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Cranial nerves and cranial nerve nuclei


There are 12, bilaterally paired, cranial nerves. These carry afferent and efferent nerve fibres between the brain and peripheral structures, principally of the head and neck. The cranial nerves are individually named and numbered (Roman numerals I–XII) according to the rostrocaudal sequence in which they attach to the brain ( Fig. 10.1 and Table 10.1 ):

  • I olfactory

  • II optic

  • III oculomotor

  • IV trochlear

  • V trigeminal

  • VI abducens

  • VII facial

  • VIII vestibulocochlear

  • IX glossopharyngeal

  • X vagus

  • XI accessory

  • XII hypoglossal

Fig. 10.1
The base of the brain illustrating the locations of the cranial nerves.
The points of attachment are shown, except for the trochlear nerve, which arises from the dorsal aspect of the brainstem.

Table 10.1
Summary of components, connections and functions of the cranial nerves
Cranial nerve Component fibres Structures innervated Central connections Functions
I Olfactory Sensory Olfactory epithelium Olfactory bulb Olfaction
II Optic Sensory Retina Lateral geniculate nucleus; pretectal nucleus Vision; pupillary light reflex
III Oculomotor Motor Superior, inferior and medial rectus muscles, inferior oblique muscle; levator palpebrae superioris muscle Oculomotor nucleus Movement of eyeball; elevation of upper eyelid
Parasympathetic Sphincter pupillae and ciliary muscle of the eyeball, via ciliary ganglion Edinger–Westphal nucleus Pupillary constriction and accommodation
IV Trochlear Motor Superior oblique muscle Trochlear nucleus Movement of eyeball
V Trigeminal Sensory Face, scalp, cornea, nasal and oral cavities, cranial dura mater Trigeminal sensory nucleus General sensation
Motor Muscles of mastication; tensor tympani Trigeminal motor nucleus Opening and dosing mouth; tension on tympanic membrane
VI Abducens Motor Lateral rectus muscle Abducens nucleus Movement of eyeball
VII Facial Sensory Anterior two-thirds of tongue Nucleus solitarius Taste
Motor Muscles of facial expression; stapedius muscle Facial nucleus Facial movement; tension on bones of middle ear
Parasympathetic Salivary and lacrimal glands, via submandibular and pterygopalatine ganglia Superior salivatory nucleus Salivation and lacrimation
VIII Vestibulocochlear Sensory Vestibular apparatus; cochlea Vestibular nuclei; cochlear nuclei Vestibular sensation (position and movement of head); hearing
IX Glossopharyngeal Sensory Pharynx, posterior third of tongue, Eustachian tube, middle ear Trigeminal sensory nucleus General sensation
Posterior third of tongue; carotid body, carotid sinus Nucleus solitarius Taste; chemoreception, baroreception
Motor Stylopharyngeus muscle Nucleus ambiguus Swallowing
Parasympathetic Parotid salivary gland, via otic ganglion Inferior salivatory nucleus Salivation
X Vagus Sensory Pharynx, larynx, trachea, oesophagus, external ear Trigeminal sensory nucleus General sensation
Thoracic and abdominal viscera; aortic bodies, aortic arch Nucleus solitarius Visceral sensation: chemoreception, baroreception
Motor Soft palate, pharynx, larynx, upper oesophagus Nucleus ambiguus Speech, swallowing
Parasympathetic Thoracic and abdominal viscera Dorsal motor nucleus of vagus Innervation of cardiac muscle. Innervation of smooth muscle and glands of cardiovascular system, respiratory and gastrointestinal tracts
XI Accessory (spinal roots) Motor Sternomastoid and trapezius muscles Spinal cord Movement of head and shoulder
XII Hypoglossal Motor Intrinsic and extrinsic muscles of tongue Hypoglossal nucleus Movement of tongue
The components are colour-coded according to their embryological origin (see also Fig. 1.11 and Fig. 10.2 ).

The first two cranial nerves attach directly to the forebrain, while the rest attach to the brainstem. The olfactory system is closely associated, both structurally and functionally, with parts of the forebrain collectively referred to as the limbic system; these, including cranial nerve I, are considered together in Chapter 16 . The visual system and cranial nerve II are described in Chapter 15 . Cranial nerves III–XII are directly connected to various neuronal cell groups, located ipsilaterally within the brainstem and called the cranial nerve nuclei; these either receive cranial nerve afferents or contain the cell bodies of efferent neurones that have axons leaving the brain in cranial nerves. The locations of these nuclei are illustrated schematically in Fig. 10.2 . Some can readily be seen in stained sections of the brainstem (see Fig. 9.5, Fig. 9.6, Fig. 9.7, Fig. 9.8, Fig. 9.9, Fig. 9.10, Fig. 9.11, Fig. 9.12, Fig. 9.13 ).

Fig. 10.2
The brainstem viewed from the dorsal aspect.
The diagram illustrates the locations of the afferent cranial nerve nuclei (left) and the efferent cranial nerve nuclei (right). On the right, nuclei shaded in the same colour share a common embryological origin (see also Fig. 1.11 ).

Cranial nerve nuclei

Afferent nuclei

Afferent fibres carrying general sensory information (touch, pressure, pain, temperature) from the head enter the brain through the trigeminal nerve at the level of the pons and terminate in the trigeminal sensory nucleus . This is a large nucleus that runs the whole length of the brainstem and extends caudally into the cervical spinal cord. Fibres conveying the special senses of motion/positional sense and hearing run in the vestibulocochlear nerve. They terminate in the vestibular and cochlear nuclei , respectively, which are located in the medulla, in and near to the lateral part of the floor of the fourth ventricle (sometimes referred to as the vestibular area). Visceral afferents, including taste fibres, terminate in the nucleus solitarius of the medulla.

Efferent nuclei

On the basis of their embryological derivation, the efferent cranial nerve nuclei can be divided into three groups, each lying in a discontinuous longitudinal column.

Nuclei of the somatic efferent cell column

The somatic efferent cell column lies near to the midline and consists of the nuclei that send motor fibres into the III, IV, VI and XII nerves. The oculomotor nucleus lies in the ventral apex of the periaqueductal grey of the midbrain at the level of the superior colliculus (see Fig. 9.13 ). Its efferent fibres run in the oculomotor nerve to innervate the levator palpebrae superioris muscle and all of the extraocular muscles, except the superior oblique and lateral rectus. The trochlear nucleus also lies in the midbrain, at the ventral border of the periaqueductal grey, but at the level of the inferior colliculus (see Fig. 9.12 ). Fibres leave in the trochlear nerve, to innervate the superior oblique muscle of the eye. The abducens nucleus is located in the caudal pons beneath the floor of the fourth ventricle (see Fig. 9.8 ). Its efferents run in the abducens nerve and they innervate the lateral rectus muscle. In the medulla lies the hypoglossal nucleus (see Fig. 9.7 ), which innervates the intrinsic and extrinsic muscles of the tongue via the hypoglossal nerve.

Nuclei of the branchiomotor cell column

The branchiomotor cell column innervates striated muscles derived from the embryonic branchial (pharyngeal) arches. In the tegmentum of the mid-pons is located the trigeminal motor nucleus , which supplies fibres to the trigeminal nerve and innervates the muscles of mastication, tensor tympani, tensor veli palatini, mylohyoid and the anterior belly of the digastric muscle. In the caudal pontine tegmentum lies the facial motor nucleus . This innervates the muscles of facial expression and the stapedius muscle via the facial nerve. Within the medulla lies the nucleus ambiguus . This long nucleus sends motor fibres into the glossopharyngeal, vagus and cranial part of the accessory nerve to innervate the muscles of the pharynx and larynx.

Nuclei of the parasympathetic cell column

The parasympathetic cell column consists of preganglionic parasympathetic neurones that send axons into the III, VII, IX and X cranial nerves. The most rostral cell group constitutes the Edinger–Westphal nucleus , which lies in the midbrain periaqueductal grey matter adjacent to the oculomotor nucleus (see Fig. 9.13 ). Its axons leave the brainstem in the oculomotor nerve and pass to the ciliary ganglion in the orbit, within which they synapse; from here postganglionic fibres innervate the sphincter pupillae and ciliary muscles within the eye.

In the pontine tegmentum lie two parasympathetic cell groups, the superior and inferior salivatory nuclei . The superior salivatory nucleus supplies preganglionic fibres to the facial nerve that terminate in the pterygopalatine and submandibular ganglia. Postganglionic fibres from the pterygopalatine ganglion innervate the lacrimal gland and the nasal and oral mucous membranes. Those from the submandibular ganglion innervate the submandibular and sublingual salivary glands. The inferior salivatory nucleus sends preganglionic fibres into the glossopharyngeal nerve. These terminate in the otic ganglion, which in turn sends postganglionic axons to the parotid salivary gland.

The largest preganglionic parasympathetic cell group lies in the medulla and constitutes the dorsal motor nucleus of the vagus (see Fig. 9.7 ). Its rostral portion lies immediately beneath the floor of the fourth ventricle, lateral to the hypoglossal nucleus. Fibres leave in the vagus nerve and are widely distributed to thoracic and abdominal viscera.

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