There are twelve pairs of cranial nerves. They are named so because they come from inside the cranial vault. The cranial nerves (CN) for the most part control the sensory organs, muscles and vegetative functions of the head. In contrast to the other CNs, the vagus nerve leaves the cranial vault and travels to all the organs of the chest and abdomen. Only the first and second pair originate from the cerebrum, the rest emerge from the brain stem. For CN functions scroll to the bottom of the page.
The picture below shows the underside of the brain and the cranial nerves are labeled by their number.
CN 0 -S
CN I Olfactory-S
CN II Optic-S
CN III Oculomotor-MM
CN IV Trochlear-MM
CN V Trigeminal-B
CN VI Abducens-MM
CN VII Facial-B
CN VIII Vestibulocholear-MS
CN IX Glossopharyngeal-B
CN X Vagus-B
CN XI Accessory-MM
CN XII Hypoglossal-MM
S-sensory MS-mostly sensory M-Motor MM-mainly motor B-both
The first four CN arise from above the buldge of the pons. The nerve cell bodies of the first CN, for the nose, called the olfactory nerve arises beneath the forebrain just in back and above the bridge of the nose. The nose is part of the primitive root brain structures called the rhinencephalon.
The second cranial, the optic nerve, which is for sight comes from the lateral geniculate body of the thalamus and the superior colliculus.
CN three and four which control muscles of the eye come from the midbrain. CN five through eight arise from the pons. CN nine through twelve arise from the medulla of the brainstem.
The brain’s anatomy and the location of the CNs makes them susceptible to compression from the bones of the floor and foramen in the base of the skull, as well as from too much CSF volume in the cisterns. Moreover, the CNs are particularly susceptible to chronic ischemia due to compression of the vertebral-basilar arteries as they pass through the cervical spine, suboccipital cavernous sinus and foramen magnum. Shifts in the normal position of the head and neck can affect tension and compressive forces acting on the brain inside the cranial vault. It can also cause fluid shifts.
Most of the foramina in the base of the skull are neurovascular tunnels for the passage of both nerves and blood vessels. Therefore, increased pressure in the foramen can affect nerves and blood vessels. As a result of their location, size and orientation, some foramen and foramina and their neurovascular contents are more susceptible to problems with compression due to shifts in brain position.