Schizophrenia: a psychiatric disorder characterized by hallucinations, delusions, disordered thinking, unusual speech or behavior and social withdrawal that impair the affected person’s ability to interact with others.

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.

Sedation: An induced state of quiet or sleep.

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.

Sequelae: Plural of sequela, which is any abnormal condition that occurs subsequent to and/or is caused by disease, injury, or treatment.

Sequence: As a noun, a sequence is a series of chemical bases—adenine (A), thymine (T), cytosine (C) and guanine (G)—in DNA or a string of amino acids in a protein. As a verb, sequence refers to the process of determining the order of the bases or amino acids.

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.

Sialidosis: A lysosomal storage disease in which deficiency of the enzyme neuraminidase leads to abnormal accumulation of certain complex carbohydrates (sialyloligosaccharides) in particular tissues and organs. There are different variants of the disorder, based upon age of onset, severity, and other factors. Sialidosis type I, also known as "cherry-red-spot myoclonus syndrome," usually becomes apparent during the second decade of life. Associated symptoms include the development of characteristic, cherry-red circular areas within the middle layers of the eyes; gradual loss of visual clarity; and sudden, involuntary, "shock-like" contractions (myoclonus) of muscles of the arms and legs. The myoclonus is progressive in nature and may be triggered by voluntary movements (action myoclonus) or certain external stimuli, such as sound (reflex myoclonus). Sialidosis is inherited as an autosomal recessive trait.

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.

Single photon emission computed tomography (SPECT): A noninvasive scanning procedure during which a radioactive substance known as a radionuclide is introduced into the body to help evaluate the function and structure of certain organs or tissues. The amount of the substance taken up by particular tissues may depend upon the amount of blood flow within such regions. For example, absence of radionuclide uptake in a targeted region may indicate a lack of blood flow in certain areas. Following intravenous administration of the radioactive compound, a specialized rotating camera detects the radiation emanating from the radionuclides in the form of particles known as protons. The recorded images may produce colorized, horizontal and vertical cross sections and be reconstructed by computer to create three-dimensional images. By evaluating the blood supply to particular tissues, SPECT may be particularly helpful in detecting certain changes within the central nervous system or the heart.

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

Sleep fragmentation: Sleep fragmentation is a continual disruption of sleep, which often leads to excessive daytime sleepiness. This disruption can occur as the result of a variety of factors, including sleep disorders, the need to get up to use the bathroom, pain, and a noisy or uncomfortable sleeping environment.

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.

Spasmolytic: Antispasmodic; referring to agents that may eliminate or relieve spasms, typically of involuntary (i.e., smooth) muscle, such as within the arteries, the intestine, the ring-shaped muscles around certain natural openings or passages (sphincters), the bladder, the muscular tubes that carry urine from the kidneys to the bladder (ureters), etc.

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.

Spinal cord: The cylindrical structure of nerve tissue that, together with the brain, comprises the central nervous system. The spinal cord is an extension of the medulla oblongata--which is part of the lowest region of the brain (brainstem)--and is contained within a central canal in the spinal column. The spinal cord and the brain are surrounded by a protective, 3-layered membrane (meninges). Cerebrospinal fluid (CSF) flows through the cavities (ventricles) of the brain, the spinal cord's central canal, and the space between the middle and inner layers of the meninges (subarachnoid space). The spinal cord consists of an inner core of gray matter and surrounding areas of white matter, composed of bundles of myelinated nerve fibers (axons) known as spinal tracts. These include ascending tracts that carry sensory impulses up the spinal cord to the brain and descending tracts that transmit motor impulses from the brain down the spinal cord. Nerves emerge from both sides of the spinal cord (i.e., spinal nerves) through the narrow gaps (foramina) between bones of the spinal column (vertebrae). The spinal nerves, which are attached to the spinal cord by specialized nerve bundles (spinal nerve roots), contain both motor and sensory neurons.

Spinocerebellar ataxias (SCA): This group of disorders, of which there are now 17 genetically identified types, are inherited as an autosomal dominant genetic trait and appear to be mostly due to increased numbers of repeats in the various genes involved (as in HD). All types of SCA involve degeneration of the cerebellum, causing impaired balance, walking, and coordination. Each type of SCA has its own typical pattern of symptoms; however, these may vary greatly from person to person. Some forms of SCA affect eye movements, swallowing, thinking, reflexes, or cause parkinsonism, chorea, or dystonia. In most types of SCA, the onset of symptoms occurs during adulthood; symptoms slowly become worse over the years.

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.

Stimulus: A stimulus is something that creates a response in a muscle, nerve, gland or other excitable tissue or organ of the body. The plural is stimuli.

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.

Substrate: A chemical substance that is acted upon by an enzyme is called a substrate.

Subthalamic nucleus: The subthalamic nucleus is an oval mass of gray matter located beneath the thalamus.

Sydenham's chorea: A usually self-limited condition in which chorea develops in association with an inflammatory disease caused by certain strains of streptococci bacteria. This disease, known as rheumatic fever, is characterized by the sudden onset of fever and joint pain, with subsequent inflammation of the heart (carditis), chest pain, skin rash, and other symptoms. If rheumatic fever involves the nervous system, Sydenham’s chorea may develop. This condition commonly affects children aged 5 to 15 or women during pregnancy. Sydenham's chorea involves involuntary, uncontrollable, jerky movements that gradually worsen in severity, potentially affecting arm movements, the manner of walking (gait), and speech. In most patients, the condition spontaneously resolves in weeks or months.

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.

Synthesis: The formation of a complex chemical compound through the union of simpler substances.

Systemic lupus erythematosus: an autoimmune disease involving multiple organ systems that is defined clinically and associated with antibodies directed against cell nuclei.

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