cranial nerves are directly attached to the brain, they can be sensory, motor or mixed nerves. we use roman numerals to name the nerves from I to XII. they send electrical signals between your brain and different parts of your face, neck, head and torso. these signals help us see, smell, taste, hear and move our facial muscles. the cranial nerves start at the back of our brain and play a key part in our nervous system. we have 12 cranial nerve pairs, each pair splits to serve the two sides of our brain and body.
CN I - olfactory nerve: sense of smell. we have one pair of this nerve, one is on the left side of our brain and the other is on the right side. it is the shortest sensory nerve, it starts in our brain and ends in the upper inside part of our nose. this nerve is also apart of the autonomic nervous system (ANS) which regulates body functions. it detects odors, scents and aromas. substances that have smell give off tiny molecules that we inhale through our nose, special cells called olfactory receptors detect these molecules and relay the information to the brain through the olfactory nerve and allows us to perceive smell. the olfactory system turns on our sense of smell in two ways; first is nostrils as mentioned before and the back of the throat, drinking or chewing also releases those molecules, they travel up the throat to the olfactory receptors in the back of the nose. olfactory mucosa also plays an important role, it is located in the upper part of the nasal cavity and contains 3 different types of cells: olfactory receptor cells that support two processes: dendritic process which propels cells towards tiny hairs in the olfactory mucosa where they stimulate olfactory cells and central process which directs cells in the opposite direction. sustentacular cells which provide support to nearby tissue and basal cells which both olfactory receptor cells and sustentacular cells can develop from. the olfactory nerve is one of two nerves (visual nerve/CN II) that originate directly from the cerebrum which is the upper part of the brain. olfactory nerve fibers travel to an area in the upper part of the nose called the olfactory bulb but before it does the nerve passes through the cribriform plate which is a spony, lightweight skull bone that separates the nasal area from our brain.
CN II - optic nerve: vision. the optic nerve is the connection that lets the eyes send signals to the brain describing what they see, the brain takes those signals and processes them and then uses them to construct what we see. this nerve also contributes to certain eye reflexes and our circadian rhythm which is our body's internal clock. it is comprised of millions of nerve fibers that send visual messages to the brain to help us see. we have an optic nerve at the back of each eye that directly connects to our brain. each nerve is a one-way connection and it only carries signals from the eyes to the brain. the retinas at the back of each eye detects light and converts that into electrical signals. the optic nerve carries those signals to the brain. this nerve is extra special because of how it forms, it's the only cranial nerve that is also part of the central nervous system (CNS). to get to the brain the optic nerves extend out from the retina and travel a route that includes the optic canal which is a bony opening that your optic nerves pass through to enter the skull and reach the brain. then it goes through the optic chiasm which is a y-shaped junction where nerve fibers meet up to travel together, some of the nerve fibers from both optic nerves switch sides. that crossover is a part of how the brain organizes left and right sided input from both eyes so it can merge them into the single and seamless pictures we see. the brain is just after the separate optic nerves that join at the optic chiasm. once in the brain they head straight to the visual cortex which is apart of the occipital lobe at the back of the brain which is where most visual processing happens. as the nerve fibers travel through the brain a small fraction of them branch off into other places in the brain. they support abilites like pupil reflexes, our pupils automatically adjust to let more or less light in, the reflex needs to be fast which is why it involves nerve fibers that branch off before they reach the visual cortex. next is the accommodation reflex which involves muscles in the ciliary body of each eye, those muscles adjust the shape of each eye's lens and thats how the eyes automatically focus on whatever we are looking at. last is the circadian rhythm which manages our natural sleep/wake cycle and contributes to many processes like body temperature, blood pressure and blood sugar. the optic nerve fibers tell the brain about the light they detected. most brains use that to help anchor the circadian rhythms to daytime and nighttime.
CN III - oculomotor nerve: eye muscle command signals.