Further Studies upon Development of Somatic Activity in Albino Rat Fetuses

Abstract

As previously reported 1 the early development of response to stimulation in albino rat fetuses passes through 3 stages: myogenic, neuromotor, and sensory motor. Experiments are here reported which were designed to test in a more definite manner the first phase of the development of the response to stimulation, the myogenic stage.

The myogenic response was described 1 as follows: “The movements elicited by strong mechanical stimulation are slow and feeble, but maintained. The responses elicited by light tactile stimulation, reflex activity, are also slow and feeble, but quickly followed by relaxation.” One may justly infer that maintenance of contractions is indicative of purely myogenic activity. This is not the case. The above statement is true only in relation to the strength of the stimuli used, not to the stage of development. In further experiments in which 80 fetuses from 12 different litters were studied, the mechanical stimulation was applied to the fetus with different degrees of strength. The result shows that when the stimulus was just strong enough to elicit responses, these responses were not maintained, but instead were quickly followed by relaxation. If the strength of the stimulus was increased, the contractions began to be maintained and the length of maintenance increased with the increased strength of the stimulus. However, a limit was reached beyond which this period of maintenance ceased to increase, even when the stimulus applied was strong enough to seriously damage the tissue.

Since with adequate though light stimuli the contractions were followed by quick relaxation, there seems to be, upon this basis, no marked difference between the nature of myogenic and reflex activity. Thus, at this early stage of response to stimulation the following properties occur: Latent period, summation, and contractions followed by quick relaxation are common to both myogenic as well as reflex activity. One might conclude that the earliest reaction, previously described as myogenic, might be, in fact, reflex.