A term to describe the different facial muscles involved in an expression. The facial action codingsystem (FACS) of Ekman and Friesen (1978) gives detailed descriptions of all the changes in appearance that accompany each of 44 action units they identified in the face. Attempts have been made to automatically track facial action units by means of motion analysis systems. …
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Action syntax
Rules of sequential order among actions that are often hierarchically organised into units and larger blocks, rather as letters, words and phrases in language. See Action sequences, Passive sentences, Serial ordering, Syntax
Action theory
A theoretical approach that views individuals as both products and producers of their developmental histories. Vygotsky‘s theory of social development is a prime example of such an action theory. More generally, action theory addresses processes giving rise to intentional patterns of movement. See Action, Intentionality, Life course analysis, Own-sex schema
Action sequences
Organization of behaviour in time, where specific actions follow one another in various temporal patterns. See Action, Action syntax, Serial ordering
Action
A goal-directed behavior, consisting of different movement components, that are carried out intentionally or under voluntary control. Thus, an action is goal-directed while a movement is not. An example is reaching and grasping movements in a prehensile action. Moreover, in action there are variable means to the same end state. Actually, two generally classes can be identified: exploratory (or investigative) …
Action potential
MotorA momentary change or electrical excitation in the voltage difference across the membrane of a neuron that is usually triggered by nerve impulses from a number of other neurons at synapses. It results in an influx of positively charged sodium ions through the cell membrane producing the impulse being propagated along the axon of the neuron. This …
Actin
A protein that interacts with myosin to form the contractile protein actomyosin in muscles, which is involved in other physiological processes (e.g., cellular movement, maintenance of cell shape) in the form of other contractile elements such as flagella. It can exist in a globular or a fibrous form, and is the most abundant protein in …
Acrosome
A membrane-bound organelle at the tip of a sperm derived from the Golgi apparatus, which contains lytic enzymes that digest the outer coatings around the egg so that it can enter the ovum and inject its haploid DNA (see figure below). Acrosome at tip of sperm and related structures See DNA (deoxyribonucleic acid), Enzyme, …
Acidosis
See Respiratory acidosis
Acetylcholinesterase (AcHE)
Also called cholinesterase, it is a hydrolytic enzyme that regulates the level of acetylcholine at cholinergic synapses and neuromuscular junctions. It serves to stop further postsynaptic nerve impulses by destroying acetylcholine after it is released from the terminal boutons of presynaptic neurons and postsynaptic receptors. See Action potential, Acetylcholine (AcH), Boutons, Cholinergic neurotransmitter system, Enzyme, …