Cranial Nerves: Origin, Sites of Innervation and Functions, Practice Problems, and FAQs

We all know that the brain is the master organ in the human body that controls and coordinates all body activities. But how does it do so? By communicating with the different parts of the body with the help of nerves which carry neuron fibres that help in transmitting impulses to and from the brain.

What happens when you are walking down the street and someone calls you from the back? You hear the sound, stop and turn your head to see who called and if it's your friend, your face will break into a smile to greet him or her. All these events occur within a matter of minutes and are well coordinated by your brain with the help of a set of nerves that arise from the brain itself and innervate the regions of the face, neck and torso. These nerves are known as the cranial nerves.

In fishes and amphibians, 10 pairs of cranial nerves are present. But in humans, 12 pairs of cranial nerves are present. Let us discuss each of these 12 pairs of nerves in detail.

Table of contents

Introduction
Origin and sites of innervation of cranial nerves
Functions of cranial nerves
Major complications associated with cranial nerves
Practice Problems
FAQs

Introduction

The human nervous system is divided into a central nervous system comprising the brain and the spinal cord and a peripheral nervous system which consists of all the nerves arising from and coming to the central nervous system. The peripheral nervous system is further divided into a somatic nervous system consisting of the cranial and spinal nerves arising from the brain and spinal cord, respectively, and the autonomic nervous system which consists of nerves innervating the visceral organs and responsible for controlling the involuntary actions of the body.

The 12 pair of nerves connected to the brain are known as the cranial nerves. These cranial nerves transmit electrical signals between the brain and the regions of the face and neck. These nerves aid in taste, smell, hearing, and sensations and also help in the movement of the tongue, eyes, and facial expressions.

The cranial nerves emerge from the brain above the level of the atlas which is the first vertebrae of the vertebral column. Cranial nerves have intracranial paths within and extracranial paths outside the skull. They leave the skull through many holes present in it called foramina. Each cranial nerve is paired and occurs on both the sides of the body.

The 12 pairs of cranial nerves are described using Roman numerals I–XII based on the order of their emergence from the brain and brainstem, from front to back. These cranial nerves also have special names, for example, the cranial nerve that transmits impulses of smell from the nose to the brain is called the olfactory nerve.

These cranial nerves can be of three types:

Sensory or afferent nerves carrying sensory nerve fibres that transmit impulses from the receptors to the CNS.
Motor or efferent nerves carrying motor nerve fibres that transmit impulses from the CNS to the effector organs (muscles or glands).
Mixed nerves that carry both sensory and motor nerve fibres.

The given table lists the names of the various cranial nerves in the order of their numbering and the type of nerve that they are. An easy mnemonic to remember the type of the 12 pairs of cranial nerves is ‘ Some Say Money Matters But My Brother Says Big Books Matter More’. Refer to the table for a better understanding.

Fig: 12 pairs of cranial nerves

Origin and sites of innervation of cranial nerves

The cranial nerves arise from different parts of the brain or brainstem and innervate various effector organs depending on if they are motor or mixed nerves. These sites of origin and innervation can be summarised as given below.

Olfactory nerve

It originates from the olfactory mucosa of the nose and terminates at the olfactory bulb of the brain.

Optic nerve

It originates from the retina of the eye and terminates at the visual cortex of the brain.

Oculomotor nerve

It originates from the midbrain and innervates four out of the six eyeball muscles and the ciliary muscles of the eyes.

Trochlear nerve

It originates from the backside of the midbrain and terminates at the superior oblique muscle of an eye.

Trigeminal nerve

The trigeminal nerve has three branches -

Ophthalmic
Maxillary
Mandibular

The ophthalmic and maxillary branches carry sensory nerve fibres whereas the mandibular branch has both sensory and motor nerve fibres. The sensory fibres of the ophthalmic branch carry impulses from the eyeball and the sensory fibres of the maxillary branch carry impulses from the upper jaw, upper teeth, cheeks and lower eyelids. The sensory and motor nerve fibres of the mandibular branch innervate the teeth and gums of the lower jaw, lower lip, chin, tongue and the muscles of mastication. The sensory nerve fibres from these branches converge at a trigeminal ganglion which contains the cell bodies of the sensory fibres coming into the brain. It serves as a sensory root which enters the brainstem at the level of the pons and adjacent to this point of entry, at the same level, a motor root of the trigeminal nerve emerges.

Abducens nerve

It is a motor nerve that originates from the pons region of the brainstem and innervates the lateral rectus muscles of the eyeball.

Facial nerve

It is a mixed nerve that arises from the pons of the brainstem. The sensory nerve fibres originate from the taste buds on the anterior part of the tongue and the motor nerve fibres innervate the superficial muscles of the face, submaxillary and sublingual salivary glands.

Auditory nerve

It has a cochlear branch and a vestibular branch. These two branches of the auditory nerve originate from separate areas of the brain. The cochlear portion originates from the inferior cerebellar peduncle of the brain, whereas the vestibular portion originates from the pons and medulla.

The sensory fibres present in the auditory nerve originate from the vestibular region (utriculus and sacculus), ampulla (semicircular canals) and organ of Corti (cochlea) of the inner ear.

Glossopharyngeal nerve

It originates from the medulla oblongata of the brain stem. The sensory fibres of the nerve arise from the taste buds on the posterior part of the tongue and lining of pharynx and the motor nerve fibres innervate the muscles of pharynx and the parotid glands.

Vagus nerve

It originates from the medulla oblongata and is a mixed nerve whose sensory and motor nerve fibres innervate the pharynx, oesophagus, larynx, trachea, thoracic and abdominal viscera.

Spinal Accessory nerve

It originates from the medulla oblongata and innervates the skeletal muscles in the neck region.

Hypoglossal nerve

It originates from the medulla oblongata and innervates the muscles of the tongue.

Fig: Cranial nerves

Functions of cranial nerves

Each cranial nerve has its own specific function and is listed below:

Olfactory nerve

The olfactory nerves are responsible for transmitting sensory information about smell to the brain. A person inhales the aromatic molecules, such as the molecules of scent that dissolve in the moist lining of the nasal cavity. This moist lining of the nasal cavity is known as the olfactory epithelium. As a result, the receptors are stimulated that generate nerve impulses. These signals are transmitted to the olfactory bulb via the olfactory nerve. It is an oval-shaped structure that houses specialised nerve cells. Nerves travel from the olfactory bulb into the olfactory tract, which is found beneath the frontal lobe of the brain. The nerve signals are then received by the areas of the brain that are associated with memory and smell recognition.

Optic nerve

The optic nerve is one of the sensory nerves that are responsible for vision. There are two types of cells present in the retina, rods and cones cells. The light enters the eye and interacts with these special receptors. Rod cells are highly sensitive to light and are present in large amounts. They specialise in night vision. On the other hand, cone cells are present in small amounts. They are involved in colour vision because they have low sensitivity toward light.

The rods and cones cells receive sensory information from the retina and transmit it to the optic nerve. Both the optic nerves meet inside the skull and this forms an optic chiasm. At this point, two separate optic tracts are formed by the nerve fibres from each half of the retina. The nerve impulses finally go through each optic tract to the visual cortex, which then interprets the data. The visual cortex is situated in the back of the brain.

Oculomotor nerve

The oculomotor nerve begins in the brainstem, specifically in the front portion of the midbrain and reaches to the area of the eye sockets. This nerve performs two motor functions:

Muscle function
Pupil response

Muscle function

The oculomotor nerve provides motor function to four muscles out of the six muscles around the eyes. These muscles support eye movement and focusing on objects at different distances.

Pupil response

The oculomotor nerve also aids in regulating the size of the pupil when it responds to light.

Trochlear nerve

The trochlear muscle is responsible for regulating the superior oblique muscles of the eyeball. These muscles control the downward, inward, and outward movements of an eye. It originates from the back portion of the midbrain.

Trigeminal nerve

The trigeminal nerve is mainly involved with sensation of touch, pain, temperature and controlling muscles movements needed for chewing, biting and swallowing.

Ophthalmic division

The ophthalmic division transmits sensory data from the scalp, forehead, eye orbit, eyelids, conjunctiva of eye, lachrymal glands, nose, nasal mucosa and meninges of the brain.

Maxillary division

This division transmits sensory data from the cheekbones, upper lip, upper gum, palate lining and nasal cavity, which are in the centre of the face.

Mandibular division

The mandibular division has sensory as well as motor function. It transmits sensory data from the chin, lower lip, gums and teeth of the lower jaw, parts of the external ears, parts of the meninges. It also regulates the motor functions of muscles involved with mastication in the mouth and the muscles in the middle ear which helps in dampening loud sounds produced due to biting, chewing, etc.

Abducens nerve

This nerve originates in the pons region of the brainstem and enters the eye sockets, where it regulates the lateral rectus muscle. The lateral rectus muscle is associated with eye movement. This muscle also controls the outward eye movements.

Facial nerve

The facial nerve originates in the pons area of the brainstem where it possesses both sensory as well as motor roots. The two nerve fibres arising from these roots meet eventually and form a facial nerve. It provides sensory as well as motor functions, such as

Muscle movements involved in facial expressions
Providing a sense of taste
Supply salivary glands and tear-producing glands in the head or neck area
Sending sensations from the outer ear

Auditory nerve

The auditory nerve has sensory functions that involve hearing and balance. It is composed of two parts:

Cochlear branch
Vestibular branch

Cochlear branch

The sensory nerve fibres of the cochlear branch of the auditory nerve arise from the sensory receptor cells present in the organ of Corti of the cochlea in the inner ear. They carry sound impulses from the organ of Corti to the brain for interpretation.

Vestibular branch

The sensory nerve fibres of the vestibular branch of the auditory nerve arise from the sensory receptor cells present in the utricle and saccule of the vestibular region and the ampulla of the semicircular canals in the inner ear. As the head moves from side to side while we are in rest or in motion, the fluid in the inner ear also moves and vibrates. These vibrations serve as stimuli for the receptor cells which generate impulses and send those to the brain via the sensory nerve fibres of the vestibular branch of the auditory nerve.

The cerebellum of the brain interprets the nerve impulses and helps the body to attain equilibrium while at rest or in motion. Utricle senses the vertical movements and saccule senses the sideways movement of the head while we are at rest. The semicircular canals sense movements while we are in motion.

Glossopharyngeal nerve

The medulla oblongata, a region of your brainstem, is where the glossopharyngeal nerve begins. It gradually penetrates the throat and neck region. It has both, motor as well as sensory functions, such as:

Transmits sensory information from the back of the throat, the inner ear, the sinuses, and the back of the tongue.
Provides a sense of taste to the back part of the tongue.
Stimulates voluntary movements of a muscle that is present in the back side of the throat to help in swallowing.
Stimulates secretion of saliva from parotid salivary glands.

Vagus nerve

Out of all the cranial nerves, the vagus nerve is the only one that has the longest pathway. It extends from the head to the abdomen. It originates from the medulla and is considered a very diverse nerve. It possesses both sensory as well as motor functions, such as:

Transmits sensory information from the ear canal to the throat.
Transmits sensory information from the internal organs of the thoracic and abdominal cavities, such as the heart and intestines.
Allows motor control of the throat’s muscles.
Helps in stimulating the muscles that are involved in the food movement through the digestive tract.

Fig: Vagus nerve

Spinal accessory nerve

The accessory nerve is a type of motor nerve that regulates the muscles of the neck region. These muscles allow the neck and shoulder to rotate, flex and extend.

Hypoglossal nerve

The hypoglossal nerve is the 12th cranial nerve that allows the movement of most of the muscles present in the tongue.

Major complications associated with cranial nerves

Some conditions or injuries can happen that damage the part of the brain where cranial nerves are situated. These disorders or injuries that affect the cranial nerves are listed below:

Amyotrophic lateral sclerosis (ALS)
Bell’s palsy
Trigeminal neuralgia
Hemifacial spasm
Stroke

Amyotrophic lateral sclerosis (ALS)

It is a condition that worsens with time and causes muscle paralysis due to degeneration and breakdown of the motor neurons of the cranial and spinal nerves..

Bell’s palsy

In this condition, sudden muscle weakness occurs and half of the face starts drooping due to inflammation and swelling in the facial nerve which can be caused by viral infections.

Trigeminal neuralgia

Contact between a normal blood vessel and the trigeminal nerve at the base of the brain, puts pressure on it, causing it to malfunction. This results in chronic painful sensations, similar to that of an electric shock, on one side of the face.

Hemifacial spasm

During this condition, involuntary contractions occur on one side of the face. This can be caused due to injury to the facial nerve, tumours or contact between a blood vessel and a facial nerve.

Stroke

During a stroke, interruptions to the blood supply occurs in the brain because of the presence of blood clots or ruptured blood vessels.

Practice Problems

Q1. Identify the correct statement regarding cranial nerves.

In humans, 21 pairs of cranial nerves are present
Only 9 cranial nerves originate from the brainstem
Cranial nerves exit the skull through apertures called foramina
Optic nerves originate from the midbrain

Solution: Cranial nerves are nerves that originate in the brain. They exit the skull through openings known as foramina. In humans, there are 12 pairs of cranial nerves. Hence, the correct option is c.

Q2. Which structure is not included in the brainstem?

Midbrain
Medulla oblongata
Pons Varolii
Cerebellum

Solution: The distal portion of the brain is known as the brainstem. It houses the medulla oblongata, the pons, and the midbrain (mesencephalon). Cerebellum, a component of the hindbrain, is not a part of the brainstem. Hence, the correct option is d.

Q3. Identify the smallest cranial nerve.

Trochlear nerve
Optic nerve
Olfactory nerve
Vagus nerve

Solution: The trochlear nerve is the smallest cranial nerve out of the 12 pairs. This is the only nerve that originates from the dorsal part of the brainstem. Hence, the correct statement is a.

Q4. Identify the cranial nerve that is responsible for vision.

Olfactory nerve
Optic nerve
Accessory nerve
Auditory nerve

Solution: The optic nerve is one of the sensory nerves that are responsible for vision. There are two types of cells present in the retina, rods and cones cells. The light enters the eye and interacts with these special receptors. Rod cells are highly sensitive to light and are present in large amounts. They specialised in night vision. On the other hand, cone cells are present in small amounts. They are involved in colour vision because they have low sensitivity toward light. Hence, the correct option is b.

FAQs

Q1. Which cranial nerve is the longest one?
Answer: The vagus nerve is the longest cranial nerve that extends from the head to the neck, thorax, and abdomen regions.

Q2. Which cranial nerve is responsible for regulating speech?
Answer: The hypoglossal nerve is responsible for regulating speech via hyoglossus, intrinsic, genioglossus, and styloglossus muscles. These muscles aid in speaking, swallowing, and movement of substances in the mouth.

Q3. How can cranial nerves be differentiated from spinal nerves?
Answer: The cranial nerves originate from the brain and the brainstem, whereas the spinal nerves originate from the spinal cord. Moreover, the cranial nerves transmit information between the brain and the human body. On the other hand, the spinal nerves transmit signals from the spinal cord to the body.

Q4. Which cranial nerve regulates the heartbeat?
Answer: The vagus nerve is the longest cranial nerve in the human body that controls the heartbeat.