Brain Development

The basic structure of the brain reflects the way it forms during early (embryonic) development. It begins as the neural tube that gives rise to the central nervous system. The portion that becomes the brain has three major cavities, or vesicles, at one end—the forebrain (prosen-cephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) (fig. 11.14). Later, the forebrain divides into anterior and posterior portions (telencephalon and diencephalon, respectively), and the hindbrain partially divides into two parts (metencephalon and myelen-cephalon). The resulting five cavities persist in the

- Prosencephalon (forebrain) -Mesencephalon (midbrain)

- Rhombencephalon (hindbrain) -Neural tube the mature brain. It consists of two large masses or cerebral hemispheres (ser'e-bral hem'i-sferz), which are essentially mirror images of each other (fig. 11.16 and reference plate 49). A deep bridge of nerve fibers called the corpus callosum connects the cerebral hemispheres. A layer of dura mater called the falx cerebri separates them.

Cerebral hemispheres

Diencephalon Midbrain

Pons and Cerebellum

Medulla oblongata Spinal cord

-Telencephalon -Diencephalon

Mesencephalon Metencephalon Myelencephalon

Neural tube

Cerebral hemispheres

Diencephalon Midbrain

Pons and Cerebellum

Medulla oblongata Spinal cord

(a) The brain develops from a tubular structure with three cavities.

(b) The cavities persist as the ventricles and their interconnections.

(c) The wall of the tube gives rise to various regions of the brain, brain stem, and spinal cord.

A fetus or newborn with anencephaly has a face and lower brain structures, but lacks most higher brain structures. A newborn with this anomaly survives only a day or two, and sometimes the parents donate the organs.

Anencephaly is a type of neural tube defect (NTD). It occurs at about the twenty-eighth day of prenatal development, when a sheet of tissue that normally folds to form a neural tube, which develops into the central nervous system, remains open at the top. A less-serious NTD is spina bifida, in which an opening farther down the neural tube causes a lesion in the spine. The most serious form of this condition results in paralysis from that point downward. Sometimes spina bifida can be improved or even corrected with surgery.

The precise cause of neural tube defects is not known, but it involves folic acid; taking supplements of this vitamin sharply cuts the recurrence risk among women who have had an affected child. Most pregnant women take a blood test at the fifteenth week of pregnancy to detect fluid leaking from an NTD.

mature brain as the fluid-filled ventricles and the tubes that connect them. The tissue surrounding the spaces differentiates into the structural and functional regions of the brain.

The wall of the anterior portion of the forebrain gives rise to the cerebrum and basal nuclei whereas the posterior portion forms a section of the brain called the diencephalon. The region the midbrain produces continues to be called the midbrain in the adult structure, and the hindbrain gives rise to the cerebellum, pons, and medulla oblongata (fig. 11.15 and table 11.4). Together, the midbrain, pons, and medulla oblongata comprise the brain stem (bran stem), which attaches the brain to the spinal cord.

On a cellular level, the brain develops as specific neurons attract others by secreting growth hormones. Apoptosis (programmed cell death) destroys excess neural connections.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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