The brain is the center of the nervous system in all
vertebrate and most invertebrate animals—only a few invertebrates such as
sponges, jellyfish, adult sea squirts and starfish do not have one, even if
diffuse neural tissue is present. It is located in the head, usually close to
the primary sensory organs for such senses as vision, hearing, balance, taste,
and smell. The brain of a vertebrate is the most complex organ of its body. In
a typical human the cerebral cortex (the largest part) is estimated to contain
15–33 billion neurons, each connected by
synapses to several thousand other neurons. 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
targeting specific recipient cells.
From an evolutionary-biological point of view, the function
of the brain is to exert centralized control over the other organs of the body.
The brain acts on the rest of the body either by generating patterns of muscle
activity or by driving secretion of chemicals called hormones. This centralized
control allows rapid and coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the
spinal cord or peripheral ganglia, but sophisticated purposeful control of behaviour
based on complex sensory input requires the information-integrating
capabilities of a centralized brain.
From a philosophical point of view, what makes the brain
special in comparison to other organs is that it forms the physical structure
that generates the mind. As Hippocrates put it: "Men ought to know that
from nothing else but the brain come joys, delights, laughter and sports, and
sorrows, griefs, despondency, and lamentations." In the early part of psychology, the mind was
thought to be separate from the brain. However, after early scientists
conducted experiments it was determined that the mind was a component of a
functioning brain that expressed certain behaviours based on the external
environment and the development of the organism. The mechanisms by which brain
activity gives rise to consciousness and thought have been very challenging to
understand: despite rapid scientific progress, much about how the brain works
remains a mystery. The operations of individual brain cells are now understood
in considerable detail, but the way they cooperate in ensembles of millions has
been very difficult to decipher. The most promising approaches treat the brain
as a biological computer, very different in mechanism from electronic
computers, but similar in the sense that it acquires information from the
surrounding world, stores it, and processes it in a variety of ways.
This article compares the properties of brains across the
entire range of animal species, with the greatest attention to vertebrates. It
deals with the human brain insofar as it shares the properties of other brains.
The ways in which the human brain differs from other brains are covered in the
human brain article. Several topics that might be covered here are instead
covered there because much more can be said about them in a human context. The
most important is brain disease and the effects of brain damage, covered in the
human brain article because the most common diseases of the human brain either
do not show up in other species, or else manifest themselves in different ways.
