<P> The above account deals with the response of a single nerve fiber . If a nerve trunk is stimulated, then as the exciting stimulus is progressively increased above a threshold, a larger number of fibers respond . The minimal effective (i.e., threshold) stimulus is adequate only for fibers of high excitability, but a stronger stimulus excites all the nerve fibers . Increasing the stimulus further does increase the response of whole nerve . </P> <P> Heart muscle is excitable, i.e., it responds to external stimuli by contracting . If the external stimulus is too weak, no response is obtained; if the stimulus is adequate, the heart responds to the best of its ability . Accordingly, the auricles or ventricles behave as a single unit, so that an adequate stimulus normally produces a full contraction of either the auricles or ventricles . The force of the contraction obtained depends on the state in which the muscles fibers find themselves . In the case of muscle fibers, the individual muscle fiber does not respond at all if the stimulus is too weak . However, it responds maximally when the stimulus rises to threshold . The contraction is not increased if the stimulus strength is further raised . Stronger stimuli bring more muscle fibers into action and thus the tension of a muscle increases as the strength of the stimulus applied to it rises . </P>

One motor unit obeys a principle called the all-or-none law