CRANIAL NERVES

ORIGIN OF CRANIAL NERVE FIBERS

Cranial nerve fibers with motor (efferent) functions arise from collections of cells (motor nuclei) that lie deep within the brain stem

they are homologous to the anterior horn cells of the spinal cord.

Cranial nerve fibers with sensory (afferent) functions have their cells of origin (first-order nuclei) outside the brain stem, usually in ganglia that are homologous to the dorsal root ganglia of the spinal nerves

Second-order sensory nuclei lie within the brain stem

Functional groups

CN I,II and VIII are devoted to sensory input

CN II,IV, VI control eye movements and pupillary constriction

CN XI, XII are purely motor CN V,VII, IX are mixed CN III, IV, VII, X carry

parasympathetic fibres

FUNCTIONAL COMPONENTS OF THE CRANIAL NERVES Somatic efferent fibers, also

called general somatic efferent fibers, innervate striated muscles that are derived from somites and are involved in eye (nerves III, IV, and VI) and tongue (nerve XII) movements.

Special visceral efferent fibers are special somatic efferent

components. They innervate muscles that are

derived from the pharyngeal arches and are involved in chewing (nerve V), making facial expressions (nerve VII), swallowing (nerves IX and X), producing vocal sounds (nerve X), and turning the head (nerve XI).

Visceral efferent fibers

also called general visceral efferent fibers (preganglionic parasympathetic components of the cranial division)

they travel within nerves III (smooth muscles of the inner eye), VII (salivatory and lacrimal glands), IX (the parotid gland), and X (the muscles of the heart, lung, and bowel that are involved in movement and secretion

Visceral afferent fibers

also called general visceral afferent fibers, convey sensation from the alimentary tract, heart, vessels, and lungs by way of nerves IX and X.

A specialized visceral afferent component is involved with the sense of taste; fibers carrying gustatory impulses are present in cranial nerves VII, IX, and X.

Somatic afferent fibers

often called general somatic afferent fibers, convey sensation from the skin and the mucous membranes of the head.

found mainly in the trigeminal nerve (V).

A small number of afferent fibers travel with the facial (VII), glossopharyngeal (IX), and vagus (X) nerves; these fibers terminate on trigeminal nuclei in the brain stem.

Special sensory fibers

found in nerves I (involved in smell), II (vision), and VIII (hearing and equilibrium).

Ganglia Related to Cranial Nerves

Two types first type -contains cell bodies of

afferent (somatic or visceral) axons within the cranial nerves.

somewhat analogous to the dorsal root ganglia that contain the cell bodies of sensory axons within peripheral nerves.

The second type contains the synaptic terminals of visceral efferent axons, together with postsynaptic (parasympathetic) neurons that project peripherally

Sensory ganglia

semilunar (gasserian) ganglion (nerve V)

geniculate ganglion (nerve VII) cochlear and vestibular ganglia

(nerve VIII), inferior and superior

glossopharyngeal ganglia (nerve IX)

superior vagal ganglion (nerve X), and inferior vagal (nodose) ganglion (nerve X).

Parasympathetic ganglia

ciliary ganglion (nerve III) the pterygopalatine and

submandibular ganglia (VII) otic ganglion (IX) intramural ganglion (X).

The first four of these ganglia have a close association with branches of CN V

the trigeminal branches may course through the autonomic ganglia

Cranial Nerve IX: Glossopharyngeal Nerve contains several types of fibers Special visceral efferent fibers from

the nucleus ambiguus pass to the stylopharyngeal muscle.

Visceral efferent (parasympathetic preganglionic) fibers from the inferior salivatory nucleus pass through the tympanic plexus and lesser petrosal nerve to the otic ganglion, from which the postganglionic fibers pass to the parotid gland.

The inferior salivatory nucleus receives cortical impulses via the dorsal longitudinal fasciculus and reflexes from the nucleus of the solitary tract.

Visceral afferent fibers arise from unipolar cells in the inferior ganglia.

Centrally, they terminate in the solitary tract and its nucleus, which in turn projects to the thalamus (VPM nucleus) and then to the cortex

Peripherally, the visceral afferent axons of nerve XI supply general sensation to the pharynx, soft palate, posterior third of the tongue, fauces, tonsils, auditory tube, and tympanic cavity.

Through the sinus nerve, they supply special receptors in the carotid body and carotid sinus that are concerned with reflex control of respiration, blood pressure, and heart rate.

Special visceral afferents supply the taste buds of the posterior third of the tongue and carry impulses via the superior ganglia to the gustatory nucleus of the brain stem.

A few somatic afferent fibers enter by way of the glossopharyngeal nerve and end in the trigeminal nuclei.

CLINICAL CORRELATIONS

rarely involved alone by disease processes (eg, by neuralgia)

generally involved with the vagus and accessory nerves because of its proximity to them.

Pharyngeal (gag) reflex

Depends on nerve IX for its sensory component, whereas nerve X innervates the motor component.

Stroking the affected side of the pharynx does not produce gagging if the nerve is injured

Carotid sinus reflex

Depends on nerve IX for its sensory component.

Pressure over the sinus normally produces slowing of the heart rate and a fall in blood pressure.

Cranial Nerve X: Vagus Nerve Special visceral efferent fibers from

the nucleus ambiguus contribute rootlets to the vagus nerve and the cranial component of the accessory nerve (XI).

Those of the vagus nerve pass to the muscles of the soft palate and pharynx.

Those of the accessory nerve join the vagus outside the skull and pass, via the recurrent laryngeal nerve, to the intrinsic muscles of the larynx

Visceral efferent fibers

From the dorsal motor nucleus of the vagus course to the thoracic and abdominal viscera.

Their postganglionic fibers arise in the terminal ganglia within or near the viscera.

They inhibit heart rate and adrenal secretion and stimulate gastrointestinal peristalsis and gastric, hepatic, and pancreatic glandular activity

Somatic afferent fibers

from unipolar cells in the superior (formerly called the jugular) ganglion send peripheral branches

1. via the auricular branch of CN X to the external auditory meatus and part of the earlobe

2. via the recurrent meningeal branch to the dura of the posterior fossa.

Central branches pass with CN X to the brain stem and end in the spinal tract of the trigeminal nerve and its nucleus.

Visceral afferent fibers

From unipolar cells in the inferior (formerly nodose) ganglion-

peripheral branches to the pharynx, larynx, trachea, esophagus, and thoracic and abdominal viscera

few special afferent fibers to taste buds in the epiglottic region

Central branches run to the solitary tract and terminate in its nucleus.

The visceral afferent fibers of CN X carry the sensations of abdominal distention and nausea and the impulses concerned with regulating the depth of respiration and controlling blood pressure.

A few special visceral afferent fibers for taste from the epiglottis pass via the inferior ganglion to the gustatory nucleus of the brain stem.

CLINICAL CORRELATIONS

CN X lesions near the skull base often involve CN IX and CN Xi and sometimes CN XIIas well.

Complete bilateral transection of CN X is fatal.

Unilateral lesions within the cranial vault or close to the base of the skull, produce widespread dysfunction of the palate, pharynx, and larynx

The soft palate is weak and may be flaccid so the voice has a nasal twang.

Weakness or paralysis of the vocal cord may result in hoarseness

There can be difficulty in swallowing, and cardiac arrhythmias may be present.

Damage to the recurrent laryngeal nerve can occur as a result of invasion or

compression by tumor or as a complication of thyroid surgery

may be accompanied by hoarseness or hypophonia but can be asymptomatic.

Cranial Nerve XI: Accessory Nerve two separate components: cranial

and spinal In the cranial component, special

efferent fibers (from the nucleus ambiguus to the intrinsic muscles of the larynx) join CN XI inside the skull but are part of CN X outside the skull

In the spinal component, the special efferent fibers from the lateral part of the anterior horns of the first 5 or 6 cervical cord segments ascend as the spinal root of CN XI through the foramen magnum and leave the cranial cavity through the jugular foramen

They supply the sternocleidomastoid muscle and partly supply the trapezius muscle.

Central connections of the spinal component These are those of the typical lower

motor neuron: voluntary impulses via the

corticospinal tracts postural impulses via the basal

ganglia reflexes via the vestibulospinal and

tectospinal tracts.

CLINICAL CORRELATIONS

Interruption of the spinal component leads to

1. paralysis of the sternocleidomastoid muscle, causing the inability to rotate the head to the contralateral side

2. paralysis of the upper portion of the trapezius muscle, which is characterized by a wing-like scapula and the inability to shrug the ipsilateral shoulder.

Cranial Nerve XII: Hypoglossal Nerve Somatic efferent fibers from the

hypoglossal nucleus in the ventromedian portion of the gray matter of the medulla emerge between the pyramid and the olive to form CN XII

leaves the skull through the hypoglossal canal and passes to the muscles of the tongue

A few proprioceptive fibers from the tongue course in the hypoglossal nerve and end in the trigeminal nuclei of the brain stem

CN XII distributes motor branches to the geniohyoid and infrahyoid muscles with fibers derived from communicating branches of C1 nerve.

A sensory recurrent meningeal branch of CN XII innervates the dura of the posterior fossa of the skull.