The nervous system consists of a set of bodies to coordinate the body's vital functions. It can be classified into central nervous system and peripheral nervous system. The first comprises the brain, composed of the brain, the cerebellum and brain stem (all of these organs in the cranial cavity), and spinal cord that is cylindrical, about 45 cm long and 1 cm in diameter. In the brain are the nerve centers that capture and respond to stimuli through impulses. Act out a complex network through which a nerve stimulation leads to a neuron and another carries the impulse response. The function of the spinal cord is mainly transported stimuli and impulses to and from the extremities, as well as intervene in the regulation of certain acts, and the excretion of urine and feces. The peripheral nervous system central nervous system communicates with the rest of the body, and also with the outside world. It consists of a network of nerves and ganglia.
The nerves of the peripheral nervous system are grouped in pairs: 12 pairs of cranial nerves and 31 pairs or spinal cords. The first originate in the brainstem, the latter in the spinal cord.
Neurons have two types of extensions: dendrites and axons. Dendrites receive chemical signals from other nerve cells and axons conduct those signals in the form of electrical impulses.