With the exception of some primitive creatures such as the sea jelly and the sponge, animals have some form of central nervous system. The anatomy of the central nervous system varies between different organisms but always contains a brain which is made up of billions of neurons. More advanced animals, including humans, also have a spinal cord that helps regulate body functions.
The brain is the most important organ in the anatomy of the central nervous system. The brainstem, which forms the entirety of the brain in many animals, regulates functions needed to keep an organism alive such as metabolism, breathing, and heartbeat. More advanced evolutionary features include the cerebellum, which is found in reptiles and animals that evolved from them. It is involved in movement, coordination and memory. The cerebrum, which is common to all mammals, is the center of more complex processes, such as those that control the voluntary muscles and cognition. This section of the brain is divided into a number of lobes such as the frontal and the parietal lobe that are often closely associated with a small set of responsibilities.
The anatomy of the central nervous system in some animals also includes a spinal cord. The presence or absence of this organ has historically been a method for classifying animals, though a better understanding of evolutionary relationships has made it one of a few features that help place animals into groups called phyla. In animals with lateral symmetry, such as humans, fish, and reptiles, the spinal cord runs down the length of the animal's body, along the center of the animal's back. Though the spinal cord is composed of a network of neurons, like the brain, it functions as more of a relay station between the peripheral and the central nervous system and has direct control of only a few key systems.
Neurons are the building blocks of the anatomy of the central nervous system. These cells are unlike other animal cells both in structure and in use. Like other cells, they have organelles and deoxyribonucleic acid inside them, but unlike other cells, they lack the ability to repair themselves if they become damaged. A neuron has two ends, a cell body that has many dendrites branching off from it in order to receive information from other cells, and a terminal end with axons that transmit information to other neurons. The information sent through a neuron is in the form of a chemical and electrical signal.