Latent Period

Latent Period, or hidden period, is the interval of time that passes between the moment of action on a given object and the reaction of that object to this action. Initially noted in muscle upon its stimulation, the term "latent period" has become widely used in biology as one of the characteristics defining the relationship of an object to stimulation. The magnitude of the latent period is to some extent a measure of the processes occurring in the stimulated object as a result of the given stimulation, which is why this magnitude is not constant but variable, since the biological processes themselves are variable depending on different causes. When stimulated by an induction shock, the muscle responds with contraction. This response does not coincide with the moment of stimulation; there is always some interval of time during which the muscle remains at rest. This latent period of excitation (Helmholtz) lasts for fractions of a second in striated muscles, and its duration varies widely not only in muscles of different animals but also in the same muscle depending on the nature of the stimulation, load, temperature, etc. In human muscles, the duration of the latent period ranges between 0.004 and 0.01 seconds; in frogs, a minimum of 0.048 seconds has been recorded, although these values can be significantly reduced if the muscle remains connected to the body and is normally supplied with blood. Increasing stimulation within submaximal limits, as well as warming the muscle, shortens its latent period; increasing load and cooling lengthen it. In a fatigued muscle, the latent period is also prolonged. It makes a considerable difference whether the stimulator acts directly on the muscle or through a nerve; in the latter case, the latent period is longer even after deducting the time for the propagation of excitation along the nerve, since the transmission of stimulation through the final motor apparatus in turn requires a time loss for the "latent period of nerve endings." In this case, there is an example of the summation of the latent period of the muscle itself and those apparatuses through which the excitation traveling along the nerve is transformed into an impulse acting on the muscle. The latent period of smooth muscles is considerably greater than that of striated muscles and ranges from 0.75 to 1 second. The method for studying the latent period in muscles is essentially the method for studying muscle contraction by means of a myograph that records on a moving surface the entire course of muscle contraction—the myogram. Here only one type of muscle stimulation has been considered—the shock of an induction current. Any other effect—mechanical, chemical, or thermal—can serve as such a stimulator. Each of these, when applied to a given muscle, has its own latent period peculiar to it. As stated above, the concept of the latent period is not limited to defining a known characteristic of living muscle; it is also applicable to other biological objects. The response to stimulation of an individual cell is a component in the overall reaction of a given organ, and its latent period is the sum of the latent periods of its constituent cells. Not only individual organs but also an entire system of them, incorporated into a given biological mechanism, responds to stimulation with a reaction specific to it alone, and the time that passes between the moment of stimulation and the reaction it causes is also a characteristic defining the stimulated object. Thus, one can speak of the latent period of such responses as reflex acts of the most varied types. The best studied is the latent period of the glands of the digestive tract. According to experiments on dogs, the salivary glands do not immediately respond with secretion when their centrifugal nerves are stimulated. For the submaxillary gland, when the chorda tympani is stimulated by an induction current, the latent period equals approximately 5 seconds. With artificial stimulation of the vagus nerves, the pancreas begins to secrete after 2-3 minutes. In Khizhin's experiments (also on dogs), the latent period of the gastric department during meat eating averaged 8 minutes, during bread eating-6 minutes, and when milk was given-9 minutes. Changes in the latent period are often an important diagnostic sign. For example, the latent period of muscles is prolonged in muscular atrophy, progressive ataxia, etc. In spastic tabes and contractures, it is shorter.