Cranial nerves are the 12 nerves that emerge directly from the brain. 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:
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).
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.
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.
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.
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.
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.
The abducent nerve is 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.
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.
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.
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.
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.
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.
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.
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.
- Special visceral efferent (SVE) fibres. 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.
- General visceral afferent (GVA) fibres. They carry general sensations of distension and ischemic pain from viscera.
- Special visceral afferent (SVA) fibres. 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.
- 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.
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