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motor neuron

a neuron whose axon connects directly to muscle fibers. Because motor neurons are the final stage of output from the nervous system and are the only means of stimulating muscle fibers, they are known as the final common path. There are two types: lower motor neurons (or alpha motor neurons), found in the cranial nerves and the anterior horn of the spinal cord and that are responsible for muscle contraction; and upper motor neurons (or gamma motor neurons), found in the corticospinal tract and that modulate the sensitivity of muscle spindles, thus influencing activity of the lower motor neurons. Also called motoneuron.

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Psychology term of the day

March 15th 2025

microaggression

microaggression

n. brief and commonplace verbal, behavioral, or situational indignities that communicate hostile, derogatory, or negative slights or insults, especially toward members of minority or oppressed groups. Microaggression may be intentional (e.g., calling a transgender person a “she-male”) or implicit (e.g., a White employee asking a Black colleague how he or she got a certain job, implying that the colleague may have obtained it through affirmative action or a quota system). Three subtypes have been identified: microassaults, which are purposefully discriminatory actions (e.g., uttering a racial slur, displaying a swastika); microinsults, which are subtle snubs that devalue a person’s identity; and microinvalidations, which are unintentional exclusions or negations of an individual’s thoughts, feelings, or experiences. Research on microaggression is limited but shows that this form of bias can cause recipients to feel that they are abnormal, inferior, invisible, powerless, or untrustworthy. [introduced in 1970 by U.S. psychiatrist Chester M. Pierce (1927–  ); the subtypes were proposed in 2007 by U.S. psychologist Derald Wing Sue]