Anatomic and Physiologic Overview
The nervous system consists of two divisions: the central nervous system (CNS), including the brain and spinal cord, and the pe-ripheral nervous system, made up of the cranial and spinal nerves. The peripheral nervous system can be further divided into the somatic, or voluntary, nervous system, and the autonomic, or in-voluntary, nervous system. The function of the nervous system is to control all motor, sensory, autonomic, cognitive, and behav-ioral activities. The nervous system has approximately 10 million sensory neurons that send information about the internal and ex-ternal environment to the brain and 500,000 motor neurons that control the muscles and glands. The brain itself contains more than 20 billion nerve cells that link the motor and sensory path-ways, monitor the body’s processes, respond to the internal and external environment, maintain homeostasis, and direct all psy-chological, biologic, and physical activity through complex chemical and electrical messages (Bradley, Daroff, Fenichel & Marsden, 2000).