Wednesday, April 13, 2011

Neuroanatomy- Post #3- Development, the Spinal Cord, etc.

         This blog is going to be divided into three section: more divisions of the brain, spinal cord overview, and Cerebrospinal fluid. This is a semi-continuation of the last blog, but this one is going to be shorter than the last one. I got kinda carried away with that. ha. Hope you enjoy!
        The first topic that I would like to mention is how the brain develops, and it's embryonic origins. This is important to understand because each of the brain structures rise from some common origins. There are three main divisions that are associated with development: the Hindbrain, the Midbrain, and the Forebrain. Each of these is comprised of embryonic originations that give rise to individual brain structures. The image below gives a very simplistic view, but it is the most straighforward of any I've been able to find.





The Hindbrain (Rhombencephalon)


   Myelencephalon (Medulla oblongata): The base of the brain, which is formed by the enlarged top of the spinal cord. This part of the brain directly controls breathing, blood flow, and other essential functions.


   Metencephalon (Pons & Cerebellum): The metencephalon develops from the hindbrain, and is differentiated from the myelencephalon in the embryo by approximately 5 weeks of age. By the third month, the metencephalon differentiates into its two main structures, the pons and the cerebellum.


The Midbrain (Mesencephalon)


   Mesencephalon (Tectum, Tegmentum (Substantia Nigra) comprises the tectum and tegmentum. The mesencephalon is considered part of the brain stem. The substantia nigra is closely associated with motor system pathways of the basal ganglia (as mentioned in the last blog).


Forebrain (Prosencephalon)


   Diencephalon (Thalamus, Hypothalamus): The diencephalon (“interbrain”) is the region of the brain that includes the thalamus, hypothalamus. It combines with the telencephalon. The diencephalon is located near the midline of the brain.
   Telencephalon (Cerebral Cortex): The cerebrum or telencephalon, together with the diencephalon, constitute the forebrain. It is the most anterior or, especially in humans, most superior region of the vertebrate central nervous system.


     All of these areas combine to form the brain. This can be important because it allow us to pinpoint when certain structures will develop, and if something goes wrong, it can be determined when this might have happened. If you refer to the table below, you can see when certain milestones are for neural development.









Table 28-2 Timetable of Growth and Nervous System Development in the Normal Embryo and Fetus


Age, Days
Size (Crown–Rump Length), mm
Nervous System Development
18
1.5
Neural groove and tube
21
3.0
Optic vesicles
26
3.0
Closure of anterior neuropore
27
3.3
Closure of posterior neuropore; ventral horn cells appear
31
4.3
Anterior and posterior roots
35
5.0
Five cerebral vesicles
42
13.0
Primordium of cerebellum
56
25.0
Differentiation of cerebral cortex and meninges
150
225.0
Primary cerebral fissures appear
180
230.0
Secondary cerebral sulci and first myelination appear in brain
8–9 months
240.0
Further myelination and growth of brain (see text)
 






      As you can see there are many stages in which something can go wrong in this development. One such example is Anencephaly. Anencephaly is a neural tube defect that occurs when the cephalic (head) end of the neural tube fails to close, usually between the 23rd and 26th days of pregnancy, resulting in the absence of a major portion of the brain, skull, and scalp. Infants with this disorder are born without a forebrain. The remaining brain tissue is often exposed - not covered by bone or skin. Unfortunately it is not known what causes this disorder, but it is suggested that women who are pregnant should take folic acid supplements. Recent studies have shown that the addition of folic acid to the diet will significantly reduce the incidence of neural tube defects.




     
         The next topic that I would like to discuss is the spinal cord, in a general overview sense. You probably already know what is to follow, but it may be a good review for you anyway.  The spinal cord is the the major column of nerve tissue that is connected to the brain and lies within the vertebral canal and from which the spinal nerves emerge. Thirty-one pairs of spinal nerves originate in the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The spinal cord and the brain constitute the central nervous system (CNS). The spinal cord consists of nerve fibers that transmit impulses to and from the brain. Like the brain, the spinal cord is covered by three connective-tissue envelopes called the meninges. The space between the outer and middle envelopes is filled with cerebrospinal fluid (CSF), a clear fluid that protects the spinal cord from blunt impact.

         Of course there are many pathologies that can be associated with the spinal cord, but one of the most obvious would be damage to a spinal nerve. There are cases in which the nerve can either be "pinched" or completely severed. Depending on the extent of the nerve involvement, the damage can have widespread effects. Usually the injuries are work related, and can leave a person permanently injured. Usually treatment will focus on the pain management aspect of medicine.


References:

1.)      Moore Keith L., Dalley Arthur F., Agur Anne M.R., "Chapter 6: Upper Limb". Lippincott Williams & Wilkins. Clinically Oriented Anatomy, 6e. 2010
2.)      Larson, J.L., Hurston, M.T., & Felli, G.J., "Chapter 3: Cerebrum". McGraw-Hill Inc. Neuroanatomy Through Clinical Cases. 2011.
3.)      Waxman SG, "Chapter 4. The Relationship Between Neuroanatomy and Neurology" (Chapter). Waxman SG: Clinical Neuroanatomy, 26e: http://www.accessmedicine.com/content.aspx?aID=5271560. 4.)      Lomen-Hoerth Catherine, Messing Robert O, "Chapter 7. Nervous System Disorders" (Chapter). McPhee SJ, Hammer GD: Pathophysiology of Disease, 6e: http://www.accessmedicine.com/content.aspx?aID=5368376.
5.)     Ropper AH, Samuels MA, "Chapter 28. Normal Development and Deviations in Development of the Nervous System" (Chapter). Ropper AH, Samuels MA: Adams and Victor's Principles of Neurology, 9e: http://www.accessmedicine.com/content.aspx?aID=3634622.

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