Brain

Brain, complex organ coordinating nerve activity and responsible for thought in higher animal forms. Invertebrates have only a rudimentary brain, most highly developed in the octopus. Vertebrates have brains more fully differentiated, consisting of forebrain, midbrain, and hindbrain. In highly developed vertebrate animals, the forebrain has developed into a large and highly differentiated cerebrum. The human brain is composed of some billions of interconnecting nerve cells and many more supporting cells (neuroglia). Together with the spinal cord, the brain makes up the central nervous system (CNS), which governs and coordinates the operations of all tissues and organs and is the physical basis of all mental activities: consciousness, sensation, thought, speech, memory, emotion, character, and skill.

The nervous system carries information in the form of electric signals. All the outlying nervous pathways—the peripheral nervous system, or PNS—converge on the CNS. Nerve fibers conduct in 1 direction only: Those that carry information from the sensory organs to the CNS are called afferent, or sensory pathways; those that carry impulses outward from the CNS to the muscles and glands are called efferent, or motor, pathways. Nervous pathways are made up of separate units—nerve cells known as neurons. The main parts of the neuron are the cell body, the axon, and the dendrites. Dendrites generally carry an impulse toward the cell body, and the axon carries it away from the cell body to another cell. The ends of the axon lie adjacent to, but not quite touching, the dendrites or cell bodies of other neurons; the region where the 2 nearly touch is called a synapse. Electrical impulses cannot cross synapses; the transfer is made by chemical substances called neurotransmitters. These substances have become extremely important because of the part they play in the pharmacological treatment of some psychiatric and neurological conditions. The full-grown brain is described as having 3 parts.

(1) The brain stem is chiefly a relay station for nervous pathways between the higher parts of the brain and the rest of the body; if it is damaged, sensory and motor functions are greatly impaired. It is also responsible for subvoluntary activities like digestion and respiration.

(2) The cerebellum, divided into hemispheres, each controlling a side of the body, is responsible for the coordination of voluntary muscular movements and for posture.

(3) The cerebrum, the highest center of the brain and the latest in evolutionary development, is responsible for sensation, thought, and the initiation of voluntary motor activity. It, too, is divided into hemispheres, each consisting of 2 main parts: (1) the basal ganglia, made up of a complicated collection of bunches of gray matter that clusters about the top of the brain stem, and (2) the cerebral cortex, also made of gray matter, separated from the basal ganglia by tracts of white matter. The cerebrum is customarily divided into 4 lobes: (1) The frontal lobe (lobus frontalis) controls voluntary motor patterns, the organization of the motor units necessary for speech, original thinking, and the evaluation of ideas. (2) The parietal lobe (lobus parietalis) is mainly concerned with the reception of body sensations and memory in regard to language and learning. The region also has a role in spatial organization. (3) The temporal lobe (lobus temporalis) receives auditory sensations (the sense of hearing), participates in speech through auditory monitoring, plays a role in spatial organization, and is a memory mechanism. The temporal lobe has also been variously claimed to be concerned with memory and dreams. (4) The occipital lobe (lobus occipitalis), in the back of the head, is the primary center for vision. The limbic system contains components of the frontal, parietal, and temporal lobes, and controls behavioral reactions to the external environment, possibly influenced by the internal environment, which may alter the excitability of the nervous system. It seems to operate in preserving the individual (feeding, fleeing, or fighting) or the species (reproduction).

The corpus callosum, a large white bundle of fibers that connects the cerebral hemispheres, appears concerned with the transfer of learning from 1 hemisphere to another. The functions of the corpus callosum include (1) correlation of images in the left and right halves of the visual field, (2) integration of sensations from paired limbs or for learning that requires motor coordination of the limbs, and (3) unification of cerebral processes of attention and awareness. The absence of a corpus callosum slows down the rate of learning.