Anatomy of the olfactory (CN I) and optic (CN II) nerves

The cranial nerves are made up of sensory fibers, motor fibers, or both, that innervate the majority of structures in the head and neck.In addition to general sensory and motor innervation, they also transmit unique sensations, via special sensory fibers.You know these as taste, smell, sight and hearing!So to stop and smell the roses, but also to see the roses, we need to focus on the Olfactory and Optic nerves, also known as cranial nerves I and II.So first, there’s cranial nerve I, or the olfactory nerve, which only contains special sensory fibers responsible for the sense of smell.These fibers transmit olfactory impulses from the olfactory epithelium of the nasal cavity to the brain, where they are perceived as the sense of smell.Now, the olfactory epithelium is located on each side of the roof of the nasal cavity where the nasal mucosa contains olfactory receptor neurons.Olfactory receptor neurons are bipolar neurons that each have a dendrite on their apical surface that gives rise to many olfactory cilia, which possess receptors for odorant molecules.The basal surfaces of these neurons give rise to central processes - or axons - that are collected into bundles to form approximately 20 olfactory nerves on each side, that, together, we call the right and left olfactory nerves.The fibers pass through small foramina in the cribriform plate of the ethmoid bone, and enter the olfactory bulb in the anterior cranial fossa.Here, the olfactory nerve fibers synapse on second order neurons, called mitral cells.The axons of these cells form the olfactory tract, which travels posteriorly to several olfactory areas including the primary olfactory cortex within the temporal lobe.Moving on to the cranial nerve II, or the optic nerve, which is another special sensory nerve that transmits visual impulses from the retina to the brain.Now, the optic nerve is a paired extension of the forebrain, so it’s actually CNS fiber tracts formed by axons of retinal ganglion cells with their cell bodies located in the retina.The optic nerve starts when the retinal ganglion axons pierce the fibrous external coat of the eyeball posteriorly, also known as the sclera, on its way to the brain.Now, the nerve passes posteromedially in the orbit, exiting through the optic canal to enter the middle cranial fossa, where it communicates with the other optic nerve to form the optic chiasm.Chiasm means “crossing”, and, sure enough, fibers from the medial half of each retina cross in the chiasm and join fibers from the lateral halves of the retina that do not cross over to form the optic tracts.This crossing of optic nerve fibers in the chiasm is what allows us to have binocular vision which is the ability to focus on an object with both eyes, and allow us depth-of-field perception which is also known as 3D vision.So, before the optic chiasm there are the optic nerves which carry sensory fibers from the medial and lateral areas of their respective retina.After the optic chiasm, there are the optic tracts.Specifically, the right optic tract transmits impulses from the left visual field, while the left optic tract transmits impulses from the right visual field.Most fibers in the optic tract terminate in the lateral geniculate bodies of the thalamus, and from here, the axons are relayed to the visual cortices of the occipital lobes of the brain.Alright, as a quick recap.

The first cranial nerve, or the olfactory nerve is a special sensory nerve which contains sensory fibers that transmit impulses from the olfactory epithelium of the nasal cavity to the brain, where the perception of smell occurs.The second cranial nerve, or the optic nerve, is a special sensory nerve that transmits impulses from the retina to the brain which allow