Rheobase and chmnaxie Imagine that a nerve fiber is stimulated by a current of very low intensity, no response develops

maximal confracfion. Therefore, although in a given (single) nerve fiber, the degree of excifafion does no! change by increasing the strength of stimulus from threshold to further, in a nerve frunk, where many nerve fibers are present, the effect varies upto a point Graded potential (GP) When a subthreshold stimulus is applied to a nerve, there is no development of fhe AP. However, although there is no development of AP, some changes in the value of membrane potential, where fhe sfimulus is applied, do occur. For example, a subthreshold stimulus can cause a voltage drop in the area where the stimulus is applied. But this voltage drop is not AP (see below). This change is - local phenomenon, that is, unlike the AP. (i) it does no! propagate Again, unlike AP, (ii) its magnitude depends on the duration and intensity of the stimulus, that is, the response is a graded one (hence fhe name). The phenomenon is also called local response' for obvious reasons. The exacf role, the graded potential plays in our body, or fhe exact mechanism of its production are unknown. A recently developing view is that the GP (like the AP) have very definite roles fo play. When the nerve fiber sends a signal to a long distance, it does so by the AP, whereas transactions between short disfance might be through graded potential. Most authors, now a days, use the term, graded potential (GP) in a much broader sense as follows: recall the cardinal feafures of action potential (AP). An AP, (i) obeys all or none law, and (ii) it propagates According to current usage, GPs are those localised potential changes which neither shows all or none law (i.e, they increase when the stimulus is stronger) and nor propagates. Viewed in this way, the synaptic potential (EPSP & IPSP), EPP & MEPP and generator potential all can be clubbed together as graded potential. Action potential (AP) When a threshold stimulus is applied fo a nerve fiber, the fiber develops an AP which propagates onwards without any reduction of its amplitude until it reaches the end of the fiber. This, according to the classical teaching, is the mode of signalling by the nerves. The mechanism of development of AP (Hodgkin- Huxley fheory) has been discussed in chap.2, sec.I. Some highlights of the AP, in short, are restated: (i) The AP shows, all or none phenomenon That is, once the nerve fiber receives a threshold strength stimulus, it develops, an AP. If the strength of the stimulus is increased, there is no augmentation of the amphtude of AP. However, this does not mean tha the amplitude of the AP cannot be altered by other means. For example, if the ECF contains a higher concentration of Na+, the amphtude of AP rises. (ii) When the AP propagates, there is no diminution of the amplitude of it.(iii) Stimulation A single subthresholod stimulus fails to produce an AP. But if a second stimulus, which, also, is subthreshold, is applied sufficiently quickly after the first one, the two stimuli are summated and an excitation results. The first subthreshold stimulus produces a graded response, and on the top of it the second one is applied which again produces another graded response and the fwo stimuli are summated. This type of summation is called temporal (that is in relation to time) summation. Spatial summations (spatial= in relation to space) are also known. In spatial summation, two subthreshold stimuli when applied geographically (topographically) closely, but simultaneously, evoke a response. Refractory period When a nerve fiber is producing an AP, the fiber becomes refractory to a second (or any further) stimulus. The first phase of this refractory period is called, absolute refractory period, (ARP) that is, no matter, how strong is the stimulus, the fiber does not respond (or in other words, the threshold is infinitely high). During the later phase, only a very strong stimulus can produce a response (that is, the threshold is now higher than normal, but no longer infinite); this phase is called relative refractory period (RRP). Rheobase and chmnaxie Imagine that a nerve fiber is stimulated by a current of very low intensity, no response develops (because the stimulus intensity is severely subthreshold). As the current strength is gradually raised, a point is reached, when the current strength is such, that when it is applied for indefinite duration, a response is produced. This current strength, which, when applied for an indefinite period, can produce a response in the nerve, is called 'rheobasic current strength' or simply 'rheobase' (Fig. 10A.3.1). Now apply a current whose strength is just double the rheobasic current strength. The time taken to respond (with this current strength) is called the 'chronaxie'. Rheobase is therefore a current strength, whereas chronaxie is expressed in seconds; chronaxie varies from tissue fo tissue. An easily excitable tissue has a shorter chronaxie and vice versa. Clinically, chronaxie may be determined in suspected cases of nerve damage. Fig. 10A3.1. Sfrength duration curve. Note, rheobasic current strength and chronaxie CONDUCTIVITY Propagation of AP In a non myelinated nerve, the AP propagates as follows: Firsf, a spot of the nerve fiber is depolarized. Because of the fact, that the nerve membrane (axolemma) is both excitable and resistant and separates the two conductors (see cable properties of nerve, chap.2, sec XA), potential drop across the membrane shall now develop in a neighbouring point. As the space between the axolemma and Schwann cell membrane communicates with the ECF throughout the length of the axon, entry of Na+ and exit of K+ are no problems In short, once AP has developed at a point it causes potential drop fo develop at neighbouring regions