CRANIAL NERVES I.

I. OVERVIEW ‑ Cranial nerves vs. Spinal nerves.

A. Cranial nerves contain inflow/outflow of brain; spinal nerves contain inflow/outflow of spinal cord.

B. Cranial nerves often contain types of neurons that are similar to types of neurons found in spinal nerves; ex. sensory axons to skin.

C. Cranial nerves can contain types of neurons not found in spinal nerves; ex. taste fibers.

D. Many cranial nerves contain more than one type of neuron.

E. In order to analyze and remember the types of neurons found in different cranial nerves we have a system of classification based upon anatomy, embryology and homologies; TYPES OF NEURONS are called FUNCTIONAL COMPONENTS.

II. BASIS OF CLASSIFICATION ‑ three letter system.

A. First letter

G = General = types of neurons found both in spinal nerves and cranial nerves.

S = Special = types of neurons only found in cranial nerves not spinal nerves.

B. Second letter

S = Somatic = types of neurons innervating structures derived from somites.

V = Visceral = types of neurons innervating gut, structures derived from or associated with gut and branchial arches; also vascular system, smooth muscle, internal organs and glands.

C. Third letter

A = Afferent = sensory neurons.

E = Efferent = motor neurons to skeletal and smooth muscle; also secretomotor neurons to glands.

III. CLASSIFICATION OF INNERVATION AS FUNCTIONAL COMPONENTS

A. General Innervation ‑ like spinal cord

1. GSE (General Somatic Efferent) ‑ motor to somatic voluntary skeletal muscles (derived from somites).

2. GSA (General Somatic Afferent) ‑ sensory to skin, joints, muscle and tendon receptor endings, nasal and oral cavity.

3. GVE (General Visceral Efferent) = Autonomic Efferents ‑ motor to smooth muscles in general and smooth muscles of skin (arrector pilae muscles) and blood vessels, secretomotor to glands.

4. GVA (General Visceral Afferent) ‑ sensory to gut and its derivatives, pharynx, blood vessels, glands and internal organs.

B. Special Innervation ‑ only found in head.

1. SSA (Special Somatic Afferent) ‑ special senses of vision, auditory sensation and vestibular apparatus.

2. SVA (Special Visceral Afferents) ‑ chemical senses of taste and by analogy smell.

3. SVE (Special Visceral Efferents) ‑ motor to voluntary skeletal muscles derived from branchial arches (these muscles develop in association with pharynx, which is rostral end of gut and therefore visceral).

IV. NAMES OF CRANIAL NERVES

I. Olfactory

II. Optic

III. Oculomotor

IV. Trochlear

V. Trigeminal

VI. Abducens

VII. Facial

VIII. Vestibulo‑cochlear (Stato‑acoustic)

IX. Glossopharyngeal

X. Vagus

XI. Accessory (Spinal Accessory)

XII. Hypoglossal

V. SOMATIC CRANIAL NERVE COMPONENTS OF CRANIAL NERVES ‑ like spinal cord.

A. GSE (General Somatic Efferents) ‑ innervate voluntary skeletal muscles derived from somites; two groups of muscles.

1. Eye (Extraocular) muscles ‑ derived from pre‑otic somites; innervated by

a. III (Oculomotor) ‑ to Superior, Inferior and Medial Rectus, Inferior Oblique and Levator Palpebrae Superioris (skeletal part).

b. IV (Trochlear) ‑ to Superior Oblique muscle.

c. VI (Abducens) ‑ to Lateral Rectus muscle.

2. Intrinsic and Extrinsic Muscles of Tongue ‑ derived from occipital somites ‑ all innervated by XII (Hypoglossal).

B. GSA (General Somatic Afferents) ‑ innervate skin, oral cavity, nasal cavity, joints, muscles; sensory cell bodies in sensory ganglia attached to cranial nerves as they enter central nervous system, like dorsal root ganglia. (Atlas Figs. pages 605, 806)

1. All of face, forehead, temporal region, oral cavity, temporo‑mandibular joint innervated by V (Trigeminal); cell bodies in Trigeminal ganglion.

2. Exception: skin of outer ear, external auditory meatus is innervated by V (Trigeminal), plus branches of VII (Facial), IX (Glossopharyngeal) and X (Vagus).

(note: sensory cell bodies of VII in sensory ganglion called Geniculate ganglion)

Note: In Bell's Palsy (paralysis of VII) patients can complain of ear ache due to GSA innervation of outer ear by Facial nerve.

VI. AUTONOMIC INNERVATION OF HEAD (Snell Fig. 1‑30)

A. GVE (General Visceral Efferent) ‑ two neuron arcs.

1. Sympathetic innervation (thoracolumbar outflow)

a. First neuron arises from spinal cord levels T1, T2; axon exits via ventral roots and white communicating rami, ascends in paravertebral sympathetic chain to synapse in Superior Cervical Ganglion.

b. Second neuron in Superior Cervical Ganglion; axon joins plexuses associated with branches of Internal and External Carotid arteries; these give off branches in two ways: i) small unnamed branches close to target; ii) small named branches that come off arterial plexuses and join other nerves (ex. deep petrosal nerve).

2. Parasympathetic innervation (craniosacral) ‑ first neuron in brainstem; axon goes out with cranial nerve to synapse in named ganglion located close to target; second neuron innervates target.

Nerve Ganglion Innervates

III (Oculomotor) Ciliary ganglion Pupillary sphincter muscle, ciliary muscle

VII (Facial) Pterygopalatine Lacrimal gland, mucus glands of nose and ganglion palate

Submandibular Submandibular and sublingual salivary ganglion glands

IX (Glossopharyngeal) Otic Ganglion Parotid gland

X (Vagus) (Many ganglia in Provides parasympathetic innervation to thorax, abdomen) many organs in thorax and abdomen.

B. GVA (General Visceral Afferents) ‑ distributed with both parasympathetic and sympathetic innervation.

1. Sympathetic ‑ sensory to blood vessels, pharynx and its derivatives; cell bodies in dorsal root ganglia of spinal cord; axons travel with sympathetic efferents.

2. Parasympathetic ‑ more localized, specific

Nerve Innervates

VII (Facial) Nasopharynx

IX (Glossopharyngeal) Sensation (touch, pressure) to posterior third of tongue, oropharynx, tympanic cavity and auditory tube, carotid sinus.

X (Vagus) Sensation to laryngopharynx, larynx in head (also innervates many organs in thorax and abdomen).

VI. SPECIAL INNERVATION ‑ only found in head.

A. SSA (Special Somatic Afferents) (Atlas Figs. 9.4, page 814)

1. II (Optic nerve) ‑ vision (actually a brain tract); primary receptors (rods and cones) in retina; axons of ganglion cells of retina form optic nerve; half of axons cross over to opposite side at optic chiasm.

2. VIII (Vestibulocochlear nerve) ‑ auditory and vestibular sensation; cell bodies in cochlear and vestibular apparatus.

B. SVA (Special Visceral Afferents) ‑ Smell and taste.

1. I (Olfactory nerve) ‑ smell; cell bodies in olfactory epithelium; axons project through fila olfactoria to olfactory bulb. (Atlas Fig. page 800)

2. Taste ‑ more complex ‑ distributed over several cranial nerves. (Atlas Fig. 7.52B)

Nerve Taste sensation from

VII (Facial) Anterior two thirds of tongue

IX (Glossopharyngeal) Posterior third of tongue

X (Vagus) Posterior tongue, immediately anterior to epiglottis

C. SVE (Special Visceral Efferents) ‑ motor to muscles of face, ear, pharynx and neck that are derived from branchial arches.

Nerve Innervates

V (Trigeminal) muscles of mastication

(all in V3) mylohyoid

tensor tympani

tensor palati

anterior belly of digastric

VII (Facial) muscles of facial expression

stylohyoid

posterior belly of digastric

stapedius

IX (Glossopharyngeal) stylopharyngeus

X (Vagus) all muscles of pharynx (except stylopharyngeus)

muscles of larynx

all muscles of palate (except tensor palati)

XI (Accessory) sternocleidomastoid

trapezius

VII. SUMMARY OF CRANIAL NERVE COMPONENTS

Nerve GSE SVE GVE GSA GVA SVA SSA

III. + +

IV. +

VI. +

XII. +

V. + +

VII. + + + + +

IX. + + + + +

X. + + + + +

XI. +

I. +

II. +

VIII. +