The brain has always attracted psychologists because its physiological activity governs most behaviors. Made up of billions of neurons, the human brain is more intricate than any computer. Most of the behaviors described earlier were controlled by the brain. It is the center for “higher” mental functions such as thinking and memory. It processes the messages carried to it by the peripheral nervous system and the spinal cord, makes decisions about what responses are necessary, and sends orders to the muscles to carry out the appropriate behavior. The brain can best be understood by examining its three major divisions: the hindbrain, the midbrain, and the forebrain. Keep in mind that it is the interactions of many brain structures that control behavior.
The hindbrain connects the spinal cord with the rest of the brain. It controls the basic life support functions of the body, such as breathing and heart rate. The hindbrain consists of the medulla, pons, cerebellum, and reticular formation.
The medulla, the part of the hindbrain that directly connects the spinal cord with the brain, helps regulate basic bodily functions, such as breathing and heart rate. The medulla is where a number of peripheral nervous system cranial nerves, which include sensory and motor nerves, enter the central nervous system. Lying just above the medulla is the pons which, through the cranial nerves, plays a role in controlling such behaviors as eating and facial movement. Next to the medulla is a convoluted structure known as the cerebellum, which controls body balance, movement, and muscle coordination, allowing us to walk and run without tripping.
The reticular formation is a network of neurons extending from the medulla up to the higher brain centers found in the forebrain. This structure, sometimes called the reticular activating system (RAS), acts as a gate, allowing relevant sensory information such as that involved in arousal and sleep to enter the brain. All of the structures in the hindbrain work together and are influenced by other parts of the brain.
The midbrain is a relatively small region that connects the hindbrain with the forebrain. Although the midbrain appears to function mainly as a relay station for messages coming into the brain, it also contains structures that play a role in seeing, hearing, and movement.
The forebrain is the largest and most complex part of the brain. It not only influences many of the basic life support functions controlled by the midbrain and hindbrain, but also is responsible for the uniquely human higher level behaviors such as thinking and speaking. The major structures of the forebrain include the thalamus, hypothalamus, and cerebrum.
The hypothalamus and related structures formed a circuit for emotional behavior, called the limbic system. Located in the most primitive part of the forebrain, called the rhinecephalon, it is a collection of structures that influence motivation and emotional behaviors such as sex, aggression, fear, pleasure, and pain. The structures usually included in the limbic system are the thalamus, hypothalamus, amygdale, mammillary bodies, fornix, and septal area.
The thalamus is an important relay station for sensory information, including visual, auditory, tactile (touch), pain, and temperature stimuli. Messages from these sense organs are channeled into the thalamus, and from there are carried into a specific part of the forebrain for interpretation and action.
An important part of the limbic system is the small structure called the hypothalamus. It is involved in the motivation of such behaviors as eating, drinking, having sex, sleeping, and regulating temperature, and influences the pituitary gland which regulates biochemical reactions in the body. The hypothalamus also influences the part of the peripheral nervous system that regulates the internal body organs.
Other areas in the limbic system are also important in a wide variety of behaviors. The hippocampus is important in learning and memory. The amygdale and septal area are involved in fear, aggression, and other social behaviors. Research has shown that reward sites for ESB are located throughout the brain, but are especially concentrated around the medical forebrain bundle, a tract that runs through the limbic system.
The cerebrum, the largest part of the forebrain, consists of two distinct structures called hemispheres. The two hemispheres are connected by the corpus callosum, a bundle of nerves that keeps each hemisphere informed about what is happening in the other. Basically, the left hemisphere controls the right side of the body and the right hemisphere controls the left side. For proper motor coordination, each hemisphere must know what the other is doing. An activity involving both sides of the body, such as crossing our hands or tying out shoes would be difficult if our two cerebral hemispheres were not connected.
The outermost payer of the cerebrum, the cerebral cortex, is the part of the brain that renders humans truly unique. Here is where messages from our sense organs are interpreted and stored, and where decisions about behavior are made. The cortex has distinct sections, or lobes, that control different activities. Your occipital lobes allow you to interpret what you see in the environment. Your parietal lobes are responsible for interpreting somatosensory senses such as touch, pain, and temperature.
Although each lobe of the brain is primarily responsible for one main function (frontal lobes, movement and memory; occipital lobes, vision; temporal lobes, hearing; and parietal lobes, somatosensory senses), much of the area of the cerebral cortex is involved in multiple sensory or motor functions. These association areas of the brain help integrate information for successful adaptation.
In general, each hemisphere of the cerebrum controls the opposite side of the body: the left half controls the right side of the body and the right controls the left. But the two hemispheres are not totally redundant in function. The left hemispheres are not totally redundant in function. The left hemisphere is usually considered to be the more logical, analytical, and verbal half, exerting greater control over manual dexterity, reading, language, and understanding speech. The right hemisphere tends to process nonverbal information and is more concerned with emotions, imagination, and artistic information.