The duration, amplitude, and shape of the compound action potential change with an increase in stimulus strength. When stimulation increases in strength, more fiber nerves are activated and the AP of the fibers summates a CAP and as a result, when the stimulus happens to be stronger a large segment of the fibers attain threshold. It is important to note that the threshold of fibers depends on both stimulus power and the period of the stimulus as well (Aiki.bme.duke.edu, 2010).
Since action potential has negative and positive deflections, it is referred to as biphasic and its negative point is due to the arrangement of after-hyperpolarization while the negative mode of the CAP is so because of how it is recorded. When the extreme left-hand 1-2 mm of the nerve is keenly crushed, it will make the action potential monophasic this is because the action will depolarize the membrane effectively and permanently at this end and the result would be monophasic compound action potential.
Initially, a compound action potential will not be observable because of the minimal stimulus amplitude; however, a short biphasic deflection at the commencement of the exhibit will be observed (Peripheral Nerves, 2008). When stimulus strength is increased, it means more fibers are recruited thus more action potentials are summed up to produce a bigger bell-shaped curve. Therefore, the more the stimulus strength is increased the more fibers are recruited resulting in a wider compound action potential with a longer duration (Iworx.com, 2010). Beyond a certain point, a further increase in the stimulus voltage no longer increases the amplitude of the observed action potential because it will have reached the maximal stimulus voltage. The point where there is no change in the amplitude whatsoever – increase in stimulus voltage does is not affected by an increase in the compound action potential is the point which is known as maximal stimulus voltage. At a maximal point, the compound action potential does not increase because all the fibers of the nerve have been excited and are CAPs.
The sciatic frog nerves contain numerous axons that vary in thresholds, diameters, and degree of myelination (Neuroscience Laboratory, n.d.). The variations are a recipe for the different types of waves. Type A fibers are the fibers that have the fastest conduction velocities and are the most myelinated and are further subdivided into δ, β, α, and γ types. Another type of fiber is type B which like type A is myelinated but they have slower conduction velocity and smaller diameters. The other group of fibers is type C, which has unmyelinated axons and is very small. Therefore, the waves are due to the responses of axons of the various fibers when stimulus is delivered to the nerve. When the waves move closer, it means that their velocity is increasing.
Conduction Velocity = Change of conduction distance:
- Peak latency
- = (25-0) mm/ (2.85-1.50) ms
- =25mm/1.35ms
- =18.5mm/ms
According to the McGill Physiology Virtual Lab (2010), “Absolute refractory period is the brief interval after a successful stimulus when no second shock, however maximal, can elicit another response. Its duration in mammalian A fiber is about 0.4 ms; in frog nerve, at 15oC it is about 2 ms”. This particular period will follow the absolute refractory period and it will not result until high stimulus voltage is passed through the system. It, therefore, follows that the refractoriness cannot be overcome by increasing the stimulus voltage.
When stimulus voltage is consistently increased, a second wave will be observed due to the response. It will appear very small will be a product of A-beta fibers. Axon’s response to the stimulus is reduced and after AP the axon can’t initiate another AP with the stimulus of any strength and this time that is known as the absolute refractory period. The axon can shoot a second AP after the absolute refractory period but with a stimulus of larger strength hence the relative refractory period. This relation exhibit the unique relationship between the two refractory periods that gives paramount information in either relating the periods or understanding their sequence.
There will not be any change in the action potential amplitudes impulse with nerve firing at 10impulses per second provided the stimulus strength be maintained. This is because duration, amplitude, and shape action potential are directly proportional to stimulus strength (Oakley Schafer, 200). Since there are directly proportional to stimulus strength, a decrease or increase in the stimulus strength will affect them accordingly. The local anesthetic will gradually block the conduction of action potential because during the process the transfer of charges and depolarization of membranes will be taking place to a point where there will be no more action potential. The fibers do not block at the same time since they have different properties.
References
Aiki.bme.duke.edu, 2010. Electrophysiology of the frog Sciatic nerve. Web.
Iworx.com, 2010. Recording compound Action potential with a differential amplifier.Web.
McGill Physiology Virtual Lab, (2010). Compound action potential. Web.
Neuroscience Laboratory, n.d.. The compound action potential. Web.
Oakley, B. & Schafer, B., 2001. Compound action potentials: frog sciatic nerve. Web.
Peripheral Nerves, 2008. The compound action potential. Web.