Anatomy and Terminology
- Cranial nerves emerge directly from the brain, including the brainstem
- There are conventionally twelve pairs of cranial nerves
- Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck
- Cranial nerves are considered components of the peripheral nervous system
- Olfactory (I) and optic (II) nerves emerge from the cerebrum, while the remaining ten pairs arise from the brainstem
- Cranial nerves are generally named according to their structure or function
- Many cranial nerves maintain Latin or Greek names
- Cranial nerves are numbered based on their position from front to back on the brain
- There are intracranial and extracranial paths for cranial nerves
- Cranial nerves are paired and occur on both sides of the body
- All cranial nerves have a nucleus, except for the olfactory (I) and optic (II) nerves
- Nuclei of the cranial nerves are present in the brainstem
- Each nerve may have several functions and may collect in more than one nucleus
- Cranial nerves emerge from different regions of the brainstem
- Olfactory (I) and optic (II) nerves emerge separately from other cranial nerves
- Cranial nerves give rise to sensory and parasympathetic ganglia
- Sensory ganglia correspond to the dorsal root ganglia of spinal nerves
- Sensory ganglia exist for cranial nerves V, VII, VIII, IX, X
- Parasympathetic cranial nerve ganglia include ciliary, pterygopalatine, submandibular, and otic ganglia
- Sympathetic ganglia supplying the head and neck do not belong to the cranial nerves
- Cranial nerves travel within the skull and some must leave it to reach their destinations
- Exits of cranial nerves from the skull occur through foramina
- Cranial nerves have both intracranial and extracranial paths
- Cranial nerves pass through specific ganglia before continuing their course
- Cranial nerves have ipsilateral or contralateral functions depending on the side of the body they originate from.
Function
- Cranial nerves provide motor and sensory supply to structures in the head and neck
- Sensory supply includes general sensation and special senses such as taste, vision, smell, balance, and hearing
- The vagus nerve (X) provides sensory and autonomic supply to structures in the neck, chest, and abdomen
Specific Cranial Nerves
- Terminal nerve (0) may not have a role in humans but has been implicated in hormonal responses to smell, sexual response, and mate selection
- Olfactory nerve (I) conveys information for the sense of smell and damage can cause anosmia or parosmia
- Optic nerve (II) transmits visual information and damage can result in specific vision impairments
- Oculomotor (III), trochlear (IV), and abducens (VI) nerves coordinate eye movement and damage can result in double vision and abnormal visual movements
- Glossopharyngeal nerve (IX) supplies oral sensation, taste, and salivation and damage can cause uvula deviation, failure of the gag reflex, and other symptoms
- Vagus nerve (X) provides sensory and parasympathetic supply to neck structures and organs in the chest and abdomen and damage can lead to various effects including increased blood pressure and heart rate
- Accessory nerve (XI) supplies shoulder elevation and head-turning and damage can cause winged scapula
- Hypoglossal nerve (XII) supplies tongue movement and damage can result in weakness, fasciculations, or wasting of tongue muscles
Clinical Significance and Damage
- Cranial nerve examination is part of neurological examination to examine cranial nerves
- Dysfunction of a nerve during testing may indicate a problem with the nerve or part of the brain
- Observation of the patient is the first step in the cranial nerve exam
- Various tests are conducted to assess vision, eye movement, facial sensation, hearing, uvula, shoulder shrug, head turn, and tongue movements
- Smell is not routinely tested, but can be assessed if there is suspicion of a change in the sense of smell
- Nerves may be compressed due to increased intracranial pressure, intracerebral hemorrhage, or tumor
- Loss of function of a cranial nerve may be the first symptom of intracranial or skull base cancer
- Stroke can cause specific signs and symptoms related to the damaged area
- Inflammation of a cranial nerve can occur due to infection or spontaneously
- Trauma, bone disease, and surgical injury can also cause nerve damage
Other Animals
- Cranial nerves are present in other vertebrates, including non-amphibian tetrapods
- In anamniotes (fishes and amphibians), the accessory nerve (XI) is part of the vagus nerve (X) and the hypoglossal nerve (XII) is represented by spinal nerves
- The terminal nerve (nerve N or O) exists in humans but may have a role in sexual receptivity in other animals based on pheromones
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck, including the special senses of vision, taste, smell, and hearing.
| Cranial nerves | |
|---|---|
 Left View of the human brain from below, showing origins of cranial nerves. Right Juxtaposed skull base with foramina in which many nerves exit the skull. | |
 Cranial nerves as they pass through the skull base to the brain | |
| Details | |
| Identifiers | |
| Latin | nervus cranialis (pl: nervi craniales) |
| MeSH | D003391 |
| TA98 | A14.2.01.001 A14.2.00.038 |
| TA2 | 6142, 6178 |
| FMA | 5865 |
| Anatomical terms of neuroanatomy | |
The cranial nerves emerge from the central nervous system above the level of the first vertebra of the vertebral column. Each cranial nerve is paired and is present on both sides. There are conventionally twelve pairs of cranial nerves, which are described with Roman numerals I–XII. Some considered there to be thirteen pairs of cranial nerves, including cranial nerve zero. The numbering of the cranial nerves is based on the order in which they emerge from the brain and brainstem, from front to back.
The terminal nerves (0), olfactory nerves (I) and optic nerves (II) emerge from the cerebrum, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain.
The cranial nerves are considered components of the peripheral nervous system (PNS), although on a structural level the olfactory (I), optic (II), and trigeminal (V) nerves are more accurately considered part of the central nervous system (CNS).
The cranial nerves are in contrast to spinal nerves, which emerge from segments of the spinal cord.
