Medicine

Cranial nerves in humans consist of twelve pairs of nerves that emerge from the ventral side of the brain. The first two pairs arise from the cerebrum while the other ten pairs emanate from the brainstem. As in the case of spinal nerves which are named based on their origin from the spinal cord, cranial nerves derived their name from their close association with the cranium. The holes or openings in the skull serve as their passage to reach their targets throughout the human body. Some of these nerves send messages from our senses while other cranial nerves control muscle movement, glandular secretions and our internal organs.

A cranial nerve may also be classified as sensory, motor or both according to their mode of signal transmission. Motor nerves carry impulses from the brain to target tissues such as muscles and glandular tissues while sensory nerves transmit impulses from sensory organs to the brain. The table below shows an overview of the cranial nerves, their modality and function.

Table 1.

CRANIAL NERVES	MODALITY	FUNCTION
I. Olfactory	Sensory	Smell
II. Optic	Sensory	Vision
III. Oculomotor	Motor	Eye movement and pupil constriction
IV. Trochlear	Motor	Eye movement
V. Trigeminal	Both	Controls muscles for mastication (motor); somatosensory output from face, head and neck
VI. Abducens	Motor	Eye movement
VII. Facial	Both	Controls muscles for facial expression (motor); sensory input from anterior 2/3 of tongue (taste) and ear (tympanic membrane)
VIII. Vestibulocochlear	Sensory	Hearing and balance
IX. Glossopharyngeal	Both	Controls some muscle for swallowing (motor); sensory input from posterior 1/3 of tongue, tonsil and pharynx
X. Vagus	Both	For motor, sensory and autonomic functioning of the viscera (e.g. digestion, glandular secretion, heart rate)
XI. Spinal Accessory	Motor	Controls muscles used in head movement
XII. Hypoglossal	Motor	Controls muscles of tongue except palatoglossal

 

I. Olfactory Nerve

This nerve functions primarily for the sense of smell. Its name is derived from the Latin words ‘olfactare’, which means to sniff at, and ‘olfacere’, which means to smell. Among all the cranial nerves, this is the only one capable of self-renewal since it has the property to regenerate continually through adulthood. Also, aside from being the smallest nerve, the olfactory nerve doesn’t branch out into left and right entities. Instead, it consists of a collection of sensory rootlets that extend from the olfactory epithelium to the olfactory bulb as it passes through the numerous openings of the cribriform plate in the ethmoid bone. When a person inhales, molecules of air attach themselves to the sensory parts of the olfactory nerve which are located in the olfactory mucosa at the upper part of the nasal cavity. As these nerve endings become stimulated, electrical activity is generated and is transmitted to the olfactory bulb. The cells in the olfactory bulb in turn relay the electrical signal to the brain through the olfactory tract.

II. Optic Nerve

During embryonic development, the optic nerve arises from the diencephalon, thus considered a part of the central nervous system. As with other CNS components, optic nerves are covered with Myelin sheath rather than Schwann cells and are wrapped by meninges. Unlike the olfactory nerve, optic nerves are incapable of regeneration which increases the risk of permanent blindness when severed.

Optic nerve conveys visual information from the retina to the brain. When light reaches the retina, stimulation of the photoreceptors (rods and cones) occurs which triggers the production of electrical impulses. These are relayed to the bipolar cells which contain the optic nerve endings. The bipolar cells in turn transmit the electrical impulses to the CNS via the optic nerve. The optic nerves enter the CNS at the optic chiasm where they become the optic tract. From there, the electric signal is transduced to the lateral geniculate nucleus of each thalamus before going to the visual centers of the brain for interpretation.

III. Oculomotor Nerve

Oculomotor nerve has two components: 1. Somatic motor or general somatic efferent which controls muscles responsible for precise eye movement for visual tracking and fixation on an object and 2. Visceral motor or general visceral efferent provides parasympathetic innervation of the constrictor pupillae and ciliary muscles of the eye which regulate light accommodation by controlling pupil constriction.

The somatic motor component is further divided into inferior and superior divisions. The inferior division innervates the medial rectus, inferior rectus and inferior oblique muscles while the superior division supplies the levator palpebrae superioris and superior rectus muscle. The table below shows a summary of the function of the eye muscles controlled by the somatic motor component.

Table2.

Eye Muscles	Primary action	Secondary action	Tertiary action
Medial rectus	Adduction	–	—
Inferior oblique	Extorsion	Elevation	Abduction
Inferior rectus	Depression	Extortion	Adduction
Superior rectus	Elevation	Intortion	Adduction
Levator palpebrae superioris	Controls movement of the upper eyelid

 

Table adapted from http://www.med.yale.edu/caim/cnerves/cn3/cn3_3.html

The other two eye muscles not included in Table2 are the Superior oblique (Intorsion, Depression, Abduction) and Lateral rectus (Abduction) which are controlled by the Trochlear Nerve (IV) and the Abducens Nerve (VI) respectively.

IV. Trochlear Nerve

This nerve has only a somatic motor component which controls one eye muscle – the Superior oblique. As mentioned above, superior oblique carries out three different functions: intorsion, depression and adbuction.

Though this nerve only supplies one eye muscle, it has several characteristics that distinguish it from the other cranial nerves.

1.This is the only nerve that exit the brain dorsally.

2.This is the only nerve that decussate or cross which makes the nerve fibers supply the superior oblique muscle on the side opposite their origin.

3.This has the longest intracranial route.

4.This has the smallest number of neurons.

V. Trigeminal Nerve

The trigeminal nerve, as the name implies, is consists of three large branches: Ophthalmic and Maxillary which are both sensory, and the Mandibular branch which has both motor and sensory functions. The Ophthalmic branch innervates the forehead and the eye while the Maxillary branch supplies the cheeks. The Mandibular branch on the other hand controls the lower jaw and lower portion of the face. Since this nerve has both sensory and motor functions, it is responsible for sensing facial touch, pain, and temperature and controlling the mandibular muscles during mastication.

The three branches meet in an area called the Gasserion ganglion. The fused branches extends backwards to the brainstem, inserting itself into the pons. Inside the brainstem, the signals that pass through the trigeminal nerve arrive at a structure called trigeminal nerve nucleus. From there, the information is sent to the cerebral cortex where perception of the sensation is achieved.

VI. Abducens

This nerve has only a motor component which innervate the lateral rectus muscle of the eye. To make a lateral movement, inputs from the higher centers of the brain in the form of electrical impulses synapse at the lateral gaze center. From there, the signal travels through the longitudinal fasciculus to reach the abducens nucleus, which then sends the message to the lateral rectus muscle via CN VI , causing the muscle to be abducted.

VII. Facial Nerve

The facial nerve is a mixed nerve having both the sensory and motor components. It has approximately 10,000 neurons in which 70% of these are myelinated and control facial expressions. The other 30% are somatosensory and secretomotor in function. The table below shows the nature and function of the facial nerve.

Table3.

Branchial motor (special visceral efferent)	The largest component of the facial nerve    Supplies voluntary control of the posterior belly of the digastric, stylohyoid and stapedius muscles as well as muscles involve in facial expression such as the buccinator, occipitalis and platysma muscles
Visceral motor (general visceral efferent)	Comprise the parasympathetic constituent of CN VII    Provides parasympathetic control over the submandibular, sublingual, and lacrimal glands   Innervates the mucous membranes of the nasopharynx and hard and soft palates
Special sensory (special afferent)	Consists of fibers that transmit sensation of taste from the anterior 2/3 of the tongue as well as from the hard and soft palates
General sensory (general somatic afferent)	Conveys sensory information from the skin of the concha of the auricle and from a small area of skin behind the ear    Acts as a supplement to the Mandibular component of the Trigeminal Nerve to provide sensation at the wall of the acoustic meatus and the outer surface of the tympanic membrane

 

VIII. Vestibulocochlear Nerve

This is a sensory nerve that consists of two components – vestibular and cochlear. The vestibular nerve detects head and body motion while the cochlear nerve detects sound. Thus, the vestibulocochlear nerve functions for hearing and balance. Balance is achieved by conveying information to the vestibular component of the CN VIII which are done by the semicircular canal and the otolithic organ. The former detects angular acceleration while the latter detects linear acceleration.

For us to hear and recognize the sounds around us, the cochleat component of the CN VIII should be stimulated. This happens when sound waves collected by the outer ear travel through the ear canal and cause vibrations of the ear drum. The vibration of the ear drum moves the bones of the middle ear which in turn pass the vibrations to the fluid-filled cochlea. As the fluid inside the cochlea moves, it causes the cilia to vibrate and stimulate the sensory cochlear nerve. When stimulated, the nerve sends signal to the area of the brain where the signals are processed into the sound we hear.[ad#co-1]

IX. Glossopharyngeal Nerve

As the name implies, CN IX innervates the tongue and the pharynx and has both the motor and sensory components. The functions of each component are summarized in the table below.

Table4.

Branchial motor (special visceral efferent)	Provides voluntary control of the stylopharyngeus muscle which elevates the pharynx during swallowing and speech
Visceral motor (general visceral efferent)	Parasympathetic component which innervates the parotid gland and the smooth muscle of the pharynx, larynx, and viscera of the thorax and abdomen
Visceral sensory (general visceral afferent)	Controls the baroreceptors of the carotid sinus and chemoreceptors of the carotid body
General sensory (general somatic afferent)	Transmits sensation of pain, temperature, and touch from the skin of the external ear, internal surface of the tympanic membrane, the walls of the upper pharynx, and the posterior 1/3 of the tongue
Special sensory (special afferent)	Carries sensation of taste from the posterior one-third of the tongue

X. Vagus Nerve

The term ‘Vagus’ is a Latin word which means ‘wandering’. Vagus nerve fits its name since it wanders from the brainstem and extend to organs found in the neck, chest and abdomen. Table 5 summarizes the vagus nerve’s components and their respective functions.

Table5.

Brancial motor (special visceral efferent)	Supplies voluntary control to the striated muscle of the pharynx and larynx (except for the stylopharyngeus muscle and the tensor veli palatini muscle) as well as the palatoglossus muscle of the tongue
Visceral motor (general visceral efferent)	Parasympathetic control of the smooth muscle and glands of the pharynx, larynx, and thoracic and abdominal viscera down to the splenic flexure resulting to increased GI tract secretion and motility, higher bronchiolar secretions and bronchoconstriction in the lungs and slower heart rate.
Visceral sensory (general visceral afferent)	Transmits sensory information from the larynx, esophagus, trachea, abdominal and thoracic viscera, aortic arch and the aortic bodies
General sensory (general somatic afferent)	Carries sensation of pain, temperature, and touch from from the larynx and pharynx as well as information from the skin of the back of the ear and external auditory meatus, parts of the external surface of the tympanic membrane
Special sensory (special afferent)	Transmits sensation of taste from the epiglottic region

 

XI. Spinal Accessory Nerve

This is a motor nerve consisting of cranial and spinal parts that control muscles of the neck and shoulders to move the head. The cranial part (special visceral efferent) innervates muscles of the pharynx and larynx while the spinal part (special visceral efferent) controls the trapezius and sternocleidomastoid muscles. The trapezius muscle moves the scapula, acts as a support for the arm, assists in breathing, keeps the head in position and produces bending and rotational head movement. The sternocleidomastoid muscle on the other hand flexes the neck and turns the head obliquely.

XII. Hypoglossal Nerve

As indicated in its name, this nerve innervates the muscles found below (hypo) the tongue hence consists only of the motor component. It provides voluntarily control over the three out of four extrinsic muscles of the tongue which include genioglossus, styloglossus, and hyoglossus. Genioglossus muscle is responsible for protruding (or sticking out) the tongue while styloglossus draws up the sides of the tongue to aid in swallowing. The hyoglossus muscle on the other hand retracts and depresses the tongue for chewing and speech.[ad#afterpost]