Cranial Nerves

Cranial Nerves 

The cranial nerves, also called cranial nerves, are 12 pairs of nerves that arise directly from the brain or at the level of the brainstem to distribute through the foramina at the base of the skull in the head, neck, thorax and abdomen. The International Anatomical Nomenclature includes the terminal nerve as a cranial nerve, despite it being atrophic in humans and being closely related to the olfactory nerve.


Cranial nerves have an apparent origin that is the place where the nerve exits or enters the brain. The real origin is different according to the function they fulfill. The fibers of the cranial nerves with motor function (efferent) originate from cell groups found deep in the brain stem (motor nuclei) and are homologous to the cells of the anterior horn of the spinal cord. The fibers of the cranial nerves with sensory or sensory (afferent) function have their cells of origin (first-order nuclei) outside the brain stem, usually in ganglia that are homologous to those of the dorsal root of the spinal nerves. The second-order sensory nuclei are found in the brain stem.


 The nuclei from which the cranial nerves originate are located in a generalized region known as the tegmentum that runs through the brain stem.

  • At the level of the midbrain (upper structure of the brain stem) pairs III and IV originate


  •  At the level of the brainstem pons (middle structure of the brainstem) pairs V, VI, VII and VIII originate


  •  At the level of the medulla oblongata (lower structure of the brainstem) nerves IX,


 Functional classification


 According to its functional aspect


  •  Pairs I, II and VIII are dedicated to special sensory afferents.


  •  Pairs III, IV, and VI control eye movements, photomotor reflexes, and accommodation.


  •  Pairs XI and XII are pure motor nerves (XI for the sternocleidomastoid and trapezius; and XII for the tongue muscles). However, the spinal roots of the accessory nerve (XI pair) present sensory fibers that emerge from the spinal cord, even showing a small ganglion in the conjugation of the root fillets with the trunk of the spinal root of the nerve.


  •  Pairs V, VII, IX and X are mixed.


  •  Pairs III, VII, IX and X carry parasympathetic fibers. 


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