User:Pieningd/Rheobase

Wikipedia Proposal: Rheobase
This is a project for Professor Burdo's Intro to Neuroscience course at Boston College by Daniel Piening, Cameron Perry, Lyndsey Brozyna

Introduction
Rheobase is a measure of membrane excitability. The ease with which a membrane can be stimulated depends on two variables: the strength of the stimulus, as well as the duration for which the stimulus is applied. These variables are related inversely: as the strength of the applied current increases, the time required to stimulate the membrane decreases (and vice versa) to maintain a constant effect. In neuroscience, rheobase is the minimal current amplitude of indefinite duration (in a practical sense, about 300 milliseconds) that results in the depolarization threshold of the cell membranes being reached, such as an action potential or the contraction of a muscle. The root "rheo" translates to current, and "base" means foundation: thus the rheobase is the minimum current that will produce an action potential or muscle contraction.

In the case of a nerve or single muscle cell, rheobase is half the current that needs to be applied for the duration of chronaxie. Mathematically, rheobase is equivalent to half of chronaxie, which is a strength-duration time constant that corresponds to the duration of time that elicits a response when the nerve is stimulated at twice rheobasic strength. he properties of the nodal membrane largely determine the axon's strength-duration properties, and these will change with changes in membrane potential, with temperature, and with demyelination as the exposed membrane is effectively enlarged by the inclusion of paranodal and internodal membrane.

History
Weiss developed the following formula in 1901, which can be used to calculate the value of rheobase:

Coming Soon

The term "rheobase" was coined in 1907 by the French physiologist Louis Lapicque. Lapicque developed the following formula to calculate rheobase:

Coming Soon

Strength-Duration Curve

 * Strength duration curves and chronaxie determinations were historically performed from the 1930s-1960s to assess nerve injuries prior to the common, more recent use of EMG/NCV testing. These graphs were a means of evaluating the severity and subsequent recovery of a nerve injury.

Q = I x T
 * Depolarization of an excitable membrane requires flow of electrical charge across the membrane. Because of the dominant electrical capacitance of the membrane, the relevant parameter for effective membrane depolarizaton is the total amount of charge transferred across the membrane.
 * For a short duration stimulus generating a steady transmembrane current, the charge (Q) transferred is proportional to the product of current (I) and time (T):


 * Plotting a strength-duration curve requires stimulating a muscle at its motor point with fixed pulse duration (ranging from .01 - 100 ms), recording the current strength in amperes, and plotting the mA values obtained vs. the pulse duration utilized on the X and Y axis respectively.


 * For most neural elements, the form of the strength-duration curve is typically an exponential decay.
 * The amplitude asymptote (threshold) at very long durations is called the rheobase.


 * Rheobase can be determined directly from the charge-duration plot; it is equal to the slope of the regression line.


 * The curve of a denervated muscle will be shifted to the right compared to normal innervated muscle, and will subsequently shift to the left during reinnervation.

Experimental Data

 * In a 1996 review aimed at comparing strength-druaton curves for compound sensory and muscle action potentials, Mogyoros et. al determined that regenerated motor neurons displayed increased rheobase and decreased chronaxie, which is consistent with abnormal active membrane properties.


 * Mogyoros et. al also found rheobase to be lower for sensory fibers than for motors fibers.
 * Excitability changes in human and motor axons during hyperventilation


 * More recently, in a 2005 review studying changes during the postnatal development in physiological and anatomical characteristics of rat motoneurons, Carrascal et. al determined that the current required to reach rheobase decreases in oculomotor neurons with age, which contrasts the increase observed in hypoglossal motorneurons.
 * The depolarization voltage required to generate an action potential also diminishes in oculomotor motoneurons, whereas it remains constant in hypoglossal motoneurons.


 * Since the amount of depolarization required to reach the threshold in genioglossal motoneurons remains unchanged with age, the two-fold increase in rheobase during postnatal development in these motoneurons must be a consequence of a decrease in specific resistance, producing a decrease in excitability.
 * Increased excitability in motoneurons of the oculomotor nucleus is probably carried by depolarizing currents

Clinical Significance

 * Diabetes
 * Polyneuropathy
 * CIDP
 * Machado-Joseph Disease

Plan to Split Up Work
As a group, we decided to meet every other Tuesday at 6 pm to work on the project.