Neurophysiology is a broad field of study because many different levels are involved in the overall functioning of the nervous system and its components. For example, the transmission of a nervous impulse across the synapses, or the cleft that connects nerve cells, involves chemical reactions at the cellular level of organization. Understanding how messages are relayed from the brain to the hand is best explained at the system level. This involves studying the relationship and pathways between the brain and the organs of the body and the nerves that connect them, both sensory, meaning nerves that receive input from sensors, and motor, meaning the nerves that activate muscle. Thus, neurophysiology studies nervous function ranging from individual nerve cells to the complex behaviors of the central nervous system. Additionally, the nervous system not only functions at the cell and system levels of organization, but ... at a mechanistic level, that involves the study of the control or regulatory processes that occur.