. Principles of modern biology. Biology. NORMAL DEPRESSED NORMAL Fig. 25-2. The intensity of a nerve impulse traveling from left to right along a nerve fiber is indicated by the height of the action potential outlined above the fiber. The slightly shriveled region of the fiber indi- cates the depressed portion in which the impulse in- tensity is reduced. Note that the action potential, once past the depressed (perhaps drugged) section, regains its full intensity. Fig. 25-1. Variation of excitability of a nerve after stimulation. 1, absolute re- fractory period; 2, relative refractory period; 3
. Principles of modern biology. Biology. NORMAL DEPRESSED NORMAL Fig. 25-2. The intensity of a nerve impulse traveling from left to right along a nerve fiber is indicated by the height of the action potential outlined above the fiber. The slightly shriveled region of the fiber indi- cates the depressed portion in which the impulse in- tensity is reduced. Note that the action potential, once past the depressed (perhaps drugged) section, regains its full intensity. Fig. 25-1. Variation of excitability of a nerve after stimulation. 1, absolute re- fractory period; 2, relative refractory period; 3, supernormal period; 4, normal condition. (Adapted from Keith Lucas.) tentacles of the animal. The nerve net lies in close contact with the mesoglea (p. 630), but its branches make contact with the cells of both the ectoderm and endoderm. Some of these branches are equivalent to the sensory STIMULUS-» A INDEPENDENT EFFECTOR STIMULUS-». MUSCLE PROCESSES MUSCLE PROCESSES MUSCLE PROCESSES B RECEPTOR-EFFECTOR STIMULUS STIMULUS ->c C RECEPTOR-CONDUCTOR-EFFECTOR Fig. 25-3. Responsive mechanisms of Hydra. Note gradations of complexity from the independent effec- tor to distinct receptor, conductor, and effector Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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