Cranial Nerves and Their Functions

Cranial nerves are the second part of the somatic nervous system within the peripheral nervous system that protrudes from the brainstem. There are 12 pairs of these nerves and they are denoted with Roman numerals, as I to XII (1). The division of these nerves is made based on the functions each of them has.

Classification of Cranial Nerves

These are the twelve cranial nerves
and their functions:

• I olfactory nerve - a bundle of nerves that convey the sense of smell to the central nervous system
• II visual nerve – these transmit visual stimuli from the retina of the eye and are in charge of vision
• III, IV and VI nerve – control the movements of eyes, trochlear nerve, nerve arrester – these nerves are used to move the eyeball
• V Triangular Nerve - it consists of three nerves and is used to receive information from the facial area and innervation of the chewing muscles
• VII facial nerve – it is responsible for innervation of facial musculature muscles, receiving sensory information from 2/3 of the tongue and innervating the salivary and sensory glands
• VIII tremor - “snail” nerve - conveys auditory information (hearing) and is responsible for the sense of balance and orientation in space
• IX lingual nerve - receives information from the last 1/3 of the tongue and innervates the parotid gland
• X nerve vagus - innervates the musculature responsible for ingestion and digestion, controls the muscles that create the voice and provides parasympathetic fibers that innervate the heart and smooth muscles of the respiratory system
• XI accessory nerve - enters the X nerve composition of the nerve vagus
• XII sublingual nerve - responsible for the innervation of the lingual musculature (2).

The Olfactory nerve

This nerve is not a nerve in the narrow sense, but a brain shoots because its neurons are not only located in the CNS but also in the periphery, in this case in the nasal part of the mucous membrane of the nasal cavity (and these are bipolar cells).

Some neurons transmit their axons towards the roof of the nasal cavity. The axons cluster into nerve bundles (approximately 20 axons per 1 bundle) which then pass through the lamina cribrosa ossisethmoidalis and approach the bulbus olfactorius.

After passing through the bulbus olfactorius, the fibers pass through the tractusolfactorius and they divide into trigonum olfactorium.

The Optical Nerve

This is not a nerve in the narrow sense, but also a brain shoot because its neurons are also not located only in the CNS, but can be found in the periphery too. In this case, they are located in the retina.

Four parts can be distinguished, including pars bulbaris, pars orbitalis, pars intracanalicularis, and pars intracranialis. Pars bulbaris is inside the eyeball and has 4 subdivisions. Those are pars praelaminaris, pars laminaris, pars postlaminaris, and pars orbitalis.

The Oculomotorius Nerve

After exiting the brain, the nerve located between the processusclinoideus anterior and posterior pierces the dura mater and enters the sinus wall of the cavernosus. In this sinus, we can also identify the IV and VI nerve, and nerve oculomotorius is positioned above both of these nerves in the sinus.

The innervation of the eyeball muscles is the most accurate innervation of the muscles in the body (due to the small ratio of neurons entering the motor board. The ratio is 1: 1.

The Trochlearis Nerve

This is the cranial nerve with the longest course in the subarachnoid space. After exiting the brain, the nerve pierces the dura mater between the processusclinoideus anterior and posterior. It enters the sinus wall of the cavernosus.

In fissure orbitalis superior, this nerve is located above the III cranial nerve, even though it is located below it in the sinus. We can also say that the innervation of the eyeball muscles is the most accurate innervation of the muscles in the body thanks to the small ratio of neurons entering the motor board.

The Trigeminus Nerve

This is the largest cranial nerve. Overall, it consists of 2 parts: portio major and portio minor, emerging from the pons. In impressiotrigemina there is a ganglion called semilunar ganglion that corresponds materially to the spinal ganglia, with primary neurons in it.

The motor part of the nerve has nothing to do with the ganglia. Three large branches are separated from the ganglia: nerve ophthalmicus, nerve maxillaris and nerve mandibullaris. These are the sensory branches.

The nervusophthalmicus has only
general sensory fibers, just like the nervusmaxillaris. On the other hand, the
mandibullaris nerve has both sensory and motoric fibers.

The Abducens Nerve

This nerve has a trajectory of passage through the brain in the caudal end of the pons. Positioning in the subarachnoid space is c. basalis. The puncture of the dura mater is localized to the inferior petrosal sinus. The passage through the base takes place at the fissura orbitalis superior.

After exiting the brain, the nerve pierces the dura mater at the level of the inferior petrosal sinus and it enters the sinus cavernosus (but does not enter the wall). This is the cranial nerve with the longest intracranial flow.

The Facial Nerve

Both parts of the facial nerve (motor fibers nervus intermedius) pass through the meatus acusticus internus and enter the canalisfacialis as a common branch. It is very important to point out that, in the canal, the nerve is firstly oriented laterally, then turns 90° backwards, forming a knee, geniculum n. facialis.

In that spot, there is also a ganglion geniculi. Canalisfacialis crosses above the eardrum and reaches the foramen of the stylomastoideum where the nerve protrudes from the skull.

The Vestibulocochlearis/Statoacusticus nerve

This nerve has two parts: radix cochlearis for the hearing organ and radix vestibularis for the organ of balance. The radix cochlearis has fibers that originate from bipolar cells and come from the organ of Corti. Peripheral extensions are located in the ganglion spirals of cochleae modiolus, while the central neurons form bundle of tractus spiralis foraminosus.

Radix cochlearis and radix
vestibularis are surrounded by a common envelope and they enter the meatus
acusticus internus into the cranial cavity in the pontocerebellar angle (the
cochlear part of this nerve is dorsal and the vestibular part is ventral).

Radix vestibularis has fibers that start from bipolar cells and the pericardial / peripheral extensions are located in the ganglion vestibulare, while the central neurons form radixvestibularis.

The Glossopharyngeus Nerve

After the fibers exit the medulla oblongata, they unite and pass through f. jugular. Before passing, the nerve forms the ganglion superius (ganglion jugulare), and after the passage, they form the ganglion inferius. In ganglion superius there are taste fiber cells, and in ganglion inferius (which is located in the fossulapetrosi), there are cells of viscerosensitive fibers (3).

The Nervusvagus

This is the largest parasympathetic nerve and antagonist to the sympathetic system. After the fibers exit the medulla oblongata, they unite and pass through f. jugular. Before passing, the nerve forms the ganglion superius, and after the passage the ganglion inferius.

In the ganglion superius, there are exteroceptive fibers (sensory fiber cells from dura mater and the outer ear), while in the ganglion inferius, there are viscerosensitive and palatable fibers (3).

The Accessorius Nerve

This nerve contains the radix
spinalis and the radix cranialis. The radix spinalis has multipolar cells that
lie on the lateral edge of the anterior horn of medulla spinalis. Resuscitation
cells of m. sternocleidomastoideus are located in the upper segments. Cells
exit from medulla spinalis and join into a common bundle passing through the
foramen magnum and enter the skull.

After passing through f. jugulare,
fibers of radix cranialis (now called r. internus) join nervusvagus in the
innervation of the muscles of the pharynx and larynx (3).

The Hypoglossus Nerve

This nerve is responsible for the
tongue muscles. On its path, this nerve forms an arch, called
archusnervushypoglossi. It is joined at one instant by fibers from C1 and C2
that participate in the formation of the ans cervicalis profunda by separating
later as radix superior (branches for the infrahioid muscle) and fused with
radix inferior (branches from C2 and C3) (3).

Conclusion

Cranial nerves are nerves that come from our brain or the brain stem. This is opposite to the spinal nerves, which come from parts of the spinal cord. Cranial nerves transmit information between the brain and parts of the human body, especially to and from the head and the neck area. 

Each cranial nerve is paired and is
present on both sides of the brain. Based on the definition, in humans, there
are twelve pairs of cranial nerves, marked with Roman numerals I-XII for
identification. Cranial nerve counts are based on the order in which they
appear or originate from the brain.

References

1. Romano N, Federici M, Castaldi A. Imaging of cranial nerves: a pictorial overview. Insights Imaging. 2019 Mar 15;10(1):33. doi: 10.1186/s13244-019-0719-5. PMID: 30877408; PMCID: PMC6420596.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6420596/
2. Sonne J, Lopez-Ojeda W. Neuroanatomy, Cranial Nerve. [Updated 2019 Apr 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-.  Found online at: https://www.ncbi.nlm.nih.gov/books/NBK470353/
3. Gillig PM, Sanders RD. Cranial Nerves IX, X, XI, and XII. Psychiatry (Edgmont). 2010 May;7(5):37-41. PMID: 20532157; PMCID: PMC2882282.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2882282/