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excitation(eks-ÿ-tay -shŏn) A process in which an electron bound to an atom is given sufficient energy to transfer from a lower to a higher energy level but not to escape from the atom. The atom is then in an excited state. The atom may be excited in two ways. In collisional excitation a particle, such as a free electron, collides with the atom and transfers some of its energy to it. This energy corresponds exactly to the energy difference between two energy levels. The atom rapidly returns to a lower energy level, usually its ground state, possibly passing through several intermediate levels on the way. Photons are emitted during these transitions, so that an emission line spectrum results. The photon energies are equal to the energy differences between the levels involved.
In radiative excitation, a photon of radiation is absorbed by the atom. The photon energy must correspond exactly to the energy difference between two energy levels. This process produces an absorption line in a spectrum. The atom rapidly returns to its ground state, emitting photons in the process (as before). Since the photons can be emitted in any direction and can have different (lower) energies than the absorbed photon, the absorption line dominates the spectrum. See also Boltzmann equation; ionization.
(biology), reaction of a live cell to stimulation, developed in the process of evolution. When a live system is excited, it changes from a state of relative physiological rest to a state of activity (for example, contraction of muscle fibers, secretion by glandular cells, and other activities).
Excitation is based on complex physico-chemical processes. The initial triggering factor in excitation is a change in the ionic permeability and electrical potentials of a membrane. Excitation has been most thoroughly studied in nerve and muscle cells, where it is accompanied by action potential, which can spread without being attenuated (without diminishing) along the entire cell membrane. This property of action potential ensures the rapid transmission of information from the peripheral nerves to the nerve centers and from the latter to the executor organs—the muscles and glands. In the fibers of the skeletal muscles,, action potential spreads along the membrane and deep into the fibers to the contractile apparatus of the myofibrils. Thus, a wave of contraction spreads along the muscle fiber after a wave of excitation. Action potential also stimulates the nerve endings to secrete chemical substances—mediators—which excite or inhibit innervated tissues. It obeys the all-or-nothing law: it arises only after the stimulus attains threshold force (threshold of stimulation), and it immediately acquires maximum amplitude. During the development of action potential the cell completely loses its excitability—that is, the capacity to respond with new excitation to a repetition of the stimulus. Excitability is restored gradually only after the cessation of action potential.
A spreading impulse is not the only form of excitation, which has a local character in certain parts of vertebrate nerve and muscle cells and in some invertebrate cells. Among the varieties of local excitation, the generator potentials of the receptors and the stimulating postsynaptic potentials, which arise at the site of contact between the cell and motor nerve endings, have the greatest functional significance. Like action potential, local excitation is associated with a selective increase in the ionic permeability of a membrane, and it is manifested by a negative oscillation of surface potential. However, unlike action potential, local ex-citation does not obey the all-or-nothing law: it has no threshold and its amplitude and duration vary with the intensity and duration of the stimulus. The depolarization of the membrane that accompanies local excitation stimulates the adjacent portions of the membrane, which are capable of generating spreading action potential. Thus, when local excitation (generator or postsynaptic potential) achieves threshold force, action potential arises. The differences between local and spreading excitation are very important in the transmission of information by nerve cells and their fibers. Local excitation is peculiar to those parts of the cell membrane that are specialized in the reception of stimuli coming from outside (receptor membranes) or from other nerve cells (postsynaptic membranes). Local excitation is gradual and can therefore reflect the characteristics of the stimulus—its force, duration, and rate of increase and decrease—more precisely than action potential, whose occurrence merely indicates that the stimulus has achieved threshold force. On the other hand, the capacity of action potential to spread rapidly without decreasing makes it particularly suitable for transmitting information on long conductors. During transmission, information about the intensity, duration, and sharpness of changes in the stimulus is coded by the frequency of the nerve impulses, the change in this frequency over time, and the duration of the entire volley of action potential.
The mechanism of the generation and conduction of nerve impulses improved in the course of evolution, reaching its highest development in warm-blooded animals and man. The conduction of excitation in the medullated (myelinized) fibers of warm-blooded animals attained a rate of 100-120 m per sec, with action potential lasting 0.2-0.4 milliseconds. In cold-blooded vertebrates (for example, amphibians) the rate of conduction of action potential along medullated fibers of the same diameter (20 microns) does not exceed 20-30 m per sec, with action potential lasting 1.5-2 milliseconds.
Excitation is a complex reaction in which not only electrical but also structural, chemical (including enzymatic), physical (temperature), and other components play an essential role. Changes in membrane potential during excitation are the result of a selective increase in membrane permeability to extra- and intracellular ions, which is caused by molecular rearrangement of the membrane. Increased heat production during the ascending phase of action potential and heat absorption in the descending phase are assumed to be related to these changes in the membrane. The penetration of Na+ and (or) Ca2+ ions into the protoplasm during excitation stimulates the enzymatic processes aimed at restoring the original unequal concentrations of the Na+, K+ and Ca2+ ions on both sides of the membrane and synthesizing the proteins and phosphatides required for constant renewal of the membrane structure and protoplasm of the cell. Stimulation of the metabolic processes is accompanied by intensified oxygen consumption by the tissues and a new increase in heat production, which persists in the nerve for many minutes after the impulse has passed.
REFERENCESFiziologiia cheloveka, 2nd ed. Edited by E. B. Babskiy. Moscow, 1936.
Eccles, J. Fiziologiia nervnykh kletok. Moscow, 1959. (Translated from English.)
Hodgkin, A. Nervnyi impul’s. Moscow, 1965. (Translated from English.)
Katz, B. Nerv, myshtsa, sinaps. Moscow, 1968. (Translated from English.)
Khodorov, B. I. Problema vozbudimosti. Leningrad, 1969.
B. I. KHODOROV