Cranial nerve pathways

Not only are there 12 cranial nerves with unique functions, each cranial nerve also has its own unique pathway from where it begins to where it ends.

All this information can surely make your brain explode, but we are going to break it down for you! Simply put, cranial nerves connect centrally to cranial nerve nuclei which is where cranial motor nerves begin, or where cranial sensory nerves terminate.

Remember, nerve fibers leaving the brain are considered efferent fibers, where nerves fibers entering the brain are considered afferent fibers.

These centrally located nuclei contain the cell bodies of motor and sensory neurons of the cranial nerves, and the nuclei are where these neurons will synapse with the neurons of higher brain centers such as the thalamus and cerebral cortex.

So essentially, these nuclei act as a pit stop for the exchange of signals travelling between higher brain centers such as our cerebral cortex, to all of the structures our cranial nerves go on to innervate, ultimately facilitating the function of the cranial nerves.

This is important as the cranial nerves will carry different types of motor and sensory information, so the nuclei help organize all this information.

In short, cranial nerves can carry two types of motor information, the first being somatic and branchial motor information, and the second being visceral motor or parasympathetic motor information.

Then there are the three types of sensory information, the first being somatic sensory information, the second is visceral sensory information, and the third is special sensory information.

So, we used the analogy before that cranial nerve pathways are like highways, so let’s talk about these highways and how they transmit the information carried along cranial nerves from the brain centrally to our head, neck, and body peripherally.

Let’s begin by talking about the cranial nerves that carry somatic motor fibers and branchial motor fibers, which eventually go on to innervate voluntary muscles.

Of note, the difference between somatic and branchial motor is that branchial motor fibers are just fibers that innervate structures derived from the pharyngeal arches, which are also sometimes called the branchial arches.

So if all information in our nerves is like cars travelling on a road, then it has to come from somewhere. First, there are upper motor neurons found in the motor cortex.

Their axons start in either the left or right cerebral hemisphere, and decussate, or simply put, cross the midline, typically at the same level of their cranial nerve nuclei prior to synapsing.

By decussating, these nerves can then go on to eventually innervate some structure on the contralateral side of the head.

After decussating, they then synapse with the lower motor neuron found in the motor nuclei for each specific cranial nerve.

These nuclei are centers where the upper motor neurons will synapse with the lower motor neurons, thereby connecting the higher functioning centers of the brain with the cranial nerves and their peripheral destinations.

Overall, there are five somatic motor nuclei, and three branchial motor nuclei. Each motor nucleus is made of the cell bodies of the lower motor neuron, and the axons of these lower motor neurons then exit the brainstem to make the motor fiber components of their respective cranial nerves.The cranial nerve will arise from the brainstem near the corresponding nuclei, so when it comes to the somatic motor nerves they arise from the brainstem near the midline because the somatic motor nuclei are located in the medial portion of the brainstem.

To remember this, there is an M in Motor to represent Medial. Then, these cranial nerves carry their motor information to the muscles that they innervate.

The association between a nucleus and the cranial nerve is easy to figure out, as most of the cranial nerve motor nuclei have the same name as the corresponding nerve.

For example, the somatic motor nucleus for the oculomotor nerve is the nucleus of the oculomotor nerve. The other somatic motor nuclei are the nucleus of the trochlear nerve, the nucleus of the abducens nerve, the nucleus of the hypoglossal nerve, and the spinal portion of the nucleus of the spinal accessory nerve which is not in the brainstem but lies in gray matter anterior horns of the upper cervical spinal cord.

When it comes to branchial motor nuclei, we have the motor nucleus of the trigeminal nerve, the motor nucleus of the facial nerve, and the nucleus ambiguus which corresponds to and receives information from the the glossopharyngeal nerve, the vagus nerve, and the spinal accessory nerve.

An example of a general overall motor pathway would be an upper motor neuron from the cortex sending a signal to the motor nucleus of the facial nerve, transmitting the signal to the facial nerve which would act to close your eyes during a horror movie, which very well may be one of these osmosis anatomy videos on the cranial nerves!

Next we have visceral motor or parasympathetic nerve fibers. Now, with visceral or parasympathetic motor neurons the pathway gets a bit tricky.

There are 4 cranial nerves that carry parasympathetic information: 3rd, 7th, 9th and 10th, and the nuclei where the preganglionic nerve fibers come from are called the Edinger-Westphal nucleus, the superior salivatory nucleus, the inferior salivatory nucleus and the dorsal motor nucleus of the vagus, respectively.

These contain the cell bodies of the parasympathetic preganglionic nerves. Their axons travel with the cranial nerve to their respective ganglia, where they synapse with the cell bodies of the postganglionic portion of the pathway which will go on to provide autonomic innervation to things such as muscles and glands.So, looking at these four pathways, the Edinger-Westphal nucleus sends its preganglionic PNS fibers to synapse in the ciliary ganglion to innervate the sphincter pupillae muscle; the superior salivatory nucleus sends fibers to the pterygopalatine ganglion to innervate glands such as the lacrimal gland, and the submandibular ganglion to innervate salivary glands around the mouth; the inferior salivatory nucleus sends fibers to the otic ganglion to supply the parotid gland; and lastly the dorsal motor nucleus of vagus sends its preganglionic fibers to the cardiac, pulmonary, and myenteric ganglia.

As a note, don’t confuse ganglia with the nuclei. Even though both are made of cell bodies, nuclei are in the brainstem, while ganglia are enlargements more peripherally along the cranial nerve.

Think of them like rest stops along the cranial nerve highway! Now it’s time to talk about the sensory information travelling along the cranial nerves, starting with how the sensory system is organized.

It consists of three neuronal groups: the first being primary first order neurons such as those found with the sensory receptors in the skin, whose cell bodies are found within sensory ganglions similar to the dorsal root ganglions found along the spinal cord.

These first order neurons are pseudounipolar, meaning that they have a single axon that has a peripheral process going from the sensory receptor, and an additional central process going to sensory nuclei in the brainstem.

Second order neurons then carry that information from the sensory nuclei in the brainstem to synapse in processing areas of the brain such as the thalamus.

Then, finally, the third order neurons carry information from areas like the thalamus to the cortex.Now, somatic sensory fibers carry information about pain, temperature and tactile sensation as well as proprioception from the skin of the head, the temporomandibular joint, and associated muscles.

To understand this pathway better, let's use the trigeminal nerve as an example as it is the major cranial nerve responsible for conveying sensory information.

It carries most of the somatic sensory fibers from the head, and logically cell bodies of these nerve cells are found in the trigeminal ganglion.

So the information travels from the receptor in the skin via the peripheral process to the cell body in the trigeminal ganglion, then it synapses with the second order sensory neuron in one of two trigeminal sensory nuclei: the chief (or principal) sensory nucleus that receives tactile sensation, or the spinal trigeminal nucleus that receives pain and temperature sensation.

Information may also go to the mesencephalic nucleus that receives proprioceptive sensation. Information then travels from these nuclei to eventually reach the cortex.

Just like how the motor nuclei in the brainstem were found most medial, the somatic sensory nuclei in the brainstem are more lateral.

Ok, that sums up the somatic sensory information. Now we can take a look at visceral sensory information, which is somewhat similar to somatic sensory information, but mainly conveys fibers from mucosa of the pharynx, chemoreceptors, and thoracoabdominal viscera.

But again, we typically have the same sensory system organisation with first, second, and third order neurons. There are two cranial nerves involved with detecting visceral sensory information, the Glossopharyngeal nerve and the Vagus nerve.

Just like with the trigeminal nerve, each of them has its own ganglion where pseudounipolar nerve cell bodies are located, but unlike the trigeminal nerve and trigeminal ganglion, these ganglia don’t always have the same names as the nerves.

So first off, visceral sensory innervation from the oropharyngeal mucosa, as well as pressure receptors from the carotid sinus and chemoreceptors from the carotid body, travel via peripheral processes that make up the glossopharyngeal nerve, whose cell bodies are found in the petrosal or inferior ganglion of the glossopharyngeal nerve and travel to the nucleus of the solitary tract.

Next, visceral sensory information from the the mucosa of the hypopharynx and larynx; chemoreceptors of the aortic bodies; and thoracoabdominal viscera travels through the peripheral processes that make the vagus nerve, whose cell bodies are found in the inferior ganglion of the vagus nerve and through its central processes ends up - you guess it!

In the nucleus of the solitary tract! After synapsing in the nucleus of the solitary tract, second order neurons take this information to the thalamus, where third order neurons eventually carry this information to the cortex.

Now, it is time to go through our last subject for this video - special visceral sensory information, which is the area of expertise for six cranial nerves:the Olfactory, Optic, Facial, Vestibulocochlear, Glossopharyngeal, and Vagus nerve.

Recall that the first two nerves, Olfactory and Optic, are not really cranial nerves but rather they are brain projections, which is why their anatomy is somewhat different.So, let’s start with the Olfactory nerve.

As with other sensory nerves there are first order neurons, but this time their bodies are not in a ganglion. They are bipolar, meaning that they have 2 extensions: an axon and a dendrite.

Their cell bodies are found in the olfactory epithelium in the nasal cavity. They receive the information through dendrites which are also in the olfactory epithelium.

Their axons travel from the epithelium and make the olfactory nerve fibers, which pass through the cribriform plate of the ethmoid bone and synapse with the second order neurons, called mitral cells in the olfactory bulb.

Now axons of the second order neurons make the olfactory tract which goes to the parts of the brain called the amygdala and uncus which are involved in olfactory processing and emotions.Now it’s time for the Optic nerve.

Just like with the Olfactory nerve, primary first order neurons are bipolar. They are found in the part of the eye called the retina, where they synapse with the second order sensory neurons called ganglion cells.

Now, axons of the ganglion cells form the optic nerve, which eventually synapse in the lateral geniculate body of the thalamus and end up in the primary visual cortex of the brain.Another famous special sense is taste.

This information travels with three different cranial nerves depending on the area where the taste buds are located. So taste from the anterior two thirds of the tongue and the palate travel through peripheral processes that course with the facial nerve, whose cell bodies are found in the geniculate ganglion, which then proceed through its central process to synapse in the nucleus of the solitary tract.

Special sensory information from the taste buds on the posterior third of the tongue will travel with the glossopharyngeal nerve, whose cell bodies are found in the inferior, or petrosal ganglion and travel to the nucleus of the solitary tract.

Finally, other taste buds in the area of the epiglottis and pharyngeal region travel with the vagus nerve, where again the cell bodies are found in the inferior ganglion of the vagus nerve and whose central processes end up in the nucleus of the solitary tract!And finally, now for the Vestibulocochlear nerve; this cranial nerve travels through the internal acoustic meatus, where it separates in two nerves: the vestibular nerve and cochlear nerve.

The Vestibular nerve carries information about motion and position from the semicircular canals and vestibular apparatus.

Once again there are bipolar nerve cells, whose cell bodies are located in the vestibular ganglion. The peripheral processes of these neurons receive information from the hair cells in the vestibular apparatus, and the central processes continue as the vestibular nerve to end up in the vestibular nuclei in the medulla and pons.

The Cochlear nerve is somewhat similar, but carries auditory information from the hearing receptors in the cochlea. Again there are bipolar nerve cells whose peripheral processes receive information from hair cells in the cochlear duct, their cell bodies located in the cochlear ganglion or spiral ganglion, and the central processes continue as the cochlear nerve to end up in the cochlear nuclei in the medulla.

As a quick break, can you remember which cranial nerves carry motor innervation, sensory innervation, or both? What about parasympathetic or special sensory innervation?

Alright as a quick recap. Cranial nerves are composed of efferent fibers leaving the brain, and afferent fibers entering the brain, both which carry information either to or from central nuclei mostly located in the brainstem.

Nuclei are clusters of cell bodies found centrally in the brainstem, whereas ganglia are nerve swellings containing clusters of cell bodies found along the peripheral nerves.

These cranial nerves carry many types of information. Cranial nerves carry somatic motor and branchial motor information which go on to innervate voluntary muscles.

They can carry visceral or parasympathetic motor neurons to provide autonomic innervation to things such as muscles and glands.

They also convey somatic sensory information about the outside world, sensing things such as pain and temperature, as well as visceral sensory information which carries sensory information from areas such as the pharynx mucosa and thoracoabdominal viscera, and finally they have special sensory functions which transmit information on things such as taste, smell,