The human brain weighs about 1.5 kg and has the consistency of porridge or blancmange. It lies protected within the skull. The brain is composed of many billions of nerve cells (neurons) which connect with each other and with all parts of the body via efferent and afferent nerve pathways. Efferent nerve pathways consist of motor neurons which carry impulses away from the central nervous system and afferent neurons are sensory neurons which carry nerve impulses from the rest of the body to the central nervous system.