The peripheral nervous system is the part of the nervous system located outside the brain and spinal cord. It regulates the functions of our body.

The peripheral nervous system has two functional units: afferent (or sensory) and efferent (or motor).

Here, based on the principle of functional autonomy, we can distinguish two more divisions of the PNS:

• Somatic nervous system (that controls voluntary actions and conveys sensory information from the skin, skeletal muscles and joints)
• And the autonomic nervous system (that controls the involuntary activities within the body, as the work of our stomach, for instance).

The anatomical components of the PNS are ganglia (which are the groups of cell bodies) and nerves (which are the bundles of axons running together).

The latter include 31 pairs of spinal nerves (that arise from the spinal cord) and 12 pairs of cranial nerves (that arise from the brain).

Cranial nerves

In this note, we will briefly discuss the cranial nerves (nervi craniales)

To get familiar with these nerves, let’s list all of them in one place.

• The first cranial nerve (CN I), the olfactory nerve (nervus olfactorius) is responsible for olfaction (or the sense of smell)

• The second cranial nerve (CN II), the optic nerve (nervus opticus) is responsible for vision

There are three cranial nerves in charge of eye movement:

• The third cranial nerve (CN III), the oculomotor nerve (nervus oculomotorius)

• The fourth cranial nerve (CN IV), the trochlear nerve (nervus trochlearis)

• The sixth cranial nerve (CN VI), the abducens nerve (nervus abducens)

• The fifth cranial nerve (CN V), the trigeminal nerve (nervus trigeminus) is responsible for mastication and sensation of the facial area

• The seventh cranial nerve (CN VII), the facial nerve (nervus facialis), in charge of facial expressions, sensory innervation to the taste buds of the anterior ⅔ of the tongue, salivation, and lacrimation.

• The eighth cranial nerve (CN VIII), the vestibulocochlear nerve (nervus vestibulocochlearis), supplies the hearing and equilibrium receptors

• The ninth cranial nerve (CN IX), the glossopharyngeal nerve (nervus glossopharyngeus) is responsible for multiple functions. These include speech, swallowing, salivation, sensory innervation of the pharynx and some other structures, sensory innervation to the taste buds of the posterior ⅓ of the tongue, and salivation. It also receives signals from chemoreceptors in the carotid bodies, which help regulate respiration and baroreceptors of the carotid sinus, which help monitor blood pressure.

• The tenth cranial nerve (CN X), the vagus nerve (nervus vagus), is responsible for swallowing, speech, sensory innervation of the pharynx, larynx, thoracic and abdominal organs, as well as the baroreceptors of the aortic arch, chemoreceptors in the carotid and aortic bodies, and from taste buds of the epiglottis.

• The eleventh cranial nerve (CN XI), the accessory nerve (nervus accessorius) innervates the trapezius and sternocleidomastoid muscles, which together move head and neck, and the nerve carries sensory proprioceptive information from these muscles.

• The twelfth cranial nerve (CN XII), the hypoglossal nerve (nervus hypoglossus) sends fibers to the muscles of the tongue, providing its movement.

If we take the first letter of each nerve, we can build a mnemonic to help remember the cranial nerve names:

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Now let’s discuss the terms special, general, somatic and visceral. The information is considered special if it’s related to our special senses (vision, smell, taste, hearing and balance), while general is referred to information from everywhere else. The information carried by a nerve is called somatic if it goes to or from the skeletal muscles and skin, or visceral if it travels to or from our internal organs.

Broadly speaking, the cranial nerve consists of four components:

• Somatic motor component
• Visceral motor component
• Visceral sensory component
• Somatic sensory component

Further, these components can be classified in a more comprehensive manner, but we will discuss this in greater detail in subsequent videos.

Now let’s take a look at the diagram. As we can see, some nerves have only a sensory component, others have only a motor component, and others have both. Thus, all cranial nerves can be divided into purely sensory, purely motor, while most nerves are mixed.

The easiest way to remember if a nerve is sensory, motor or both, is to use this mnemonic trick:

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Cranial nerve nuclei

If the cranial nerve has a sensory component (whether visceral or somatic), then it has a sensory nucleus. If it has a motor component, then it has a motor nucleus. Finally, if it has a visceral component (the motor part, specifically), then it has an autonomic (or visceral) nucleus.

The names of the nuclei are not crucial for now. What we should really pay attention to at this point is the distribution of the nuclei in the brain stem. The midbrain contains the nuclei of the 3rd and 4th cranial nerves, the pons contains nuclei of the 5th, 6th, 7th, and 8th cranial nerves, and the medulla oblongata contains the nuclei of the 9th, 10th, 11th, and 12th cranial nerves (this is the Rule of Fours if we conditionally consider the 1st and 2nd cranial nerves also to be part of the midbrain)

There are several exceptions to this rule, which we will discuss in greater detail in subsequent videos.

Dictionary

Cranial nerves: Overview

Peripheral nervous system

systema nervosum periphericum

Cranial nerves

nervi craniales

Olfactory nerve

nervus olfactorius

Optic nerve

nervus opticus

Oculomotor nerve

nervus oculomotorius

Trochlear nerve

nervus trochlearis

Abducens nerve

nervus abducens

Trigeminal nerve

nervus trigeminus

Facial nerve

nervus facialis

Vestibulocochlear nerve

nervus vestibulocochlearis

Glossopharyngeal nerve

nervus glossopharyngeus

Vagus nerve

nervus vagus

Accessory nerve

nervus accessorius

Hypoglossal nerve

nervus hypoglossus