The two divisions of the human nervous system are the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord. The brain enables us to think, take decisions, experience emotions and move voluntarily. It consists of the telencephalon, which includes the cerebral cortex, the limbic system and the basal ganglia; the diencephalon, which comprises the thalamus and hypothalamus; the midbrain or mesencephalon, which contains the tectum and tegmentum; the metencephalon, which includes the pons and the cerebellum; and the myelencephalon. The spinal cord, which is divided into segments each representing an area of skin (dermatome), conveys motor commands from the brain to the muscles and transmits sensory information from receptors to the brain.
The brain and spinal cord are the most protected divisions of the nervous system. They are encased in the skull and the spinal column, are covered by three membranes called meninges and are washed in cerebrospinal fluid. The brain is further protected by the blood–brain barrier.
The second division of the human nervous system, the peripheral nervous system, conveys messages to and from the central nervous system. It consists of the somatic nervous system via which we interact with the external environment, and the autonomic nervous system with which we regulate the functioning of internal organs. The somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves, whereas the autonomic nervous system has a parasympathetic division, which is largely involved in building up energy levels and preserving energy, and a sympathetic division, which is generally involved in expending energy.
Several methods can be used to study the living nervous system, particularly the brain. When the interest lays in assessing the brain structurally, CT or MRI are two of the available options to consider. Alternatively, when what we want is to be able to conduct a functional analysis to find out how the brain is working, PET, fMRI, EEG or ERPs might prove useful. There is no perfect method so the decision on which one to use depends on the questions that need answering.