Video: Medulla oblongata: Vagus nerve level
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We go about our day-to-day lives without even having to think about our basic functions like breathing or controlling our heart activity. This little structure highlighted in the image here is what... Read more
We go about our day-to-day lives without even having to think about our basic functions like breathing or controlling our heart activity. This little structure highlighted in the image here is what does this for us. A fun structure to say, the medulla oblongata is vital in maintaining these basic functions and we find it located in a structure called the brainstem. As we can see from this image, the medulla oblongata makes up most of the inferior end of the brainstem, and in our tutorial today, we'll explore the various structures that we find within it.
The medulla oblongata is found at the terminal or caudal end of the brainstem. It is situated in the posterior cranial fossa and is continuous with the spinal cord inferiorly and with the pons superiorly. The medulla oblongata is an important structure because it contains important ascending and descending nerve tracts, contains a number of cranial nerve nuclei, and is responsible for controlling important involuntary functions necessary for life such as heart rate, breathing, and blood pressure. This is because the cardiac, respiratory, vasomotor, and vomiting centers are located in the medulla oblongata.
In this tutorial, we'll be discussing the medulla oblongata as seen in cross-section at the level of the vagus nerve. So, imagine slicing through the medulla oblongata at the level of the vagus nerve and it will look like the illustration on the left of the screen from a superior view. We'll explore in detail the various nuclei and tracts that can be found in this brainstem structure at this level.
First, we'll begin by discussing all of the nuclei that are visible at this level giving a brief description of their function. These nuclei include the medial vestibular nuclei, the posterior nucleus of the vagus nerve, the hypoglossal nucleus, Roller's nucleus, the cuneate nucleus, the spinal trigeminal nucleus, the raphe nucleus, the nucleus ambiguus, the lateral reticular nucleus, and the various olivary nuclei we find.
After we have explored the nuclei, we will discuss the various tracts that are traversing the medulla oblongata, and these tracts will include the solitary tract, the inferior cerebellar peduncle, the spinal tract of the trigeminal nerve, the medial longitudinal fasciculus, the spinocerebellar tract, the spinothalamic tract, the medial lemniscus, and the pyramidal tracts. We'll also discuss the reticular formation which is composed of both nuclei and tracts.
Pictured here is the brainstem with the medulla oblongata highlighted in green. Recall that the brainstem is composed of the midbrain, the pons, and the medulla oblongata with the cerebellum located just posterior to these structures. It is apparent from this image that the medulla oblongata is the most caudal or most inferior portion of the brainstem. This picture provides us with an anterior inferior view of the brainstem where we can see almost all of the cranial nerves extending outward from the brainstem.
From superior to inferior, we have cranial nerves one, two, three, four, five, six, seven, eight, nine, ten, eleven, and twelve. The vagus nerve is cranial nerve ten. It is at this particular level that we'll be looking deep inside the medulla oblongata.
This is a cross-sectional image of the medulla oblongata at the level of the vagus nerve and we'll be going in depth on all of the structures that we see here. To orient ourselves here, our ventral or anterior surface of the medulla oblongata is here and the dorsal or posterior surface is here. Now before we dive into the various structures, let us refresh our memories on the difference between a nucleus and a tract in the central nervous system or the CNS.
A nucleus is a collection of cell bodies found in the CNS. We can see the various nuclei in the medulla oblongata pulsing in this image. Nuclei receive various information and also send out information to other parts of the central nervous system or to the rest of the body. This information travels along tracts which are collections of axon fibers that travel within the central nervous system. Nerves are the same thing, but communicate with the body outside of the CNS. We'll also find that tracts within the central nervous system can also be called lemnisci or fasciculi.
Now that we've covered the basics, let's dive into the various nuclei we find in this image, starting with this one. Highlighted here in green is the posterior nucleus of the vagus nerve, also known as the dorsal nucleus of the vagus nerve. This nucleus contributes parasympathetic fibers to the vagus nerve pointed out here. These particular fibers mainly supply vagal parasympathetic innervation to its targets like the GI tract, lungs, and vagal innervations in the abdomen and thorax. The remainder of the fibers of the vagus nerve come primarily from the nucleus ambiguus.
Just as the name suggests, this nucleus is ambiguous in that it supplies efferent fibers to more than one cranial nerve. In this image, we can see the nucleus ambiguus here highlighted in green and how it traverses through the medulla. If we take a look at it in cross-section here, we find that the nucleus ambiguus is a deeper nucleus found slightly lateral to the posterior nucleus of the vagus nerve which we looked at previously.
The nucleus ambiguus houses cell bodies that supply efferent fibers to cranial nerves nine, ten, and eleven. With respect to the vagus nerve, it contributes motor fibers to the larynx and the pharynx and together with the posterior nucleus, their fibers compose the vagus nerve.
Now that we've examined the nuclei that make up the vagus nerve, let's discuss the remainder of the nuclei found at this level starting at the midline and working our way outwards. The most medial nuclei that we find are the raphe nuclei which are the three tiny highlighted nuclei in this image. In reality, there are more than three raphe nuclei and the cluster of numerous raphe nuclei are found throughout the reticular formation which we will discuss further on in this tutorial. The raphe nuclei contains serotonergic neurons that are a major supply of serotonin to the central nervous system and act in both arousal and pain modulation.
As we move posterior to the raphe nuclei, we find Roller's nucleus seen here highlighted in green. This nucleus is also known as the sublingual nucleus or perihypoglossal nucleus due to its close proximity to the hypoglossal nucleus which we'll be discussing next. Roller's nucleus is primarily responsible for receiving afferent information from the vestibular complex, the cerebral cortex, and other oculomotor nuclei. Its efferent fibers project outward to other nuclei that are responsible for innervating extrinsic muscles of the eye and two areas of the cerebellum, thus the nucleus is responsible for movements of the eyes in relation to other visual stimuli, for example, fixating on a single stimulus while movement of either the stimulus or the individual changes.
Just dorsal to the Roller'nucleus, we have the hypoglossal nucleus. It houses the cell bodies that are responsible for the fibers that contribute to cranial nerve twelve - the hypoglossal nerve - which emerges one level below the previous nucleus we just examined. If we take a look at this image here, we see that the hypoglossal nucleus highlighted in green does in fact span multiple levels of the brainstem as do the majority of the nuclei that we're discussing today. This is why we can see the same nucleus at multiple brainstem levels. However, the actual nerve fibers generally emerge from the brainstem at only one level. The hypoglossus nucleus is responsible for sending motor efferents to the muscles of the tongue along with several muscles associated with the hyoid bone.
Next, we approach the medial vestibular nucleus highlighted here. This large nucleus is actually a part of a complex of the vestibular nuclei that receive afferents from cranial nerve eight - the vestibulocochlear nuclei. The information it receives helps this nucleus aid in both head and body orientation.
The cuneate nucleus is found just lateral to the medial vestibular nucleus at this level. The cuneate nucleus receives sensory afferents from the ipsilateral upper limbs regarding light touch, proprioception, and vibration senses. These afferents synapse within the cuneate nucleus before decussating and descending in the medial lemniscus of the brainstem, which will be discussed later in this tutorial.
The next nucleus we'll look at is the spinal trigeminal nucleus. This image here shows us a dorsal view of the brainstem giving us a view of the breadth of the cranial nerve nuclei and the nerve fibers. Highlighted in green is the spinal trigeminal nucleus and as we can see, it spans the majority of the pons, the medulla, and traverses down into the spinal cord, hence, this nucleus' name.
Looking at it in cross-section at the vagus nerve level, the spinal trigeminal nucleus is located ventral to the cuneate nucleus. This nucleus is primarily responsible for receiving the sensory afferents from the fifth cranial nerve - the trigeminal nerve - and carries information on touch, vibration, pain, and temperature from the face.
Neighboring the spinal trigeminal nucleus is the lateral reticular nucleus seen here highlighted in green, and this particular nucleus acts as an integrating nucleus receiving input from a variety of sources. It then takes the information it receives and sends it out to the ipsilateral half of the cerebellum. Thus the lateral reticular nucleus functions in aiding the cerebellum with motor planning and coordination.
The final nuclei that we'll be discussing today are the olivary nuclei. These include the dorsal accessory olivary nucleus, the inferior olivary nucleus, and the medial accessory olivary nucleus. So the first one that we'll look at is the inferior olivary nucleus highlighted here in green, and this particular olivary nucleus is great to use as a landmark to find the other nuclei at this level of the brainstem as its convoluted C-shaped structure is quite prominent. This particular nucleus has numerous connections with the cerebellum and is responsible for aiding the cerebellum in motor coordination.
Now if we look just dorsal and medial to the inferior olivary nucleus, we find the accessory olivary nuclei. The accessory olivary nuclei are highlighted in green in this image. The dorsal accessory olivary nucleus and the medial accessory nucleus are found dorsal and medial to the inferior olivary nucleus respectively.
Now that we've covered all of the nuclei at the vagus nerve level of the medulla oblongata, we can begin exploring the various tracts and fasciculi that we find at this level. We'll discuss the general location of these fasciculi at this level, the type of information we find in these tracts, and where they're heading to.
So one of the largest and most pronounced tracts that we find at this level are the pyramidal tracts. They're located most ventrally and are what give rise to the pyramids we see on the medulla oblongata externally and they're highlighted in green in this image. The pyramidal tracts contain descending motor fibers from the cerebral cortex that are traveling to synapse on the motor neurons within the ventral horns of the spinal cord.
The medial lemniscus is found adjacent to the midline and dorsal to the pyramidal tracts at the vagus nerve level and we can see it highlighted in green in this image. This particular tract carries fibers from the gracile and cuneate nuclei which provide somatosensory information from the lower and upper limbs, respectively. The medial lemniscus will ascend the entirety of the brainstem where it will synapse on the thalamus and eventually reach the primary somatosensory cortex.
Located just dorsal to the medial lemniscus and in close proximity to the raphe nuclei is the medial longitudinal fasciculus highlighted in green. This particular tract carries information regarding how the eyes should be moving relative to a visual stimulus. It acts to connect the nuclei of cranial nerves three, four, and six with higher order brain structures.
Moving laterally from the medial longitudinal fasciculus, we find the inferior cerebellar peduncle here neighboring and almost sitting on top of the cuneate nucleus. This contains the white matter fibers that are projecting to and from the spinal cord and medulla to and from the cerebellum along with the fibers traversing in the opposite direction. This peduncle contains fibers that are responsible for carrying sensory information regarding proprioception. It also contains emerging fibers from the cerebellum that are responsible for the maintenance of posture and balance.
As we move ventral to the inferior cerebellar peduncle, we run into the spinal tract of the trigeminal nerve highlighted in this image. Recall that the spinal nucleus of the trigeminal nerve is located right here, thus since the spinal tract of the trigeminal nerve carries sensory information from the face, these afferent fibers are traveling into the nucleus. This is why this tract is in such close proximity to its nucleus.
Continuing our journey to the lateral and ventral portions of the medulla oblongata, we find the spinocerebellar tract located just anterior to the lateral reticular nucleus. Like the name of the tract infers, the spinocerebellar tract contains fibers coming from the spinal cord and heading to the cerebellum. It carries sensory information regarding muscle tension that is unconscious to us.
And lastly located medially and ventral to the spinocerebellar tract is the spinothalamic tract highlighted in green. This particular tract carries cutaneous pain and temperature information about the contralateral side of the body. The spinothalamic tract ascends through the brainstem where it synapses within the thalamus before terminating in the somatosensory cortex.
The last tract that we'll explore in our tutorial is the solitary tract. The solitude tract contains a number of afferent fibers that are carrying sensory information from the facial, glossopharyngeal, and vagus cranial nerves. Thus, the solitary tract acts to convey information about taste and sensory information from the GI tract and thorax.
Now that we've covered the tracts that run through the medulla oblongata at the vagus nerve level, let's touch upon one more structure we find that is a mixture of both nuclei and fiber tracts at this level. The reticular formation highlighted here in green occupies most of the inner portions of the medulla oblongata and is composed of numerous nuclei and ascending and descending fibers. It actually spans throughout the entire brainstem and is not just situated in the medulla oblongata.
The reticular formation carries a variety of information up to the cerebral cortex and is primarily responsible for our state of consciousness or awareness. In achieving this function, the reticular formation can modulate which sensory signals or other information reaches the thalamus, the cerebrum, and several other neural structures and it's been shown that damage to the reticular formation can lead to states of coma and a lack of awareness.
Now that we've covered all of the necessary structures at the vagus nerve level in the medulla oblongata, let's touch upon one clinical syndrome that can impact a number of nuclei and tracts at this level.
Medial medullary syndrome, also known as Dejerine syndrome, arises when there is an infarct in the anterior spinal artery or vertebral arteries that prevent blood flow through the medullary arteries to the underlying medial medulla.
Let's go back and take a look at the medulla in cross-section. The anterior spinal artery supplies the pyramidal tract and medial lemniscus at the level of the vagus nerve. If blood supply to these regions is impaired, a person can exhibit symptoms related to these tracts and these symptoms include weakness of the trunk and limbs on the contralateral side due to loss of blood to the medial portions of the pyramidal tract and the patient would also experience light touch, vibration, and proprioception sensory loss on the contralateral body side due to the loss of blood supply to the medial lemniscus. If lower portions of the medial medulla are also affected, there's a ipsilateral paralysis of the tongue due to damage to the cranial nerve twelve nucleus.
Alright so that brings us to the end of our tutorial. To recap, let's quickly list off the structures we discussed and you can use this as a tool to check off what you have or have not covered.
So first we looked at several nuclei found at the vagus nerve level of the medulla oblongata. We discussed the medial vestibular nuclei, the posterior nucleus of the vagus nerve, the hypoglossal nucleus, Roller's nucleus, cuneate nucleus, spinal trigeminal nucleus, raphe nucleus, nucleus ambiguus, lateral reticular nucleus, and the various dorsal and medial accessory olivary nuclei as well as the inferior olivary nucleus.
We then touched upon the tracts that traverse through the medulla oblongata - the inferior cerebellar peduncle, the spinal tract of the trigeminal nerve, the medial longitudinal fasciculus, the spinocerebellar tract, the spinothalamic tract, the medial lemniscus, and the pyramidal tracts.
To finish off the tutorial, we examined the reticular formation composed of both nuclei and fiber tracts and touched upon medial medullary syndrome and its clinical manifestation.
And that wraps up our tutorial on the medial oblongata at the level of the vagus nerve.
Thanks for joining me and happy studying!