What causes this consistency? I read the previously asked questions on this site none of them carry an answer to this question.
Actually, it would be better to say that action potential does not travel but is regenerated anew in a sequence along the axon.
I would like to elaborate how this happens and why this is the reason why the action potential stays constant:
In rest state, there is an ion gradient across the membrane of each cell. This causes a small voltage between the outside and the inside of the membrane, the so called transmembrane potential. Once the cell membrane becomes permeable for ions (normally this occurs when an ion channel is opened), the potential causes ions to flow across the membrane, the membrane becomes depolarized.
In the axon, the membrane contains so called voltage-gated ion channels. As soon as the membrane is depolarized at the trigger zone, the depolarization of the adjacent membrane causes voltage gated ion channels to open in that adjacent memrane as well, causing ions to flow also across the adjacent piece of membrane. This process continues in a wave along the whole membrane, this is what we call the action potential. Because the ion gradient across the membrane and the density of ion channels is constant along the whole axon, the amplitude of the action potential stays constant along the whole length of the axon.
Descriptive Answer- A nerve impulse is the movement of an action potential as a wave through a nerve fibre. A wave of negative charge on the surface of an axon marks the position, at any moment, of the action potential. Action potentials are self-generated along the axon. The events that set up an action potential at one spot on the nerve fibre also transmit it along the entire length of the nerve fibre. The changes in membrane permeability at one site upset the normal membrane permeability at the adjacent area. The action potential then moves to the neighbouring region of the nerve fibre till it covers the whole length of the fibre. Action potential is propagated along a nerve fibre without any change in the amplitude. It is, thus, a nondecremental (not decreasing) phenomenon. Actually, it would be better to say that action potential does not travel but is regenerated anew in a sequence along the axon.
Explanation- To know why nerve impulse transmission is consistent, lets first know how it is transmitted, for which we should know how it is generated.
Generation- At rest (no impulse), outside of neuron i.e. ECF (extra-cellular fluid) has more positive charge and inside of neuron has relatively less positive charge or more negative charge. This is maintained by keeping Na+ outside and allowing K+ to move freely between inside outside. It is done by action of voltage-gated ion channels like closing of Na+ channels and keeping K+ channels open, and through Na+/K+ ATPase which transports 3 Na+ outside for bringing 2 K+ inside, and K+ can again freely move outside down the concentration gradient. This results in a resting membrane potential of -70mV i.e. depolarization.
When neuron receives a stimulus, its Na+ channels open and K+ channels close which causes an influx of Na+ which results in more positive charge inside and negative charge outside. This is the action potential of neuron of about +30mV (value varies, I have taken from here, for variations in AP see this). It lasts for less than 1/1000 of a second as Na+/K+ pump starts working again immediately and K+ channels open and Na+ channels close again to recover the resting potential of neuron i.e. repolarization.
Transmission- Breaking of resting potential i.e. action potential at one place stimulates action potential at the adjacent place. Actually, action potential is not transmitted through the neuron, it is regenerated at adjacent areas in a sequential manner from one point to another in the axon. Opening & closing of ion channels at one point causes opening & closing of ion channels at the next point by which action potential is transmitted through the axon.
Source This also explains why nerve impulse does not travel in reverse direction i.e. due to repolarization state of neuron at the previous site during which it cannot transmit any impulse.
Now, for why nerve impulse is consistent through the neuron i.e. why it does not decrease during its course through axon. This is because of equal charge throughout the length of axon. Resting potential is -70mV throughout the axon, and action potential is +30mV at any point in the axon, due to which no charge is dissipated and the nerve impulse remains consistent. For detail about it, refer to the all or none law of nerve impulse which states:
The all-or-none law is the principle that the strength by which a nerve or muscle fiber responds to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential, the nerve or muscle fiber will give a complete response; otherwise, there is no response.
i.e. if a stimulus has the threshold potential (-40mV to -55mV), it will always generate nerve impulse of same action potential (+30mV) otherwise no action potential will be generated and transmitted i.e. it will be consistent everywhere.
Source- Pradeep's : A textbook of Biology for Class XI, chapter 22- Neural control and coordination.
Similar answer has been posted by @Thawn too. I have added some more explanation and citation to this answer.
EDIT 1- You may find an answer to How does a change in the potential across a neuron's membrane get turned into a signal that is sent down the axon? helpful which says:
the sodium gates open the more positive the voltage is, which in turn opens more sodium gates. If a patch of neural membrane is depolarized, the charge diffuses into the nearby patches of neural membrane. This will open the sodium channels of the nearby membrane patch. Now once enough sodium channels open, the neuron will fire a action potential, at this point potassium channels open and potassium rushes out of the cell repolarizing it(making it more negative). Furthermore the sodium gates close at positive voltages which further repolarizes the neuron and prevents back propagation.
EDIT 2- Thus propagation of ions in axon can be, as pointed out by @Thawn, categorised as:
1. Perpendicular to length- this type of movement is responsible for generation of action potential. It is perpendicular to length of axon i.e. through neuron's membrane. This type of movement of ions is governed by voltage-gated ion channels, which ensure charge difference between inside and outside of axon.
2. Along the length- this type of movement is responsible for transmission of nerve impulse through the axon. It is parallel to length of axon. Since there is no membrane in length of axon, this type of movement is thus governed only by diffusion. As Na+ enters cytoplasm of axon, it diffuses to adjacent areas. But since previous (depolarized) area already has high concentration of Na+, so its net movement is towards the next (polarized) area, which becomes the cause for depolarization of that area. This is similar for K+, Cl- and other ions (like protein-derived anions).