In the previous newsletter, we explained the early stages of brain development when the embryo is only a few millimeters long. Do you remember? The neural tube extends along the entire length of the embryo and it is this structure that will later become the brain and spinal cord. Now let’s take a look at how the major areas of the nervous system develop.

At about 33 days after conception, the embryo is slightly larger at about 5 millimeters long. The cells that make up the neural tube differentiate into five major regions at the rostral end (head): the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

The first area, or telencephalon, is located at the very end of the tube and will later form the brain itself. At this stage, two lateral buds form from the telencephalon, or forebrain, and then grow to form the two hemispheres of the brain. At the same time, neurons in this area begin elongating in order to establish connections between the cerebral cortex and other parts of the nervous system, such as the spinal cord. This connecting tissue is called white matter (as opposed to gray matter, composed largely of neuronal cell bodies). The corpus callosum, another white matter structure, forms to allow communication between the two cerebral hemispheres.

As for the cerebellum, it will be located at the base of the brain and will play an important role in motor coordination and particularly speech articulation. The cerebellum arises from the fourth subdivision, the metencephalon. The tissue of the lateral walls of the neural tube thickens and forms two distinct masses that will eventually merge to form the cerebellum.

The entire caudal part of the neural tube (after the five subdivisions) becomes the spinal cord. It will then receive the neuronal extensions (axons) that transmit sensory information (touch, pain, temperature, position in space) to the brain. These are afferent neurons as the nerve impulses are carried from the body to the brain. In turn, efferent nerve fibers send messages from the brain through the spinal cord and enervate the various muscles of the body. This system will allow the body to adapt to changes in the environment (for example a loss of balance). To speed up reaction times in emergency situations, these messages will not reach the brain, but will be processed directly by the spinal cord. These quick, automatic reactions are known as reflexes.