Glossary

(work in progress)

action potentials

Electrical signals produced by the activity of nerves and muscles. Can be measured by nerve conduction velocities (NCV) and electromyography (EMG). You can read more about electromyography and nerve conduction velocities here.

allele

One version of a gene at a given location (locus) along a chromosome.

apoptosis

One of the main types of programmed cell death (PCD). As such, it is a process of deliberate suicide by an unwanted cell in a multicellular organism.

atrophy

The partial or complete wasting away of a part of the body. In Dejerine-Sottas, affected people experience muscular atrophy as a result of loss of nerve supply to the muscles.

autonomic nervous system

In contrast to the voluntary nervous system, the "involuntary" or autonomic nervous system is responsible for homeostasis, maintaining a relatively constant internal environment by controlling such involuntary functions as digestion, respiration, and metabolism, and by modulating blood pressure. Although these functions are generally outside of voluntary control, they are not outside our awareness, and they may be influenced by ones state of mind.

autosomal dominant

Describes a trait or disorder in which the phenotype is expressed in those who have inherited only one copy of a particular gene mutation (heterozygotes); specifically refers to a gene on one of the 22 pairs of autosomes (non-sex chromosomes).

axon

Long projection of a neuron.

base pair

(Abbreviated bp.) Two nitrogenous bases paired together in double-stranded DNA by weak bonds; specific pairing of these bases (adenine with thymine and guanine with cytosine) facilitates accurate DNA replication. When quantified (e.g., 8 bp), it refers to the physical length of a sequence of nucleotides.

bulbar

The bulbar region of the brain is the brainstem, the nerves coming out of the brainstem are the bulbar nerves and the muscles they innervate are the bulbar musculature. Bulbar functions include eye movements, muscle of facial expression, speaking, and swallowing.

carrier

An individual who has a recessive, disease-causing gene mutation at a particular locus on one chromosome of a pair and a normal allele at that locus on the other chromosome; may also refer to an individual with a balanced chromosome rearrangement.

creatine kinase

This muscle enzyme is used to provide a component of the energy source in muscle. Muscle breakdown leads to elevated levels of CK in the blood.

codon

In DNA or RNA, a sequence of three nucleotides that codes for a certain amino acid or signals the termination of translation (stop or termination codon)

consanguinity

Literally meaning common blood, describes a familial relationship between two individuals.

deletion

Absence of a segment of DNA; may be as small as a single base or as large as one or more genes.

demyelinating

Resulting in removal or degradation of myelin already formed.

de novo mutation

An alteration in a gene that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents or in the fertilized egg itself. De novo is Latin for new.

distal

Anatomical term. On the limbs, a point closer to the main body is "proximal"; a point farther away is "distal".

duplication

The presence of an extra segment of DNA, resulting in redundant copies of a portion of a gene, an entire gene, or a series of genes, usually caused by unequal crossing-over during gene replication when gametes are formed in meiosis.

dysmyelinating

Causing deficient or defective myelin development or maintenance.

EGR2

EGR2 is the name of one of the genes that can cause Dejerine-Sottas. The acronym stands for Early Growth Response 2.

electromyography (EMG)

This is a diagnostic test for many types of neuromuscular disorders. It involves insertion of a small needle into several muscles to obtain information about the severity and the cause of weakness. It is usually combined with nerve conduction studies (NCS) which allow direct measurement of the integrity of various motor and sensory nerves. The skin overlying certain nerves is electrically stimulated and the response is measured over skin or muscle. You can read more about electromyography and nerve conduction velocities here.

endoplasmic reticulum

A membrane network within the cytoplasm of cells involved in the synthesis, modification, and transport of cellular materials.

eponym

A name of a drug, structure, or disease based on or derived from the name of a person.

exon

A sequence of messenger RNA that is translated into an amino acid chain, three bases at a time, each triplet sequence coding for a single amino acid. Synonym: reading frame.

fasciculation

An arrangement of fasciculi (see below), or a form of muscular contraction consisting of involuntary contractions or twitchings of groups of muscle fibers.

fasciculus

A bundle of anatomical fibers, as of muscle or nerve. Also called fascicle. Plural fasciculi.

foot drop

This describes the condition when a person cannot flex their ankle upwards towards the knee (opposite of pointing your toe). It may develop suddenly or slowly and may or may not be painful depending on the underlying condition.

frameshift mutation

An insertion or deletion involving a number of base pairs that is not a multiple of three and consequently disrupts the triplet reading frame, usually leading to the creation of a premature termination (stop) codon and resulting in a truncated protein product. Synonyms: out-of-frame deletion, out-of-frame mutation.

genotype

The genetic constitution of an organism or cell; also refers to the specific set of alleles inherited at a locus.

glycoprotein

Any of a group of conjugated proteins that contain a carbohydrate as the nonprotein component.

Hereditary Motor Sensory Neuropathies (HMSN)

The group of disorders known as Charcot-Marie Tooth disease (CMT) has come to be known as the Hereditary Motor Sensory Neuropathies (HMSN) as genetic research has uncovered the relationships between the different diseases that have previously fallen under the category of Charcot-Marie-Tooth; the two terms are used more or less interchangeably, although technically the HMSN category includes a few disorders that were not previously called CMT but have related etiologies.

heterozygote

An individual who has two different alleles at a particular locus, one on each chromosome of a pair; one allele is usually normal and the other abnormal. Such an individual may also be referred to as a carrier.

homozygote

An individual who has two identical alleles at a particular locus, one on each chromosome of a pair.

idiopathic

Cause unknown.

insertion

A chromosome abnormality in which material from one chromosome is inserted into another nonhomologous chromosome; a mutation in which a segment of DNA is inserted into a gene or other segment of DNA, potentially disrupting the coding sequence.

lamella

A thin layer of tissue (Pl. lamellae.)

locus

The physical site or location of a specific gene on a chromosome.

mesaxon

The plasma membrane of the neurolemma that surrounds a nerve axon.

missense mutation

A single base pair substitution that results in the translation of a different amino acid at that position.

motoneuron

Neurons which activate muscle cells.

MP0

See MPZ below.

MPZ

MPZ (sometimes written as MP0) is the name of one of the genes that can cause Dejerine-Sottas. The acronym stands for Myelin Protein Zero.

mutation

Any alteration in a gene from its natural state; may be disease-causing or a benign, normal variant.

myelin sheath

The fatty layer surrounding the axons of nerves that provides electrical insulation so that impulses can travel. In the peripheral nervous system, myelin is produced by Schwann cells. For more information, see the Wikipedia article.

nerve conduction velocity (NCV)

Measures action potentials conducted by axons. You can read more about electromyography and nerve conduction velocities here.

neurilemma or neurolemma

The delicate membranous covering of a nerve fiber. Also called sheath of Schwann.

neuron

Cell of the nervous system. May be motor, sensory or specialized for other purposes.

neuropathy

A disease of the peripheral nervous system. Strictly speaking, neuropathy is a general term for damage or destruction of the peripheral nerves. Also see polyneuropathy.

nomenclature

Derived from the Latin nomenclatura, meaning "name-calling". The system of naming and categorizing objects in a given category.

nucleotide substitution

onion-bulb formation

Onion bulb formations develop following repeated episodes of demyelination and remyelination. Abundant connective tissue, basal lamina and Schwann cells form around thinly myelinated axons. The size of the onion bulb gives an idea of disease progression, in that larger bulbs indicate more demyelination / remyelination cycles. Onion bulbs may contain 0, 1, or several axons. Large onion bulbs are frequently found in HMSN Types IB and III (Dejerine-Sottas), CIDP, and Refsum disease; smaller and less frequent onion bulbs are also found in CMT Type IA, Krabbe's disease and MLD.

peripheral nervous system

The peripheral nervous system (PNS) is the part of the nervous system that consists of the nerves and neurons that reside or extend outside the central nervous system--to serve the limbs and organs, for example. The peripheral nervous system is further divided into the somatic nervous system and the autonomic nervous system.

phenotype

The observable physical and/or biochemical characteristics of the expression of a gene; the clinical presentation of an individual with a particular genotype.

plasmalemma

Cell membrane.

PMP22

PMP22 is the name of one of the genes that can cause Dejerine-Sottas. The acronym stands for Peripheral Myelin Protein 22.

point mutation

An alteration in DNA sequence caused by a single nucleotide base change, insertion, or deletion.

polyneuropathy

Though the terms neuropathy and polyneuropathy are often used interchangeably, polyneuropathy is more specific subset of neuropathy that implies a systemic process causing damage to multiple peripheral nerves.

proband

The affected individual through whom a family with a genetic disorder is ascertained; may or may not be the consultand (the individual presenting for genetic counseling). (Synonyms: index case, propositus)

PRX

PRX is the name of one of the genes that can cause Dejerine-Sottas, short for Periaxin.

reading frame

A sequence of messenger RNA that is translated into an amino acid chain, three bases at a time, each triplet sequence coding for a single amino acid. Synonym: exon.

Schwann cells

Cells that wrap around the axons of peripheral nerves and form the insulating myelin sheath, discovered and named after Theodor Schwann. For more information, see the Wikipedia article.

somatic nervous system

Also called the voluntary nervous system. The part of the peripheral nervous system associated with the voluntary control of body movements through the action of skeletal muscles. The somatic nervous system consists of afferent fibers which receive information from external sources, and efferent fibers which are responsible for muscle contraction.

tomacula

Thickening of myelin sheath. (pl. tomaculae) From the Latin tomaculum, meaning sausage.

translation

The process of synthesizing an amino acid sequence (protein product) from the messenger RNA code.

wild-type allele

The normal, as opposed to the mutant, gene or allele.