How does stimulus trigger action potential

There is no such thing as a "strong" or "weak" action potential. Instead, it is an all-or-nothing process. This minimizes the possibility that information will be lost along the way. This process is similar to the action of pressing the trigger of a gun. A very slight pressure on the trigger will not be sufficient and the gun will not fire. When adequate pressure is applied to the trigger, however, it will fire. The speed and force of the bullet are not affected by how hard you pull the trigger.

The gun either fires or it does not. In this analogy, the stimulus represents the force applied to the trigger while the firing of the gun represents the action potential. The body still needs to determine the strength or intensity of a stimulus. It's important to know, for example, how hot a cup of coffee is as you take an initial sip, or to determine how firmly someone is shaking your hand.

In order to gauge stimulus intensity, the nervous system relies on the rate at which a neuron fires and how many neurons fire at any given time.

A neuron firing at a faster rate indicates a stronger intensity stimulus. Numerous neurons firing simultaneously or in rapid succession would also indicate a stronger stimulus. If you take a sip of your coffee and it is very hot, the sensory neurons in your mouth will respond at a rapid rate.

A very firm handshake from a co-worker might result in both rapid neural firing as well as a response from many sensory neurons in your hand. In both cases, the rate and number of neurons firing provide valuable information about the intensity of the original stimulus. The all-or-none law was first described in by physiologist Henry Pickering Bowditch. In his descriptions of the contraction of the heart muscle, he explained, "An induction shock produces a contraction or fails to do so according to its strength; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time.

While the all-or-none law was initially applied to the muscles of the heart, it was later found that neurons and other muscles also respond to stimuli according to this principle. Action Potential Learning Outcomes Explain the stages of an action potential and how action potentials are propagated. Practice Question The formation of an action potential can be divided into five steps, which can be seen in Figure 1. Figure 1. Action Potential. Show Answer Potassium channel blockers slow the repolarization phase, but have no effect on depolarization.

This video presents an overview of action potential. Try It. Did you have an idea for improving this content? Licenses and Attributions. CC licensed content, Shared previously. A totally new type of signal is initiated; the action potential. Note that if the size of the battery is increased even more, the amplitude of the action potential is the same as the previous one Figure 1.

The process of eliciting an action potential in a nerve cell is analogous to igniting a fuse with a heat source. A certain minimum temperature threshold is necessary. Temperatures less than the threshold fail to ignite the fuse.

Temperatures greater than the threshold ignite the fuse just as well as the threshold temperature and the fuse does not burn any brighter or hotter. If the suprathreshold current stimulus is long enough, however, a train of action potentials will be elicited. In general, the action potentials will continue to fire as long as the stimulus continues, with the frequency of firing being proportional to the magnitude of the stimulus Figure 1.

Action potentials are not only initiated in an all-or-nothing fashion, but they are also propagated in an all-or-nothing fashion. An action potential initiated in the cell body of a motor neuron in the spinal cord will propagate in an undecremented fashion all the way to the synaptic terminals of that motor neuron.

Again, the situation is analogous to a burning fuse. Once the fuse is ignited, the flame will spread to its end. The action potential consists of several components Figure 1. The threshold is the value of the membrane potential which, if reached, leads to the all-or-nothing initiation of an action potential. The initial or rising phase of the action potential is called the depolarizing phase or the upstroke. The region of the action potential between the 0 mV level and the peak amplitude is the overshoot.

The return of the membrane potential to the resting potential is called the repolarization phase. There is also a phase of the action potential during which time the membrane potential can be more negative than the resting potential. This phase of the action potential is called the undershoot or the hyperpolarizing afterpotential. In Figure 1. Before examining the ionic mechanisms of action potentials, it is first necessary to understand the ionic mechanisms of the resting potential.

The two phenomena are intimately related. The story of the resting potential goes back to the early 's when Julius Bernstein suggested that the resting potential V m was equal to the potassium equilibrium potential E K. The key to understanding the resting potential is the fact that ions are distributed unequally on the inside and outside of cells, and that cell membranes are selectively permeable to different ions. Thus, there will be an electrical force directed inward that will tend to counterbalance the diffusional force directed outward.

The potential at which that balance is achieved is called the Nernst Equilibrium Potential. An experiment to test Bernstein's hypothesis that the membrane potential is equal to the Nernst Equilibrium Potential i. Also shown is the line that is predicted by the Nernst Equation. The experimentally measured points are very close to this line. Note, however, that there are some deviations in the figure at left from what is predicted by the Nernst equation.

Such deviations indicate that another ion is also involved in generating the resting potential. There is also an electrical driving force because the inside of the cell is negative and this negativity attracts the positive sodium ions. When a membrane is permeable to two different ions, the Nernst equation can no longer be used to precisely determine the membrane potential.

It is possible, however, to apply the GHK equation. If the GHK equation is applied to the same data in Figure 1. The value of alpha needed to obtain this good fit was 0.

Click here to go to the interactive Membrane Potential Laboratory to experiment with the effects of altering external or internal potassium ion concentration and membrane permeability to sodium and potassium ions.

Predictions are made using the Nernst and the Goldman, Hodgkin, Katz equations. A change in permeability would depolarize the membrane potential since alpha in the GHK equation would equal one. Initially, alpha was 0. Try substituting different values of alpha into the GHK equation and calculate the resultant membrane potential.