All contents Copyright © WE MOVE 1999.   Last Update: December 29, 2000

Acetylcholine (ACh): A neurotransmitter present at junctions of nerve and muscle cells and various sites of the central nervous system, including the cerebral cortex and the basal ganglia. Primary functions of acetylcholine include regulating the delivery of messages from neurons to skeletal muscle fibers, smooth (involuntary) muscle fibers, and effector organs as well as between nerve cells in the brain and spinal cord. Acetylcholine also functions as a vasodilator and triggers certain actions within the parasympathetic nervous system, such as lowering blood pressure and slowing heart and breathing rates.

Action tremor: a tremor that occurs during the performance of voluntary movements. Such tremors include postural, isometric, kinetic, and intention tremors.

Activities of daily living (ADL): functions that are typically performed as part of a person's daily routine, such as dressing, bathing, eating, toileting, leisure activities, socialization, and other functions of daily living.

Agonist: A muscle whose contraction executes an intended movement.

Akinesia: absence of movement or loss of the ability to move such as temporary or prolonged paralysis or "freezing in place."

Alpha 2-adrenergic agonist: A drug that reduces the activity of the neurotransmitter norepinephrine by stimulating certain receptors within the central nervous system (central presynaptic alpha 2-adrenergic receptors). The release of norepinephrine triggers action within the sympathetic nervous system. This part of the nervous system regulates certain involuntary activities during stress, such as increasing the heart rate, deepening breathing, and raising blood pressure. Norepinephrine also plays a role in regulating mood and emotion. The administration of alpha 2-adrenergic agonists may result in a reduced heart rate and lowered blood pressure and have depressive or sedative effects.

Amplitude: the "size" or "height" of a tremor; the extend or breadth of a tremor's range.

Antagonist: (1) A drug that blocks a receptor, preventing stimulation. (2) A muscle whose contraction opposes an intended movement.

Anticholinergics: Drugs that block the action of acetylcholine, a neurotransmitter whose effects oppose dopamine. By blocking acetylcholine's action, these drugs increase dopamine's ability to control movement.

Anticonvulsant medication: An agent that prevents or arrests convulsive seizures.

Apraxia: Loss of the ability to sequence, coordinate, and execute certain purposeful movements and gestures in the absence of motor weakness, paralysis, or sensory impairments. Apraxia is thought to result from damage to the cerebral cortex, such as due to stroke, brain tumors, head injury, or infection. It may also occur as a result of impaired development of the cortex as in certain neurodevelopmental disorders, including Rett syndrome. Apraxia may affect almost any voluntary movements, including those required for proper eye gaze, walking, speaking, or writing.

Archimedes spirals: a relatively simple test used to evaluate tremor severity. During this test, the patient is asked to draw increasingly wider circles on a piece of paper.

Asterixis: involuntary, jerking or flapping movements, especially of the hands. Extending the patient's arm with the wrist bend in a backward position may induce this form of tremor, which may be associated with advanced liver disease.

Ataxia: impaired ability to coordinate voluntary movement, e.g., that seen in gait disturbances.

Athetosis: Involuntary, relatively slow, writhing movements that essentially flow into one another. Athetosis is often associated with chorea, a related condition characterized by involuntary, rapid, irregular, jerky movements. Although athetosis may be most prominent in the face, neck, tongue, and hands, the condition may affect any muscle group. Athetosis may occur in association with certain neurodegenerative diseases, such as Huntington's or Wilson's disease, or cerebral palsy (CP). It may also result from infections affecting the brain or the use of particular medications.

Augmentation: A phenomenon that may occur as a result of the use of certain medications (particularly levodopa). Augmentation is characterized by the emergence of worsening symptoms earlier in the day (e.g., early evening, afternoon, or morning). Many people who take levodopa and some who take other dopamingeric agents develop augmentation, especially those who have severe symptoms or are taking high doses of the drug.

Autosomal dominant trait: a genetic characteristic that may be expressed when carried by only one of a pair of matched genes on a chromosome. A dominant gene "masks" or overrides the effect of the other gene in the pair.

Axons: Nerve fibers. Axons are the relatively slender extensions of neurons that transmit nerve impulses away from nerve cell bodies. The ends of the axons or "terminals" release chemical substances known as neurotransmitters, enabling the transmission of nerve impulses to other neurons or effector organs. The whitish, fatty, protein-containing substance called myelin forms an insulating, protective, cylindrical sheath around some axons, serving to increase the speed and efficiency of nerve impulse transmissions.

Baclofen: A mainstay of antispastic drug therapy, baclofen is a GABA agonist that stimulates the GABAB autoreceptor. When stimulated, this autoreceptor inhibits muscle activity. In experimental systems, baclofen inhibits the release of noradrenaline, and acetylcholine from peripheral nerve endings, and acetylcholine, noradrenaline, dopamine, serotonin, and glutamate from CNS neurons.

Basal ganglia: Specialized nerve cell clusters of gray matter deep within each cerebral hemisphere and the upper brainstem, including the striate body (caudate and lentiform nuclei) and other cells groups such as the subthalamic nucleus and substantia nigra. The basal ganglia assist in initiating and regulating movement.

Benzodiazepines: a class of drugs that interferes with chemical activity in the nervous system and brain, serving to reduce communication between nerve cells and to a "slowing down" the central nervous system. Such medications promote sleep, relieve anxiety, reduce restlessness, and relax muscles.

Beta-adrenergic receptor: a specialized molecular structure on the surface membrane of a neuron that selectively receives the neurotransmitter norepinephrine. Reception of this neurotransmitter causes changes in the neuron that increase its likelihood of "firing" or sending its own signal to other neurons. The activities of norepinephrine affect that part of the nervous system involved in the control of some involuntary body functions, such as blood pressure regulation, etc. (sympathetic nervous system).

Biosynthesize: To form or produce (during normal physiologic functions) a chemical compound in the body.

Biphasic: having two distinct stages or phases.

Bradykinesia: The gradual loss of spontaneous movement; slowness of voluntary movements.

Brainstem: The region of the brain consisting of the medulla oblongata, pons, and midbrain. The brainstem primarily contains white matter interspersed with some gray matter. This area of the brain serves as a two-way conduction path, conveying nerve impulses between other brain regions and the spinal cord. In addition, most of the 12 pairs of cranial nerves from the brain arise from the brainstem, regulating breathing, digestion, heartbeat, blood pressure, pupil size, swallowing, and other basic functions.

Carnitine: A natural substance found in skeletal and cardiac muscle and the liver. Carnitine serves to transport fatty acids across mitochondrial membranes, thereby playing an important role in energy production and the metabolism of fatty acids.

Central nervous system (CNS): The brain and spinal cord. The CNS, which receives sensory impulses from and sends motor impulses to the peripheral nervous system (i.e., nerves outside the CNS), plays an essential role in the coordination and control of the entire body.

Central oscillators: nerve cells that discharge signals that result in alternating fluctuations of electrical impulses along certain tracts of the central nervous system.

Cerebral cortex: The outer region of the brain's cerebral hemispheres. Comprised of gray matter, the cerebral cortex contains several deep folds (gyri) and grooves (sulci or fissures). Two sulci divide the surfaces of both cerebral hemispheres into four distinct lobes that are named for overlying bones of the skull. These include the frontal, temporal, occipital, and parietal lobes. The cerebral hemispheres are joined by a thick band of nerve fibers known as the corpus callosum. The cerebral cortex is responsible for integrating higher mental functioning and conscious thought, sensations, and general movements.

Chemodenervation: the local relaxation of muscle activity due to deactivation of the muscle's nerve supply through injection of a chemical substance such as phenol or botulinum toxin type A.

Circadian: A rhythm of biological functions occurring in a 24-hour periodic cycle (e.g., sleeping, eating, etc.).

Clonus: movements characterized by alternate contractions and relaxations of a muscle, occurring in rapid succession. Clonus is frequently observed in conditions such as spasticity and certain seizure disorders.

Co-contraction: The simultaneous contraction of agonist and antagonist muscles.

Cogwheel phenomenon: rhythmic brief increase in resistance during passive movement about a joint.

Computerized tomography (CT) imaging or scanning: An advanced diagnostic scanning technique during which cross-sectional images of tissues and organs are produced by passing x-rays through the body at various angles. In some cases, a contrast medium, which is opaque on x-rays, may be injected intravenously to produce enhanced images of certain tissues, organs, or blood vessels.

COMT inhibitors: Drugs that block catechol-O-methyltransferase, an enzyme that breaks down dopamine. COMT inhibitors include entacapone (Comtan®) and tolcapone (Tasmar®).

Congenital: Existing at birth.

Contracture: Shortening of a muscle due to chronic lack of stretching.

Contralateral: the opposite side. For example, during brain surgery for essential tremor, if the surgery is performed on the right side of the brain, the left side of the body will be affected.

Controlled-release formulation: A form of a drug (e.g., L-dopa) that is absorbed slowly by the digestive system, prolonging the duration of effect of each dose.

DC inhibitors: Drugs that block decarboxylase, one type of enzyme that breaks down dopamine. Also called AADC inhibitors, they include carbidopa and benserazide.

Decarboxylase: A category or subclass of enzymes that removes or eliminates a molecule of carbon dioxide from a carboxylic group. Carbidopa is an inhibitor of the decarboxylation of levodopa.

Dendrites: The relatively narrow, branching projections that extend from the cell bodies of neurons. Nerve cells may contain multiple dendrites, which are stimulated by neurotransmitters, receive impulses from the nerve fibers (axons) of other neurons, and convey them toward their nerve cell bodies.

DNA: Deoxyribonucleic acid. DNA is a spiraling, ladder-like (helical) molecule that is the carrier of the genetic code. Nucleic acids are the primary components of the chromosomes within the nuclei of cells. DNA is also found in mitochondria, the rod-like structures outside the nuclei of cells that function as a primary source of cellular energy.

Dopamine: A neurotransmitter that controls movement and balance and is essential to the proper functioning of the central nervous system (CNS). Dopamine assists in the effective transmission of electrochemical signals from one nerve cell (neuron) to another.

Dopamine agonist (DA): A drug that acts like dopamine. DAs combine with dopamine receptors to mimic dopamine actions. Such medications stimulate dopamine receptors and produce dopamine-like effects.

Dopamine autoreceptor: A type of dopamine receptor that acts like a thermostat, preventing excess dopamine release as levels rise.

Dopamine receptor: A molecule on a receiving nerve cell (neuron) that is sensitive (or receptive) to stimulation (arousal) by dopamine or a dopamine agonist. At least five types have been identified including D1, D2, D3 receptors and the dopamine autoreceptor.

Dopamine receptor antagonist: A pharmacologic agent that binds to and blocks the action of dopamine receptors, essentially hindering receptor activity by preventing stimulation by dopamine.

Dopaminergic dysfunction: Malfunction of dopamine receptors.

Dysarthria: disordered speech or impaired articulation of speech.

Dysesthesias: Unpleasant sensations that are produced in response to normal stimuli.

Dyskinesia: An abnormal involuntary movement that results in jerky or repetitive motions. Dyskinesias are common in many neurologic movement disorders including late Parkinson's disease (PD), restless legs syndrome, etc.

Dyskinesias while awake (DWA): Uncontrolled, sporadic movements of the legs and, in some cases, the arms. These movements may be very rapid (myoclonic) or quite slow and prolonged (dystonic); they usually disappear upon voluntary action. Some researchers suspect that these movements may represent a wakeful form of periodic limb movements in sleep (PLMS).

Dyspraxia: Partial loss of the ability to coordinate and perform certain purposeful movements and gestures in the absence of motor or sensory impairments.

Dystonic: Referring to dystonia; sudden jerky or repetitive movements and muscle spasms due to impaired muscle tone and abnormal muscle rigidity.

Effector organs: Organs that produce a nerve-stimulated effect, such as a muscle contraction or glandular secretion.

Electrical stimulation (ES): A test in which a small electrical charge is applied to a muscle via a needle to determine the level of muscle responsiveness.

Electrocardiogram (ECG, EKG): A noninvasive, diagnostic test that assesses the electrical activity of the heart muscle (myocardium). The heart's conduction system transmits electrical impulses that serve to coordinate contractions of the heart. During an ECG, the electrical activity of the cardiac conduction system is translated into recordings that assist in evaluating any abnormal alterations. The test may be administered while a patient is at rest or performing certain designated exercises. ECGs may help to detect or characterize cardiac conduction defects, abnormal heart rhythms (arrhythmias), enlargement (cardiac hypertrophy), or localized damage in heart muscle.

Electroencephalography (EEG): A noninvasive, diagnostic technique that records the electrical impulses produced by brain cell activity. An EEG reveals characteristic brain wave patterns that may assist in the diagnosis of particular neurologic conditions, such as seizure disorders, impaired consciousness, and brain lesions or tumors.

Electromyography (EMG): A diagnostic test that records the electrical responses of skeletal muscles while at rest and during voluntary action and electrical stimulation. During this test, a small needle is inserted into a muscle to record the level of activity.

Electrophysiologic studies: The study of the electrical activity associated with a specific body function (e.g., EMG, EEG, etc.).

Endogenous opiate system: A morphine-like substance that is produced within the body.

Epidemiological study: examination of the distribution of disease as well as the determining factors related to specific diseases or health-related problems in a specific population.

Ergot: A plant alkaloid produced by the fungus Claviceps purpurea.

Ergot-derived medication: A medication that has a chemical structure based on ergot, a plant alkaloid produced by a fungus called Claviceps purpurea. Permax® and Parlodel® are examples of ergot-derived medications that may be used to treat certain neurologic movement disorders.

Esophageal atony: Lack of normal muscle tone within the esophagus, the muscular tube that transports food from the throat to the stomach.

Flexion: The act of bending (as opposed to extending) a joint.

Free radicals: Unstable molecular fragments that can damage cells. One hypothesis holds that free radical formation in the substantia nigra causes the loss of nigral cells (leading to Parkinson's disease).

Frequency: number of cycles or repetitions within a fixed unit of time such as the number of cycles per second (Hertz or Hz). For example, essential tremor is typically 4 to 12 Hz.

Froment's sign: Increased resistance to passive movements of a limb that may be detected upon voluntary activity of another body part.

Functional Magnetic Resonance Imaging (fMRI): A noninvasive, diagnostic scanning procedure that produces detailed, computerized images. The use of a "contrast agent" or dye enhances the detail of the images. Sequencing of these images may help physicians visualize the body's functioning, thus aiding differential diagnosis.

Gait: the style or manner of walking.

Gait apraxia: Loss of the ability to consciously sequence and execute the movements required to coordinate walking. Gait apraxia may result in unsteady walking patterns; "toe-walking"; a widely based, jerky gait; and balance difficulties.

Gamma knife radiosurgery: a highly specialized technique using a device that produces ionizing radiation to produce a lesion in the target tissue. This device focuses a beam of high intensity irradiation to a targeted area and is used as localized therapy to treat individuals with certain brain diseases (e.g., brain tumors, certain movement disorders, etc.).

Gastroesophageal reflux: Backflow of stomach contents into the esophagus. This condition may be chronic and cause weakness of the lower esophageal sphincter, the ring-shaped muscle located at the junction of the esophagus and stomach.

Gastrostomy tube: A plastic tube inserted into the stomach through a surgical incision in the abdomen. A gastrostomy tube is used to deliver liquified food to the digestive system when swallowing becomes dangerous or difficult.

Glutamate: An amino acid that is a primary excitatory neurotransmitter in the central nervous system. This chemical agent plays an essential role in initiating and transmitting nerve impulses, crossing synapses to stimulate postsynaptic neurons.

Gray matter: Nerve tissue that primarily consists of nerve cell bodies, dendrites, and unmyelinated axons, thus having a gray appearance. In contrast, white matter predominantly contains myelinated nerve fibers.

Hallucination: An illusion or sensory perception that is not caused by an environmental or external stimulus and occurs while one is awake. Hallucinations are a possible side effect of high dosages of certain drugs used to treat neurologic movement disorders.

Hemizygote: Refers to a cell, organism, or individual with only one of a pair of genes for a specific trait. This term is often used to describe males who inherit one copy of an X-linked disease trait. It may also refer to the state in which genetic material is deleted from one member of a chromosomal pair.

Hertz (Hz): cycles per second.

Hyperkinetic: Characterized by excessive movement because of abnormally increased motor activity or function. Certain movement disorders are termed "hyperkinetic" such as tics or essential tremor.

Hyperventilation: Abnormally rapid breathing usually accompanied by air swallowing. Hyperventilation results in excessive intake of oxygen and increased elimination of carbon dioxide, which may eventually lead to a disturbance in the body's acid-base balance.

Hypokinetic: Diminished movement and decreased motor function. Some movement disorders are hypokinetic, such as Parkinson's disease.

Idiopathic: Refers to a disease of unknown cause that appears to be spontaneous in origin.

Implantable Pulse Generator (IPG): A device that is placed under the skin near the collarbone as part of a surgical procedure known as deep brain stimulation. Wire leads from electrodes implanted in the brain are connected to the pulse generator, which then delivers continuous high frequency electrical stimulation to the thalamus via the implanted electrodes. This form of stimulation probably "jams" the nucleus and therefore modifies the message in the movement control centers of the brain, serving to suppress tremor.

Joint contractures: Permanent flexing or extension of joints in fixed postures due to shortening of muscle fibers. Contractures, abnormal fixation of the limbs, and associated deformity may result from prolonged immobility of developing joints.

Magnetic resonance imaging (MRI): A diagnostic scanning technique during which radio waves and an electromagnetic field are used to help create detailed, cross-sectional images of specific organs and tissues. MRI is often considered a particularly valuable imaging technique for studies of the brain and spinal cord because of the MRI’s ability to scan images from various angles and provide strong contrast between healthy and abnormal tissues.

MAO-B inhibitors: Inhibitors of the enzyme monoamine oxidase B. MAO-B helps break down dopamine; inhibiting it prolongs the action of dopamine in the brain.

Monotherapy: A drug used alone to treat a disease (opposite of combination therapy).

Muscle tone: The low level of contraction in a muscle not being intentionally contracted.

Mutation: A change in a gene, such as loss, gain, or substitution of genetic material, that alters its function or expression. This change is passed along with subsequent divisions of the affected cell. Gene mutations may occur randomly for unknown reasons or may be inherited.

Myoclonic: Pertaining to myoclonus or irregular, involuntary, shock-like contractions or spasms of a muscle or muscle group.

Nerve conduction velocity (NCV) test: A diagnostic study during which both sensory and motor nerves are repeatedly stimulated in order to measure the speed at which nerve impulses are conducted. Unusually slow conduction velocities suggest damage to nerve fibers (e.g., loss of the protective covering surrounding certain nerve fibers [demyelination] or other disease process).

Neuron: An individual nerve cell.

Neuroprotective effect: Having the ability to prevent or slow the death of neurons. The drug selegiline (Eldepryl®) may have a neuroprotective effect, possibly by preventing formation of free radicals.

Neuroreceptor: Specific sites on the surface of a nerve cell to which certain special substances (neurotransmitters) bind, initiating the conduction of impulses (or signals) to other nerve cells.

Neurotransmitter: A specialized substance (such as norepinephrine or acetylcholine) that transfers nerve impulses across spaces between nerve cells (synapses). Neurotransmitters are naturally produced chemicals by which nerve cells communicate.

Nigrostriatal system: Referring to the substantia nigra, the striatum, and the connection between them.

Non-ergotoline medication: A dopamine agonist medication that has action similar to that of pergolide (Permax®) and bromocriptine (Parlodel®) but is not derived from an ergot. Examples of nonergotoline medications that are used to treat some neurologic movement disorders include pramipexole (Mirapex®) and ropinirole (Requip™.)

Noradrenaline (norepinephrine): A vasoconstrictor whose release triggers action within the sympathetic nervous system, the part of the nervous system that regulates certain involuntary responses during times of stress. Noradrenaline serves as a neurotransmitter that stimulates receptors (alpha- and beta-adrenergic receptors) at effector organs supplied or innervated by certain sympathetic nerve fibers (postganglionic adrenergic fibers). In addition to its production by neurons, noradrenaline is also secreted by the inner region of the adrenal glands (adrenal medulla). The release of noradrenaline serves to deepen breathing, raise blood pressure, and increase the heart rate. It also plays a role in regulating mood.

NREM sleep: Non-REM (nonrapid eye movement) sleep, which is the normal period of dreamless, lighter sleep as compared to the deeper REM sleep. NREM sleep accounts for the major portion of sleep.

Obstructive sleep apnea: A sleep disorder characterized by episodes of temporary cessation of breathing due to obstruction of the airway.

Opiate: Any preparation or derivative of opium.

Opioids: Means "like or similar to opium" and refers to medications with opium-like effects. This term is used to describe any synthetic drug that possesses the characteristic properties of opiate narcotics but is not derived from opium.

Orthostatic hypotension: A sudden decrease in blood pressure that occurs when the affected individual sits up or stands. In some cases, it may occur as a side effect of certain medications.

Palate: The bony and muscular structure that forms the roof of the mouth and separates the oral and nasal cavities.

Parasympathetic nervous system: Part of the nervous system that, together with the sympathetic nervous system, forms the autonomic nervous system (ANS). The ANS controls the functioning of involuntary structures, including the heart, glands, and smooth muscle. The parasympathetic nervous system regulates nerve transmissions to certain effector organs under normal conditions, as opposed to times of stress. It serves to "steady" involuntary activities and conserve or restore energy. Parasympathetic responses may include slowing of the heart and breathing rates, contraction of the pupils, an increase in glandular activity, and an acceleration in the rate of peristalsis.

Paresthesias: An abnormal sensation, such as prickling, tingling, or burning that is frequently subjective and experienced without environmental stimulation.

Parkinsonism: A constellation of the following symptoms: tremor, rigidity, bradykinesia (slow movements), and loss of postural reflexes. Although classically seen in Parkinson's disease, parkinsonism may have other causes. In the elderly, parkinsonism may be caused by dopamine-blocking drugs, multiple system atrophy, striatonigral degeneration, Shy-Drager syndrome, cortico basal degeneration, diffuse Lewy body disease, and Alzheimer's disease with parkinsonism. In younger people, parkinsonism may be caused by juvenile-onset dystonia/parkinsonism, Westphal variant of Huntington's disease, Wilson's disease, L-dopa-responsive dystonia, Hallervorden-Spatz disease, and progressive pallidal degeneration.

Penetrance: The regularity or frequency with which a specific gene yields its effect or "is expressed." For example, if a specific gene produces a disease in all individuals who carry the gene, it is termed 100% penetrant. If a gene produces the disease less than 100% of the time, it is not fully penetrant.

Periodic apnea: Episodes of the temporary cessation of spontaneous breathing. Periodic apnea may be characterized by absence of airflow, absence of chest wall movements, or airway obstruction that may result from poor control of tongue movements, impaired coordination of upper airway muscles, or other abnormalities.

Periodic limb movements in sleep (PLMS): Repeated stereotypic movements of the limbs (usually the legs) that occur during sleep. These movements typically consist of upward extension of the great toe and foot as well as flexion of the ankle, knee, or hip; they occur every 15 to 40 seconds and 0.5 to 6.0 seconds, usually during NREM sleep and have a duration of 0.5 to 6.0 seconds.

Peripheral edema: unusual fluid accumulation, resulting in swelling of the arms or legs.

Peristalsis: Rhythmic, wave-like contractions of smooth or involuntary muscle fibers that propel food through the digestive tract.

Physiologic tremor: A form of rapid tremor that may occasionally occur in any individual. Physiologic tremor is typically the result of fear, anxiety, or excitement. Physiologic tremor may affect the arms, legs, and, in some patients, the face or neck.

Positron emission tomography (PET): An advanced, computerized imaging technique that uses radioactively-labels substances (e.g., glucose) to demonstrate chemical and metabolic activities in the brain as well as track other brain functions. Brain structures are also visualized on PET scans.

Positron emission tomography (PET) scanning: A noninvasive, diagnostic procedure used to record the uptake and distribution of certain substances in the tissues and organs of the body. Thus, PET assists in evaluating various metabolic and physiological activities in the body. During this procedure, three-dimensional, color-coded images are created based upon the detection of positively charged particles (positrons). The positrons are produced by certain biochemicals (e.g., glucose) carrying radioactive substances that have been introduced into the body (via intravenous injection). PET scanning may help to detect abnormal biochemical patterns associated with certain neurologic conditions, such as Parkinson’s disease, brain tumors, seizure disorders, and psychiatric abnormalities.

Postural tremor: Any tremor that is present while an individual voluntarily maintains a position against gravity, such as holding the arms outstretched.

Precursor: Literally a "forerunner," such as a substance that precedes another in a biochemical reaction.

Pulmonary: Referring to the lungs.

Range of motion (ROM): The extent of a joint's free movement. The normal ROM of the elbow, for instance, carries the forearm through a half-circle. Passive ROM is tested while the limb is relaxed. Active ROM is movement controlled by the patient.

Receptor: A molecule on a neuron that receives a neurotransmitter. Reception of the neurotransmitter causes changes in the neuron which increase or decrease its likelihood of "firing," or sending its own signal to other neurons. Dopamine receptors are located on corpus striatum neurons, and on nigral cells.

Reflex: Involuntary, predictable response to a particular stimulus.

REM sleep: The period of sleep that is associated with dreaming, rapid eye movements (REM), and certain involuntary muscle movements.

Restorative sleep: A refreshing sleep, i.e., receiving a sufficient amount of rest to feel refreshed and to engage in the activities of daily living without experiencing excessive daytime sleepiness (EDS).

Retina: The nerve-rich membrane that forms the innermost region of the eye. As light passes through other areas of the eye (including the cornea, pupil, and lens), it is bent or refracted to focus on the retina, which contains nerve cells that respond to light (photoreceptors). Images formed on the retina are converted into nerve impulses that are transmitted to the brain via the optic nerve (second cranial nerve).

Rhythmical myoclonus: Involuntary, shock-like contractions or spasms of a muscle or muscle group that occur in a rhythmical pattern. This usually occurs as a result of a lesion in the central nervous system.

Rigidity: Stiffness and resistance to movement. May be a symptom of a neurologic movement disorder such as Parkinson's disease.

Scoliosis: Lateral or "sideways" curvature of the normally vertical line of the spine. Progressive spinal deformity may be associated with numerous neuromuscular and neurodevelopmental disorders (neurogenic scoliosis), such as cerebral palsy, spinal muscular atrophy, and Rett syndrome. The severity of the deformity varies, depending upon the degree of weakness, the nature and progression of the underlying disorder, or other factors.

Seizures: Episodes of uncontrolled electrical activity in the brain. These abnormal electrical disturbances may lead to involuntary jerking, spasms, or rhythmic contraction and relaxation of certain muscle groups and impaired control of involuntary functions such as breathing or bladder or bowel control. There may also be loss of consciousness or sensory or behavioral abnormalities.

Sensorimotor: Pertaining to both the sensory and motor aspects of a bodily function.

Serotonin (3-[2-aminoethyl]-5-indolol): A vasoconstrictor found in many tissues of the body that is present in relatively high concentrations in portions of the central nervous system (e.g., hypothalamus, basal ganglia, etc.). Serotonin functions as a neurotransmitter, regulating the delivery of messages between nerve cells (neurons). This neurotransmitter is thought to play some role in regulating consciousness and mood states. Serotonin is also present in other tissues of the body such as the intestines and blood platelets.

Sialorrhea: Excess production of saliva, or increased retention of saliva in the mouth, due to difficulty swallowing.

Side effect: An effect of a drug that is not the main or intended effect. Side effects may be of no concern, or they may be bothersome or even dangerous, in which case they may limit the upper dose a patient can tolerate. Side effects are also called adverse effects.

Sinus bradycardia: an abnormally slow heart rate (i.e., of less than 60 beats per minute).

Sleep latency: The interval of time between "settling in" to go to sleep and the onset of sleep.

Sleep maintenance: Once asleep, the ability to remain asleep.

Spasmodic dysphonia (SD): A manifestation of dystonia. SD involves the muscles of the larynx and surrounding muscles and therefore involves speech. In individuals with SD, speech in blocked by intermittent spasms of the voice box (larynx).

Spasmodic torticollis (ST): A form of dystonia involving the muscles of the neck, and therefore called "cervical dystonia." As a result of the abnormal involuntary contractions of the neck muscles, the head may be rotated, tilted, flexed, extended, or any combination of these postures. The movements may be quick, sustained, or patterned and, therefore, may be associated with tremor.

Spasticity: An abnormal increase in muscle tone that may be caused by certain types of damage to the nerve pathways regulating muscles. Spasticity is a common complication of cerebral palsy, brain injuries, spinal cord injuries, multiple sclerosis, and stroke. Spasticity can lead to incoordination, loss of function, pain, and permanent muscle shortening, or contracture.

Sporadically: Occurring intermittently, randomly, or in isolation.

SSRIs: Selective serotonin reuptake inhibitors. Drugs belonging to this class are antidepressant agents that selectively inhibit the absorption of serotonin at certain nerve membranes (e.g., presynaptic neuronal membranes). These drugs increase the concentration of serotonin within the central nervous system and enhance serotonin's neurotransmission activities.

Stereotactic: refers to use of precise coordinates to identify deep structures of the brain. The coordinates may be obtained by fitting a patient's head with a special frame and taking a CT or MRI scan. The position of the brain structures relative to the frame permits fine localization of the deep brain structures. Stereotactic methods are used during brain surgery for tremor, Parkinson's disease, and dystonia. These brain structures are located with precise, three-dimensional coordinates.

Stereotypic: Inappropriate, persistent repetition of particular bodily postures, actions, or speech patterns. These are typically involuntary, rhythmic, coordinated, and purposeless movements, postures, or vocalizations that may appear ritualistic or purposeful in nature. Stereotypies may be associated with a variety of neurologic and behavioral disorders, such as Tourette syndrome, obsessive-compulsive disorders, Rett syndrome, restless legs syndrome, schizophrenia, and autism.

Stereotypical: Conforming to a repetitive pattern as in repetition of particular movements or gestures.

Stretch reflex: Contraction of a muscle stimulated by rapid stretching.

Stretch-loop circuits: Pathways of electrical impulses along specific nerve fibers (alpha motoneurons) that result in a "stretch" reflex in a muscle.

Striatum: An area of the brain that controls movement and balance. It is connected to and receives signals from the substantia nigra.

Substantia nigra: A dark band of gray matter deep within the brain where cells manufacture the neurotransmitter dopamine for movement control. Degeneration of cells in this region may lead to a neurologic movement disorder such as Parkinson's disease.

Sympathetic nervous system: Part of the nervous system that along with the parasympathetic nervous system forms the autonomic nervous system (ANS). The ANS regulates the functioning of involuntary structures, such as the glands, smooth muscle, and heart. The sympathetic nervous system regulates certain involuntary responses during times of strong emotion, such as fear or anger; exercise; or other forms of stress. These responses, sometimes referred to as the "fright-or-flight response," include widening of the pupils; increased heart and breathing rates; constriction of most blood vessels, raising blood pressure; widening of those blood vessels that supply skeletal muscles; and reduction in the rate of peristalsis.

Synapse: The junction between two neurons or between a neuron and an effector organ. As a nerve impulse reaches a synapse, the terminal or end of the "presynaptic" neuron's axon releases neurotransmitters, which diffuse across the gap and bind to receptors of the "postsynaptic" neuron or the effector organ (i.e., muscle or gland). As the electrical impulse is conducted across the gap, electrical changes are triggered that serve to continue or hinder transmission of the impulse.

Tendon: A tough fibrous cord of tissue that attaches muscle to bone (or other structures of the body).

Teratogenic: Possessing the ability to disrupt normal fetal development and causing fetal abnormalities.

Thalamus: A crucial area of the brain consisting of two relatively large masses gray matter. The thalamus receives and processes messages from the body regarding heat, cold, pain, pressure, smell, taste, and movement. Input from all the sensory organs (expect smell) enter the deeper areas of the brain through the thalamus. In addition, the thalamus is crucial to sensorimotor function.

Tone: Resistance or tension in a muscle when a limb or other body part is moved passively and in a relaxed state about a joint. A state of muscle tension balanced by partial contraction or alternate contraction and relaxation.

Toxic metabolites: Potentially harmful substances formed as the result of normal body functions.

Tremor: Rhythmic, involuntary, oscillatory (or to-and-fro) movements of a body part.

Upper motor neurons: Nerve cells extending from the brain to the spinal cord that control movement.

Vasoconstrictor: A chemical substance whose actions result in the narrowing of blood vessels.

Vasodilator: A chemical substance whose actions cause an increase in the diameter of blood vessels.

Vasomotor: Pertaining to the muscles and nerves that control blood vessel diameter, thereby regulating or modulating blood pressure.

Ventral intermediate (VIM) nucleus: A specific region of the thalamus. This area of the brain is involved in the control of movement and is the "target" area for thalamotomy and deep brain stimulation when treating patients with tremor.

White matter: Bundles of myelinated nerve fibers or axons. These nerve fibers have a creamy white appearance due to myelin, a whitish substance that primarily contains fats and proteins. Myelin forms a protective, insulating sheath around certain axons, functioning as an electrical insulator and ensuring efficient nerve conduction. The breakdown, destruction, or loss of myelin from a nerve or nerves (demyelination), such as seen in certain neurodegenerative diseases, results in impaired nerve impulse transmission.

X-linked dominant trait: Human traits, such as an individual's specific blood group, eye color, or expression of certain diseases, result from the interaction of one gene inherited from the mother and one from the father. In X-linked dominant disorders, the gene mutation for the disease trait is transmitted as a dominant gene on the X chromosome and therefore may "override" the instructions of the normal gene on the other chromosome, resulting in expression of the disease. Because females have two X chromosomes yet only one functioning copy is necessary, a percentage of X chromosomes with the mutated gene may not be expressed (random X chromosome inactivation). As a result, females often have less severe symptoms than affected males. In contrast, because males have one X chromosome from the mother and one Y chromosome from the father, those who inherit an X-linked dominant disease trait typically fully express the mutated gene on the X chromosome, causing a more severe form of the disorder that may result in lethality before or shortly after birth. Fathers with an X-linked dominant trait transmit the gene to their daughters but not to their sons. Mothers with a single copy of an X-linked dominant gene have a 50 percent risk of transmitting the gene to their daughters as well as to their sons.