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Brain
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6. 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 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 b
itself contains neural circuitry capable of generating reflex responses as well as simple motor patterns such as swimmin
or walking.[6] 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, bu
how individual cells give rise to complex operations is unknown.
