Neurons are the basic unit of nervous system. These highly specialized cells have a multitude of terminals which provide intricate anatomical connections that allow complex functional relationships. Neurons generate, receive and transmit impulses to other neurons across connections by means of microrelease of neuro transmitter substances. The nervous system is a network of neurons organized into special reflex centers, serving different functions. The brain consists of many such reflex centers and circuits that comprise large clusters interconnecting neurons. Reflex arc is the basic unit of nervous organization which have afferent (carrying into) and afferent (carrying out) nerves. Efferent neurons convey impulses to muscles or glands to contract or secrete. Reflexes have a sensory and motor component. It is comparable to a door bell in which the sensory component is the switch while its motor expression is the ringing bell. In humans the clitoris is the sensory organ, its motor expression is felt in the contractions of the vagina. The brain have an advanced upper center and a primitive lower center. Upper center is a late evolutionary development along with the mammalian transformation. Most of the lower centers are located in the spinal cord. It is called reptilean brain because during this stage no higher centers were evolved to process raw impulses, reflexes were passed directly to the motor muscles. The higher system was evolved with highly specialized functions like storing memories and reproducing them, differentiating virtue from evil etc. Evolution of thalamus, hypothalamus, the limbic system with its cortex which mediates reasoning and social judgements were important developments. The central nervous system elaborated higher integrative centers dominating the original ones. But it never discarded some of its primitive structures. Thus reflexes for erection and ejaculation in male and vaginal lubrication and orgasm in female were retained in the primitive lower center, the sacral segments of the vertebrae numbers S2 S3 and S4.