welcome to section three of Neurology this section is divided into two parts in the first part of this section we'll be introducing cranial nerves and going through several examples and the second part will focus on examples and we will integrate the remaining material so let's get started the first topic is the corticobulbar tract this is figure 8.5 from your text which shows a coronal view of the brain for step 1 you need to know that the corticobulbar tract is a cranial nerve pathway that connects the upper motor neurons to the lower motor neurons remember all motor information originates from upper motor neurons in the primary motor cortex this remains true when we talk about the cranial nerves so an upper motor neuron originates in the primary motor cortex which we'll assume is about right here this region where I'm drawing circles and these neurons send axons down through the brain and through the internal capsule so down through the internal capsule recall from section 1 that the corticobulbar tract passes through the genu of the internal capsule once past the internal capsule the axons decussate just above each lower motor neuron in the brain stem let's imagine that this represents the brainstem we'll say this is the midbrain the pons and the medulla so again the lower motor neurons of the cranial nerves are located within the brainstem which I'll draw here these are also called cranial nerve nuclei it's important to know that many of the cranial nerve nuclei spend large segments of the brainstem so for example while the trigeminal cranial nerve may emanate from the pons the nucleus of the trigeminal cranial nerve may actually be seen in the pons medulla and even part of the cervical spinal cord okay once the axons from the upper motor neurons past the internal capsule decussate directly superior to the cranial nerve nucleus that they innervate so for example the third and fourth cranial nerve nuclei are in the midbrain right third and fourth so the axons innervating these nuclei would cross over right above that region so again this is the left side of the brain and this is the right side of the brain stem so this blue circle would represent the third and fourth cranial nerve nuclei on the right side of the brain stem obviously the opposite side of the brain would do the same thing it would cross over directly above the lower motor neuron that is innervated this same principle holds true for all the cranial nerve nuclei receiving motor information from the primary motor cortex finally it's important to recognize that the corticobulbar tract is quite complicated and lesions of the tract are fairly unpredictable so please don't spend time memorizing all of the intricacies of the cortical bulbar tract if you understand the general idea and the pathway we just covered you'll be well prepared for step one okay with this in mind let's look at cross-sections of the different levels of the brainstem to look at each of the cranial nerve nuclei this is figure eight point one six which shows a transverse section of the midbrain this is anterior and this is posterior for step one you need to be able to recognize a transverse section of each level of the brainstem the midbrain can be easily identified by the cerebral Kerra which are right here I use this to remind me that the midbrain looks like two legs so kind of like this you could also use the substantial which is this dark pigmented region right here or the red nucleus which is this region right here these will both help remind you that you're looking at a section of the brain okay with this in mind let's look at the cranial nerve nuclei in the midbrain again as I mentioned on the previous slide many of the cranial nerve nuclei span large segments of the brainstem however as a general rule of thumb the nucleus of the cranial nerve can be found wherever the peripheral branch emanates from the brainstem so for example the peripheral branches of cranial nerves three and four emanate from the midbrain so we can deduce that the nuclei for these cranial nerves must also be in the midbrain the peripheral branches of cranial nerves five through eight emanate from the pons so we can deduce that the nuclei for these cranial nerves must also be in the pons the peripheral branches of cranial nerves nine through twelve emanate from the medulla so we can deduce that the nuclei for these cranial nerves must also be in the medulla one exception to this rule is the nucleus of cranial nerve 11 the nucleus of cranial nerve 11 resides in the cervical spinal cord but the peripheral branch of this nerve emanates from the medulla finally remember that cranial nerves wanted to do not emanate from the brainstem so in this case we're looking at a section of the midbrain so we should expect the nuclei of cranial nerves three and four to be present here I find it helpful to keep in mind that most of the cranial nerve nuclei are directly adjacent to the cerebral aqueduct or fourth ventricle we'll talk about this in more detail later but recall that the cerebral spinal fluid moves from the third ventricle into the fourth ventricle via the cerebral aqueduct in this image we can see the cerebral aqueduct right here with this logic we'd expect a nuclei for cranial nerves three and four to be close to the cerebral aqueduct which they are from this figure we can see the nucleus of the oculomotor nerve or cranial nerve three right here we can't see the nucleus for the trochlear nerve or cranial nerve 4 because it's directly inferior to the oculomotor nucleus however if we took a cross-section of the midbrain directly below this region we'd have a very similar figure and we'd be able to see the trochlear nucleus for your reference this is a myelin stain of an actual microscopic cross-section of the midbrain for step 1 it's important for you to be familiar with these myelin stains of the brainstem you need to be able to recognize what segments of the brainstem you're looking at and have a general idea of the location of the major landmarks ok now let's look at a cross-section of the pons this is figure eight point one eight which shows a transverse section of the pons again this is anterior and this is posterior the pons can be easily identified by the transverse pontine fibers these little fibers right here if you can't find the transverse pontine fibers then you should still be able to deduce that you're looking at the pons by process of elimination this is because the midbrain and the medulla have a very distinct appearance ok with this in mind let's look at the cranial nerve nuclei in the pons recall that cranial nerves 5 through 8 emanate from the pons remember most of the cranial nerve nuclei are directly adjacent to the cerebral aqueduct or fourth ventricle I'm not going to go through each of the cranial nerve nuclei but I'd like to briefly mention the importance of the solitary nucleus for step 1 you need to know that the solitary nucleus is a large nucleus that spans much of the brainstem and contains sensory information for cranial nerves 7 9 and 10 I use the letter s in the word solitaire to help remind me that this nucleus contains sensory information rather than memorizing all of the details of cross-sections of the brain I find it helpful to use a landmark called the sulcus limit hands as a guide to answering many of the cross-sectional you'll see the sulcus limit ants which is approximately right here is useful because it divides the sensory nuclei from the motor nuclei those nuclei lateral to the sulcus limit ends our sensory and those nuclei medial to the sulcus limit ins our motor I use the letter M in the word medial to help me remember that the nuclei medial to the sulcus limit hands are motor nuclei so in summary if you can identify what section of the brainstem you're looking at and you know that the nuclei located laterally are associated with sensory information then you should be able to get all the step one questions correct that are associated with this topic for your reference this is a myelin stain of an actual microscopic cross section of the pons we can clearly see the transverse pontine fibers right here