The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. 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, linked with up to 10,000 synaptic connections each. Each cubic millimeter of cerebral cortex contains roughly one billion synapses. 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. 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. Sponges, which lack a central nervous system, are capable of coordinated body contractions and even locomotion. 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. However, sophisticated control of behavior on the basis of 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 give rise to complex operations is unknown.

Quotes about the brain.

Sourced

• Aristotle taught that the brain exists merely to cool the blood and is not involved in the process of thinking. This is true only of certain persons.
  • Will Cuppy, The Decline and Fall of Practically Everybody, 1950

• Brain, n. An apparatus with which we think we think.
  • Ambrose Bierce, The Devil's Dictionary

• Do we trivialize a sublime feeling if we appreciate its dependence on the brain? Not in the least. Its significance does not depend on its being a soul state or a brain state...Humility bids us to take ourselves as we are; we do not have to be cosmically significant to be genuinely significant.
  • Brain-Wise by Patricia Churchland

• "We do have an organ for understanding and recognizing moral facts. It is called the brain.
  • A Neurocomputational Perspective by Paul Churchland

• "If the human brain were so simple that we could understand it, we would be so simple that we couldn't."
  • Emerson M. Pugh, As quoted in The Biological Origin of Human Values

Unsourced

• The Human Brain starts working the moment you are born and never stops until you stand up to speak in public!
  • Sir George Jessel

• I was taught that the human brain was the crowning glory of evolution so far, but I think it’s a very poor scheme for survival.
  • Kurt Vonnegut

• My Brain: it's my second favorite organ.
  • Woody Allen

From Wikiquote under the GNU Free Documentation License.
Sun Aug 29 07:39:02 2010