Cranial nerves

Definitions & Key takeaways

The cranial nerves are a set of 12 nerves that originate in the brain and the brainstem. They supply the muscles and skin of the head, and some of the muscles within the neck, and provide some visceral innervation in the chest and abdomen. The cranial nerves are numbered I through XII, starting with the nerve closest to the brainstem. The olfactory nerve (I) is unique in that it doesn't supply any muscles or skin; it's responsible for smell perception. The remaining 11 cranial nerves supply various muscles and skin in the head, as well as some mucous membranes and glands. Each nerve has a specific function, such as tasting food (II), controlling blinking (III), or supplying sensation to part of the face (V).

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Introduction0:00–0:17

The cranial nerves consist of 12 pairs of nerves originating directly from the brain and brainstem. They supply both motor and sensory information from the brain to other parts of the body, primarily supplying the region of the head and neck.

Nervous system0:17–1:04

The nervous system is divided into the central nervous system, so the brain and the spinal cord, and the peripheral nervous system, which is further divided into the somatic and the autonomic nervous systems.
Broadly speaking, the nervous system is split into an afferent and an efferent division. The afferent division brings sensory information - like vision, smell, touch, and proprioception - from the outside world into the brain.
On the other hand, the efferent division brings motor information from the brain to the periphery, ultimately resulting in contraction of skeletal muscles to trigger movement through the somatic nervous system, as well as the contraction of the smooth muscles to trigger activity of the glands and organs through the autonomic nervous system.
Now, neurons are the main cells of the nervous system. And they’re composed of a cell body, which contains all of the cell’s organelles, and when there’s a group of neuron cell bodies found in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies that are located outside of the central nervous system is called a ganglion.

Neurons1:04–1:49

Neurons also have nerve fibers that extend out from the neuron cell body - these are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.
Where two neurons come together is called a synapse, and that’s where one end of an axon releases neurotransmitters, further relaying the signal to the dendrites or directly to the cell body of the next neuron in the series.Now, the peripheral nervous system consists of nerves, which are enclosed bundles of axons that connect the central nervous system to every other part of the body.

Nerves1:49–3:20

Some nerves are purely sensory or purely motor, while most nerves are mixed. Nerves are either classified as cranial nerves - which are ones that exit from the skull and innervate the head and neck - or spinal nerves, which exit from the spinal cord and innervate the rest of the body.
Now we’re going to focus on cranial nerves. There are twelve cranial nerve pairs that are numbered based on the order in which they arise from different nuclei in the brain, except for cranial nerves eleven and twelve, which are inverted.
In order from one to twelve, the cranial nerves are called olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal.
And a quick way to remember them is with the acronym “On Old Olympus’ Towering Top A Finn Van German Viewed A Hop.” The cranial nerves exit the skull through different openings or foramina.
The primary functions of the cranial nerves - so sensory, motor, or both for mixed - can be remembered with the acronym “some say marry money, but my brother says big brains matter more.” The first cranial nerve, the olfactory nerve, arises from the olfactory neurons in the nasal cavity, which has specific receptors that can bind to a variety of odor molecules.

CN1: olfactory nerve3:20–3:55

They send tiny sensory nerve fibers through the cribriform plate of the ethmoid bone of the skull to synapse with neurons in the olfactory bulb.
Fibers from the olfactory bulb neurons form the olfactory tract of the olfactory nerve, which runs to the primary olfactory cortex located at the temporal lobe of the brain.The second cranial nerve, called the optic nerve, actually emerges from the retina of the eye.

CN2: optic nerve3:55–4:58

The optic nerve then passes through the optic canal and meets the optic nerve coming from the other eye at the optic chiasm.
Here, partial decussation happens, meaning that the nasal fibers that carry input from the temporal visual fields of the eye cross to the opposite side.
After decussating, the optic nerve fibers form the optic tracts. So each optic tract carries information from the opposite visual field of each eye.
Then, the optic fibers travel and synapse at different nuclei – some synapse at the suprachiasmatic nucleus found in the hypothalamus and help regulate the sleep wake cycle, some synapse at the pretectal nucleus in the midbrain and help regulate eye reflexes, and most fibers synapse at the lateral geniculate nucleus in the thalamus.
From here, thalamic fibers form the optic radiations, which run to the occipital visual cortex where the brain interprets what you see.

CN3: oculomotor nerve4:58–6:16

There are three purely motor cranial nerves in charge of eye movement: the third cranial nerve, called the oculomotor nerve, the fourth cranial nerve, called the trochlear nerve, and the sixth cranial nerve, called the abducens nerve.
The oculomotor nerve arises from the ventral midbrain, and then it runs through the superior orbital fissure - a foramen in the sphenoid bone of the skull, to enter the orbit.
Here, the oculomotor nerve splits into a superior and inferior branch. The superior branch innervates the levator palpebrae superioris muscle - which raises the upper eyelid – and the superior rectus muscle that elevates the eye.
The inferior branch innervates the inferior oblique, and the inferior and medial rectus muscles, which help elevate and abduct, depress and adduct, or just adduct the eyeball, respectively.
Some fibers of the inferior branch go to the ciliary ganglion, and synapse with autonomic nerve cell bodies. From here, the short ciliary nerves arise, carrying autonomic motor fibers to the sphincter pupillae muscles - which cause pupil constriction or miosis - as well as to the ciliary muscle, which helps with lens accommodation for visual focusing.

CN4: trochlear nerve6:16–6:35

The trochlear nerve arises from the dorsal midbrain and runs around the midbrain, and then follows the oculomotor nerve through the superior orbital fissure to enter the orbit.

CN6: abducens nerve6:35–6:49

The trochlear nerve innervates the superior oblique muscle, which abducts, depresses, and internally rotates the eye. Then there’s the abducens nerve which emerges from the pons and runs through the superior orbital fissure to enter the orbit.

CN5: trigeminal nerve6:49–7:40

The abducens nerve innervates the lateral rectus muscle, which abducts the eye. The fifth cranial nerve, called the trigeminal nerve, emerges from the pons and then travels to the trigeminal ganglion.
From here, three branches arise. The ophthalmic nerve exits through the superior orbital fissure to give sensory information to the skin of the upper eyelid, the nose, the forehead, and the scalp.
The maxillary nerve exits the skull through the foramen rotundum to give sensory innervation to the maxilla, nasal cavity, the palate and the skin of the cheeks.

CN7: facial nerve7:40–8:53

Finally, the mandibular nerve exits the skull through the foramen ovale. It is the largest trigeminal branch and has both motor and sensory fibers.
The motor fibers innervate the chewing muscles, while the sensory fibers innervate the lower lip, the lower teeth, the chin, and the temporal region of the scalp.The seventh cranial nerve, called the facial nerve, emerges from the pons and enters the temporal bone of the skull through the internal acoustic meatus.
Then the nerve runs within the bone and reaches the geniculate ganglion. Here, the facial nerve gives rise to its three intracranial branches: the greater petrosal nerve - which provides autonomic fibers to the lacrimal, nasal, palatine, and pharyngeal glands - the stapedius nerve - which sends motor fibers to the stapedius muscle of the middle ear - and the chorda tympani, which gives sensory innervation to the taste buds of the anterior ⅔ of the tongue.
The remaining part of the facial nerve exits the skull through the stylomastoid foramen. The facial nerve then gives rise to five extracranial branches: temporal, zygomatic, buccal, mandibular, and cervical.
These five branches spread across the face a bit like five fingers of a hand, and innervate the muscles in charge of facial expressions, like smiling or frowning.

CN8: vestibulocochlear nerve8:53–9:13

These muscles are located at the forehead, the nose, around the eyes, at the cheeks, around the lips, and at the chin.The eighth cranial nerve, called the vestibulocochlear nerve, emerges from the pons and exits the skull through the internal acoustic meatus.

CN9: glossopharyngeal nerve9:13–10:08

Then, it splits into two branches: the cochlear nerve – which supplies the hearing receptors in the cochlea – and the vestibular nerve, which supplies the equilibrium receptors in the vestibule.
The ninth cranial nerve, called the glossopharyngeal nerve, arises from the medulla, and exits the skull through the jugular foramen.
Then, it runs down and sends branches to innervate the tongue and pharynx. The glossopharyngeal nerve sends motor fibers to the stylopharyngeus muscle, which elevates the pharynx in swallowing.
It also provides parasympathetic motor fibers to the parotid salivary glands, as well as sensory fibers that convey taste from the posterior ⅓ of the tongue, and general sensory impulses – like touch and pain - from the pharynx.

CN10: vagus nerve10:08–10:53

Additionally, the glossopharyngeal nerve conveys information from chemoreceptors in the carotid bodies - which help regulate respiration by monitoring the levels of oxygen and carbon dioxide in the blood.
It also conveys information from baroreceptors of the carotid sinus, which help monitor blood pressure.The tenth cranial nerve, called the vagus nerve emerges from the medulla, passes through the skull via the jugular foramen, and then dips down into the thorax and abdomen - it’s the only cranial nerve that extends beyond the head and neck.
It sends somatic innervation to the muscles of the pharynx and larynx involved in swallowing, and sends parasympathetic fibers to the heart, lungs, and abdominal organs to help regulate heart rate, breathing, and digestion.

CN11: accessory nerve10:53–11:26

The vagus nerve also brings in sensory information from thoracic and abdominal organs, as well as the baroreceptors of the aortic arch, chemoreceptors in the carotid and aortic bodies, and from taste buds on the epiglottis - yep, the epiglottis has taste buds!The eleventh cranial nerve, called the accessory nerve, gets its name because it is considered an accessory part of the vagus nerve.
The accessory nerve actually forms from rootlets that emerge from the spinal cord, run upward and enter the skull via the foramen magnum.

CN12: hypoglossal nerve11:26–11:46

Then, the accessory nerve emerges from the medulla and exits the skull through the jugular foramen. The accessory nerve innervates the trapezius and sternocleidomastoid muscles, which together move head and neck, and the nerve carries sensory proprioceptive information from these muscles.Finally, the twelfth cranial nerve, called the hypoglossal nerve, arises from the medulla and exits the skull through the hypoglossal foramen.

Review11:46–12:05

Then, this nerve runs below the tongue and sends motor fibers to the muscles of the tongue, and the nerve carries sensory proprioceptive information from the tongue.
Alright, as a quick recap… to help