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 human brain contains roughly 100 billion neurons, 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.
From a philosophical point of view, it might be said that the most important function of the brain is to serve as the physical structure underlying the mind. From a biological point of view, though, the most important function is to generate behaviors that promote the welfare of an animal. Brains control behavior either by activating muscles, or by causing secretion of chemicals such as hormones. 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, but these methods do not have the resolution to reveal the activity of individual neurons.
The brain is composed of two broad classes of cells, neurons and glia. Neurons receive more attention, though glial cells overall have equal numbers to neurons in the whole brain they outnumber them by roughly 4 to 1 in the cerebral cortex. Glia come in several types, which perform a number of critical functions, including structural support, metabolic support, insulation, and guidance of development.
The property that makes neurons so important is that, unlike glia, they are capable of sending signals to each other over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects, usually with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body. The extent of an axon can be extraordinary: to take an example, if a pyramidal cell of the neocortex were magnified so that its cell body became the size of a human, its axon, equally magnified, would become a cable a few centimeters in diameter, extending farther than a kilometer. These axons transmit signals in the form of electrochemical pulses called action potentials, lasting less than a thousandth of a second and traveling along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials constantly, at rates of 10–100 per second, usually in irregular temporal patterns; other neurons are quiet most of the time, but occasionally emit a burst of action potentials.
Axons transmit signals to other neurons, or to non-neuronal cells, by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections. When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Some types of neuronal receptors are excitatory, meaning that they increase the rate of action potentials in the target cell; other receptors are inhibitory, meaning that they decrease the rate of action potentials; others have complex modulatory effects on the target cell.
Axons actually fill most of the space in the brain. Often large groups of them are bundles together in what are called nerve fiber tracts. In many cases, each axon is wrapped in a thick sheath of a fatty substance called myelin, which serves to greatly increase the speed of action potential propagation. Myelin is white in color, so parts of the brain filled exclusively with nerve fibers appear as white matter, in contrast to the gray matter that marks areas where high densities of neuron cell bodies are located. The total length of these myelinated axons in the human brain is considerable: 176,000 km in a 20 year old male and 149,000 km in a female.
The illustration on the right shows a thin section of one hemisphere of the brain of a Chlorocebus monkey, stained using a Nissl stain, which colors the cell bodies of neurons. This makes the gray matter show up as a dark blue, and the white matter show up as a paler blue. Several important forebrain structures, including the cortex, can easily be identified in brain sections that are stained in this way. Neuroanatomists have invented hundreds of stains that color different types of neurons, or different types of brain tissue, in distinct ways; the Nissl stain shown here is probably the most widely used.