Threshold stimulus

Fig. 17.3. (a) An action potential produced by a nerve-cell membrane in response to a depolarization (above the threshold) stimulus, (b) Sketch of time-dependent conductivity change of a nerve axon membrane (from Ref. 22... 

Threshold There is no satisfactory definition or quantitative description of threshold that will work for all conditions. A working definition of threshold would be the state the cell has to reach in order to produce an action potential. It is usually characterized by the strength of an external stimulus required to bring the cell from some initial state to the threshold state. The problem with this definition is that the threshold stimulus will vary considerably depending on the initial state of the cell the cell may be in a resting state or it may be undergoing repolarization or depolarization. 

The physiological actions of magnesium include a direct membrane stabilizing effect by reducing early after depolcirizations and increasing the threshold stimulus required to provoke either ventricular fibrillation or ventricular tachycardia, but magnesium sulfate has not previously been used to treat persistent wide QRS complex and hemodynamic instability. 

Saturation is the concentration of a stimulus above which no increase in perception can be detected. It is tme that Weber-Stevens laws can predict the relationship between stimulus intensity and sensory response with some precision however, they do not describe the very common situation of stimuli at or near the threshold or point of saturation. 

Generalizations. Several generalizations can be made regarding taste (16,26). A substance must be in water solution, eg, the Hquid bathing the tongue (sahva), to have taste. Water solubiUty is the first requirement of the taste stimulus (12). The typical stimuli are concentrated aqueous solution in contrast with the Hpid-soluble substances which act as stimuli for olfaction (22). Many taste substances are hydrophilic, nonvolatile molecules (15). Taste detection thresholds for lipophilic molecules tend to be lower than those of their hydrophilic counterparts (16). 

The odor detection-threshold values of organic compounds, water, and mineral oil have been determined by different investigators (Table 2 and 3) and may vary by as much as 1000, depending on the test methods, because human senses are not invariable in their sensitivity. Human senses are subject to adaption, ie, reduced sensitivity after prolonged response to a stimulus, and habituation, ie, reduced attention to monotonous stimulation. The values give approximate magnitudes and are significant when the same techiriques for evaluation are used. Since 1952, the chemistry of odorous materials has been the subject of intense research (43). Many new compounds have been identified in natural products (37—40,42,44—50) and find use in flavors. 

Odors are measured by their intensity. The threshold value of one odor to another, however, can vary greatly. Detection threshold is the minimum physical intensity necessary for detection by a subject where the person is not required to identify the stimulus, but just detect the existence of the stimulus. Accordingly, threshold deterrninations are used to evaluate the effectiveness of different treatments and to estabflsh the level of odor control necessary to make a product acceptable (8). Concentration can also produce different odors for the same matenal. For example, indole (qv) in low concentrations has the smell of jasmine and a low threshold of perception. In high concentrations, it has a strong odor of feces and CX-naphthyl amine as well as a considerably higher threshold of perception. 

Increased responsiveness to noxious stimuli is termed hyperalgesia. It occurs following injury or disease and encompasses enhanced responses as well as reduced thresholds to a given noxious stimulus. Primary hyperalgesia occurs in the damaged area whereas secondary hyperalgesia occurs in the area surrounding it. 

Lidocaine (Xylocaine), die representative class I-B drug, raises the threshold of the ventricular myocardium. Threshold is a term applied to any stimulus of the lowest intensity that will give rise to a response in a nerve fiber. A stimulus must be of a specific intensity (strength, amplitude) to pass along a given nerve fiber (Fig. 40-2). 

To further illustrate the threshold phenomenon using plain figures instead of precise electrical values, a certain nerve fiber has a threshold of 10. If a stimulus rated as 9 reaches the fiber, it will not pass along the fiber because its intensity is lower than die fiber s threshold of 10. If another stimulus reaches the fiber and is rated 14, it will pass along the fiber because its intensity is greater than the fiber s threshold of 10. If the threshold of a fiber is raised from 10 to 15, only the stimuli greater dian 15 can pass along the nerve fiber. 

A stimulus must reach the threshold to cause a response in a nerve fiber. Note that stimuli a, b, and d do not reach the threshold therefore, they do not cause a response in a nerve fiber. Stimuli c, e, f, and g do reach and surpass the threshold, resulting in stimulation of nerve fiber. 

Hyperpathia A painful syndrome characterized by increased reaction to a stimulus, especially a repetitive stimulus, as well as an increased threshold. 

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