12 cranial nerves and their Functions

There are in total 12 pairs of cranial nerves that arise from the brain and brain stem , respectively. The cranial nerves are paired nerves with the first two namely the olfactory and optic nerves arising directly from the cerebrum meaning they are somewhat the extension of the brain itself. The other 10 cranial nerves arise from the brain stem.

List of the Cranial Nerves

1. I Olfactory
2. II Optic
3. III Oculomotor
4. IV Trochlear
5. V Trigeminal
6. VI Abducens
7. VII Facial
8. VIII Vestibulocochlear
9. IX Glossopharyngeal
10. X Vagus
11. XI Accessory
12. XII Hypoglossal

ORIGIN

Cranial nerves III – XII originate from the midbrain, pons or medulla or a junction between these parts.

• Midbrain – trochlear nerve (IV)  It has the longest route to travel inside the cranial cavity due to the fact that it has to come from the posterior side of the midbrain.
• Midbrain-pontine junction – oculomotor nerve (III).
• Pons – trigeminal nerve (V).
• Ponto-medullary junction – abducens, facial and vestibulocochlear nerves (VI-VIII).
• Medulla Oblongata – glossopharyngeal, vagus, accessory, hypoglossal (IX-XII)

Cranial Nerves and their exit points in the Skull

1. Olfactory (CRIBRIFORM PLATE OF ETHMOID)
2. Optic  (OPTIC FORAMEN)
3. Oculomotor  (SUPERIOR ORBITAL FISSURE)
4. Trochlear (SUPERIOR ORBITAL FISSURE)
5. Trigeminal (SUPERIOR ORBITAL FISSURE)
6. Abducens  (SUPERIOR ORBITAL FISSURE)
7. Facial        (STYLOMASTOID FORAMEN)
8. Vestibulocochlear  (INTERNAL ACOUSTIC MEATUS)
9. Glossopharyngeal (JUGULAR FORAMEN)
10. Vagus    (JUGULAR FORAMEN)
11. Accessory  (JUGULAR FORAMEN)
12. Hypoglossal  (HYPOGLOSSAL CANAL)

CRANIAL NERVES, SENSORY, MOTOR OR MIXED

1. Olfactory nerve. (sensory)
2. Optic Nerve (sensory)
3. Occulomoter nerve (Mainly Motor)
4. Trochlear nerve (Motor)
5. Trigeminal nerve (Both Sensory and Motor/ Mixed)
6. Abducens nerve (Mainly Motor)
7. Facial nerve  (Both sensory and motor/ Mixed)
8. Vestibulocochlear nerve (Mainly Sensory)
9. Glossopharyngeal nerve (Both sensory and motor/ Mixed)
10. Vagus nerve (Both sensory and motor/ Mixed)
11. Accessory nerve nerve  (Mainly Motor)
12. Hypoglossal nerve (Mainly Motor)

FUNCTIONS OF 12 CRANIAL NERVES :

1. OLFACTORY NERVE

It is the shortest cranial nerve. Olfactory nerve is related to the sense of smell. It is part of the cerebrum similar to the  optic nerve. It is capable of regeneration. It consists of the peripheral olfactory receptors and the central processes that relay the information transmitted by the peripheral processes back to the brain. Damage to the olfactory nerve can result in anosmia which can occur in infections of the nose, head injury, certain tumors e.g meningiomas or in neurodegenerative conditions such as Parkinson disease or Alzheimer.

2. OPTIC NERVE

The optic nerve is related to sight / vision. It transfers visual information in the form of electrical signals/impulses to the visual center in the brain. It comprises of ganglionic cells with more than one million nerve fibers. Its originated from the optic vesicle, an extension of the forebrain/ diencehalon. So, it is basically part of the central nervous system with all three meningeal coverings. Opthalmoscopy gives us a unique snapshot of the intracranial vitality and health due to representation of optic nerve.

3. OCULOMOTOR NERVE

Its a mixed cranial nerve, but mainly/predominantly  motor in its function.  The oculomotor nerve is the third cranial nerve. It enters the orbit via the superior orbital fissure and controls the various eyeball movements, elevates the eyelids, adjusts the lens of the eye for near vision (accommodation) and constriction of the pupil of the eye.

Parasympathetic fibres 

Constriction of pupil and accommodation of lens

Fibres outside the eye

Innervate the  upper eye-lid and extrinsic muscles namely the superior ,medial and inferior recti plus the inferior oblique muscle.

4. TROCLEAR NERVE

Its also a mixed nerve like oculomotor but having predominantly motor function. It supplies the superior oblique which depresses and abducts the eyeball rotating it in its socket. It coordinates these movements with the oculomotor and abducens.

5. TRIGEMINAL NERVE

It is a mixed nerve which  splits into 3 divisions, each having both motor and sensory fibres

Ophthalmic branch/division

General sensation to the cornea, forehead and scalp

Maxillary branch/division

Maxillary portion of the face, maxillary teeth, palate, nasal  cavity

Mandibular branch/division

Mandibular face/teeth, muscles of mastication, anterior 2/3rds sensation from the tongue

**The motor fibers from all 3 divisions contribute to the the facial muscles involved in chewing.

6. ABDUCENS NERVE

• It is a motor nerve.
• It supplies the lateral rectus muscle which abducts the eyeball.

7. FACIAL NERVE

• Its a mixed nerve.
• Muscles of facial expression (motor)
• Anterior 2/3rd of tongue (taste sensations)
• Sublingual and submandibular glands (Salivation)
• Nasal and oral mucosa (Lacrimation)

8. VESTIBULOCOCHLEAR NERVE

• Two branches, vestibular and cochlear branch
• Vestibular nerve: Assists in maintaining equilibrium / balance / posture by contributing towards the balance of linear and angular acceleration from the semicircular canals.
• Cochlear nerve: Carries audition and inner ear impulses (from cochlear). Also known as the nerve of hearing.

9. GLOSSOPHARYNGEAL NERVE

• Mixed nerve
• sensations to pharyngeal mucosa
• posterior 1/3rd tongue (taste sensations)
• Parotid gland (Salivation)
• Stylopharyngeus muscle (motor)
• Maintain blood pressure and levels of CO2/O2 in the blood (sensory fibres)
• Aids/modulates swallowing (motor)

10. VAGUS NERVE

• Mixed nerve
• Motor: Muscles of the palate, larynx and pharynx (Swallowing, Speech)
• Sensory: Laryngeal mucosa
• Parasympathetic : Heart , lungs, foregut and mid gut

11. ACCESSORY NERVE

• Nerve fibers from the cranial root  join the vagus nerve.
• They form the recurrent laryngeal nerve which supplies the internal laryngeal muscles.
• Motor fibers supply the sternocleidomastoid and trapezius controlling the head and shoulder movements

12. HYPOGLOSSAL NERVE

• Muscles of the tongue
• Mixed nerve , mainly motor
• tongue movements involved in speech and swallowing