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.
Globus pallidus: A major substructure
of the basal ganglia deep within the brain. Specialized groups of nerve cells in the globus pallidus function as an "intermediate relay system." This system processes and transmits information from the basal ganglia by way of the thalamus to areas of the brain that regulate complex motor functions (e.g., motor cortex, premotor area of frontal lobe).
A slowly progressive hereditary disorder characterized by abnormal accumulations of iron pigments in certain regions of the basal ganglia (e.g., globus pallidus and substantia nigra). This autosomal recessive disorder typically becomes apparent during late childhood or adolescence.
Associated symptoms may include progressive stiffness and resistance to movement (rigidity); increasing difficulties speaking and eating; sustained muscle contractions that result in repetitive twisting motions or unusual postures or positions (dystonia); the presence of multiple, irregular, jerky motions that may combine to appear flowing or writhing in nature (choreoathetosis); and/or other abnormalities.
Hallucinations: Perceptions that occur in the absence of external stimuli and while an individual is awake. Hallucinations may involve any of the senses, including hearing (auditory hallucinations), vision (visual hallucinations), smell (olfactory hallucin-ations), taste (gustatory hallucinations), and touch (tactile hallucinations).
Hallucinations may occur with high-dose
administration of certain medications or the abuse of particular drugs (hallucinogens) that cause increased stimulation or excitation of the central nervous system.
They may also be caused by chronic alcohol abuse; sensory deprivation; certain types of seizures due to impairment or damage of a limited area of the brain (e.g., temporal lobe epilepsy); psychotic illnesses characterized by disturbances in thought, behavior, and emotional reactions (schizophrenia); or certain organic disorders, such as Huntington's disease.
Hemichorea: Irregular, abrupt, relatively rapid involuntary (i.e., choreic) movements that are unilateral or involve muscle groups on one side of the body.
Hemiparesis: Weakness that affects one side of the body.
Hereditary: Inherited; inborn; referring to the genetic transmission of a trait, condition, or disorder from parent to offspring.
Heredodegenerative: Pertaining to genetically transmitted (hereditary) neurologic disorders characterized by progressive neuronal loss and other neurodegenerative changes (e.g., associated gliosis). For example, the term "heredo-degenerative dystonias" is used to refer to several, usually hereditary disorders in which neurodegenerative changes may lead to dystonia as a primary feature and other neurologic symptoms are typically present.
However, in some patients with such disorders, dystonia may not always develop and other neurologic features may be primary findings.
Disorders that have been categorized as heredodegenerative dystonias include ataxia-telangiectasia, Hallervorden-Spatz disease, Huntington's disease, Parkinson's disease, progressive supranuclear palsy,
neuroacanthocytosis, and Wilson's disease.
Homocystinuria: A metabolic abnormality characterized by excessive amounts of the amino acid homocystine in the urine. Homocystinuria, which may be transmitted as an autosomal recessive trait, may result from deficient activity of certain enzymes involved in the metabolic conversion of the amino acid methionine to cysteine.
hemorrhage: Bleeding; (such as in brain hemorrhage)
Huntington's disease (HD):
A hereditary, progressive, neuro-degenerative disorder primarily characterized by the development of emotional, behavioral, and psychiatric abnormalities; gradual deterioration of thought processing and acquired intellectual abilities; and movement abnormalities, including involuntary, rapid, irregular jerky movements (chorea) of the face, arms, legs, or trunk.
HD may be inherited as an autosomal dominant trait or, less commonly, appear to occur randomly for unknown reasons (sporadically). The disorder results from abnormally long sequences or "repeats" of certain coded instructions (i.e., unstable expanded CAG repeats) within a gene (located on chromosome p16.3). Progressive nervous system dysfunction associated with HD results from loss of neurons in certain areas of the brain, including the basal ganglia and cerebral cortex.
Hyperkalemic: Pertaining to hyperkalemia, a condition characterized by unusually high concentrations of potassium in the blood. Potassium, a positively charged ion, helps to regulate water balance, nerve conduction, and muscle contraction.
Hyperkalemia may lead to various signs and symptoms, such as diarrhea, nausea, abdominal cramps, weakness, and, with increasing severity, abnormal heart rhythms and muscle paralysis.
Hyperkinetic: Characterized by excessive movement because of abnormally increased motor activity or function. Certain movement disorders are termed "hyper-kinetic" 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.
An endocrine condition characterized by insufficient secretion of parathyroid hormone due to injury, disease, or congenital abnormalities of the parathyroid glands. Parathyroid hormone plays an essential role in regulating calcium balance in the body.
Hypoparathyroidism results in low calcium blood levels (hypocalcemia), leading to various symptoms and findings. For example, hypoparathyroidism may cause a condition known as tetany in which increased excitability of nerves and muscles results in uncontrollable, painful muscle spasms and cramping; abnormal involuntary movements resembling chorea; spasmodic closure of the larynx (laryngospasm); and other findings.
Hypoparathyroidism may also lead to seizures; abnormal calcium deposits (calcifications) in the basal ganglia; loss of transparency of the lenses of the eyes (cataracts); dry, scaly skin; weakened tooth enamel; and/or other abnormalities.
Idiopathic: A disorder or condition of spontaneous origin; self-originated or of unknown cause. The term is derived from the prefix "idio-" meaning one's own and "pathos" indicating disease.
Idiopathic epilepsy: Recurrent seizures of unknown origin or cause; repeated episodes of uncontrolled electrical activity in the brain that occur in the absence of detectable brain lesions. Also known as essential or primary epilepsy, these conditions are often thought to have a genetic component. This may suggest that affected individuals have an inherited predisposition for recurrent seizures.
Immunogenicity: The ability to or the degree to which a particular substance may provoke an immune response; having the properties of an antigen or any substance that may trigger a particular immune reaction, such as the production of anti-bodies.
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.
Inhibition: The restraint, suppression, or arrest of a process or the action of a particular cell or organ; the prevention or slowing of the rate of a chemical or
an organic reaction. The term "reciprocal inhibition" refers to the restraint or "checking" of one group of muscles upon stimulation (excitation) and contraction
of their opposing (antagonist) muscles.
Inhibitor: A substance that blocks, restricts, or interferes with a particular chemical reaction or other biologic activity.
Innervate: To supply a body part, tissue, or organ with nerves or nervous stimulation.
Of subtle, gradual, or imperceptible development; referring to the development of symptoms that may not be recognized by an affected individual until the disorder in question is established.
Ion: An electrically charged atom or group of atoms due to the loss or gain of one or more electrons. Positively charged ions, known as cations, include potassium, calcium, sodium, hydrogen, ammonium, and magnesium. Negatively charged ions, called anions, include chloride, phosphate, and bicarbonate.
The transfer of ions across cellular membranes plays an essential role in vital bodily processes, including enzyme activation, protein metabolism, acid-base balance, nerve impulse transmission, and muscle contraction.
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.
Juvenile myoclonic epilepsy: A form of idiopathic epilepsy or recurrent seizures of unknown origin, with symptom onset typically occurring from approximately
12 to 16 years of age. The condition is characterized by sudden, involuntary, "shock-like" muscle jerks (myoclonus) that primarily occur during the morning
or with stress, fatigue, or alcohol consumption. Patients may later develop generalized tonic-clonic seizures associated with loss of consciousness and rhythmic contraction and relaxation of all muscle groups.