Cranial Nerves: Organization, Functions, and Passageways 15.0

Organization of Nerves that Bud from the Brain

Introduction

Cranial nerves are specialized nerves that emerge directly from the brain and brainstem, passing through openings in the skull to innervate structures of the head and neck. Understanding their organization, functions, and anatomical passageways is essential for the study of the nervous system in Anatomy & Physiology.

Functional Organization of the Nervous System

Central vs. Peripheral Nervous System

• Central Nervous System (CNS): Composed of the brain and spinal cord; integrates information.

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves; links CNS to the rest of the body and performs motor and sensory functions.

Divisions of the PNS

• Sensory (afferent) division: Transmits sensory signals to the CNS.

• Motor (efferent) division: Transmits motor signals from the CNS to effectors.

• Somatic sensory division: Carries signals from skin, muscles, bones, joints, and special senses.

• Visceral sensory division: Carries signals from organs.

• Somatic motor division: Controls skeletal muscles.

• Autonomic nervous system (ANS): Controls smooth muscle, cardiac muscle, and glands.

Anatomy of the Peripheral Nervous System

Classification of Nerves

• Location:

  • Cranial nerves: 12 pairs

  • Spinal nerves: 31 pairs

• Function:

  • Sensory nerves: Contain only sensory axons; send information to the CNS.

  • Motor nerves: Contain only motor axons; send information away from the CNS.

  • Mixed nerves: Contain both sensory and motor axons.

Location of Cranial Nerves

Overview

There are 12 pairs of cranial nerves, each with specific functions and anatomical passageways. They are grouped by their origin in the brain:

• Cerebrum:

  • CN I (Olfactory)

  • CN II (Optic)

• Midbrain:

  • CN III (Oculomotor)

  • CN IV (Trochlear)

• Pons:

  • CN V (Trigeminal)

  • CN VI (Abducens)

  • CN VII (Facial)

  • CN VIII (Vestibulocochlear)

• Medulla Oblongata:

  • CN IX (Glossopharyngeal)

  • CN X (Vagus)

  • CN XI (Accessory)

  • CN XII (Hypoglossal)

Cranial Nerves: Number, Name, Function, and Passageway

CN I (Olfactory Nerve)

• Function: Sensory; transmits olfactory (smell) information to the brain.

• Passageway: Olfactory foramina of the cribriform plate of the ethmoid bone.

CN II (Optic Nerve)

• Function: Sensory; transmits visual information from the retina to the brain.

• Passageway: Optic canal of the sphenoid bone; fibers cross at the optic chiasma.

CN III (Oculomotor Nerve)

• Function: Motor; controls most extrinsic eye muscles (levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, inferior oblique). Parasympathetic fibers innervate pupil and ciliary muscle.

• Passageway: Superior orbital fissure of the sphenoid bone.

CN IV (Trochlear Nerve)

• Function: Motor; innervates the superior oblique muscle of the eye.

• Passageway: Superior orbital fissure of the sphenoid bone.

CN V (Trigeminal Nerve)

• Function: Mixed; sensory from face, motor to muscles of mastication.

• Branches:

  • V1 Ophthalmic: Sensory from forehead, nose; superior orbital fissure and supraorbital foramen.

  • V2 Maxillary: Sensory from midface, upper jaw; foramen rotundum and infraorbital foramen.

  • V3 Mandibular: Sensory from lower jaw, anterior tongue; motor to chewing muscles; foramen ovale, mandibular foramen, mental foramen.

CN VI (Abducens Nerve)

• Function: Motor; innervates lateral rectus muscle of the eye.

• Passageway: Superior orbital fissure of the sphenoid bone.

CN VII (Facial Nerve)

• Function: Mixed; taste sensation from anterior 2/3 of tongue, motor to facial expression muscles, parasympathetic to salivary glands.

• Passageway: Internal acoustic meatus and stylomastoid foramen of the temporal bone.

CN VIII (Vestibulocochlear Nerve)

• Function: Sensory; balance (semicircular canals) and hearing (cochlea).

• Passageway: Internal acoustic meatus of the temporal bone.

CN IX (Glossopharyngeal Nerve)

• Function: Mixed; sensory from pharynx, tonsils, posterior 1/3 of tongue; taste for posterior 1/3; motor to stylopharyngeus muscle; parasympathetic to parotid gland.

• Passageway: Jugular foramen.

CN X (Vagus Nerve)

• Function: Mixed; sensory and taste from pharynx/larynx, visceral sensory from thoracic/abdominal organs; motor to swallowing muscles; parasympathetic to heart, lungs, digestive system.

• Passageway: Jugular foramen.

CN XI (Spinal Accessory Nerve)

• Function: Motor; innervates sternocleidomastoid and trapezius muscles.

• Passageway: Jugular foramen.

CN XII (Hypoglossal Nerve)

• Function: Motor; controls tongue muscles (genioglossus, palatoglossus, hyoglossus, styloglossus, intrinsic muscles).

• Passageway: Hypoglossal canal of the occipital bone.

Extrinsic Eye Muscles and Their Innervation

Overview

Extrinsic eye muscles control the movement of the eyeball and eyelid. Each muscle is innervated by specific cranial nerves.

Extrinsic Eye Muscle	Action	Innervation
Levator palpebrae superioris	Elevates upper eyelid	CN III (Oculomotor)
Superior rectus	Elevates and medially rotates eye	CN III (Oculomotor)
Inferior rectus	Depresses and medially rotates eye	CN III (Oculomotor)
Medial rectus	Adducts eye	CN III (Oculomotor)
Lateral rectus	Abducts eye	CN VI (Abducens)
Superior oblique	Depresses and laterally rotates eye	CN IV (Trochlear)
Inferior oblique	Elevates and laterally rotates eye	CN III (Oculomotor)

Cranial Nerve Functions Table

Nerve	Sensory Function	Motor Function	Parasympathetic Function
I: Olfactory	Smell	-	-
II: Optic	Vision	-	-
III: Oculomotor	-	Eye movement	Pupil constriction, lens accommodation
IV: Trochlear	-	Eye movement (superior oblique)	-
V: Trigeminal	Facial sensation	Mastication	-
VI: Abducens	-	Eye movement (lateral rectus)	-
VII: Facial	Taste (anterior 2/3 tongue)	Facial expression	Salivary/lacrimal glands
VIII: Vestibulocochlear	Hearing, balance	-	-
IX: Glossopharyngeal	Taste (posterior 1/3 tongue), pharynx	Swallowing	Parotid gland
X: Vagus	Visceral sensation, taste	Swallowing, speech	Heart, lungs, digestive tract
XI: Spinal Accessory	-	Neck muscles	-
XII: Hypoglossal	-	Tongue movement	-

Cranial Nerve Passageways Table

Nerve	Opening in the Skull	Bone
I: Olfactory	Olfactory foramina	Ethmoid
II: Optic	Optic canal	Sphenoid
III: Oculomotor	Superior orbital fissure	Sphenoid
IV: Trochlear	Superior orbital fissure	Sphenoid
V1: Ophthalmic	Superior orbital fissure, supraorbital foramen	Sphenoid, Frontal
V2: Maxillary	Foramen rotundum, infraorbital foramen	Sphenoid, Maxilla
V3: Mandibular	Foramen ovale, mandibular foramen, mental foramen	Sphenoid, Mandible
VI: Abducens	Superior orbital fissure	Sphenoid
VII: Facial	Internal acoustic meatus, stylomastoid foramen	Temporal
VIII: Vestibulocochlear	Internal acoustic meatus	Temporal
IX: Glossopharyngeal	Jugular foramen	Temporal/Occipital
X: Vagus	Jugular foramen	Temporal/Occipital
XI: Accessory	Jugular foramen	Temporal/Occipital
XII: Hypoglossal	Hypoglossal canal	Occipital

Clinical Implications

Bell's Palsy

• Definition: Paralysis of facial muscles due to damage of the facial nerve, often resulting in loss of taste sensation.

• Example: Stress can trigger Bell's palsy; congenital damage can cause characteristic facial features (e.g., upturned lip).

Check Your Understanding

• What is the number, name, function, and passageway of each cranial nerve?

• Identify and name all cranial nerves responsible for blinking and movement of the eyeball.

• What cranial nerve is associated with the organs found in the ventral body cavity?

• What are the cranial nerves associated with the special sensory organs in the head?

• Create a list of cranial nerves that are sensory only, motor only, and both sensory and motor.

• What specific lobe of the brain is associated with receiving or sending information given the cranial nerve function?

• What cranial nerve(s) are associated with tongue and swallowing muscles?

• What cranial nerve(s) are associated with flexion and extension of the neck? What muscles do they innervate?

• List all cranial nerves that pass through the superior orbital fissure of the sphenoid bone.

• List all structures that pass through the jugular foramen.

• What cranial nerve(s) serve the tongue for the sensation of taste?

• What cranial nerve sends sensory information from the head to the CNS?

• What cranial nerve sends motor information to facial expression muscles? What about chewing muscles?

Additional info:

• The cranial nerves are numbered I-XII in order from anterior to posterior as they emerge from the brain.

• Mnemonic for cranial nerve functions: "Some Say Marry Money, But My Brother Says (It's) Bad Business (to) Marry Money" (S = Sensory, M = Motor, B = Both).

• Special sensory cranial nerves include Olfactory (I), Optic (II), Vestibulocochlear (VIII), and parts of Facial (VII), Glossopharyngeal (IX), and Vagus (X) for taste.