By Dr Ananya Mandal, MD
The cells of the nervous system are of a specific type and form over the course of an organism’s life. The neurons form and form connections from the time when the organism is an embryo or fetus. The appropriate neurons develop in appropriate numbers and migrate to their necessary locations before birth. The axons and dendrites that form the connections then extend from these nerve cells so they reach the targets.
The connections that form initially mature over time and this process begins when the baby is in the womb itself. The degree of complexity in the brain means that this development takes years before it is mature. Across different species the initial development is similar but due to the complexity of the human nervous system it changes and becomes more complex in humans.
Knowledge of these steps helps in the prevention and treatment of various developmental disorders like mental retardation etc. Studies are helping to understand how the brain is able to reorganize in response to external influences or injury. These studies also shed light on brain functions such as learning and memory.
Beginning of the nervous system
After the fetus is conceived it takes around three to four weeks before one of the two cell layers of the gelatinlike human embryo, about one-tenth of an inch long, begins to thicken and build up along the middle.
The cells grow and form a flat area called the neural plate with parallel ridges across its surface. Over a few days these ridges fold in toward each other and fuse to form the hollow neural tube.
The tube thickens at the top and forms three bulges that form the hindbrain, midbrain, and forebrain. The first signs of the eyes and the hemispheres of the brain appear later in development.
Development of the nervous system
The embryo consists of three layers that undergo many changes to form organ, bone, muscle, skin, or neural tissue. Skin and neural tissue arise from one layer called the ectoderm. This occurs in response to the adjacent layer, the mesoderm.
Once the ectoderm starts becoming neural tissues due to specific signals, more signalling interactions determine which type of brain cell forms. Some form the neurons while others form the glial cells.
Formation of the neurons
The immature neurons then migrate and start making transient connections with other neurons before reaching their destination. A single neuron uses a glial fiber as a guiding wire to move using adhesion molecules, which recognize the pathway, and also uses contractile proteins to propel it along.
The fate of the neural tube
Neurons are initially produced along the central canal in the neural tube. Then they move to the brain. They collect together to form each of the various brain structures. Their axons grow long distances to find and connect with other neurons. Once the required circuit is established there is a process of sculpting that removes redundant or improper connections.
The neurons move from the neural tube’s ventricular zone, or inner surface, to near the border of the marginal zone, or outer surface. Once they stop dividing they form an intermediate zone where they gradually accumulate as the brain develops.
Migration is most at the cerebral cortex in primates, including humans. Many external forces, such as alcohol, cocaine, or radiation exposure of the fetus while in the mother’s womb affects this neuronal migration leading to problems with mental growth after birth.
Once the neurons reach their final location, they must make the proper connections for a particular function to occur. Axon growth is directed by growth cones. These are enlargements of the axon’s tip that seek out the destination for growth.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)