The nervous system consists of the central nervous system (the brain and the spinal cord) and its outlying neural network, the peripheral nervous system; the latter communicates with the glands and muscles and includes the sensory receptors for seeing, hearing, smelling, tasting, touching, feeling pain, and sensing stimuli within the body. The circulatory system circulates the blood and also carries the important chemical agents known as hormones from the glands to all parts of the body. Both these communication systems are very important in overall human behavior.

The smallest unit of the nervous system is the single nerve cell, or neuron. When a neuron is properly stimulated, it transmits electrochemical signals from one place in the system to another. The nervous system has 12.5 billion neurons, of which about 10 billion are in the brain itself.

One part of the peripheral nervous system, the somatic system, transmits sensations into the central nervous system and carries commands from the central system to the muscles involved in movement. Another part of the peripheral nervous system, the autonomic system, consists of two divisions that have opposing functions. The sympathetic division arouses the body by speeding the heartbeat, dilating the pupils of the eye, and releasing adrenaline into the blood. The parasympathetic division operates to calm the body by reversing these processes.

A simple example of communication within the nervous system is the spinal arc, which is seen in the knee-jerk reflex. A tap on the patellar tendon, just below the kneecap, sends a signal to the spinal cord via sensory neurons. This signal activates motor neurons that trigger a contraction of the muscle attached to the tendon; the contraction, in turn, causes the leg to jerk. Thus, a stimulus can lead to a response without involving the brain, via a connection through the spinal cord.

Circulatory communication is ordinarily slower than nervous-system communication. The hormones secreted by the body’s endocrine glands circulate through the body, influencing both structural and behavioral changes . The sex hormones, for example, that are released during adolescence effect many changes in body growth and development as well as changes in behavior, such as the emergence of specific sexual activity and the increase of interest in the opposite sex. Other hormones may have more direct, short-term effects; for instance, adrenaline, which is secreted when a person faces an emergency, prepares the body for a quick response—whether fighting or flight.

3. Psychoanalysis

Psychoanalysis, name applied to a specific method of investigating unconscious mental processes and to a form of psychotherapy. The term refers, as well, to the systematic structure of psychoanalytic theory, which is based on the relation of conscious and unconscious psychological processes.

Theory of Psychoanalysis

The technique of psychoanalysis and much of the psychoanalytic theory based on its application were developed by Sigmund Freud. His work concerning the structure and the functioning of the human mind had far-reaching significance, both practically and scientifically, and it continues to influence contemporary thought.

The Unconscious

The first of Freud’s innovations was his recognition of unconscious psychiatric processes that follow laws different from those that govern conscious experience. Under the influence of the unconscious, thoughts and feelings that belong together may be shifted or displaced out of context; two disparate ideas or images may be condensed into one; thoughts may be dramatized in the form of images rather than expressed as abstract concepts; and certain objects may be represented symbolically by images of other objects, although the resemblance between the symbol and the original object may be vague or farfetched. The laws of logic, indispensable for conscious thinking, do not apply to these unconscious mental productions.

Recognition of these modes of operation in unconscious mental processes made possible the understanding of such previously incomprehensible psychological phenomena as dreaming. Through analysis of unconscious processes, Freud saw dreams as serving to protect sleep against disturbing impulses arising from within and related to early life experiences. Thus, unacceptable impulses and thoughts, called the latent dream content, are transformed into a conscious, although no longer immediately comprehensible, experience called the manifest dream. Knowledge of these unconscious mechanisms permits the analyst to reverse the so-called dream work, that is, the process by which the latent dream is transformed into the manifest dream, and through dream interpretation, to recognize its underlying meaning.

Instinctual Drives

A basic assumption of Freudian theory is that the unconscious conflicts involve instinctual impulses, or drives, that originate in childhood. As these unconscious conflicts are recognized by the patient through analysis, his or her adult mind can find solutions that were unattainable to the immature mind of the child. This depiction of the role of instinctual drives in human life is a unique feature of Freudian theory.

According to Freud’s doctrine of infantile sexuality, adult sexuality is an end product of a complex process of development, beginning