Chapter 5: Discussion Questions

Ben Schottenstein

Chapter 5  –   Getting Ahead

1.) Why are the trigeminal and facial cranial nerves both complicated and strange in the human body?

The trigeminal and facial cranial nerves at first observation appear to just be a mass of tangled muscle and nerves, seeming to be chaotic and without order, but they actually demonstrate “a pattern that reveals the order in what initially seems chaotic” (Shubin 82). Cranial nerves are those coming directly from the brain, 12 arising from the base of the brain to be exact, and these nerves are employed to control facial actions such as chew, talk, and move our eyes and head. The majority of these nerves are rather simple to track, and their path from the brain to the various muscles and areas of function that they are to control is can be easily seen. Such is the case with the cranial nerve that goes to our nasal structures, the olfactory, and others such as the “optic nerve involved with vision [and] the acoustic nerve involved with hearing, both serving the basic function of taking information from the various area it is connected, to the brain (Shubin 84). The trigeminal and facial cranial nerves are the two cranial nerves that are the source for this misconception of chaos, for the have “very complex functions and take tortuous paths through the head to do their job” (Shubin 84). The trigeminal and facial nerve deserve special mention when it comes to complicated nervous pathways. The trigeminal  nerve branches out into an array of endings and branches forming a network of nerves, all serving the main function of relaying sensory information from our face back to our brain. The branches of the trigeminal nerve however also serve for feeling sensation in the roots of some teeth, along with providing feeling to the entire face through their complicated network of nerve endings. The facial cranial nerve too controls muscles and relays information to and from the brain, and it is “the main nerve that controls muscles of facial expression” (Shubin 85). There seems to be no rhyme or reason to the numerous and varied functions of these nerves, but if you want to understand the “plumbing” in our heads, our history needs to be reflected upon (Shubin 86). Both of these nerves also travel to adjacent muscles in the ear, a fact that seems redundant to Shubin, when these functions could essentially be serviced by a single nerve.
2.) List the structures that are formed from the four embryonic arches (gill arches) during human development.
There are four main arches involved in embryonic development: the first arch forms the upper and lower jaws, two tiny ear bones called the malleus and the incus, and all the muscles and vessels that supply them, the second arch forms a small ear bone called the stapes, a throat bone, and most muscles involved in facial expression control, the third arch froms bones bones, muscles, and nerves deeper in the throat that we will ultimately use to swallow, and the fourth arch forms the deepest parts of the throat. (Shubin 87).
3.) T or F. Homeobox genes are conserved segments of DNA found within the DNA sequence of Hox genes. What are Hox genes and why are they so important?
This is a true statement. Hox genes are so important because they “instruct cells to make the different portions of our head” (Shubin 93). Each gill arch has a different compliment of Hox genes active in it, allowing it to develop into the desired structure of the head, whether that is the jaw, throat, nose, or other structures. With knowledge of how Hox genes work, we can make a map of our gill arches and the constelation of genes active in making each.
4.)Amphioxus is a small invertebrate yet is an important specimen for study – why?  Be sure to include characteristics that you share with this critter!
The Amphioxus is and invertebrate, but it shares many characteristics with vertebrates, making it a good specimen for study when analyzing the differences and similarities between vertebrates and invertebrates. Despite not having a backbone, it shares the characteristic of having a nerve cord that runs along its back with other vertebrates. Rather than having a spine for support, the Amphioxus has a notchcord that is filled with a jelly-like substance, a anatomical feature that we too have as developing embryos. While the Amphioxus keeps its notchcord, ours breaks up and ultimately becomes “part of the disks that lie between or vertebrate” (Shubin 94). Amphioxus too have gill arches in abundance, demonstrating that the “essence of our head goes back to worms, organisms that do not even have a head” (Shubin 96).
The pharygenal slits are gills.
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  1. Well done! Note the need for the complicated nerve system to allow jaws to work the way they do, derived characters seen in more recently evolved animals.
    Nice illustrations to complement the content. Note the shared characteristics among animals regarding the gill arches and homeobox genes.
    Amphioxus (should be Italicized, but I don’t have that option here) is Tiktaalik’s “inner worm!”

  2. I think the nervous system is also complicated to increase redundancy. That way, if a part of the brain is damaged somehow, other parts are still connected and can serve the function that damaged part served. Of course, this is only possible to a certain extent of brain damage. Cephalization, in my opinion, is really fascinating (it was also an AP Bio essay one year).

  3. Hox genes are clearly a recurring theme in animal development, as they control the organization of an animal’s body plan. Perhaps the redundancy of the nervous system is due to a topic mentioned in chapter 10, that of the gill arches? It is possible that the trigeminal and facial nerves serviced different areas of the jaw that developed from the gill arches, but as parts of the gill arch structures evolved to become the bones of the mammalian inner ear, the two nerves developed a certain redundancy in that they service the same area.


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