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stimulus, pl. stimuli (stim´yu-lus, -lI)

1. A stimulant. 2. That which can elicit or evoke action (response) in a muscle, nerve, gland or other excitable tissue, or cause an augmenting action upon any function or metabolic process. [L. a goad]
adequate s.a s. to which a particular receptor responds effectively and that gives rise to a characteristic sensation; e.g., light and sound waves that stimulate, respectively, visual and auditory receptors.
aversive s.a noxious stimulus such as an electric shock used in aversive training or conditioning. See also aversive training.
conditioned s. 1. a s. applied to one of the sense organs (e.g., receptors of vision, hearing, touch) which are an essential and integral part of the neural mechanism underlying a conditioned reflex; See classical conditioning, higher order conditioning. 2. a neutral s., when paired with the unconditioned s. in simultaneous presentation to an organism, capable of eliciting a given response.
discriminant s.a s. which can be differentiated from all other s. in the environment because it has been, and continues to serve as, an indicator of a potential reinforcer.
heterologous s.a s. that acts upon any part of the sensory apparatus or nerve tract.
heterotopic s.any electrical activation from an abnormal locus.
homologous s.a s. that acts only on the nerve terminations in a special sense organ.
inadequate s.a s. too weak to evoke a response.subliminal s., subthreshold s;
liminal s.threshold s
maximal s.a s. strong enough to evoke a maximal response.
square wave stimulielectrical stimulation in which the intensity of the current is brought suddenly to a given level and maintained at that level until it suddenly is cut off; this type of s. is particularly useful in obtaining a strength-duration curve.
subliminal s.inadequate s
subthreshold s.inadequate s
supramaximal s.a s. having strength significantly above that required to activate all of the nerve or muscle fibers in contact with the electrode; used when response of all the fibers is desired.
threshold s.a s. of threshold strength, i.e., one just strong enough to excite. See also adequate s.liminal s;
train-of-four s.a method for measuring magnitude and type of neuromuscular blockade, based upon the ratio of the amplitude of the fourth evoked mechanical response to the first one, when four supramaximal 2-Hz electrical currents are applied for 2 seconds to a peripheral motor nerve.
unconditioned s.a s. that elicits an unconditioned response; e.g., food is an unconditioned s. for salivation, which in turn is an unconditioned response in a hungry animal. See classical conditioning.


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