Zygote

zygote The cell that results when a spermatozoon (SPERM cell) penetrates an ovum (egg cell) during fertilization. Each GAMETE (sperm and ovum) is a haploid cell; it contains one half the genetic material necessary to support life. The zygote is a diploid cell; it contains all the DNA necessary to create a new life. Fertilization typically takes place in the fallopian tube. The zygote grows and divides as it makes its way through the fallo-

pian tube to the UTERUS, a journey of about five days. When it reaches the uterus the zygote implants into the endometrium, the thick, BLOOD-rich lining of the uterus. By the time implantation is complete the zygote has become a two-layered structure called a blastocyst.

For further discussion of the zygote within the context of the structures and functions of reproduction and sexuality, please see the overview section "The Reproductive System."

See also ASSISTED REPRODUCTIVE TECHNOLOGY (ART); CELL STRUCTURE AND FUNCTION; EMBRYO; FALLOPIAN TUBE; FETUS; OVA.

Psychiatry and psychology are disciplines within the practice of health care that deal with mental illness. Health-care professionals who provide care for people who have mental illnesses include psychiatrists, who are physicians (MDs or DOs); psychologists (PhDs); master's level therapists such as counselors (sometimes also called psychologists); social workers; and clinical registered nurse practitioners (CNRPs) who specialize in mental health. In the United States licensing requirements and practice limitations for mental health practitioners vary among states, though in all states only psychiatrists may prescribe medications.

This section, "Psychiatric Disorders and Psychologic Conditions," presents an overview discussion of mental health and mental illness and includes entries about psychiatric disorders, psychologic conditions, and their treatments. The section "The Nervous System" contains overview discussion and comprehensive entries about conditions that affect cognition, memory, and thought processes that arise from disease or injury to BRAIN structures that alters brain function.

Finding the Line between Mental Health and Mental Illness

Psychiatric disorders and psychologic conditions are those that doctors generally define as illnesses that arise from disrupted thought processes and their corresponding behaviors. However, the causes of mental illness remain poorly understood. Most psychiatric disorders reflect a mix of biochemical, behavioral, and genetic components. About 80 percent of people who have BIPOLAR DISORDER, for example, have other family members who have either bipolar disorder or clinical DEPRESSION that requires treatment.

The diagnosis of mental illness is often a significant challenge. One issue is that many of the symptoms that characterize mental illness are thoughts, feelings, and behaviors that everyone experiences to certain degree. However, in many situations it is less the symptoms themselves and more the level of dysfunction the symptoms cause

in the person's life that defines the line between mental health and mental illness.

For example, a person may sometimes engage in speaking as though in conversation with another person when no one else is in fact present or talking, but knows the dialogue is a process of thinking aloud and can willingly start and stop the behavior. The behavior may appear amusing or quirky to others but does not interfere with the person's ability to interact with other people and to function in the world and thus in itself does not constitute mental illness.

It is a different picture when a person engages in dialogue with voices no one else can hear, but believes the voices are real and thus cannot control his or her interactions with them. In such a situation the person may believe the voices provide instruction or guidance, and behaves as though following directions the voices give. The messages from the voices may be nonsensical, confusing, demanding, or demeaning to the person and characteristically interfere with the person's ability to function in the world. This behavior represents a clear break with reality and interferes with the person's ability to function in the world and thus constitutes mental illness.

The Diagnostic Path for Psychiatric Disorders and Psychologic Conditions

The diagnostic path for mental illness begins with the elimination of potential physical or physiologic causes for symptoms. Loss of inhibition, for example, may occur as a consequence of stroke, Alzheimer's disease, or brain tumor. There is a high correlation between untreated hypothy-roidism and bipolar disorder. When the doctor can eliminate physical causes, the emphasis shifts to psychologic evaluation to identify the disorder. Because many psychologic conditions and psychiatric disorders have overlapping symptoms, distinguishing among them is often a challenge.

In the United States most providers and insurance companies follow the diagnostic criteria and treatment algorithms detailed in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, commonly called the DSM. The current version of the DSM includes a Roman numeral in the title to differentiate it from previous versions; for example, DSM-IV indicates the fourth version. Though other diagnostic criteria and treatment methods are available and valid, the DSM reflects the conventional perspective and is widely accepted as the standard.

Treatment and Outlook for Psychiatric Disorders and Psychologic Conditions

The mainstays of treatment for psychiatric disorders and psychologic conditions are medications and therapy. As is the case with numerous physical health conditions, treatment often cannot cure the condition and so aims to instead manage symptoms. A person who has hypertension (high blood pressure) often needs lifelong antihyperten-sive medications and lifestyle modifications to

support cardiovascular health (such as WEIGHT LOSS AND WEIGHT MANAGEMENT and nutritious EATING HABITS). This therapeutic approach regulates the body's functions in regard to BLOOD pressure though does not end the underlying factors causing the hypertension. Likewise, a person who has bipolar disorder may need lifelong mood stabilizing medications in combination with PSYCHOTHERAPY and lifestyle measures to limit exacerbations of symptoms.

Combinations of different types of medications are common because the boundaries of many mental disorders are not clear cut and the biochemical functions that underlie them are similar. Both depression and anxiety often respond to treatment with selective serotonin reuptake inhibitor (SSRI) antidepressants, for example, even though they present different symptoms. As well, some mental disorders, such as SCHIZOPHRENIA and bipolar disorder, have multiple components that often require different types of medication to moderate symptoms.

Because most psychiatric disorders and psychologic conditions result from multiple factors, including genetic and physiologic, there is usually no way to prevent them from occurring. Many conditions respond to treatment to manage symptoms so that the condition has a negligible effect on the person's QUALITY OF LIFE. Some short-term, reactive, or narrowly defined conditions such as BRIEF REACTIVE PSYCHOSIS, POST-TRAUMATIC STRESS DISORDER (PTSD), or certain phobias are curable with appropriate treatment.

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