Sonic Hedgehog

The density of neural tissue depends on several factors, including position, that define the environmental factors to which the cells are exposed. For example, a key factor in the development of the spinal cord is a protein called sonic hedgehog. This protein marks young neural cells that are directly adjacent to become a specialized class of supporting cells, called glial cells. Cells farther away are exposed to lower levels of the protein and develop into motor neurons that control muscles. An even lower concentration stimulates the formation of interneurons, neuronal cells that relay messages to other neurons.

Migration

As neurons are produced, they move from the neural tube’s inner surface (or ventricular zone) to near the border of the outer surface (or marginal zone). After the neurons stop dividing, they form an intermediate zone where they gradually accumulate as the brain develops.

Even though it happens in all parts of the brain, the migration of neurons is most prominent in the formation of the large cerebral cortex in primates, including humans. In this cortex, the neurons move from the place of origin, near the ventricular zone, along non-neuronal fibers that form a trail to their proper destination. Neuron migration requires several mechanisms, such as the recognition of the proper path and the ability to move long distances.

Many external forces prevent a proper neural migration and result in a misplacement of cells. Examples of such external influences are alcohol, cocaine or radiation, which may lead to numerous mental disorders, such as epilepsy. Furthermore, mutations in genes that regulate this migration have recently been shown to cause some rare genetic forms of mental retardation and epilepsy.

The Final Location

Once the neurons complete their complex and elaborate journey and eventually reach their final location, they are required to make the proper connections for a particular function, such as vision or hearing, to occur. This is done through their axons. These wire-like appendages can stretch out more than a thousand times longer than the cell body from which they originally arise. The journey of most axons ends when they meet thicker appendages, called dendrites, on other neurons. These target neurons can be located at a considerable distance from the cell body from which the axon originates, sometimes even at the opposite side of the brain. In the case of a motor neuron, a single axon may even travel from the spinal cord all the way down to a foot muscle.