CRANIAL NERVES

General

The cranial nerves are the components of the peripheral nervous system that are attached to the brain, rather than the spinal cord. Nerves I and II are not true peripheral nerves but fibre tracts of the brain.

Spinal nerves are mixed, but cranial nerves may be purely motor, purely sensory or mixed. Some also carry fibres of the autonomic nervous system (as do some spinal nerves), some innervate muscles of branchial origin, and some are associated with special sense organs.

There are 7 possible components in cranial nerves (as compared to 4 in spinal nerves), but no one cranial nerve contains all 7. Table 2 shows the components of the cranial nerves.

Cranial Nerve I - Olfactory (Sensory; SVA)

The cell bodies of the primary neurons are found in the nasal mucosa, axons pass through the cribiform plate of the ethmoid bone to the olfactory bulb, where they synapse (Figure 12). The axons of the secondary neurons form the olfactory tract which divides into medial and lateral olfactory striae. The medial branch is a component of the limbic system and goes to septal. The lateral branch (Figure 13) goes to the primary olfactory area constituted by the uncus and dorsmedial amgydala. Tertiary neurons extend to the olfactory association area or entorhinal cortex (area 28 on Brodmann's map - the anterior art of the parahippocampal gyrus). There is also a relay via the thalamus to the orbitofrontal cortex.

The connections of the olfactory nerve are complex and include limbic structures (eg. hippocampus) and the hypothalamus and brain stem via the medial forebrain bundle. In the brain stem there are reflex connections with other nuclei eg. salivatory nuclei, nucleus ambiguus, and autonomic centres.

Cranial Nerve II - Optic (Sensory; SSA)

The rods and cones of the retina are the first order neurons that connect with the bipolar cells of the retina - the second order neurons. These in turn synapse with the third order neurons - the ~ on cells -near the surface of the retina and the myelinated axons of the qanglion cells form the optic nerve fibres.

At the optic chiasma, fibres from the nasal half of each retina cross to the opposite side, but fibres from temporal oral half continue as before. Thus the optic tract contains fibres from the ipsilaterel halves of the retinas - constituting, in the case of the left optic tract, the ~t field of vision. (See Figure 14). The optic tract runs to the lateral geniculate nucleus of the thalamus. The visual fibres in the tract synapse and the fourth order neurons form the geniculocalcarine tract/optic radiation _and pass to the primary visual cortex in the calcarine sulcus.

Other fibres in the optic tract have different functions. Light reflex fibres go to the pretectal nucleus and thence to the Edinger-Westphal nucleus and are responsible for simple and consensual light reflexes. The parasympathetic efferent fibres run with the third cranial nerve to the ciliary ganglion, and from there to the constrictor papillae and ciliary muscles.

Other fibres in the optic tract pass to the superior colliculus and from thence to other cranial and spinal nuclei and are concerned with reflex movements of the eyes and head.

The accommodation reflex involves a relay from the calcarine cortex to the superior colliculus, possibly including a frontal lobe connection -

There are also fibres passing from the visual cortex to other cortical centres related to higher functions such as reading.

Cranial Nerve III - Oculomotor (Motor; CSE, GVE)

The motor fibres arise from a group of nuclei just ventral to the aqueduct (at the level of the superior colliculus) and run to supply the medial₁ superior and inferior rectus muscles and inferior oblique and levator palpebrae muscles (Figure 15). The parasympathetic fibres arise from Edinger-Westphal and medial oculomotor nucleus and run via the ciliary ganglion to the constrictor u illae and ciliar muscles respectively. These fibres form the efferent components of the light and accommodation reflexes.

The oculomotor nuclei are connected to the other oculogyric nuclei and the vestibular nuclei via the medial longitudinal fasciculus. This fasciculus continues caudally into the medial vestibulospinal tract and co-ordinates movements of the head, eves and neck

Cranial Nerve IV - Trochlear (Motor; GSE)

Motor fibres leave the trochlear nucleus which is just caudal to the III nucleus (at the level of inferior colliculus), decussate and emerge from the dorsal surface of the midbrain before passing to innervate the s~orobliuemuscle.

Cranial Nerve V .- Trigeminal (Mixed; GSA, SVE)

This is the largest cranial nerve.: It is sensory for the face, and motor to the muscles of the first branchial arch i.e. the muscles of mastication (and some small muscles associated with the ear and with swallowing). Parasym~athetic fibres from other cranial nerves also use branches of the V nerve to get to their effector organs.

Sensory fibres arise from unipolar cells in the semilunar or trigeminal ganglion. They supply sensation via 3 nerves - the ophthalmic, maxillary and mandibular - as shown in Figure 16.

There are 3 nuclei associated with the afferent fibres of the trigeminal nerve, and they also receive input from other cranial nerves. These nuclei are the chief sensory nucleus, the spinal tract and nucleus, and the mesencephalic nucleus. They subserve discriminatory touch, am and temperature, and proprioception respective The Proprioceptive fibres pass via the motor root of V to the muscles of mastication.

The trigeminothalamic tract passes centrally from the chief sensory and spinal nuclei of the fifth nerve to the thalamus. A large number of collateral fibres run from this tract to the reticular activating system.

The motor fibres pass from the motor nucleus at the midpontine level and accompany the mandibular division These fibres supply the muscles of mastication, tensor tympani and tensor veli palatini

and the anterior belly of digastric. The motor fibres are mainly crossed but significant numbers are uncrossed. The motor nucleus receives bilateral (but mainly crossed) cerebral connections from the corticobulbar ~and reflex connections from the spinal tract of V. Because of the bilateral cortical representation it is unusual for chewing to be paralysed following a C.V.A.

In the corneal reflex, the afferent limb is the ophthalmic division of the trigeminal nerve and the efferent limb is the facial nerve (VII).

Cranial Nerve VI - Abducens (Motor/GSE)

TheVI nerve nucleus lies in the floor of the fourth ventricle in the lower pons and the motor (uncrossed) fibres emerge from the ventral pons and have a long course to the orbit to supply the lateral rectus muscle. (Figure 15).

Cranial Nerve VII - Facial (Mixed, mainly motor; SVE, GVF, SVA, GSA)

The facial nerve is motor to the muscles of the second branchial arch - the muscles of facial expression (plus stapedius, stylohyoid and the posterior belly of digastric). The motor nucleus is in ~

The parasympathetic fibre~ derive from the superior salivatory nucleus and pass via the nervus intermedius to the glands and mucous membranes of the pharynx, palate, nas~vit and paranasal sinuses; and to the submaxillar and subling ual salivary glands and the lacrimal glands.

The facial nerve also carries taste fibres from the anterior two-thirds of the tongue via the chorda tympani. The cell bodies lie in the geniculate ganglion. Central branches pass via the nervus intermedius to t he~eus of the solitar tract

With respect to the cutaneous sensory component (GSA), the fibres are distributed to the skin of the external ear and behind the ear, and outer surface of the tempanic membrane.

The facial nerve motor fibres constitute the efferent limb of the corneal reflex.

Figure 17 shows the main components of the seventh nerve.

Cranial Nerve VIII - Vestibulocochlear (composite sensory nerve; SSA)

This nerve consists of 2 separate parts.

Cochlear or Auditory Nerve

Fibres from bipolar cells in the spiral ganglion consist of peripheral branches that end in the organ of Corti and central branches that end in the ventral and dorsal cochlear nuclei. (Figure 18).

The auditory pathways are shown in Figure 19. From the cochlear nuclei many second-order neurons cross the midline as the trapezoid body and ascend as the lateral lemniscus to the inferior colliculus. (Some fibres synapse in the superior olivary nucleus). The inferior colliculus is the. area for reflex connections to~nuclei and other motor nuclei of cranial and spinal nerves via tectobulbar and tectosninal tracts.

The auditory fibres pass from the inferior colliculus via the inferior brachium to the medial Geniculate nucleus of the thalamus. From this area the auditory radiation projects to the auditory cortex of the temporal lobe and the association cortex.

The auditory pathway is bilateral above the cochlear nuclei, although mainly crossed.

Vestibular Nerve

Fibres from bipolar cells in the vestibular ganglion consist of peripheral branches that pass to the semicircular canals, and central branches that terminate in the vestibular nuclear complex. The vestibular nuclei have reciprocal connections to the archicerebellum; some vestibular nerve fibres enter the cerebellum directly.

The vestibular nuclei have direct paths to the spinal cord - the ~ralvestibulosinaltract which is found throughout the whole cord and the medial vestibulospinal tract which is a continuation of the medial longitudinal fasciculus into the cervical cord.

In the brainstem the vestibular nuclei connect with the oculogyric nuclei through the medial longitudinal fasciculus, producing co-ordinated movement of the head and eyes. There are reciprocal connections with the reticular formation, and centres (nausea, vomiting).

Vestibular cortical connections are uncertain - may be parietal lobe behind general sensation for the head, or superior temporal gyrus in front of auditory cortex.

Cranial Nerve IX - Glossopharyngeal (Mixed; SVA, GV:A, GSA, SVF, GVF)

Sensory components - The glossopharyngeal nerve has 3 different sensory components. It has special visceral afferent fibres subserving taste for the posterior one third of the tongue and the pharynx. These fibres end centrally in the solitary nucleus. It has general visceral afferent fibres supplying receptors in the carotid body and sinus. Finally it has general

fibres supplying general sensation for the back of the tongue, {)pharynx, soft palate, tonsils and Eustachian tube. These fibres pass centrally to the spinal trigeminal nucleus.

Motor components - Branchiomotor (special visceral efferent) fibres from the nucleus ambiguus got to stylo-pharyngeus.

parasympathetic(general visceral efferent) fibres pass from the inferior salivator nucleus via ~ an lion to the parotid gland.

The pharyngeal or gag reflex depends on the ninth nerve for its sensory component, as does the carotid sinus reflex-

Figure 20 shows some of the main components of the glossopharyngeal nerve.

Cranial Nerve X - Vagus (Mixed; SVA, GVA, GSA, SVE, GVE)

Sensory components - The vagus nerve has the same sensory components as the ninth nerve. It has a few taste fibres (SVA) from thej~e.i~_9~tis and~al visceral afferent fibres from the aortic body and sinus. However in addition to the latter it carries visceral sensation from the larynx, trachea. oesophaous thoracic and abdominal'viscera. Finally it has general somatic afferent fibres from the external auditory meatus -auditory canal~and tympanic membrane. The central connections of the afferent fibres of X are as for IX.

Motor components - Special visceral efferent or branchiomotor fibres pass from the nucleus ambiguus to the muscles of the soft palate, pharynx, external larynx and upper oesophagus.

Parasympathetic fibres derive from the~alvaalnucleus and are visceromotor and secretomotor to the thoracic and abdominal viscera. T~rsalvaalnucleus comes ~rthe~nfluenceof many components including limbic, trigeminal, reticular vestibular, hypothalamic olfactory and taste.

The tenth nerve constitutes the efferent arc of the gag or pharyngeal reflex and the carotid sinus reflex.

Figure 21 shows some of the major components of the vagus nerve.

Cranial Nerve XI - Accessory (Motor; SVF)

The cranial branch of the accessory nerve passes from the nucleus ambiguus to the intrinsic muscles of the larynx and soft palate.

The spinal branch runs from the accessory nucleus to innervate trapezius and sternocfeidomastoid muscles.

Cranial Nerve XII - Hypoglossal (Motor; GS£)

Motor fibres from the hypoglossal nucleus in the medulla form the twelfth nerve and pass to the muscles of the tongue

TABLE 2

Components and Functions of Cranial Nerves

Name	Components	Functions (maj or)
I Olfactory nerve	Special visceral afferent (SVA)	Smell
II Optic nerve	Special somatic afferent (SSA)	Vision and associated reflexes
III Oculomotor nerve	General somatic efferent (GSE)	Movements of eyes
	General visceral efferent (GVE)	Pupillary constriction and accommodation (parasympathetic)
IV Trochlear nerve	General somatic efferent (GSE)	Movements of eyes
V Trigeminal nerve	Special visceral efferent (SVE)	Mastication Swallowing Movements of soft palate and auditory tube Movements of tympanic membrane and ear ossicles
	General somatic afferent (GSA)	General sensations from anterior half of head, including face, nose, mouth, and meninges
		visceral sensibility
	General visceral afferent (GVA)	movements of eyes
VI Abducent nerve	General somatic efferent (GSE)	Facial expression
VII Facial nerve	Special visceral efferent (SVE)	Elevation of hyoid bone Movement of stapes lacrimation, salivation, and (parasympathetic)
	General visceral efferent (GVE)	vasodilatation
		Taste
	Special visceral afferent (SVA)	Ext. ear
	General somatic afferent (GSA)	Hearing and equilibrium reception
VIII Vestibulocochlear	Special somatic afferent (SSA)	Swallowing movements
IX Glossopharynqeal	Special visceral efferent (SVE)
X Vagus nerve and cranial root of N.XI	Special visceral efferent(SVE)	Swallowing movements and laryngeal control
		Movements of soft palate, pharynx, and larynx
	General visceral efferent (GVE)	Parasympathetic to thoracic and abdominal viscera (parasympathetic)
	Special visceral afferent (SVA)	Taste(epiglottis)
	General visceral afferent (GVA)	Sensory from viscera of neck (larynx, trachea, and esophagus).thorax, and abdomen)
	General somatic afferent (GSA)	Auditory canal
XI Accessory nerve (spinal root)	Special visceral efferent (SVE)	Movements of shoulder and head
XII Hypoglossal nerve	General somatic efferent (GSE)	Movements of tongue

Alternative terms:

Somatosensory - General somatic afferent (GSA)

Somatomotor - General somatic efferent (GSE)

Branchiomotor - Special visceral efferent (SVE)

Parasympathetic - General visceral efferent (GVE)

TABLE 3

SUMMARY OF REFLEXES

REFLEX	AFFERENT nerve	CENTRE	EFFERENT nerve
Superficial
Corneal	V - Trigeminal	Pons	VII - Facial
Nasal (sneeze)	V	Brain stem	V, VII, IX, X +
		Upper cord	spinal n^vs of
			expiration
Pharyngeal	Ix - Glossophar.	Medulla	x
Upper abdominal	J7, 8, 9, 10	T7, 8, 9, 10	T7, 8, 9, 10
Lower abdominal	TlO, 11, 12	T10, 11, 12	T10, 11, 12
Cremasteric	Femoral	Ll	Genitofemoral
Plantar	Tibial	ST, 2	Tibial
Deep:
Jaw	V	Pons	V
Biceps	Musculocutaneous	CS, 6	Musculocutaneous
Triceps	Radial	C6, 7	Radial
Patellar	Femoral	L2, 3, 4	Femoral
Achilles	Tibial	ST, 2	Tibial
Visceral:
Light	II- Optic	Midbrain	III-Oculomotor
Accommodation	II	Occip cortex