Cranial Nerves: What’s hidden in our brain?

We do dozens of jobs every day. But have you ever thought about what is going on in our brains while we are doing all this? The answer is no for most of us. Because it has become a kind of habit. For example, when we see a lemon, our mouth waters, but we don’t think about why. Today we are going to talk about the cranial nerves that play an important role in the functioning of our brain and body.

What are cranial nerves?

Cranial nerves are the main nerves in our body that allow communication between our sensory organs (and to some extent our internal organs) and the brain. All of these nerves start from the brain stem and the brain. These cranial nerves are all paired. Unlike many other nerve pathways, these nerves (except IV) do not cross. Each person has 12 pairs of these nerves. However, some scientists claim that the “terminal nerve” is also a cranial nerve. Many researchers have confirmed that there are 13 pairs of cranial nerves. However, in this article we will assume that there are 12 pairs.

Olfactory Nerve (I)

Let’s start with the olfactory nerve. The olfactory nerve plays a fundamental role in the olfactory process. It transmits chemical odors perceived by special sensory cells located in the nasal mucosa to the brain. This allows us to recognize environmental odors, such as the smell of food, the smell of flowers or any smell. The olfactory nerve is made up of thin fibers that extend into the nasal mucosa where odors are detected. These fibers receive information from the receptor cells that detect odors and pass through the nose to transmit this information to the brain. The fibers of the olfactory nerve connect to an area called the olfactory bulb, an area where odors are processed and recognized. The olfactory bulb is the first point in the brain to begin processing odor information. Unlike other cranial nerves, the olfactory nerve is only involved in olfactory function. While other cranial nerves perform various tasks, such as transmitting sensations from different parts of the head and neck, the olfactory nerve has a specialized sensory function. Damage or loss of the olfactory nerve can lead to loss of the ability to smell. This condition is called “anosmia” and can often be caused by head injuries, nerve diseases or problems in the nasal cavity.

Optic Nerve (II)

The optic nerve initiates the process of vision by transmitting light and image information from the eyes to the brain. Light and color information is collected from images perceived by the lens and retina and transmitted to the brain by the optic nerve. Each optic nerve starts from the retina at the back of the eye and collects visual information from the optic nerves at the back of the eye. This information crosses over at a point called the optic chiasma, where the optic nerves meet, and then continues along the optic pathways that carry visual information to the brain. This information may seem confusing. This is because we said that only the IVth cranial nerve crosses. The crossing that the optic nerve makes is a partial crossing. The optic nerve makes it possible for humans to perceive the world around them and to form images. Information from both eyes is transmitted separately to the brain via the optic nerves and the brain combines this information to create depth perception and a three-dimensional image. However, the optic nerve can be damaged by certain diseases such as “glaucoma”. Damage to this nerve leads to permanent vision loss.

Oculomotor Nerve (III)

The oculomotor nerve controls the eye muscles and thus regulates the movement, focus and position of the eye. It affects two different groups of eye muscles: the extraocular muscles (outer eye muscles) and the inner eye muscles. This allows the eye to track objects, focus and move in different directions. The oculomotor nerve affects six eye muscles (lateral rectus, medial rectus, inferior rectus, superior rectus, superior oblique and inferior oblique). These muscles control eye movements up, down, inward and outward. This nerve exits the brain stem and follows a long path to the eye muscles. Along this path, it controls eye movements through various neurons. Research has shown that this nerve is also involved in pupil reflexes. Damage or problems with the oculomotor nerve can lead to impaired eye movements. This can cause symptoms such as double vision (diplopia), the inability of the eyes to focus properly or the inability of the pupils to react normally.

Trochlear Nerve (IV)

The trochlear nerve is responsible for controlling the eye muscles and in particular for contracting an eye muscle called the upper oblique muscle. This muscle performs the function of rotating the eye downwards and inwards. This nerve, like many other cranial nerves, is made up of neurons and fibers. These fibers extend from the brain stem (mesencephalon) to the eye muscles and control the movements of the eye. There are also several features that distinguish this cranial nerve from others. For example, the trochlear nerve is the only cranial nerve that crosses. It is the cranial nerve with the fewest axons. It is the only nerve that leaves the brain stem from the dorsal side. Damage to this nerve is called “Fourth Nerve Palsy”. Although this damage is usually congenital (49%), hypertension (18%) and trauma (18%) can also be effective.

Trigeminal Nerve (V)

The trigeminal nerve carries sensory information such as touch, pain and temperature from the face, head and mouth. It also provides control of the masticatory muscles. It therefore plays an important role in functions such as chewing, speech and facial expressions. This cranial nerve is divided into 3 main branches:
a. Ophthalmic branch (V1), which includes the eye and forehead area.
b. The maxillary branch (V2), which includes the cheek and nose area.
c. Mandibular branch (V3), which includes the lower jaw region.
This is where it gets its name from. Tri means 3 in Latin and geminal (remember Gemini, the zodiac sign) means twins. The ophthalmic and maxillary branches are purely sensory, but the mandibular branch also contributes to motor functions. The trigeminal nerve is the most complex cranial nerve. One of the most serious diseases arising from this nerve is trigeminal neuralgia. People with this disease have damage to one or more branches of the trigeminal nerve. This damage causes very severe pain, usually in the lower part of the face.

Abducens Nerve (VI)

The abducens nerve innervates the muscle of the eye called the lateral rectus. This muscle allows the eye to move outwards. The abducens nerve is therefore responsible for the eye’s ability to look outwards. It is a somatic efferent nerve. Damage or dysfunction of the abducens nerve is known as “sixth nerve palsy”. This can lead to restricted or difficult outward vision and diplopia. This type of paralysis can occur as a result of head injuries, nerve inflammation or other neurological problems.

Facial Nerve (VII)

The facial nerve is the VIIth cranial nerve, which has critical functions such as controlling various muscles of the head and regulating facial expression and gestures. This important nerve is responsible for creating facial expressions such as smiling, frowning and directing facial muscles in our daily lives. Research has shown that this nerve also innervates the lacrimal gland, producing tears. The facial nerve originates from the “facial nerve nucleus” at the bottom of the brain and is transmitted by nerve fibers that extend from the brain stem to various parts of the head to send signals to the facial muscles. It is located on both sides of both faces and acts symmetrically on the facial muscles on both sides. In addition, the facial nerve receives taste sensations from the anterior two-thirds of the tongue via the chorda tympani. Taste sensations are sent to the gustatory part of the solitary nucleus (upper part). However, a condition known as facial nerve palsy can cause weakness or irregularities in the facial muscles as a result of damage or inflammation of this nerve, which can affect facial expressions and impair daily activities such as speaking, eating and blinking, and may require medical intervention.

Vestibulocochlear Nerve (VIII)

The vestibulocochlear nerve or VIIIth cranial nerve is a cranial nerve that carries the senses of hearing and balance. The hearing function is responsible for transmitting hearing information to the brain by converting sound waves into electrical signals in the structure of the inner ear called the cochlea. It also functions to maintain balance; it monitors the position and movement of the head through the vestibular system in the inner ear, helping the body to maintain balance. This nerve goes to the middle portion of the brainstem called the pons. Damage to this nerve can lead to problems with balance in dark places, dizziness and motion sickness. This is often caused by infection or serious head injuries.

Glossopharyngeal Nerve (IX)

The glossopharyngeal nerve (IXth cranial nerve) performs several tasks. It receives afferent sensory information about taste from the back 1/3 of the tongue. It also receives visceral sensory information from the carotid sinus and carotid trunk (these structures detect arterial blood pH, temperature, pressure and regulate blood pressure). The glossopharyngeal nerve provides parasympathetic stimulation to the parotid gland (the major glands of salivary secretion) via the otic ganglion. When it is damaged, taste cannot be obtained from the 1/3 behind the tongue and swallowing problems occur.

Vagus Nerve (X)

The vagus nerve, or cranial nerve X, is a nerve containing both sensory and motor fibers that acts as an important bridge between the central and peripheral nervous system. It is the longest nerve of the autonomic nervous system in the human body. This nerve exerts various effects on the internal organs of the body, contributing to the regulation of digestion, respiration, heartbeat, immune system, bowel movements and other automatic functions. It is also involved in the ability to speak by controlling the vocal cords. The vagus nerve is also part of the parasympathetic nervous system and is responsible for relaxation from stress situations. This nerve also has an effect on the immune system and regulates immune responses. Damage to the vagus nerve can lead to very serious digestive problems called gastroparesis.

Accessory Nerve (XI)

The accessory nerve (cranial nerve XI) is a cranial nerve that provides motor control of some muscles in the head and neck. It regulates turning and tilting the head, raising the shoulders and pushing the head backwards. The accessory nerve has two main branches between the brain stem and the spinal cord, so it is considered both a cranial and a spinal nerve. Its damage can lead to weakness or paralysis of these muscles, usually as a result of trauma, surgery or nerve-related diseases. Treatment may include nerve rehabilitation, physical therapy and muscle strengthening exercises, severe damage may require surgery.

Hypoglossal Nerve (XII)

The hypoglossal nerve is known as the XIIth cranial nerve and is an important cranial nerve that controls the movement of the tongue. Originating in the brain stem, this nerve innervates the muscles in the tongue and plays a key role in regulating basic functions such as speaking, swallowing and chewing. Damage or dysfunction of the hypoglossal nerve can lead to loss of control of the tongue, tongue curvature or muscle weakness, which can cause speech disorders, swallowing difficulties and other tongue-related dysfunctions. It also ensures the smooth passage of food during the swallowing process and supports the correct movement of the tongue during speech.

Sources & Attributions

Andrewmeyerson, CC BY-SA 3.0, via Wikimedia Commons
Patrick J. Lynch, medical illustrator, CC BY 2.5, via Wikimedia Commons
Romano, N., Federici, M. & Castaldi, A., CC BY 4.0, via Wikimedia Commons
Romano, N., Federici, M. & Castaldi, A., CC BY 4.0, via Wikimedia Commons
The original uploader was Btarski at English Wikipedia., CC BY-SA 3.0, via Wikimedia Commons
Patrick J. Lynch, medical illustrator, CC BY 2.5, via Wikimedia Commons
Yeh, F. C., Panesar, S., Fernandes, D., Meola, A., Yoshino, M., Fernandez-Miranda, J. C., … & Verstynen, T. (2018). Population-averaged atlas of the macroscale human structural connectome and its network topology. NeuroImage, 178, 57-68., CC BY-SA 4.0, via Wikimedia Commons
OpenStax College, CC BY-SA 3.0, via Wikimedia Commons


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