What is the relevance of neurons and the brain on behaviour?

The early development of the brain and nervous system is largely determined by biological factors, but the influence that experience has is also fairly large. The nervous system begins to develop shortly after conception, as the neural plate (a group of cells) thickens, folds in and forms the neural tube. The cells start to migrate to fixed locations. The brain contains millions of multifunctional cells, glial cells, and neurons. Neurons carry messages within the nervous system and to and from other body parts. The extensions of these neurons, called nerves, get a layer of myelin, a white substance that promotes the efficiency of communication in the brain. An excess of neurons and connections is produced both before and after birth to ensure the flexibility of the brain. Some parts of the brain develop faster than others. For example, the development of brain parts for vision and hearing is faster than the development of the frontal brain area, which is involved in complex thinking.

There are many developments in the brain during adolescence. In this way the connections between brain areas increase. Also, the amount of grey matter in the frontal brain area decreases, while the white matter shows an increase, which is a reflection of constant myelination. These changes have implications for psychological and behavioural functioning.

The development of the brain depends on the interaction between biological predisposition and experiences (activity-dependent processes). There is pruning occurring both before and after birth, which means unnecessary cells and connections between cells are broken down. This process is probably the cause of the decline in grey matter in adolescence.