Anatomy, function and related conditions

Cranial nerves are made up of 12 pairs of nerves that originate in the brain. They provide sensory, motor and autonomic control of the structures of the head, neck and trunk.

When something goes wrong with the cranial nerves, it can affect the senses or the ability to speak, chew, or swallow.

This article explains all about the cranial nerves and their anatomy, function, and disorders.

Where are the cranial nerves?

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Cranial nerves originate in the brain. They get their name from the fact that they exit through openings in the skull or skull. There are 12 pairs of cranial nerves, often denoted by Roman numerals.

Cranial nerves III–XII (3–12) originate in the brainstem. This structure is the upper part of the spinal cord which is inside the skull. It is responsible for reflexes, autonomic and unconscious processes and certain voluntary movements.

Learn about the structure and function of the brainstem.

Cranial nerves I and II arise from the brain. The cerebrum is the most developed part of the brain. Higher functions take place there, such as language, logic, memory and understanding.

Learn about the structure and function of the brain.

Cranial nerves carry signals between the brain and the structures they control. There are three categories of functions:

  • sensory: detects a stimulus, such as smell, light, touch, or sound, and sends the information to the brain for processing
  • engine: moves skeletal and smooth muscles based on signals from the brain
  • autonomous : controls “automatic” functions such as salivation, heart rate and digestion

Cranial nerves can be sensory, motor, autonomic, or a combination.

Changes in cranial nerve function

Cranial nerve disorders and conditions can involve one or more nerves. The causes and symptoms of cranial nerve disorders often overlap. Conditions that can affect the cranial nerves include:

  • degenerative disorders
  • infections
  • inflammatory conditions
  • neuropathies
  • head trauma
  • tumors

Cranial nerve function tests

Because cranial nerves come in pairs – for each side of your body – cranial nerve function tests assess symmetry: is the change on both sides equal or is one pair of nerves more affected that the other ? Assessments include:

  • cranial nerve I: identify odors one nostril at a time
  • cranial nerve II: visual acuity, color perception and visual field test, one eye at a time
  • cranial nerve III, IV and VI: observing symmetry, testing pupil light response, and tracking a moving target, such as a finger or a penlight
  • cranial nerve V: feel the jaw muscles and test facial sensation with pinpricks or a cotton swab
  • cranial nerve VII: observation of symmetry when speaking or smiling, taste tests and agreement test for muscle weakness
  • cranial nerve VIII: hearing tests and vestibular function tests
  • cranial nerve IX and X: gag reflex test, observation of the throat while saying “ah” and laryngoscopy to see the vocal cords directly
  • cranial nerve XI: move head and shoulders
  • cranial nerve XII: examination of tongue movements

Treatments for Cranial Nerve Dysfunction

Treatment for cranial nerve dysfunction depends on the cause. Generally, doctors recommend non-surgical treatments first. Sometimes surgery is necessary.

Frequently Asked Questions

Seunggu Han, MD, has taken a look at the following frequently asked questions.

What is cranial nerve 2?

Cranial nerve 2 is the optic nerve. It is responsible for your sense of sight. It connects to the retina of the eye, the light-sensitive part at the back of the eyeball. The optic nerve carries signals from the retina to the brain. The brain interprets the signals as vision.

What is the largest cranial nerve?

The biggest is the trigeminal nerve. This is cranial nerve V (5th cranial nerve). The longest cranial nerve is the vagus nerve or cranial nerve X. “Vagus” means wandering or wandering in Latin. The nerve branches and spreads to innervate several organs and systems.

Which cranial nerve is vision?

Vision is possible through the optic nerve. This is cranial nerve II, also known as the 2nd cranial nerve.

The 12 pairs of cranial nerves come directly from the brain. They supply the innervation of the organs and structures of the head and neck. However, the vagus nerve plays a role in various autonomic bodily functions and supplies power to several organs. Learn more about the autonomic nervous system.

The cranial nerves are responsible for our senses. They are also the source of movement for the face, neck and shoulders.

Learn more about the central nervous system here.

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