Altricial

Species that are born immature at birth, requiring parental care for a relatively long time in order to survive.   Human newborns have been classified as motorically altricial and perceptually precocial, but there is mounting evidence against this simplified distinction. See Newborn, Ontogenetic adaptation, Precocial, r- and k-selection

Alphabet

A writing system in which the letters for the most part represent single phonemes, although some letters (e.g., ‘x’) represent two phonemes and some phonemes are sometimes represented by digraphs (by more than one letter) such as ‘ph’ as in ‘philosophy’.  Transliteration (the mapping of one script onto another) works by extensive use of digraphs. …

Alphabetic writing systems

Those in which there are systematic relationships between letters and sounds at the level of the phoneme.  Other writing systems are logographic (e.g., Hanzi in China; Kanji in Japan) and syllabic (e.g., Cherokee syllabary; Katakana in Japan). See Alphabet, Kana scripts, Logographic writing systems, Orthography, Phoneme, Syllabary

Alpha (α) motoneuron

One of two types of somatic motoneurons, and in fact the largest spinal motoneuron, which innervates extrafusal muscle fibers located throughout a muscle.  In addition to voluntary skeletal muscle contraction, alpha motoneurons also play a role in muscle tone, the continuous force generated by non-contracting muscle to oppose stretching.  When a muscle is stretched, sensory …

Allocentric search

Search for objects based on external cues from the objects independently of one’s body position or locations of previous successful searches for the objects.  Also referred to as allocentric spatial coding.  Animal studies have shown that the dorsalateral prefrontal cortex mediates this sort of spatial cognition.  In contrast, egocentric search involves a viewer-centered (or self-referential), …

Allometry

In biology, the study of relationships between size and shape.  During mammalian growth, different parts of the body grow at different rates, but in a proportional manner so that body shape is noticeably altered during this period without distorting a recognizable species-characteristic form.  There are two forms of allometry: negative allometry in which one body …

Allele

Pronounced ‘ah-lee-al’ and short for allelemorph (Greek word for ‘of another form’), it is a specific gene sequence that is one of a number of different mutated or natural forms of a particular gene or DNA sequence.  A single allele for each locus on a chromosome is inherited from each parent.  When the alleles of …

Akinesia

Absence of movement, used in reference to conditions or drugs (e.g., curare) that result in immobilisation of the embryo or fetes and cessation of spontaneous motor activity.  It is also a feature of severe Parkinson’s disease. See α-bungarotoxin (α-BTX), Fetal akinesia deformation sequence (FADS), Parkinson’s disease, Spontaneous motor activity