Brain, EARLY FORMATION OF THE NERVOUS SYSTEM

 

The Brain

One of the most remarkable aspects of the prenatal period is the development of the brain (Nelson, 2011). By the time babies are born, they have approximately 100 billion neurons,or nerve cells, which handle information processing at the cellular level in the brain. During prenatal development, neurons spend time moving to the right locations and are starting to become connected. The basic architecture of the human brain is assembled during the first two trimesters of prenatal development. In typical development, the third trimester of prenatal development and the first two years of postnatal life are characterized by connectivity and functioning of neurons (Moulson & Nelson, 2008).

Figure 3.4 EARLY FORMATION OF THE NERVOUS SYSTEM. The photograph shows the primitive…

As the human embryo develops inside its mother’s womb, the nervous system begins forming as a long, hollow tube located on the embryo’s back. This pear-shaped neural tube, which forms at about 18 to 24 days after conception, develops out of the ectoderm. The tube closes at the top and bottom ends at about 24 days after conception. Figure 3.4 shows that the nervous system still has a tubular appearance six weeks after conception.

 

Two birth defects related to a failure of the neural tube to close are anencephaly and spina bifida. The highest regions of the brain fail to develop when fetuses have anencephaly or when the head end of the neural tube fails to close, and they die in the womb, during childbirth, or shortly after birth (Levene & Chervenak, 2009). Spina bifida results in varying degrees of paralysis of the lower limbs. Individuals with spina bifida usually need assistive devices such as crutches, braces, or wheelchairs. Both maternal diabetes and obesity place the fetus at risk for developing neural tube defects (McQuire, Dyson, & Renfrew, 2010; Yazdy & others, 2010). A strategy that can help to prevent neural tube defects is for women to take adequate amounts of the B vitamin folic acid, a topic we will further discuss later in the chapter (Rasmussen & Clemmensen, 2010; Shookhoff & Ian Gallicano, 2010).

In a normal pregnancy, once the neural tube has closed, a massive proliferation of new immature neurons begins to takes place about the fifth prenatal week and continues throughout the remainder of the prenatal period. The generation of new neurons is called neurogenesis (Kronenberg & others, 2010). At the peak of neurogenesis, it is estimated that as many as 200,000 neurons are being generated every minute.

At approximately 6 to 24 weeks after conception, neuronal migration occurs (Nelson, 2011). This involves cells moving outward from their point of origin to their appropriate locations and creating the different levels, structures, and regions of the brain (Cozzi & others, 2010; Kuriyama & Mayor, 2009). Once a cell has migrated to its target destination, it must mature and develop a more complex structure.

At about the 23rd prenatal week, connections between neurons begin to occur, a process that continues postnatally (Moulson & Nelson, 2008).