period

1. A certain duration or division of time. 2. One of the stages of a disease, e.g., p. of incubation, p. of convalescence. SEE ALSO: stage, phase. 3. Colloquialism for menses. 4. Any of the horizontal rows of chemical elements in the periodic table. [G. periodos, a way round, a cycle, fr. peri, around, + hodos, way]

- absolute refractory p. the p. following excitation when no response is possible regardless of the intensity of the stimulus.

- amblyogenic p. p. during early visual development when the visual neurosensory system is vulnerable to developing amblyopia from blurred retinal image formation, bilateral cortical suppression (as in strabismic amblyopia), or both. SYN: critical p. (3).

- critical p. 1. in the first hours after birth, the p. of maximum imprintability; the p. before and after which imprinting is difficult or impossible; 2. in animals, a p. following birth when the processes underlying the capacity for socialization are activated or stamped in; 3. SYN: amblyogenic p..

- eclipse p. the time between infection by (or induction of) a bacteriophage, or other virus, and the appearance of mature virus within the cell; an interval of time during which viral infectivity cannot be recovered. SYN: eclipse phase.

- effective refractory p. the p. during which impulses may appear but are too weak to be conducted; the longest interval between adequate stimuli, falling just short of the time necessary to allow a propagated response to be evoked in a tissue by the second stimulus; it differs from the functional refractory p. in that it is a measure of stimulus interval rather than response interval of time.

- ejection p. SYN: sphygmic interval.

- extrinsic incubation p. (eks-trin′sik) time required for the development of a disease agent in a vector, from the time of uptake of the agent to the time when the vector is infective.

- fertile p. the p. in a regularly menstruating woman's cycle, during which conception is most likely.

- functional refractory p. the minimum interval possible between successive responses to stimulation of a tissue.

- gap₁ p. the p. of the cell cycle after cell division when there is synthesis of RNA and protein; it may last for a few hours in rapidly growing tissue or a lifetime in non-renewing cells such as nerve cells. SYN: gap₁ phase, postmitotic phase.

- gap₂ p. the p. in the cell cycle when synthesis of DNA is completed but before mitosis begins. SYN: gap₂ phase, premitotic phase.

- gap₀ p. phase of a cell no longer in the cell cycle and thus at least temporarily incapable of division. SYN: gap₀ phase.

- incubation p. 1. time interval between invasion of the body by an infecting organism and the appearance of the first sign or symptom it causes; SYN: incubative stage, latent p. (2), latent stage, stage of invasion. 2. in a disease vector, the p. between entry of the disease organism and the time at which the vector is capable of transmitting the disease to another human host.

- induction p. the p. required for a specific agent to produce a disease; the interval from the causal action of a factor to initiation of disease, e.g., the interval between exposure to radiation and the onset of leukemia; the interval between an initial injection of antigen and the appearance of demonstrable antibodies in the blood.

- intrapartum p. in obstetrics, the p. from the onset of labor to the end of the third stage of labor.

- isoelectric p. an abnormal p. occurring in the electrocardiogram between the end of the S wave and the beginning of the T wave during which electrical forces are acting in directions so as to neutralize each other so that there is no difference in potential under the electrodes. SYN: abnormal ST segment.

- isometric p. of cardiac cycle that p. in which the muscle fibers do not shorten although the cardiac muscle is excited and the pressure in the ventricles rises, extending from the closure of the atrioventricular valves to the opening of the semilunar valves (isovolumic constriction) or the reverse (isovolumic relaxation). SYN: isovolumic p..

- isometric contraction p. the time between closure of the atrioventricular valves and opening of the semilunar valves.

- isometric relaxation p. early ventricular diastole beginning with closure of the aortic and pulmonic valves and preceding opening of the atrioventricular valves.

- isovolumic p. SYN: isometric p. of cardiac cycle.

- latency p. SYN: latency phase.

- latent p. 1. the p. elapsing between the application of a stimulus and the response, e.g., contraction of a muscle; 2. SYN: incubation p. (1).

- masticatory silent p. a pause in electromyographic patterns associated with tooth contacts during chewing and biting; a part of the complex feedback mechanism of mandibular control involving receptors in the periodontal ligament and muscles.

- menstrual p. SYN: menses.

- missed p. the failure of menstruation to occur in any month at the expected time.

- mitotic p. the p. of the cell cycle in which all phases of mitosis occur. SYN: M phase.

- oedipal p. SYN: oedipal phase.

- preejection p. the interval between onset of QRS complex and cardiac ejection; electromechanical systole minus ejection time.

- prepatent p. in parasitology, the p. equivalent to the incubation p. of microbial infections; it is biologically different, however, because the parasite is undergoing developmental stages in the host.

- prodromal p. the time during which a disease process has begun but is not yet clinically manifest.

- puerperal p. the p. elapsing between the termination of labor and the return of the generative tract to its normal condition; the 6 weeks following the completion of labor.

- pulse p. the reciprocal of the repetition rate; e.g., the interval between leading edges of successive pulses.

- quarantine p. the time during which an infected individual or an area is kept isolated, avoiding contact with uninfected individuals; can be any specified p. of time, varying with the disease in question. The term is derived from the Italian word for 40, since the p. of isolation of individuals suspected of plague in the Middle Ages was 40 days.

- refractory p. 1. the p. following effective stimulation, during which excitable tissue such as heart muscle and nerve fails to respond to a stimulus of threshold intensity ( i.e., excitability is depressed); 2. a p. of temporary psychophysiologic resistance to further sexual stimulation that occurs immediately following orgasm.

- refractory p. of electronic pacemaker the time required to restore full sensitivity after detecting cardiac activity or delivering a pacing impulse.

- relative refractory p. the p. between the effective refractory p. and the end of the refractory p.; fibers then respond only to high-intensity stimuli and the impulses conduct more slowly than normally.

- silent p. 1. the time during which there is no electrical activity in a muscle following its rapid unloading; 2. any pause in an otherwise continuous series of electrophysiologic events.

- synthesis p. the p. of the cell cycle when there is synthesis of DNA and histone; it occurs between Gap₁ and Gap₂. SYN: S phase.

- total refractory p. the absolute refractory p. plus the relative refractory p..

- vulnerable p., vulnerable p. of heart a brief time during the cardiac cycle when stimuli are particularly likely to induce repetitive activity like tachycardia, flutter, or fibrillation which persists after the stimulus has ceased; for the ventricle, it occurs during the latter part of systole, during the relative refractory p. coincident with the inscription of the latter half of the T wave of the electrocardiogram.

- Wenckebach p. a sequence of cardiac cycles in the electrocardiogram ending in a dropped beat due to AV block, the preceding cycles showing progressively lengthening PR intervals; the PR interval following the dropped beat is again shortened.

* * *

pe·ri·od 'pir-ē-əd n

1 a) a portion of time determined by some recurring phenomenon

b) a single cyclic occurrence of menstruation

2) a chronological division <the \period of incubation of a disease>

* * *

pe·ri·od (pērґe-əd) [Gr. periodos a going around, circuit, period] an interval or division of time; the time for the regular recurrence of a phenomenon.