Neuronneuralnerve Theory

The neuron (or neurone, nerve cell) is the basic structural and functional unit of the nervous system and consists of three main parts: a cell body ("soma") that contains the nucleus, an axon, and one or more dendrites. A distinction is made between the terms neuron and nerve where a neuron is a single cell consisting of three parts (one of which is an axon), whereas a nerve is a bundle of many neural axons [cf., Waller's law - formulated by the English physician/physiologist Augustus V. Waller (1816-1870) in 1850 - which states that if posterior roots of the spinal cord are cut on the central side of the ganglia, those portions of the cut nerves that lie within the spinal cord degenerate, whereas the peripheral portions of the same nerves (not being severed from the ganglia) do not degenerate]. Neurons are cells that transmit information throughout the body, as well as within the brain [cf., Dale's law/principle - formulated by the English physiologist Sir Henry H. Dale (18751968) - proposes that only one kind of neuro-transmitter substance is produced by a given neuron; however, today, it is known that there are exceptions to this principle; the drainage/diversion hypothesis asserts that facilitation of neural conduction over certain neurons and its inhibition over others are due to a drainage of energy from paths of higher resistance into those of lower resistance; the chemoaffinity hypothesis proposes that molecules of particular chemical substances guide neurons in the formation of appropriate synapses during development and regeneration; and Du Bois-Reymond's law - formulated by the German electrophysiologist Emil Du Bois-Reymond (1818-1896) - states that the excitatory efficiency of an electric current that passes through neural tissue is dependent on the rate of current density change and not on the current's absolute value]. Many of the neurons in the human nervous system are extremely small (some axons are only about 0.1 millimeter long; other axons stretch up to a meter through the adult nervous system). It is estimated that the nervous system contains about 100 billion neurons. The neuron theory holds that any sensorimotor neural pathway is not a continuous tissue but consists of separate nerve cells (the neurons) that are merely con tiguous end-to-end (cf., the law of forward conduction - states that nerve impulses always travel from the postsynaptic membrane of the dendrites to the terminal knob of the axon; the law of neurobiotaxis states that dendrites of developing nerve cells grow in the direction of the axons of nearby active neurons; and the motor-primacy theory posits that bodymecha-nisms associated with motor-nerve functions develop before sensory-nerve mechanisms, and their degree of maturation governs their ability to respond to stimulation). According to the neuron theory, the neuron is the basic and essential histological/metabolic unit of the nervous system. The neuron theory (also called the neuron doctrine) was first named by the German histologist and anatomist Wilhelm Waldeyer-Hartz (1839-1921) - who also coined the words neuron and chromosome; the theory is founded on a viewpoint of the nervous system held by the Spanish physician and histologist Santiago Ramon y Cajal (18521934) whose major work was on the microstructure of the nervous system (cf., avalanche law - the principle that neural impulses spread from a stimulus receptor site to a number of other neurons, resulting in an effect that is disproportionate to the initial stimulus, such as may be observed in an epileptic seizure; and autocorrelation theory - suggests that groups of individual nerve fibers collaborate to transmit auditory nerve impulses, based on the finding that because a single fiber is unable to transmit impulses faster than 1,000 cycles per second, fibers must function as a team with one firing at one cycle, another at the next cycle, and so on; cf., volley theory of audition). Ramon y Cajal utilized the specialized histological staining techniques of the Italian histologist Camillo Golgi (1843-1926). However, the two men disagreed in their interpretations of neural structure; Ramon y Cajal maintained that nerve cells are discrete and that there is no physical continuity between one cell and another. The revolutionary "Gol-gi stain technique" (that impregnates neural tissue with silver) was crucial, histologically, for the eventual confirmation of the neuron theory, which holds that neurons act as units and communicate via synapses rather than as a continuous network-like circuit (this latter notion, called the reticularist theory, is disre garded today). The dispute over this neural issue between Ramon y Cajal and Golgi became so heated that Golgi's 1906 Nobel Prize acceptance speech consisted of a fiery denunciation of the neuron doctrine. The controversy over the neuron theory/doctrine continued for more than 25 years afterward, despite the accumulation of evidence in its favor. Only with the advent of electron microscopic pictures in the 1950s were the opponents finally satisfied. The neuron theory is one of the most important neurological contributions for the history of psychology because it brought together numerous data concerning the nature of nervous physiology that psychologists could apply in their own disciplinary specialties and interests, notably in the areas of learning theory and the theory of association ; cf., the theory of psychoneural parallelism which holds that every fact of consciousness is concomitant with some neural change without implication of the reverse relation, namely, that all neural conditions are concomitant with conscious processes. Other relevant issues and theories related to functioning of the neurons, nerves, and the nervous system are the synaptic theory of facilitation and inhibition, and the membrane theory of nerve conduction (cf., segmental theory - posits that each segment of the nervous system, in segmented animals, controls and regulates largely the activities of the corresponding segment of the body; and stimulation effects on neurons -refers to physiological changes created when a stimulus changes the electrical potential of a neuron by producing an irritating effect on the cell membrane which, in turn, disrupts the ionic balance on either side of the membrane; the potential change travels along the nerve fiber to a terminal point or synapse which then passes the impulse along to a nearby fiber; when neurons are exposed to electrical stimulation in test tube experiments, increased temperature and oxygen consumption, as well as other metabolic effects, are observed). According to the synaptic theory, the actual pathway activated depends on the physiological properties of the synapse at the time, and the choice between alternatives that nerve impulses can take depends on slight and momentary factors such as refractory phases and summation (cf., metabotropic effect - refers to a neurological event at a synapse in which neurotransmitters generate a slow, pro-longed effect via metabolic, rather than ionic, changes). In 1895, the neurologist/psychoanalyst Sigmund Freud set out his assumptions about how the nervous system works (cf., Jackson's law - named after the English neurologist John Huglings Jackson (1835-1911) and formulated in 1898 - states that when mental abilities are lost because of a neurological disorder, the abilities that appeared last in the course of evolution are lost first because it is the higher nervous centers (that is, those appearing last phylogenetically) that are first affected, and the lower/older centers are the last affected; also, Jackson's mixed cerebral dominance theory states that speech disorders and some other maladjustments may be due to the fact that one cerebral hemisphere does not lead consistently the other hemisphere in controlling bodily movement). Freud hypothesized that neural elements are separated from one another by "contact barriers" (the notion of synapses was contested hotly when Freud proposed this idea), and one element can excite the next one only when the "contact barrier" (i.e., synapse) is crossed (cf., Hebb's theory). With the English neurophysiologist Sir Charles Scott Sherrington's (1857-1952) contributions, as well as those made earlier by the Austrian physiologist Sigmund Exner (1846-1926), the supposition was greatly strengthened that processes of facilitation and inhibition are synaptic functions [cf., drainage hypothesis - posited by the British-American psychologist William McDougall (18711938), states that facilitation of neural conduction over particular neurons, and its inhibition over certain others, is due to a "drainage of energy" (analogous to a hydraulic mod-el) from paths of higher resistance into those of lower resistance]. The nature of synaptic function, however, was not disclosed completely by Sherrington's methods. The work of W. H. Nernst, R. S. Lillie, and K. Lucas added theoretical and empirical understanding to the neural-related issues of depolarization, refractory phase, hyperexcitability, inhibition, and facilitation. The membrane theory of nerve conduction was developed along with the other discoveries of the nature of synapses and conduction (cf., sensitization theory - states that once a synapse has fired repeatedly it eventually becomes more active or more effective in exciting the postsynaptic cell). The German physical chemist Wilhelm Ostwald (1853-1932) first proposed the membrane theory in 1890; Julius Bernstein amplified and established it in 1902; and R. S. Lillie began a series of experiments that supported it in 1909. The membrane theory of conduction is an explanation of the propagation of the nerve impulse in terms of the electrochemical properties of surface films or membranes [cf., Forbes-Gregg hypothesis - named after the American physiologist Alexander Forbes (1882-1965) and the American physician Alan Gregg (1890-1957), states that stimulus strength is translated by nerve fibers into frequency of discharge; this hypothesis was offered to explain how the nervous system handles varying stimulus intensities in spite of the honored all-or-none law that precludes variability of the strength of discharge in a nerve fiber; and the rate law - states that the strength of a stimulus is indicated by the rate of firing of the affected axons]. The eponymous Nernst-Lillie theory of excitation and conduction - named after the German physical chemist Walther H. Nernst (1864-1941) and the American physiologist R. S. Lillie (? - ?), holds that excitation of a living cell results from a change in the electrical polarization of a protoplasmic membrane, following local change of ionic concentration at the membrane surface (cf., myelinogenetic law - states that a nerve is not ready to function, usually, until its myelin sheath has developed). The effect in the Nernst-Lillie theory is transmitted automatically because of resulting secondary changes, such as permeability, in the properties of the membrane itself. The membrane theory accounts for the facts of refractory phase and all-or-none transmission, and was well on its way toward general acceptance among physiologists by 1920. See also ALL-OR-NONE LAW/PRINCIPLE; ASSOCIATION, LAWS/PRINCIPLES OF; AUDITION/HEARING, THEORIES OF; HEBB'S THEORY OF PERCEPTUAL LEARNING; HYDRAULIC THEORY; LEARNING THEORIES/LAWS; MIRROR NEURONS THEORY; REFLEX ARC THEORY/CON

CEPT; SPECIFIC NERVE ENERGIES, LAW OF.

Brain Blaster

Brain Blaster

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