Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
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.
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!
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.
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.