Psychologists study the brain and the nervous system because these parts of the body are essential to the way that people behave, think and feel. Psychology is the scientific study of people's behavior and their mind so studying the brain and nervous system is always going to be a crucial element for psychological study.
A 2013 study from the University of Virginia found that the human brain is wired to connect so strongly with other humans that it experiences what other humans are experiencing even though it is not going through the same experience. This was shown when participants underwent fMRI brain scans. The correlation between self and friend was strong in the study. This is a psychological phenomenon as it affects a person's mind and their behavior.
Studying the nervous system means studying its two major systems: the central nervous system, which consists of the brain and spinal cord, and the cerebral cortex, which is involved in higher cognitive, emotional, sensory and motor functions. The peripheral nervous system is divided into two additional sub-systems. These sub systems are the somatic nervous system and the autonomic nervous system. The somatic nervous system has the primary function of regulating the actions of the skeletal muscles while the autonomic nervous system works to regulate involuntary activity, such as the heart rate or breathing.