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| The human nervous system is a complex network of neural cells that carries messages and regulates bodily functions and personal behavior. The individual cells of the nervous system are called neurons. The parts of a neuron include the cell body, dendrites and axons. Neurons are separated from each other by a tiny synaptic gap. Chemical substances called neurotransmitters transmit electrical messages across the synapse. The central nervous system is composed of the brain and the spinal cord. The peripheral nervous system carries messages to and from the rest of the body. It is composed of both the somatic and autonomic nervous system. The autonomic nervous system consists of the sympathetic division, which prepares the body for stress, and the parasympathetic division, which plays its primary role under peaceful conditions. Scientists have developed exciting brain-imaging techniques that can create images of the activities of the living brain. These techniques include the EEG, the PET scan, and the MRI. The brain has three major parts. The first of these, the hindbrain, contains a) the medulla, which controls breathing and a variety of reflexes; b) the pons, which is concerned with balance, hearing, and some parasympathetic functions; and c) the cerebellum, which is chiefly responsible for maintaining muscle tone and coordinating muscular movements. A second part of the brain, the midbrain, is a center for reflexes related to vision and hearing. The forebrain, the third major part of the brain, includes two distinct areas. The first area contains the thalamus, a switching station that routes sensory information to the appropriate areas of the brain, the hypothalamus, which is involved with our motives and emotions, and most of the limbic system. The second area contains the cerebral cortex, which controls conscious experience, intellectual activities, the senses, and voluntary actions. The cortex contains two cerebral hemispheres connected by the corpus callosum. Each hemisphere of the cortex contains four lobes: the frontal lobe is involved with speaking and voluntary movement; the parietal lobe is involved with the sense of touch; the temporal lobe is involved with hearing and understanding language; and the occipital lobe is involved with vision. The cerebral cortex is also important in processing emotions. The brain is viewed as a developing system. Each part of the brain interacts with the entire nervous system, and the parts work together in intellectual, physical, and emotional functions. The endocrine system contains glands that secrete hormones. This system influences emotional arousal, metabolism, sexual functioning, and other bodily processes. The pituitary gland is referred to as the master gland, since its hormones help to regulate the activity of the other glands. The adrenal glands secrete epinephrine and norepinephrine, which are involved in emotional arousal and which affect the metabolic rate and sexual arousal. The islets of Langerhans secrete glucagon and insulin, which control blood sugar and energy levels. The gonads produce sex cells for human reproduction and also estrogen and testosterone, which are important to sexual functioning and the development of secondary sex characteristics. The thyroid gland secretes thyroxin, which helps control the metabolic rate. The parathyroid glands secrete parathormone, which controls the level of nervous activity. The pineal gland, attached to the top of the thalamus, secretes melatonin. Human characteristics and behaviors are influenced by genetic inheritance. The cells contain structures called chromosomes, which consist of strands of DNA. Genes are made up of varying sequences of adenine, thymine, guanine, and cystine; genes are the basic units of inheritance. Most normal human cells contain 46 chromosomes arranged in 23 pairs. The sex cells, however, each contain only 23 chromosomes and are capable of combining into a new zygote with a unique set of chromosomes. Researchers interested in hereditary influences on behavior have used twin studies and adoption studies. Although inheritance plays a significant role in influencing human behavior, environmental and other personal factors also play important roles. Research into disorders such as schizophrenia and Alzheimer's disease underscores the influence of the brain in both normal and disturbed behavior. |