In the anatomy of animals, the brain, or encephalon, is the supervisory center of the nervous system. Although the brain is usually cited as the supervisory center of vertebrate nervous systems, the same term can also be used for the invertebrate central nervous system. In most animals, the brain is located in the head.

The brain controls and coordinates most movement, behavior and homeostatic body functions such as heartbeat, blood pressure, fluid balance and body temperature. Functions of the brain are responsible for cognition, emotion, memory, motor learning and other sorts of learning.

The brain is primarily made up of two types of cells: glia and neurons. Glia function primarily to support and protect the neurons. The neurons carry information in the form of electrical pulses known as action potentials. They communicate with other neurons in the brain and throughout the body by sending various chemicals called neurotransmitters across gaps known as synapses. Small invertebrates such as insects may have a million neurons in the brain, larger vertebrate brains have over one hundred billion neurons. The human brain is particularly complex and large in comparison to human body size.

Brains in nature

Although many classes of animals have nervous systems, three groups of animals, with some exceptions, have notably complex brains: the arthropods (for example, insects and crustaceans), the cephalopods (octopuses, squid, and similar mollusks), and craniates (vertebrates and their cousins). The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. The arthropod brain consists of large optical lobes behind each eye for visual processing and a central brain with three divisions. The cephalopod brain has a central group of lobes known as circumesophageal lobes that are flanked by two large optical lobes on the left and right (Butler, 2000).

The brains of craniates develop from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord. In craniates, the brain is protected by the bones of the skull. Vertebrates are characterized by increasing complexity in the cerebral cortex as one moves up the phylogenetic and evolutionary tree. Primitive vertebrates, like fish, reptiles, and amphibians have cortices with less than six layers of neurons, a structure known as allocortex (Martin, 1996). More complex vertebrates like mammals have developed six-layered neocortex in addition to having some parts of the brain that are allocortex (Martin, 1996). In mammals, increasing convolutions of the brain, called gyri, are characteristic of animals with more advanced brains. These convolutions evolved to provide more surface area for a greater number of neurons while keeping the volume of the brain compact enough to fit inside the skull.

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