Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals.[1] Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, balance, taste, and smell.

Brains can be extremely complex. The cerebral cortex of the human brain contains roughly 15–33 billion neurons, perhaps more, depending on gender and age,[2] linked with up to 10,000 synaptic connections each. Each cubic millimeter of the cerebral cortex contains roughly one billion synapses.[3] These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells.

The brain controls the other organ systems of the body, either by activating muscles or by causing secretion of chemicals such as hormones and neurotransmitters. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness are possible without a brain: even single-celled organisms may be capable of extracting information from the environment and acting in response to it.[4] Sponges, which lack a central nervous system, are capable of coordinated body contractions and even locomotion.[5] In vertebrates, the spinal cord by itself contains neural circuitry capable of generating reflex responses as well as simple motor patterns such as swimming or walking.[6] However, sophisticated control of behavior based on complex sensory input requires the information-integrating capabilities of a centralized brain.

Despite rapid scientific progress, much about how brains work remains a mystery. The operations of individual neurons and synapses are now understood in considerable detail, but the way they cooperate in ensembles of thousands or millions has been very difficult to decipher. Methods of observation such as EEG recording and functional brain imaging tell us that brain operations are highly organized, while single unit recording can resolve the activity of single neurons, but how individual cells cause complex operations is unknown.[7]

The brain is the most complex biological structure known,[8] and comparing the brains of different species based on appearance is often difficult. Nevertheless, there are common principles of brain architecture that apply across a wide range of species. These are revealed mainly by three approaches. The evolutionary approach compares brain structures of different species, and using the principle that features found in all branches that have descended from a given ancient form were probably present in the common ancestor as well. The developmental approach examines how the form of the brain changes during the progression from embryonic to adult stages. The genetic approach analyzes gene expression in various parts of the brain across a range of species. Each approach complements and informs the other two.

From : http://en.wikipedia.org/wiki/Brain