Glossary
M
Machado-Joseph disease:
Machado-Joseph disease, also known as spinocerebellar ataxia (SCA) type III, is probably the most common autosomal dominant SCA. This disorder was previously known as Azorean disease, as a large number of Portuguese families from the Azores Islands were found to be affected; however, SCA type III has now been reported in many other ethnic groups. In addition to unsteadiness of gait (ataxia), symptoms may involve parkinsonism, dystonia, and chorea. The affected protein is called ataxin-3; however, its function is not yet known.
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.
Magnetic resonance imaging (MRI):
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.
Mean:
An average; intermediate or middle position in value, quantity, or degree. The “arithmetic mean” is a statistical measure derived by adding a set of values and dividing the total by the number of values.
Mechanical pain:
Mechanical pain arises from pressure or touch.
Medially:
Toward the middle; closer to or oriented toward the midline or median plane of a body or structure. The “median plane” refers to the imaginary plane that divides a body into symmetrical parts.
Membrane lipid peroxidation:
the oxidative deterioration of polyunsaturated fatty acids in which polyunsaturated fatty acids are converted to lipid hydroperoxides.
Meningitis
Meningitis is an infection of the meninges, the thin three-layer membranes that cover and protect the brain and spinal cord.
MERFF syndrome (myoclonus epilepsy with ragged-red fibers):
A rare hereditary disorder characterized by neurologic and muscle abnormalities due to defects of genetic material (DNA) in mitochondria, the rod-like structures outside the nuclei of cells that serve as a primary source of cellular energy. Removal and microscopic examination of minute samples of muscle fibers reveal structurally abnormal mitochondria (a finding known as “ragged-red fibers”). Associated symptoms include muscle weakness; sudden, involuntary, “shock-like” muscle contractions induced by voluntary movements or in response to external stimuli (action or reflex myoclonus); repeated seizures (epilepsy); progressively impaired coordination of voluntary movements (ataxia); and/or gradual deterioration of intellectual functioning (dementia). Some affected individuals may also have additional abnormalities, such as hearing loss; degeneration of the nerves (i.e., optic nerves) that convey impulses from the retinas to the brain (optic atrophy); short stature; heart muscle disease (cardiomyopathy); or other findings.
Metabolism:
Refers to the ongoing chemical processes of cells of the body, including catabolism and anabolism. Catabolism or so-called “destructive metabolism” is the breakdown of complex chemical compounds into simpler substances, typically liberating or releasing energy. In contrast, anabolism or “constructive metabolism” refers to the “building up” or conversion of simple substances into more complex chemical compounds, requiring energy consumption (provided by catabolic processes).
Mini-Mental State Examination
This short clinician-administered test measures cognitive status (the ability to think, calculate, and reason) in adults. This test is used to measure the presence of impaired cognition or a change in cognition over time.
Mitochondrial:
Referring to mitochondria or rod-like structures inside cells of the body whose primary functions include the production and regulation of energy.
Monoamine:
A molecule containing one amine group. An amine is an organic compound containing nitrogen. The so-called “biogenic monoamines” are natural, biologically active compounds that often function as neurotransmitters, including dopamine, serotonin, noradrenaline, and epinephrine.
Monotherapy:
A drug used alone to treat a disease (opposite of combination therapy).
Motor fluctuations:
Motor fluctuations occur when levodopa is used to treat Parkinson’s disease. As the disease becomes worse, the number of cells in the brain that store dopamine decreases, the symptoms of Parkinson’s disease worsen, and levodopa is not as effective in controlling the symptoms. When this happens, a person is said to have “off” episodes.
Multiple sclerosis (MS):
A progressive disease of the central nervous system characterized by destruction of myelin (demyelination), the fatty substance that forms a protective sheath around certain long nerve fibers (axons). Myelin serves as an electrical insulator, enabling the effective transmission of nerve signals. Patients with MS may develop paresthesias, such as numbness or tingling; muscle weakness and stiffness; impaired coordination; abnormal reflexes; an inability to control urination (urinary incontinence); slurred speech; visual disturbances; and/or other symptoms and findings.
Multiple System Atrophy:
A neurodegenerative disorder characterized by parkinsonism, ataxia, and dysfunction of the autonomic nervous system.
Muscle tone:
The low level of contraction in a muscle not being intentionally contracted.
Mutated:
Altered; a mutation is a change in a gene.
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.
Myelin:
The whitish, fatty substance forming the segmented, multilayered wrappings or “sheaths” around certain long nerve fibers or axons. Myelin sheaths electrically insulate axons, serving to speed the transmission of nerve signals (action potentials).
Myelinated:
Referring to long nerve fibers (axons) that have myelin sheaths. Consisting of segmented, multilayered wrappings of myelin, a whitish protein, myelin sheaths wrap around certain nerve fibers, providing electrical insulation and serving to speed the transmission of nerve signals.
Myoclonic:
Pertaining to myoclonus or irregular, involuntary, shock-like contractions or spasms of a muscle or muscle group.
Myoclonus:
A neurologic movement disorder characterized by brief, involuntary, twitching or “shock-like” contractions of a muscle or muscle group. These jerk-like movements may be accompanied by periodic, unexpected interruptions involuntary muscle contraction, leading to lapses of sustained posture (known as “negative myoclonus”). So-called “positive” and “negative” myoclonus are often seen in the same individuals and may affect the same muscle groups. Myoclonus is often a nonspecific finding, meaning that it may occur in the setting of additional neurologic abnormalities and be associated with any number of underlying conditions or disorders. In other patients, myoclonus appears as an isolated or a primary finding. Depending on the underlying cause and other factors, the shock-like muscle jerks may occur repeatedly or infrequently; may tend to appear under specific circumstances (e.g., with voluntary movements or in response to specific external sensory stimuli); and may affect any body region or regions.