Cranial Nerves

The Anatomy of Cranial Nerves

The cranial nerves are a set of twelve pairs of nerves that arise directly from the brain. The first two nerves, olfactory and optic, develop from the cerebrum, and the remaining ten arise from the brainstem. The names of each cranial nerve are related to their function, and the nerves are numbered in roman numerals from I to XII.

In this article, we shall summarise the anatomy of the cranial nerves, including their origin, course, and functions.

Origin of Cranial Nerves

There are twelve cranial nerves in total. The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum, while cranial nerves III to XII arise from the brainstem (Figure 1). They can arise from a specific part of the brainstem (midbrain, pons, medulla), or from a junction between two parts, as follows:

• Midbrain – the trochlear nerve (IV) comes from the posterior side of the midbrain and has the longest intracranial length of all the cranial nerves.
• Midbrain-pontine junction – oculomotor (III).
• Pons – trigeminal (V).
• Pontine-medulla junction – abducens, facial, vestibulocochlear (VI-VIII).
• Medulla oblongata – posterior to the olive; glossopharyngeal, vagus, accessory (IX-XI); anterior to the olive; hypoglossal (XII).

The cranial nerves are numbered based on their location on the brainstem (superior to inferior and then medial to lateral) and the order of their exit from the cranium (anterior to posterior) (Figures 1 & 2).

Modalities

Each cranial nerve can be classified as sensory, motor, or both. They can more specifically transmit seven types of information, three of which are only unique to cranial nerves (SSS, SVS and SVM). The following are the sensory and motor modalities of the cranial nerves:

• Sensory (afferent) modalities-
• General somatic sensory (GSS) – general sensation from skin.
• General visceral sensory (GVS) – general sensation from viscera.
• Special somatic sensory (SSS) – senses derived from ectoderm (i.e. sight, sound, balance, etc.).
• Special visceral sensory (SVS) – senses derived from endoderm (i.e. taste, smell, etc.).
• Motor (efferent) modalities-
• General somatic motor (GSM) – skeletal muscles.
• General visceral motor (GVM) – smooth muscles of gut and autonomic motor.
• Special visceral motor (SVM) – muscles derived from pharyngeal arches.

The anatomy of cranial nerves is of extreme importance in understanding the nervous system. Knowing the origin, course, and functions of the cranial nerves provides insight into the workings of the nervous system, which in turn helps in diagnosis and treatment of diseases of the nervous system.

It is also important to know the sensory and motor modalities of the cranial nerves, so that any anomalies can be identified and the appropriate course of action taken. This is especially important for the special somatic, special visceral, and special visceral motor modalities that are unique to cranial nerves.

By understanding and recognizing the anatomy of cranial nerves, we can effectively diagnose and treat conditions of the nervous system that can be caused by malfunctions of these nerves.

Cranial Nerve Overview

The twelve cranial nerves exit the skull through various foramen and fissures. The modality and function of each nerve is specified below, as well as the nerve's exit point and the muscles it innervates. It is important to understand the unique features of the 12 cranial nerves in order to effectively diagnose and treat various medical conditions.

CN IV: Trochlear

• Exit: Superior orbital fissure
• Modality: Motor (GSM)
• Function: Superior oblique

CN V: Trigeminal

• Ophthalmic Exit: Superior orbital fissure
• Modality: GSS
• Function: Scalp, forehead, and nose

• Maxillary Exit: F. rotundum
• Modality: GSS
• Function: Cheeks, lower eye lid, nasal mucosa, upper lip, upper teeth, and palate

• Mandibular Exit: F. ovale
• Modality: GSS SVM
• Function: GSS- anterior 2/3 tongue, skin over mandible, lower teeth. SVM- muscles of mastication

CN VI: Abducens

• Exit: Superior orbital fissure
• Modality: Motor (GSM)
• Function: Lateral rectus

CN VII: Facial

• Exit: Internal acoustic meatus > stylomastoid f.
• Modality: Both- GSS SVS SVM GVM
• Function: GSS- sensation to part of ext. ear. SVS- taste from ant. 2/3 tongue, hard and soft palate. SVM- muscles of facial expression. GVM- lacrimal, submandibular, sublingual glands, and mucous glands of mouth and nose.

CN VIII: Vestibulocochlear

• Exit: Internal acoustic meatus
• Modality: Sensory (SSS)
• Function: Hearing and balance

CN IX: Glossopharyngeal

• Exit: Jugular f.
• Modality: Both- GSS GVS SVS GVM SVM
• Function: GSS- post. 1/3 tongue, ext. ear, and middle ear cavity. GVS- carotid body and sinus. SVS- taste from post. 1/3 tongue. GVM- parotid gland. SVM- stylopharyngeus

CN X: Vagus

• Exit: Jugular f.
• Modality: Both- GSS GVS SVS GVM SVM
• Function: GSS- ext. ear, larynx and pharynx. GVS- larynx, pharynx and, thoracic & abdominal viscera. SVS- taste from epiglottis region of tongue. GVM- smooth muscles of pharynx, larynx, and most of the Gastrointestinal Tract (GIT). SVM- most muscles of pharynx and larynx.

CN XI: Spinal Accessory

• Exit: Jugular f.
• Modality: Motor (GSM & SVM)
• Function: GSM- trapezius and sternocleidomastoid. SVM- a few fibers run with CNX to viscera.

CN XII: Hypoglossal

• Exit: Hypoglossal canal
• Modality: Motor (GSM)
• Function: Intrinsic and extrinsic tongue muscles (except the palatoglossus)

Understanding the individual features of each of the twelve cranial nerves is an important part of effective medical diagnosis and treatment. Knowing the exit point, modality, and function of each nerve allow healthcare providers to quickly identify any issues with specific nerves and address them accordingly. Additionally, understanding the muscles innervated by each nerve is important in determining the type and extent of any motor issues.

Having a clear understanding of the anatomy and physiology of the twelve cranial nerves is essential for any healthcare provider, especially those working in neuroanatomy. Being able to accurately identify, diagnose, and treat any issues related to cranial nerves is what separates a competent healthcare provider from an excellent one.