Prior to fully understanding how the nervous system (NS) mediates behaviour, psychobiologists have to comprehend the structure and function of its individual components. Over time, psychobiologists have come up with different, but equally valid, ideas on how to study the organization of the NS; one method distinguishes by gross anatomy and the other, by function. Three broad, but interconnected, categorizations include central, autonomic, and somatic nervous systems. The autonomic NS is, literally, automatic and consists of sympathetic (flight or fight) and parasympathetic (rest and digest) divisions. Meanwhile, the somatic NS mediates conscious motor functions. On top, however, is the CNS consisting of the brain and spinal cord. The spinal…show more content… A thought provoking question occurs regarding these layers: How did these layers develop over evolutionary time and why, of all numbers, are there six? It is well-known that evolution is a gradual and cumulative process, taking place in the order of millions of years. It would be interesting to know whether these layers were brought forth simultaneously, or, was there a time lapse between each layer. Moreover, evolution is facilitated by certain pressures such as genetic mutations and a key to understanding the development would be in identifying the circumstances under which layering took place. Another peculiar observation is the number of cortical layers: six. This peculiarity demands to be answered; what is the evolutionary advantage of synthesising six bulky layers rather than just having one. Is it not true that there would be more surface area to volume ratio by having one, large spread out layer rather than six of them stacked together? Six different layers means more interconnecting neurons have to be formed and more vascularization is needed to transport nutrients to and wastes away from the vicinity. How is that more