Are all action potentials the same shape and amplitude when graphed with respect to time?

Question

The most common visualization of an action potential is a graph of the difference in membrane potential (y axis) at a particular time (x axis).

According to my textbook Cognitive Psychology by E. Bruce Goldstein, an action potential sent from neuron down the axon remains the same. That is, if we plot the action potential as described above, then at each as it is propagated down it will have the same shape on the graph (within some range of error obviously, but he is saying the error is trivial). But he then says on page 34

One way to answer the question of how action potentials determine different qualities [(e.g. taste of something sweet)] is to propose that the action potentials for each quality might look different. However, [Edgar] Adrian rules out that possibility by determining that all action potentials have basically the same height and shape.

My Question

Can someone confirm these two facts:

1. Once an action potential is sent from a given neuron down the axon, does the shape and amplitude remain constant as it is propagated?
2. Do all action potentials have the same amplitude and shape?

Answer 1

Short answer
Action potentials differ in shape between neuronal cell types, and action potentials may even change shapes during action potential propagation within one and the same axon.

Background

1. Once an action potential is sent from a given neuron down the axon, does the shape and amplitude remain constant as it is propagated?

Although the textbooks will typically say action potentials are transmitted without their amplitude being changed this claim is theoretically virtually impossible due to the many variables encountered in and around the axon. For example, axons in the sciatic nerve may extend to a meter and it is virtually impossible to keep the exact conditions along that length exactly identical. The amplitude of the action potential is mainly dependent on the influx of Na⁺. Slight variations in membrane potential, concentration of sodium, or channel (subtype) densities may therefore change the amplitude. In addition, temperature affects action potential amplitude (Hodgkin & Katz, 1949) and slight temperature differences along long axons may therefore be expected to alter amplitude. Likewise, shape may alter as well. For example, glutamate released alongside axons of hippocampal pyramidal neurons results in widening of action potentials (Sasaki et al., 2011).

2. Do all action potentials have the same amplitude and shape?

Therefore, given the answer under question 1 no, they do not have to. Even a propagated action potential in a given axon may change amplitude and shape, as said. Most notably, different neuronal types may in fact be classified according to their action potential morphology, such as the various neuronal types in the dorsal root ganglia of the spine that can be differentiated based on their duration (Villiere & McLachlan, 1996).

<sub>References
- Hodgkin & Katz, J Physiol; 109: 240–9
- Sasaki et al., Science 2011; 331: 599-601
- Villiere & McLachlan, J Physiol 1996; 76: 1924-41</sub>

Answer 2

1. Generally speaking, yes. For reference see this paper in Nature This is Fig. 7 from it with comment:

Reason is that most axons are not passive tubes of electrolyte with leakage (in which case AP would be attenuated along it) but rather active media with membrane potential across that have ion channels that reproduce initial action potential along the axon. However, in more passive tubes, like in dendrites, action potentials might be attenuated.

2. Generally, no. Amplitude of action potential depends on properties of ion channels, at what potential, for example, $K^+$ channels will open to start repolarization (peak of AP). Duration also depends on properties of ion channels. But since there are not a lot of variability in ion channels and extracellular ion composition is mainly same in animals, most of APs will have same shape/duration/amplitude.