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.

Excitation

(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.

REFERENCES

Fiziologiia 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

excitation

[‚ek‚sī′tā·shən] (atomic physics) A process in which an atom or molecule gains energy from electromagnetic radiation or by collision, raising it to an excited state. (control systems) The application of energy to one portion of a system or apparatus in a manner that enables another portion to carry out a specialized function; a generalization of the electricity and electronics definitions. (electricity) The application of voltage to field coils to produce a magnetic field, as required for the operation of an excited-field loudspeaker or a generator. (electronics) The signal voltage that is applied to the control electrode of an electron tube. Also known as drive. Application of signal power to a transmitting antenna. (neuroscience) A change in the electrical state of a neuron leading to an action potential. (quantum mechanics) The addition of energy to a particle or system of particles at ground state to produce an excited state.

excitation

[ek″si-ta´shun] an act of irritation or stimulation; a condition of being excited or of responding to a stimulus; the addition of energy, as the excitation of a molecule by absorption of photons.anomalous atrioventricular excitation Wolff-Parkinson-White syndrome.indirect excitation electrostimulation of a muscle by placing the electrode on its nerve.

ex·ci·ta·tion

(ek'sī-tā'shŭn), 1. The act of increasing the rapidity or intensity of the physical or mental processes.
See also: stimulation. 2. In neurophysiology, the complete all-or-none response of a nerve or muscle to an adequate stimulus, ordinarily including propagation of excitation along the membranes of the cell or cells involved.
See also: stimulation.

excitation

(ĕk′sī-tā′shən)n.1. The act or process of exciting or an instance of it.2. The state or condition of being excited.3. Physiology The activity produced in an organ, tissue, or part, such as a nerve cell, as a result of stimulation.

excitation

An MRI term for the delivery—i.e., induction or transfer—of energy into spinning nuclei via radio-frequency pulse(s), which bumps the nuclei into a higher energy state. By producing a net transverse magnetisation, an MRI system can evoke a response from the excited system.

ex·ci·ta·tion

(ek'sī-tā'shŭn) 1. The act of increasing the rapidity or intensity of physical or mental processes. 2. neurophysiology The complete all-or-none response of a nerve or muscle to an adequate stimulus, ordinarily including propagation of excitation along the membranes of the cell or cells involved.
See also: stimulation3. The process whereby radiation causes a bound electron to vibrate or oscillate within its orbit.

excitation

the process by which the electrical stimulation of

a surface membrane results in contraction of the muscle, or
brings about secretion of a transmitter substance at a nerve ending.

ex·ci·ta·tion

(ek'sī-tā'shŭn) Act of increasing the rapidity or intensity of the physical or mental processes.

Patient discussion about excitation

Q. I am excited to know in what way diet helps in preventing osteoporosis? my mother is suspecting to be having osteoporosis. She regularly complains of leg pain. Upon consultation with the doctor it was found with low calcium in her blood. She was given calcium tablets and was told to increase in the diet rich in calcium. She is taking milk and yoghurt especially. She is not well yet but shows some improvement. I am excited to know in what way diet helps in preventing osteoporosis?A. you see, our bones are a giant storage of calcium. our body needs a very steady concentration of calcium, if it'll be low the body will take it form your bones. if there's too much- it'll either build bones (but only if he'll think he needs too- that is where sport get in the picture)or you'll urinate it.any way, you rather have large amount of calcium in your diet then less. but it's not enough- sport and other medication can help.

Q. my wife is pregnant !!! i am so excited ! this is the first time out of many i hope !!! but i have concerns that i would like to share with the community here ... i am afraid of being a bad father , i am afraid i won't know how to be a father ... where do u start ... how do i support my wife after the though experince ... how do i take care of the child ... a lot of things a re running through my mind with no answer ,can any one help me to calm down ???? A. you won't be a perfect father but you'll try to be. that's what's important. thousands of years of evolution gave you the basics. you'll be fine :)
congratulations!!

Q. Hello everyone !!! are you excited with the swearing of Obama To presidency??? i sure am !!! may we all get a little joy and peaceful time wit hour fresh president ... and let us all wish him a great start and a chance to change our lives ... Good Luck Mr.President Obama !!! A. well said Mrfoot65! May all the people in America receive the healthcare treatment they need. God bless.

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Synonyms for excitation

noun intensity of feeling or reaction

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Synonyms for excitation

noun the state of being emotionally aroused and worked up

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noun the neural or electrical arousal of an organ or muscle or gland

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noun something that agitates and arouses

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