A naturally occurring quaternary ammonium cation ester, with the formula CH₃(O)COC₂H₄N(CH₃)₃⁺, which plays a prominent role in nervous system function as a neurotransmitter. Acetylcholine (Fig. 1) is an organic compound released from certain autonomic nerve endings and acts in the transmission of nerve impulses to excitable membranes. The great importance of acetylcholine derives from its role in physiology as a neurotransmitter for cholinergic neurons (nerves), which innervate many tissues, including smooth muscle, skeletal muscle, the heart, ganglia, and glands. The effect of stimulating a cholinergic nerve, for example, the contraction of skeletal muscle or the slowing of the heartbeat, results from the release of acetylcholine from the nerve endings. Other types of nerves release other transmitters, for example, norepinephrine. See also: Nerve; Nervous system (vertebrate); Neurobiology; Neuron; Synaptic transmission

A voltage or rapid sequence of changes in the movement of ions across a cellular membrane that propagates an electrical signal to other cells for purposes of communication and physiological functioning in the body. Cells that can experience action potentials are known as excitable cells. In animals, these cells include neurons, muscle cells (for instance, cardiac muscle cells), and certain endocrine cells. In neurons and, specifically, through nerves (bundles of neurons), action potentials serve as a nerve impulse: an electrical signal transmitted to send information via the nervous system to be received by bodily tissues. Action potentials can elicit physiological functions, such as movement coordinated by muscles. In this particular example, action potentials originating from motor neurons within the central nervous system are propagated to the specific motor nerves within the peripheral nervous system, where muscle cells cause contractions of the muscle fibers, generating movement. Neurons within the parasympathetic nervous system also communicate with the cardiovascular system via action potentials to control heart rate during rest. See also: Cell (biology); Cell membrane; Nerve; Nervous system (vertebrate)

A stimulating drug that affects the brain and the body. Amphetamine is a stimulant of the central nervous system. Chemically, amphetamine is a racemic mixture of the l and d isomers of α-methyl-β-phenethylamine (see illustration). The l isomer has more pronounced effects on the body, whereas the d isomer (commonly referred to as dextroamphetamine or Dexedrine) has a greater effect on the brain. The first amphetamine drug to be synthesized was known as amphetamine sulfate or Benzedrine. On the whole, the pharmacological effects of amphetamine produce an increase in blood pressure, a relaxation of bronchial smooth muscle, a constriction of the blood vessels supplying the skin and mucous membranes, and a variety of alterations in behavior. See also: Brain; Central nervous system; Pharmacology; Psychotomimetic drugs

Any of a group of drugs of diverse chemical structure and physiological effects that are used for the relief of pain. In general, to qualify as an analgesic, a pharmacological drug must selectively reduce or abolish pain without causing significant impairment of consciousness, mental confusion, incoordination or paralysis, or other dysfunction of the nervous system. In this way, analgesics are distinguished from anesthetics, which medically are used for temporary and controlled loss of sensation and potentially loss of consciousness as well, for instance as anesthesia during surgery. Analgesics (see figure) can be broadly categorized as either anti-inflammatory drugs or as opioids (though many kinds of analgesics do not conform to these categories). Anti-inflammatory drugs, which decrease perception of pain by reducing local inflammation, are generally used for mild to moderate short-term pain relief as well as antipyretics for fever reduction. Common examples include nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen. Opioid analgesics alleviate pain by binding to opioid receptors in the brain and throughout the body. Common examples include fentanyl and hydrocodone. A popular variety of analgesic, acetaminophen, has pharmacological properties of both anti-inflammatory and opioid analgesics. See also: Anesthesia; Consciousness; Inflammation; Nervous system (vertebrate); Pain; Pharmaceutical chemistry; Pharmacology; Surgery

An abundant type of glial cell of the central nervous system. Astrocytes (Fig. 1) are specialized glial cells (nonneuronal cells that surround neurons) of the central nervous system (CNS) that exert numerous essential functions in both health and disease. Astrocytes (also known as astroglia) interact closely with neurons throughout the CNS and are critical for establishing and maintaining neural circuits that implement neurological functions. Astrocyte dysfunctions are increasingly recognized as playing important roles in a variety of neurological disorders. See also: Central nervous system; Glial cell; Nervous system (vertebrate); Nervous system disorders; Neurobiology; Neuron

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as insistence on sameness, obsessions, or stereotypic behaviors (for example, hand flapping, spinning, and toe walking). The condition varies greatly in the presenting symptoms, the timing of presentation, the range and severity of symptoms, and its association with other conditions. Recognition of autism is increasingly more common (Fig. 1), but the cause of autism is still under investigation, although genetics and parental age seem to be contributing factors. With intensive early intervention, significant improvements in a large percentage of children with autism can be achieved. See also: Autism and eye gaze abnormalities; Autism and the social brain; Developmental psychology; Factors related to risk of autism; Genetics; Nervous system (vertebrate); Psychotherapy; Sociobiology

The part of the nervous system that controls visceral (internal organ) functions of the body. The autonomic nervous system is a component of the peripheral nervous system, or PNS, which, along with the central nervous system, or CNS, represents the two main divisions of the nervous system. The PNS is further divided into the somatic nervous system and the autonomic nervous system. As a distinct system, the autonomic nervous system functions primarily at a subconscious level to innervate smooth muscle, cardiac muscle, and glands. The system regulates visceral processes involuntarily including those associated with cardiovascular activity, digestion, metabolism, and thermoregulation (bodily temperature). See also: Cardiovascular system; Central nervous system; Digestion; Gland; Metabolism; Muscle; Muscular system; Nervous system (vertebrate); Peripheral nervous system; Thermoregulation

Any of the self-sustained circadian (approximately 24-hour) rhythms that regulate daily activities, including sleep and wakefulness. A biological clock is any physiologic factor that functions in regulating innate organismal rhythms. Biological clocks were described as early as 1729 by the French scientist Jean Jacques d'Ortous de Mairan. He placed a plant in constant darkness and noticed that leaf movements continued to occur at specific times of day, despite the absence of the day-night cycle. The notion that this rhythmicity was still driven in some fashion by the Earth's rotation was resolved in the midtwentieth century, when it became clear that the period of self-sustained (free-running) oscillations usually does not match that of the environmental cycle (that is, the Earth's rotation); therefore, the expression "approximately 24 hours" is used. Moreover, the free-running period varies among species and also somewhat from one individual to another. Circadian rhythmicity is often referred to as the biological clock (Fig. 1). See also: Circadian clock (plants); Photoperiodism; Plant movements

A major mental disorder in which there are lifelong episodes of both mania and depression; also known as manic-depressive illness. Bipolar disorder, also termed manic-depressive illness or manic depression, is characterized by sudden and often unexplained mood swings, ranging from delirious mania to severe depression (see illustration). These mood changes are regularly accompanied by other mental and behavioral symptoms, including fluctuations of volition, activity level, and cognitive functioning. The first recognizable descriptions of mania and depression date back to the writings of Aretaeus of Cappadocia (a Greek physician who lived around 150–200 CE). The modern history of bipolar disorder begins in the mid-nineteenth century with the concept of folie circulaire (“circular insanity”), proposed by the French psychiatrist Jean-Pierre Falret. Later, around the beginning of the twentieth century, it was defined by the work of the German psychiatrist Emil Kraepelin. See also: Affective disorders; Depression; Mental disorders

A specialized biological barrier unique to the vasculature supplying the brain. Brain tissue is highly vascularized by a dense and hierarchical network of arteries, capillaries, and veins that supply essential nutrients and oxygen to the brain. The blood-brain barrier (BBB) is a critical feature of brain vasculature that protects brain cells called neurons, which, compared to other cells, are particularly sensitive to their environment, possess little capacity to regenerate or repair, and are highly metabolically active. The BBB forms a physical and metabolic barrier that is selectively permeable to essential biological substrates while preventing the entry of potentially harmful substances. These barrier properties limit the ability of many agents to cross the BBB, which makes delivery of therapeutic drugs to the brain difficult. During disease, BBB dysfunction can exacerbate this difficulty and result in a wide range of clinical impairments. See also: Blood vessel; Brain; Neuron