nerve (CNII) is the optic (sometimes referred to as the ophthalmic) nerve. This nerve is a sensory nerve that functions to collect stimuli (light) received by the retina of the eye, convert to electrical impulses, and transfer this data through the brain to the primary visual cortex. The first-line receptors of visual information for the optic nerve are the photosensitive (light-sensitive) cells of the retina: rods and cones. Rods are critical for low-light conditions, while cones are active for high-light, color, fine-detail conditions. Together rod and cones help generate signals and transmit impulses within the retina. Signals are passed to bipolar cells, which synapse with ganglion cells. Ganglion cells join together and exit the eye at the optic disc, the make their way back to their nuclei.
All cranial nerves (all nerves for that matter) have a nucleus (cell body) and axon (that carries nerve impulses away from the nucleus to other structures). The main nucleus of CNII is located in the lateral geniculate body of the thalamus. Fibers that end here are used for visual perception. Three smaller nuclei also receive input from ganglion cells, these are the superior colliculus of the midbrain - control of eye movements and reflexes associated with sight; the pretectum of the midbrain - control of pupillary reflex; the suprachiasmatic nucleus of the hypothalamus - control of diurnal rhythms and some hormones.
The axons of the optic nerve make a unique crossover behind the eye. In an area called the optic chiasm, fibers of the optic nerve that originate from ganglion cells in the nasal (nose-side) of the retina cross over to join the temporal (ear-side) fibers of the fellow eye. Alternatively stated, fibers from the NASAL retina of the LEFT eye cross over to join fibers from the TEMPORAL retina of the RIGHT eye and synapse in the RIGHT LGB. The opposite is also true, fibers from the NASAL retina of the RIGHT eye cross over to join fibers from the TEMPORAL retina of the LEFT eye and synapse in the LEFT LGB.
A quick review of the location and destination of CNII:
Damage the optic nerve causes vision loss to some degree and can occur anywhere along its path. The clinical signs of damage can be helpful to localize where damage occurs.
Pupillary responses are altered if vision loss is asymmetric and damage occurs to one optic nerve more than the other. Damage that would cause pupillary abnormalities occurs in the anterior (nearer the eye) portion of the nerve. The result is called an afferent pupillary defect (APD), which is discovered by comparing how the pupil responds to light between the right and left eye.