Brain | Research & Encyclopedia Articles

Brain

With its hundreds of billions of interconnectednerve cells, or neurons, continuously transmitting electrochemical signals to each other, the human brain is the most complex object yet discovered. Fortunately, our understanding of the brain has been greatly aided in recent years by various technologies that allow scientists to "see" the brain, or "map" its electrochemical activity, without the surgery or autopsy required in the past.

In higher animals, the brain and spinal cord constitute the central nervous system. Not all animals have brains, though all vertebrates do. The brain of the adult human being weighs between three and four pounds (1.35-1.8 kg). The human brain is protected by he cranium and three membranes, called meninges; the space between the two inner meninges is filled with cerebrospinal fluid. The fundamental building block of the brain in all animals is the nerve cell. This cell conducts electrical and chemical signals through an axon extending from the cell body. Shorter branches, called dendrites, conduct signals back to the cell body. A fatty substance called myelin sheaths the axons, protecting them from other nerve impulses nearby. Between any two nerve cells is an infinitesimal gap, called a synapse, across which electrochemical signals are transmitted in the continuously active brain.

Although astoundingly complex, the brain has been described in simple terms as having evolved from the overdevelopment of one end of the nervous system, becoming generally larger and more complex in vertebrates, especially mammals. In human beings, the brain begins developing in the four week old embryo starting as bulges at one end of the neural tube and forming into the hindbrain, midbrain, and forebrain. These three divisions are shared by all vertebrate brains, whether of sharks, cats, or humans.

The most primitive part of the brain from an evolutionary standpoint, the hindbrain (or brainstem) consists of the medulla, the pons, and the cerebellum (or "little brain"). These structures regulate several autonomic functions not consciously controlled but necessary for survival. For instance, the portion of the hindbrain at the top of the spinal cord regulates breathing, the heartbeat, and the diameter of blood vessels. All of the nerves running between the spinal cord and the brain pass through the medulla, an important junction for controlling deliberate movement. When signals for deliberate movement pass through the medulla they cross over from one side of body's movements on the right side, and vice versa. Besides its role in deliberate movement, the medulla contains sets of nerves for organs in the chest and abdomen, for head and shoulder movements, and for such functions as salivating, swallowing, tasting, hearing, and maintaining equilibrium. Just above the medulla at the brainstem's top, the pons bridges the lower brainstem and the midbrain. It connects the cerebellum and cerebral cortex, the brain's largest, most recently evolved part. In the pons originate many facial and head nerves that regulate some eyeball movements and facial expressions. Working with nerves from the medulla, nerves from the pons also control breathing and maintain equilibrium. The hindbrain's largest part and the second largest part of the human brain the cerebellum conveys signals for movement from the cerebral cortex to the spinal cord, and from there to the muscles. Simultaneously, the cerebellum receives signals from activated muscles and joints and compares these against the movement signals from the cerebral cortex, thereby enabling adjustments to be made. The cerebellum alone does not initiate movement, but reroutes and refines signals for movement received from the cortex.

The midbrain, located between the hindbrain and forebrain, serves mainly to relay both sensory and motor nerve impulses passing between the pons and spinal cord and the thalamus and cerebral cortex. In many animals the midbrain is important in processing visual and olfactory information.

The forebrain comprises the cerebrum, with its thin outer cortex, and the limbic system. The human cerebrum consists of left and right hemispheres, each divided 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 houses Broca's area, which handles language production. The parietal lobe deals with sensations. The brain's two hemispheres communicate through a thick nerve bundle called the corpus callosum, which enables one side of the brain to transmit what it learns to the other side. Nerve tracts from the right and left sides of the cerebellum connect with the opposite cerebral hemisphere.

The limbic system, which is also called the "emotional brain," forms a loose circuit of linked structures throughout the brain. Among the limbic system's structures are the olfactory bulb, by which odors are received; the amygdala, which receives input from both the olfactory system and cerebral cortex; the hypothalamus, which controls hunger, thirst, sex, and rage, as well as other vital drives and emotions; the pituitary gland, which secretes at least six hormones vital for growth and other functions; the hippocampus, which plays an important role in the memory's consolidation of recently acquired information; and the thalamus, the "great relay station" located at the forebrain's base. The thalamus consists of two oval nerve masses that sort information about sight, sound, taste, and touch. Together with other sensory systems of the brain, neurons from the thalamus make up the reticular activating system. This system appears to filter means of maintaining consciousness. Overall, the limbic system is responsible for the basic drives, emotions, and involuntary behavior that are crucial for an animal's survival, among them, fear, anger, pain, pleasure, and sexual stimulation.

Brain disorders such as schizophrenia, dementia, Alzheimer's Disease, and alcoholism are a leading cause of death in the United States. They are also the nation's most common cause of social, economic, and psychological disability. For those reasons, the federal government is a primary sponsor of brain research, which can lead to drugs and other medical advances for dealing with brain disorders. In 1990, President George Bush proclaimed the Decade of the Brain. It was launched with a symposium sponsored by the National Institute of Mental Health and the Institute of Medicine and was attended by the world's leading neuroscientists. These are scientists from many disciplines whose work is unveiling the intricate structures and complex process that make up the human brain, offering hope for cures to the brain's many diseases.