Download Cranial nerves

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.