The nervous system has two different major parts. The two parts are the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and the spinal cord and is the site of information processing and control whereas the PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord). All body sensations and changes in our external environment are relayed from receptors and sense organs to the CNS to be interpreted. The PNS can be divided into two subcategories: the sensory system, which consists of afferent or sensory neurons that convey information from receptors in the periphery of the body to the brain and spinal cord, and the motor system, which consists of efferent or motor neurons that convey information from the brain and spinal cord to muscles and glands. In CNS the groups of neurons are called nuclei whereas in PNS the groups of neurons are called ganglia. In CNS brain in vertebrates is protected by the cranium (brain box) which is present in the skull and spinal cord is protected by vertebral column (back bone) whereas in PNS such protective structures are not found.