Jennifer B. Esty
The first trimester is a very important period of time for the developing neurological system. The nervous system is the first major system to begin forming. As the nervous system is essential for life, it is somewhat unfortunate that the formation begins before most women even suspect they may be pregnant. As stated above, the neural plate begins forming about 16 days after fertilization, or about the time when a woman may notice that she has missed her period. This means that the system begins to form without benefit of the extra care many women will take when they realize they are pregnant.
The nervous system begins with the formation of the neural plate, a thickening of the ectoderm on what will be the back of the embryo. The neural tube develops as a groove that soon rounds into a tube. Eventually, the tube develops ventricles, or small chambers, which will become the three sections of the brain. This stage is called encephalization. The remainder of the tube will become most of the spinal cord. The first two events are described in the next two sections.
Neural Tube
At about 16 days past fertilization, a special tissue forms in the developing embryo called the ectoderm. The ectoderm is one of three layers in the developing embryo at this point. It will eventually form into most of the exterior parts of the human body, forming tissues such as skin hair, nails, mammary glands, and the entire nervous system. A line of cells (the notochord) forms in the mesoderm.
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Above the notochord, the ectoderm thickens into a structure called the neural plate. A line in the neural plate just above the notochord develops into the neural groove. The edges of the groove rise and meet in the midline at about 3 weeks, forming the neural tube. The side begin meeting at the center of the length of the tube at the site of the future neck and continue towards both ends. This neural tube will eventually become the brain and spinal cord. The entire process resembles a person rolling their tongue into a tube.
Encephalization
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Once the neural tube has closed, encephalization begins. Encephalization is the process of forming the major structures found in the brain. During this stage the head region of the neural tube divides into three chambers: the prosencephalon, most of which will become the cerebrum; the mesencephalon, which will become the midbrain; and the rhombencephalon, which will become the lower part of the brain, including the cerebellum.
The prosencephalon is the vesicle or chamber that is closest to the rostral end (or the end that will become the head) of the neural tube. The prosencephalon divides into three sections. The rostral end produces two vesicles that emerge laterally from the main axis of the neural tube. These two sections together are called the telencephalon and will later become the two cerebral hemispheres of the brain. In later life, these two hemispheres of the brain will process most of the information coming into the brain from the senses as well as controlling movement, reasoning and thought. The remainder of the prosencephalon becomes the diencephalons, which eventually becomes thalamus, hypothalamus, pituitary gland, the pineal gland, and, interestingly, the retina of the eye. Most of these sections are essential for running the involuntary systems of the body, like thirst, hunger, digestion, and sleeping. The thalamus is also an essential relay stage for messages from many of the senses; it routes the message to the correct area of the brain for further processing.
The mesencephalon does not divide into additional vesicles, the way the prosencephalon does. Instead, it develops into the midbrain. The midbrain comprises a complex set of structures related to movement, coordination, mood, pleasure and pain, and visual and auditory reflexes.
The rhombencephalon divides into two vesicles: the metencephalon and the myencephalon. The metencephalon will later become the pons and the cerebellum. The myencephalon will become the medulla oblongata. The cerebellum is essential for coordinated movements; it regulates muscle movement, but it also controls the muscle tone necessary to maintain balance and posture. The pons, as its name suggests, acts as a bridge; it is a relay station between the medulla oblongata and the cerebellum. It is also required for breathing. The medulla oblongata is the first stop in the brain for many messages coming up the spinal cord. It is responsible for many of the autonomic functions of the body like breathing, coughing and swallowing.
Inevitably in every curriculum unit, the question of "how do I teach this" arises. For this unit, there are three major concepts that my students need to understand. The first is a basic question of vocabulary. This is written for an anatomy class, so the names of the parts of the brain are an essential part of the curriculum, particularly as my students will never have seen the material before. The second concept my students need to know refers to the physiology part of the class; they need to know what the parts do. The third concept is a bit more abstract in that it requires both anatomy and physiology to understand. My students need to understand how all of the parts and pieces fit together and work together. The text book that I generally use for my class includes a workbook with black and white illustrations to be colored and labeled to help students learn the names of the various parts of the body. While this approach works very well for many of the body systems, the central nervous system is one where this approach breaks down because of the fundamental three-dimensionality of the brain. So, I suggest a slightly different approach to teaching this section. As part of the preparation for writing this unit, I tried out this section of the unit with my class. The most complex concept from this section is the three dimensional structure of the brain. As most of us do not have access to a world class anatomy lab with actual human brains to study, I suggest building a three-dimensional model of the brain. In my class we built one out of a very popular children's modeling clay. It worked pretty well to convey the general shape of the various parts of the brain, however, because it is extremely malleable, our model tended to be somewhat floppy. I would suggest using a slightly stiffer material like a professional grade modeling clay or polymer clay for this project.
For this project, the students should work on building a model brain. In my class each student worked on a section, and we put all the pieces together. In a larger class, the class could be broken into smaller teams to work on several brains, possibly the brain at different stages of growth. I think it is important that each student be required to build sections of the brain rather than having teams of students each build a section. The structures are complex enough that the students should have as much experience manipulating and forming the brain sections as possible. Once the brain sections are forms, and tested to make sure they fit together, they should be labeled with the name of the part of the brain and the function that the brain part serves.
