Cranial nerves are the 12 nerves that emerge directly from the brain, unlike other nerves that originate from the spinal cord. Out of the 12 pairs of the cranial nerves, the initial 2 originate from the forebrain and the next 10 originate from the brainstem. They are numbered 1 to 12 in the craniocaudal sequence of their connection on the brain. The cranial nerves are usually designated by Roman numerals.

The image below shows the 12 cranial nerves at the base of the brain along with their functions.

The 12 pairs of nerves are:

I Olfactory VII Facial II Optic VIII Vestibulo-cochlear III Oculomotor IX Glossopharyngeal IV Trochlear X Vagus V Trigeminal XI Accessory VI Abducent XII Hypoglossal

A tiny bundle of nerve fibres closely related to the olfactory nerves is named the 13th pair or ‘O’ pair of cranial nerves. Its precise function isn’t understood, but it’s believed to give a unique chemo-sensory nerve pathway of olfaction and impacts the secretion of luteinizing hormone-releasing factor from the hypothalamus.

Additionally, it plays an essential part in smell-mediated sex behaviour. Every nerve is connected to the cerebrum, posterior to the olfactory stria of the olfactory tract close to the anterior perforated substance and septal regions and spread to the nasal mucous membrane.

Origin of Cranial Nerves

Cranial nerves originate from cerebrum and different parts of the brain stem. The cranial nerves are numbered based on their location on the cerebrum and brain stem (superior to inferior, then median to lateral) and the order of their exit from the cranium (anterior to posterior).

Cerebrum

The cranial nerves are the 12 paired sets of nerves. The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum and are considered extensions of the main nervous system.

Midbrain

The oculomotor nerve (III) originates from the anterior aspect of the midbrain and from the posterior side of the midbrain the trochlear nerve (IV) originates. The trochlear nerve has the longest intracranial length of all the cranial nerves.

Pons

Four cranial nerves emerge from the pons, they are – trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve (V-VIII).

Medulla Oblongata

Four cranial nerves emerge from the Medulla oblongata, they are – glossopharyngeal nerve, vagus nerve, accessory nerve and hypoglossal nerve (IX-XII).

The 12 Pairs of Cranial Nerves and their Function

Now let us discuss the 12 pairs of cranial nerves individually along with their functions and get a general idea about each of them.

I- Olfactory Nerve

The olfactory nerve is the first of the 12 pairs of cranial nerves and is the shortest of all the pairs. It is originated by the cells of the olfactory bulb. The word olfactory mean something which is related to the sense of smell. The olfactory nerve is in charge of transmitting olfactory stimuli from the nose to the brain.

II- Optic Nerve

The optic nerve is the second of the 12 pairs of cranial nerves which emerge in the diencephalon. It is made of axons from the ganglion cells of the retina, that take the information of the photoreceptors to the brain, later this information is integrated and interpreted by the brain.

III- Oculomotor

The Oculomotor cranial nerve is also called the common ocular motor nerve. The oculomotor nerve originates in the midbrain and is responsible for the movement of the eyes and eyelids.

IV- Trochlear

This cranial nerve has motor and somatic functions. It controls the movement of the superior oblique muscle of the eye, and eyeballs. It has the longest intracranial course but is the smallest amongst all the cranial nerve.

V- Trigeminal

It is a mixed cranial nerve, that is it performs both sensory and motor function. It is the largest of all cranial nerves, carry sensitive information to the face, and convey information for the chewing process.

VI- Abducent

The Abducent nerve or Abducens nerveis also known as the external ocular motor cranial nerve as it is responsible for transmitting the motor stimuli to the external rectus muscle of the eye and thereby allowing the eye to move to the opposite side from where we have the nose.

VII- Facial

The facial nerve is another mixed cranial nerve since it consists of several nerve fibres that perform different functions, like ordering the muscles of the face to create facial expressions, send signals to the salivary and lacrimal glands and also to collects taste information through the tongue.

VIII- Vestibulo-Cochlear

The vestibulo-cochlear nerve is a sensory cranial nerve since it is responsible for balance and orientation in space and auditory function. It is also known as the auditory and vestibular nerve, thus forming vestibulocochlear.

IX-Glossopharyngeal

The Glossopharyngeal nerve collects information from the taste buds (tongue) and sensory information from the pharynx. It leads orders to the salivary gland and various neck muscles that help with swallowing. It also monitors blood pressure.

X-Vagus

Vagus nerve is also known as pneumogastric nerve. It supplies nerves to the pharynx, esophagus, larynx, trachea, bronchi, heart, stomach and liver. Like glossopharyngeal nerve, it influences the action of swallowing, it also sends and transmits signals to our autonomous system, to help the regulate activation and control stress levels or send signals directly to our sympathetic system.

XI-Accessory

Accessory nerve is sometimes also named as the spinal nerve or spinal accessory nerve. It is a motor nerve and controls the movements of the head and shoulders by supplying the sternocleidomastoid and trapezius muscles.

XII-Hypoglossal

The hypoglossal nerve is a motor nerve which, like the vagus and glossopharyngeal, is involved in the movement of tongue muscles, swallowing and speech. It innervates all the muscles of the tongue except palatoglossus muscle which is innervated by the vagus nerve.

Functional Component

Till now we talked about the cranial nerves from a general point of view. Now we will be getting a little bit technical, but before that, you need to get brushed up with some technical terms.

Sensory Function: The sensory system creates our mental images of the external world. These representations offer us with details and clues that guide the motor system to create movements produced by the collaborated contractions and relaxations. Motor Function: A motor nerve carries command out of the central nervous system (CNS) and toward effectors (muscles or glands) that executes the commands. Afferent nerve: Afferent nerve fibers refer to axonal projections that arrive at a particular region. Efferent nerve: efferent nerve fibers refer to axonal projections that exits the region. Somatic: The term somatic is related to the movement of the body. It is mainly involved with receiving and responding to information from the external environment. Visceral: The term visceral is related to the internal organs of the body. It is mainly concered with detecting and responding to information from the internal environment. General Somatic Afferents: Somatic sensory neurons carry information from the periphery into the CNS and are also called somatic sensory afferents or general somatic afferents (GSAs). General Somatic Efferents: Somatic motor fibers carry information away from the CNS to skeletal muscles and are also called somatic motor efferents or general somatic efferents (GSEs)

Now lets come back to the functions of the cranial nerves. A cranial nerve is composed of motor fibres (motor nerve) or sensory fibres (sensory nerve) or both the motor and sensory fibres (mixed nerve). The table below shows the functional component associated with each cranial nerve.

Functional component Abbreviation General function Cranial nerves containing component General somatic afferent GSA Perception of touch, pain, temperature Trigeminal nerve [VI facial nerve [VII]; glossopharyngeal nerve [IX]; vagus nerve [X] General visceral afferent GVA Sensory input from viscera Glossopharyngeal nerve [IX]; vagus nerve [X] Special afferent SA Smell, taste, vision, hearing, and balance Olfactory nerve [1]; optic nerve [II]; facial nerve [VII]; vestibulocochlear nerve [VIII]; glossopharyngeal nerve [IX]; vagus nerve [X] General somatic efferent GSE Motor innervation to skeletal (voluntary) muscles Oculomotor nerve [III]; trochlear nerve [IV]; abducent nerve [VI]; accessory nerve [XI]; hypoglossal nerve [XII] General visceral efferent GVE Motor innervation to smooth muscle, heart muscle, and glands Oculomotor nerve [III]; facial nerve [VII]; glossopharyngeal nerve [IX]; vagus nerve [X] Branchial efferent BE Motor innervation to skeletal muscles derived from pharyngeal arch mesoderm Trigeminal nerve [VI facial nerve [VII]; glossopharyngeal nerve [IX]; vagus nerve [X]

The motor fibres of cranial nerves can be of the following 3 types:

Somatic efferent (SE) or general somatic efferent (GSE) fibres . They supply the striated muscles which grow from somites.

or . They supply the striated muscles which grow from somites. Special visceral efferent (SVE) fibres . They supply the muscles which grow from the mesoderm of pharyngeal arches.

. They supply the muscles which grow from the mesoderm of pharyngeal arches. General visceral efferent (GVE) fibres. They supply the glands, smooth muscles of viscera and vessels. They can be preganglionic parasympathetic fibres.

The sensory fibres of cranial nerves can be of the following 4 types:

General somatic afferent (GSA) fibres . They carry general sensations of pain, feel and temperature from skin and proprioceptive sensations of shaking and muscle and joint perception.

. They carry general sensations of pain, feel and temperature from skin and proprioceptive sensations of shaking and muscle and joint perception. General visceral afferent (GVA) fibres . They carry general sensations of distension and ischemic pain from viscera.

. They carry general sensations of distension and ischemic pain from viscera. Special visceral afferent (SVA) fibres . They carry unique sensations of flavor from tongue.

. They carry unique sensations of flavor from tongue. In addition to the aforementioned 3 types, the sensory fibres could be particular somatic afferent (SSA) which carry unique sensations of smell, hearing and equilibrium.

Key Points

The motor fibres of cranial nerves originate as outgrowths of axons from motor nuclei situated inside the central nervous system (CNS).

The sensory fibres originate as outgrowths of axons from cells situated inside the sensory ganglia (situated outside the CNS) and terminate in the sensory nuclei situated inside the CNS.

The motor and sensory nuclei inside the CNS are arranged in longitudinal columns named functional columns.

Summary

Nerve Afferent Efferent Exit from skull Function Olfactory nerve [I] SA Cribriform plate of ethmoid bone Smell Optic nerve [II] SA Optic canal Vision Oculomotor nerve [III] GSE, GVE Superior orbital fissure GSE — innervates levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles.

GVE — innervates sphincter pupillae for pupillary constriction; ciliary muscles for accommodation of the lens for near vision. Trochlear nerve [IV] GSE Superior orbital fissure Innervates superior oblique muscle. Trigeminal nerve [V] GSA BE Superior orbital fissure

Ophthalmic division [V1]

Foramen rotundum — maxillary nerve [V2]

Foramen ovale — mandibular division [V3] GSA — sensory from: ophthalmic division

[V1] — eyes, conjunctiva, orbital contents, nasal cavity, frontal sinus, ethmoidal cells, upper eyelid, dorsum of nose, anterior part of scalp, dura in anterior cranial fossa, superior part of tentorium cerebelli; maxillary nerve.

[V2] — dura in middle cranial fossa, nasopharynx, palate, nasal cavity, upper teeth, maxillary sinus, skin covering the side of the nose, lower eyelid, cheek, upper lip; mandibular division.

[V3] — skin of lower face, cheek, lower lip, anterior part of external ear, part of external acoustic meatus, temporal fossa, anterior two-thirds of tongue, lower teeth, mastoid air cells, mucous membranes of cheek, mandible, dura in middle cranial fossa.

BE — innervates temporalis, masseter, medial and lateral pterygoids, tensor tympani, tensor veli palatini, anterior belly of digastric, and mylohyoid muscles. Abducent nerve [VI] GSE Superior orbital fissure Innervates lateral rectus muscle. Facial nerve [VII] GSA, SA GVE, BE Stylomastoid foramen

(nerve leaves cranial cavity through internal acoustic meatus and gives rise to branches in the facial canal of the temporal bone prior to exiting through the stylomastoid foramen; these branches leave the skull through other fissures and canals.) GSA — sensory from part of external acoustic meatus and deeper parts of auricle SA—taste from anterior two-thirds of tongue.

GVE — innervates lacrimal gland, submandibular and sublingual salivary glands, and mucous membranes of nasal cavity, hard and soft palates.

BE — innervates muscles of face (muscles of facial expression) and scalp derived from the second pharyngeal arch, and stapedius, posterior belly of digastric, stylohyoid muscles. Vestibulocochlear nerve [VIII] SA (Nerve leaves cranial cavity through internal acoustic meatus) Vestibular division, balance Cochlear division and hearing. Glossopharyngeal nerve [IX] GVA, SA, GSA GVE, BE Jugular foramen GVA — sensory from carotid body and sinus.

GSA — posterior one-third of tongue, palatine tonsils, oropharynx, and mucosa of middle ear, pharyngotympanic tube, and mastoid air cells.

SA — taste from posterior one-third of tongue.

GVE — innervates parotid salivary gland.

BE — innervates stylopharyngeus muscle. Vagus nerve [X] GSA, GVA, SA GVE, BE Jugular foramen GSA — sensory from larynx, laryngopharynx, deeper parts of auricle, part of external acoustic meatus, and dura in posterior cranial fossa.

GVA — sensory from aortic body chemoreceptors and aortic arch baroreceptors, esophagus, bronchi, lungs, heart, and abdominal viscera of the foregut and midgut.

SA — taste from the epiglottis and pharynx.

GVE — innervates smooth muscle and glands in the pharynx, larynx, thoracic viscera, and abdominal viscera of the foregut and midgut.

BE — innervates one tongue muscle (palatoglossus), muscles of soft palate (except tensor veli palatini), pharynx (except stylopharyngeus), and larynx. Accessory nerve [XI] GSE Jugular foramen Innervates sternocleidomastoid and trapezius muscles. Hypoglossal nerve [XII] ' GSE Hypoglossal canal Innervates hyoglossus, genioglossus, and styloglossus muscles and all intrinsic muscles of the tongue.

Mnemonics

On Old Olympus’s Towering Top, A Fine Veteran German Viewed A House

On Olfactory 1st cranial nerve Old Optic llnd cranial nerve Olympus's Oculomotor lllrd cranial nerve Towering Trochlear IVth cranial nerve Top Trigeminal Vth cranial nerve A Abducens Vlth cranial nerve Fine Facial Vllth cranial nerve Veteran Vestibulocochlear Vlllth cranial nerve German Glossopharyngeal IXth cranial nerve Viewed Vagus Xth cranial nerve Some Accessory Xlth cranial nerve House Hypoglossal Xllth cranial nerve

Some Say Marry Money But My Bride Says Big Balls Matter More

Some Sensory 1st cranial nerve Say Sensory llnd cranial nerve Marry Motor lllrd cranial nerve Money Motor IVth cranial nerve But Both Vth cranial nerve My Motor Vlth cranial nerve Bride Both Vllth cranial nerve Says Sensory VIllth cranial nerve Big Both IXth cranial nerve Balls Both Xth cranial nerve Matter Motor Xlth cranial nerve More Motor Xllth cranial nerve

On Old Olympus’ Towering Top, A Finely Vested German Viewed A Hawk

On Olfactory 1st cranial nerve Old Optic llnd cranial nerve Olympus' Oculomotor lllrd cranial nerve Towering Trochlear IVth cranial nerve Top Trigeminal Vth cranial nerve A Abducens Vlth cranial nerve Finely Facial Vllth cranial nerve Vested Vestibulocochlear Vlllth cranial nerve German Glossopharyngeal IXth cranial nerve Viewed Vagus Xth cranial nerve A Accessory Xlth cranial nerve Hawk Hypoglossal Xllth cranial nerve

Ooh, Ooh, Ooh, to Touch and feel very good velvet. Such heaven!