Cranial Nerves: Anatomy, Names, Functions And Mnemonics | Kenhub

Anatomy

Cranial nerves are the 12 nerves of the peripheral nervous system that emerge from the foramina and fissures of the cranium. Their numerical order (1-12) is determined by their skull exit location (rostral to caudal). All cranial nerves originate from nuclei in the brain. Two originate from the forebrain (Olfactory and Optic), one has a nucleus in the spinal cord (Accessory) while the remainder originate from the brainstem.

There's a LOT to learn about the cranial nerves. You might like to ease yourself into this topic with our cranial nerves quizzes and labeling exercises.

Cranial nerves supply sensory and motor information to structures of the head and neck, controlling the activity of this region. Only the vagus nerve extends beyond the neck, to innervate thoracic and abdominal viscera.

We’re sure that while reading textbooks, you encountered with terms such as afferent, efferent, mixed, general, visceral, special, somatic etc, these refer to modalities of the cranial nerves. They often bring confusion, so let’s explain them before proceeding.  

The function of a nerve is to carry sensory and/or motor information between the body and the brain. If the information goes from the brain to the periphery, then it is an efferent (motor) nerve. If it travels from the periphery to the brain, then it is an afferent (sensory) nerve. Nerves that do both are mixed nerves. Unlike spinal nerves which are always mixed, cranial nerves can be purely motor, purely sensory or mixed. 

Now let’s understand the terms special, general, somatic and visceral. The information is classified as special if it travels from our special senses (vision, smell, taste, hearing and balance), while general describes information to/from everywhere else. The information carried by a nerve is called somatic if it goes to/from the skin and skeletal muscles, or visceral if it travels to/from our internal organs. 

Combining these categories allows us to define the functional components of a nerve. For example, if the nerve fibers exclusively carry special sensory information, it is called a special afferent nerve. If it carries other types of sensory information, like touch, pressure, pain, temperature, then it is a general afferent nerve.

If the nerve carries information to smooth muscle, cardiac muscle or glands, then it is a visceral efferent nerve. If it carries information to skin or skeletal muscle, then it is a somatic efferent nerve. As the term visceral is often a synonym for autonomic (nervous system), note that general visceral nerves carry autonomic nerve fibers to/from the target organs. The exception to this are the special visceral efferent nerves, sometime described as branchial efferent (BE). These are motor nerves, named for the embryological origin of the fibres. Information of movement and position (proprioception) from somatic structures like muscles, tendons, and joints is carried by general somatic afferent nerves. Lastly, be aware that there is no special somatic efferent classification. 

So to conclude, considering the possible directions and modalities, cranial nerves can be:

1. General somatic afferent (GSA)
2. General somatic efferent (GSE)
3. General visceral afferent (GVA)
4. General visceral efferent (GVE)
5. Special somatic afferent (SSA)
6. Special visceral afferent (SVA)
7. Special visceral efferent (SVE)

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