Brain

The brain is the control center of an organism's nervous system. Composed of specialized cells called neurons, the brain receives information from the body's sensory systems. It processes and analyzes the information, and responds by sending out messages that control the rest of the body. All vertebrate (an animal with a backbone) brains are organized and divided into three parts.

Not all organisms have a brain. Simpler forms of life, such as invertebrates (animals without a backbone) like the sponge, jellyfish, and sea anemone have what are called "nerve nets." Animals like earthworms and grasshoppers have a larger collection of cells called "cerebral ganglia." These collections of nerve cells function as a primitive brain and perform the basic operations of receiving sensory information and acting upon it. More complex forms of life, and certainly all vertebrates, have a distinct and separate organ known as the brain. Described as the control center of the body, the brain's functions are usually described by words like organizing, coordinating, supervising, and governing.

HOW THE BRAIN WORKS

The above words are appropriate in referring to what the brain does, since unlike any other organ, the brain performs what might be called higher functions. For example, no other organ in the body is responsible for the entire organism. It is the brain that receives input, or information, from numerous sensory systems and analyzes and combines that information in order to issue commands to other body systems. The brain regulates and controls other organs and makes sure that the body's "automatic" operations are running automatically. It is the brain that controls

all of the body's movements. In humans, it is the brain that is responsible for our thought, language, and awareness or consciousness.

Each brain is able to accomplish these complex tasks because it is composed of neurons, or nerve cells. Neurons are the building blocks of the animal brain. Like any cell, neurons have a nucleus (a cell's control center), cytoplasm (a jelly-like substance in a cell), and a membrane. Yet they also have structures not found in any other cells. These structures are long, thin fibers, or threads, called axons and dendrites that extend out from the cell's body. It is these fibers that allow a neuron to receive and send electrical impulses or signals. Axons do the sending and dendrites do the receiving.

PARTS OF THE BRAIN

Although the brain is astoundingly complex, the vertebrate brain can be described in simple terms. All brains can be divided into three main parts: the hindbrain, the midbrain, and the forebrain. The hindbrain, or brain stem, is the most primitive part of the brain and consists of the medulla, the pons, and the cerebellum. Located in the rear of the brain, these regulate several of the autonomic bodily functions, like heartbeat and breathing, and coordinate our movements. The midbrain does a great deal of relaying of information and is responsible for much of the sensory data obtained by the eyes and ears.

The forebrain, in the front, contains what are referred to as the higher brain centers. The cerebrum, the site for our thinking, reasoning, and language, is located in the forebrain. In humans, the cerebrum is divided into left and right hemispheres or sides. Each side is divided again into four lobes. The occipital lobe receives and analyzes visual information. The temporal lobe deals with memory, hearing, and some language functions. The frontal lobe regulates movement and handles language production. The parietal lobe deals with sensations. These two halves communicate by means of a bundle of axons called the corpus callosum, and each side of the brain controls the opposite side of the body.

In humans, the entire brain is protected by the very hard and thick bones of the skull. Between the brain and the skull are three layers of protective tissue called meninges, and a clear, colorless fluid called cerebrospinal fluid that acts as a shock absorber. The brain also contains a group of connected structures that make up what is called the limbic system. Sometimes called the "emotional brain," this system is made up of the olfactory bulb and the amygdala, which senses smells; the hypothalamus, which controls basic drives like hunger, thirst, and sex; the pituitary gland, that secretes growth and other hormones; the hippocampus, which works for memory; and the thalamus, which coordinates sensory information. Overall, the limbic system is responsible for the basic drives, emotions, and involuntary behavior that are critical for an animal's survival.

The human brain weighs about 3 pounds (48 ounces) and contains about a trillion neurons. It is one of the largest organs in the body and its cells use about 20 percent of the body's oxygen. The human cerebrum (the large rounded structure of the brain occupying most of the cranial cavity) is bigger than all of the other parts of the brain put together. The brain does most of its growing after birth, which is part of the reason that human childhood is so long. This growth continues until about the age of 18, but learning takes place until death, and the brain is constantly "remodeling" itself. In other words, the brain creates and preserves new connections in response to new experiences.

THE BRAIN AND THE FUNCTION OF MEMORY

The phenomenon of memory is still not well understood, but scientists know that the brain has short-term and long-term memory. In the short-term memory, the brain holds on to certain information for only a certain amount of time. In the long-term memory, it stores selected information for many years. The brain also regularly enters an altered state of consciousness called sleep. While there is still much to learn about sleep, it is known that sleep allows the body to rest and repair the daily wear and tear it receives. It somehow also lets the brain "recharge" its batteries.

Brain disorders can be serious, since the brain is such an important organ. Organic diseases like Parkinson's and Alzheimer's disease are a leading cause of death. Mental disorders like dementia and schizophrenia often render individuals incapable of having a normal life. Strokes, in which the flow of blood to the brain is temporarily cut off usually by a clot, disable 500,000 Americans each year. According to the part of the brain impacted by the blockage, the individual can suffer some localized paralysis or disability. If any part of the brain is without blood for even a few minutes, its cells die.

[See alsoCirculatory System; Nervous System ]