Neuron, the most basic elements of the nervous system, allows nerve impulses to pass from one part of the body to another. Information generally enters a neuron through its dendrites, is passed on to other cells its axon, and finally exits through its terminal buttons.

A coating called the myelin sheath protects most neurons. When a neuron receives a message to fire, it releases an action potential, an electric charge that travels through the cell. Neurons operate according to an all or none law: either they are at rest or an action potential is moving through them. There is no in-between state.

Once a neuron fires, nerve impulses are carried to other neurons through the production of chemical substances, neurotransmitters, which actually bridges the gaps-- known as synapses-- between neurons. Neurotransmitters may be either excitatory, telling other neurons to fire, or inhibitory, preventing or decreasing the likelihood of other neurons firing. Among the major neurotransmitters are acetylcholine (ACh), which produces contractions of skeletal muscles, and dopamine, which has been linked to Parkinson’s disease and certain mental disorders such as schizophrenia.

Neurons are linked to other neurons via networks, groups of organized communication links between cells. Among the major kinds of networks are linear circuits, multiple source/ convergent circuits, and single source/ divergent circuits.

The nervous system is made up of the central nervous system (the brain and the spinal cord) and the peripheral nervous system (the remainder of the nervous system). The peripheral nervous system is made up of the somatic division, which controls voluntary movements and the communication of information to and from the sense organs, and the autonomic division, which controls involuntary functions such as those of the heart, blood vessels and lungs.

The autonomic division of the peripheral nervous system is further subdivided into the sympathetic and parasympathetic divisions. The sympathetic division prepares the body in emergencies, and the parasympathetic division helps the body return to its typical resting state.

The central core of the brain is made up of the medulla (which controls such functions as breathing and the heartbeat,) the pons (which coordinates the muscles and the two sides of the body) the cerebellum (which controls the balance) the reticular formation (which acts to higher awareness in emergencies) the thalamus (which communicates messages to and from the brain) and the hypothalamus (which maintains homeostasis, or body equilibrium and regulates basic survival behaviors). The functions of the central core structures are similar to those found in other vertebrates; this part of the brain is sometimes referred to as the old brain.

The cerebral cortex-- the new brain -- has areas that control voluntary movement (motor areas) the senses ( the sensory area) and thinking, reasoning, speech, and memory (the association areas). The limbic system , found on the border of the old and new brains is associated with eating, reproduction and the experience of pleasure and pain.

The brain is divided into two halves or hemispheres, each of which generally controls the opposite side of the body from that in which it is located.

However, each hemisphere can be thought of as specialized in the function it carries out: the left is best at verbal tasks, such as logic, reasoning, speaking and reading; the right is best at non-verbal tasks, such as spatial understanding, pattern recognition, and emotional expression.

The growing body of evidence suggests that the brain is organized into a series of modules, separate units that carry out precise, specific functions. These modules, which are distributed throughout the brain, work interdependently and relatively simultaneously in processing information.

Genetics

To understand the biological foundation of psychology, we need to know something about hereditary influences. The field of behavior genetics combines the methods of genetics and psychology to study the inheritance of behavioral characteristics. The hereditary units we receive from our parents and transmit to our offspring are carried by structures known as chromosomes, which are found in the nucleus of each cell in the body. Most body cells contain 46 chromosomes. At conception, the human being receives 23 chromosomes from the father’s sperm and 23 chromosomes from the mother’s ovum. Each chromosome is composed of many individual hereditary units called genes. A gene is a segment of deoxyribonucleic acid (DNA), which is the actual carrier of genetic information. The DNA molecule looks like a twisted ladder or a double stranded helix (spiral). All DNA has the same chemical composition, consisting of a simple sugar (deoxyribose), phospate, and four bases—adenine, guanine, thymine, cytosine (A, G, T, C) . The bases can occur in any sequence along a strand constitute the genetic code. The fact that many different arrangement of bases are possible is what gives DNA the ability to express many different genetic messages.

A segment of the DNA molecule, the gene, will give coded instructions to a cell, directing it to perform a specific function (usually to manufacture a particular protein).