CONCEPT I: The Body's Control System

All human behavior, from the simplest response to the most complicated thought, is regulated by the nervous system and the endocrine system. Together these two systems regulate all of human behavior.

CONCEPT II: The Brain

Particular brain regions are associated with certain functions, a phenomenon called the localization of function.

The human brain is very similar to the brains of other mammals, having been shaped in structure and function by millions of years of evolution. Yet, differences do exist. Human brains, as well as those of some animals, contain lateralized brain structures, which appear on one side of the brain but not the other.

Invertebrate animals have no backbones, and simpler members of this group, such as jellyfish, have a network of nerves, but no central nervous system. More complicated animals, such as worms, have neurons organized into a nerve cord, which includes aggregates of neurons, called ganglia, which do some processing of information. Vertebrates have a sheath of bone to surround and protect the nerve cord, and the ganglion at the head end functions as a brain. The major parts of the brain - hindbrain, midbrain, and forebrain - are often associated with stages of evolutionary development. The hindbrain and midbrain control reflexive behavior; the forebrain controls emotions, motivation, and thought.

During the prenatal period, more than 100 billion nerve cells are generated, about half of which will die during early development. These neurons organize during early development, which is accomplished by establishing extensions from the neuron. The sequence seems to be this: A growth cone leads the growth of an axon or dendrite, and chemical markers help direct the axon or dendrite toward its final target. Environmental stimulation also affects the way the neurons organize: When a certain function is used and exercised, the associated brain structure enlarges and becomes elaborated. This plasticity of the developing brain allows for constant new connections and reconnections.

Several techniques have been developed to study the functioning of the brain. The oldest method, clinical observation, involves careful observation of behavior in people with localized brain damage due to injury, tumor, or stroke. The deficits experienced by brain-damaged individuals indicate the function localized to the damaged area.

The brain can also be mapped by stimulating areas through chemicals (which are delivered to a specific area) or through weak electrical impulses (which are delivered through implanted electrodes). Surgically produced lesions in laboratory animals produce behavioral deficits. Lesions typically cause the opposite effect from chemical or electrical stimulation.

An electroencephalograph (EEG), which measures electrical response patterns in the brain by attaching to the head sensitive electrodes, can be used to reveal the brain's general response pattern. Computers are now used to average out responses over many trials, thereby reducing error. Many mental processes have their own characteristic EEG patterns. It is also possible to measure the electrical activity of a single neuron: Its activity can be recorded by inserting a microelectrode into the brain of a laboratory animal until it comes in contact with a neuron.

Scientists can map the locations and functions of various brain structures by injecting otherwise traceable neurotransmitters or chemicals which will become neurotransmitters. Other techniques of measuring activity or observing structures in the brain include the PET (positron emission tomography) scan, which detects a radioactive tracer chemical related to sugar and stored in the neurons, the CAT (computerized transaxial tomography) scan, which uses Xrays to yield three-dimensional pictures of brain structures, and MRI (magnetic resonance imaging), which makes use of magnetic fields in brain cells to prepare a recording of brain structures.

The brain consists of two regions: the central core (composed of hindbrain, midbrain, thalamus, and hypothalamus) and the cerebral hemispheres, or forebrain (which includes the limbic system and the cortex).

The central core includes several structures devoted to survival functions. The hindbrain is the top of the spinal cord and includes the medulla (which controls aspects of circulation, breathing, chewing, and salivation, and facial movements), the pons (which integrates movements from the right and left body halves, transmits information to other brain parts, and may serve some function in sleep control), and the reticular formation (which affects attention and the sleep-waking cycle). The hindbrain also includes the cerebellum (which coordinates voluntary movement, regulates balance, and is involved in the remembering of simple motor tasks).

Also included in the central core are the midbrain, thalamus, and hypothalamus. The midbrain contains important centers for visual and auditory reflexes and conveys information between brain and spinal cord. The thalamus acts as a relay and processing station between sensory receptors and the higher brain and integrates various areas of the brain. The hypothalamus is an important structure that regulates much of the body's internal environment, such as temperature and metabolism. It also is involved in many basic behaviors, such as eating, drinking, sexual activity, fear, and aggression, and it plays an important role in regulating the pituitary gland.

The cerebral hemispheres, also known as the forebrain, are the most recent of the brain's evolutionary development, and are involved in the complex processes of learning, memory, language, and reasoning.

The limbic system, which is part of the forebrain, lies above the central core and is probably involved with behaviors that satisfy certain motivational and emotional needs. Of its major structures, the hippocampus is associated with learning and memory and movement and spatial organization, the amygdala is associated with learning and memory especially as related to emotion, and the septal area is associated with emotion and pleasure.

The cerebral hemispheres are the large twin structures at the top of the brain; they are involved with the higher-level functions of learning, speech, reasoning, and memory. These hemispheres are covered by the cortex. They can be separated into four lobes: the frontal (front), parietal (top back), temporal (side), and occipital (lower back). Thecentral fissure separates the frontal from the parietal lobe; the lateral fissure marks the top boundary of the temporal lobe.

The area of the frontal lobe next to the central fissure is called the motor cortex and is responsible for regulation of voluntary movements. The same area in the parietal lobe is the somatosensory cortex, and it registers body sensations. Control in the motor cortex and somatosensory cortex is contralateral- - that is, the left side of the brain controls the right half of the body, and vice versa. The temporal lobe of each hemisphere involves integration of sensory information, especially auditory signals. Visual information is primarily processed in the occipital lobe.

Neuropsychologists attempt to understand the brain's role in complex behaviors. Many years ago, Karl Lashley demonstrated that most of the brain is involved in most of the things we do. Now we know, however, that localization of function is a basic feature of brain organization, but any behavior requires the integrated activity of numerous brain structures.

The two cerebral hemispheres each have certain special abilities they control, a phenomenon called lateralized function. For example, speech centers, including Wernicke's area and Broca's area, are generally housed in the left hemisphere. Studies with patients who have had the fibers connecting the two hemispheres, called the corpus callosum, severed to treat severe epilepsy show further evidence of the lateralized functions of the brain. Studies of these split-brain patients have shown that language, mathematics, and analytical thinking are generally left-brain activities; perception of spatial relationships and artistic abilities are generally attributes of the right hemisphere. It is important to remember, however, that in normal people with intact corpus callosums the hemispheres are in constant communication with each other. The two hemispheres have much in common, and complex activity involves many locations in both hemispheres.

Some brain disorders can be treated with drugs. The symptoms of Parkinson's disease, which involves the degeneration of dopamine-releasing axons in the substantia nigra, can sometimes be improved by giving patients L-DOPA, a building block of dopamine. Alzheimer's disease is characterized by a buildup of senile plaques, or clumps of brain proteins, which are associated with acetylcholine deficits. Researchers are exploring the use of nerve growth factor (NGF) as a means of preventing this tissue degeneration. Transplants of small sections of brain tissue may hold some promise for treatment of diseases such as Parkinson's. They may be able to serve as bridges for existing axons to grow through and also as sources for new cells. Finally, repeated practice on tasks can sometimes improve performance, perhaps by training a new brain pathway to take over the function lost to impairment.