Wait... capacitance exists along the axon inside the cell? I thought it was on the membranes.
No, you're right. I hope I didn't say otherwise
The cell membrane is said to act as a capacitor, and has a property known as capacitance. A capacitor consists of two conducting regions separated by an insulator (cytoplasm). A capacitor works by accumulating a charge on one of the conducting surfaces. As this charge builds, it creates an electric field that pushes like charges on the other side of the insulator away. This causes an induced current known as a capacitative current. It is important to realize that there is no current between the conducting surfaces of the capacitor. Capacitance then, may be defined two ways, 1) as an ability to store and separate charge, or 2) as the quantity of charge required to create a given potential difference between two conductors. Thus, given a set number of charges on each side of the membrane, a higher capacitance results in a lower potential difference. In a cellular sense, increased capacitance requires a greater ion concentration difference across the membrane.
To be honest toshiroumezawa, my skills in physics are probably no better than yours lol. However, you may find this website (
https://biology-forums.com/definitions/index.php/Neurons_As_Biological_Circuits) very helpful with explaining this phenomenon. That's is where I got the information above. According to the article:
In the neuron, the membrane is the insulator between the two conducting surfaces (represented by the aqueous intra and extracellular fluids). A neurons capacitance is proportional to it's membrane surface area, so large neurons, have larger capacitances. Capacitance also decreases with the distance between the two conducting surfaces. Capacitance plays the most important role in the axon, and is involved in action potential generation and propagation. Myelin plays a very important role in this also. Myelination not only increases the membrane resistance of the axon (there is very little ion exchange across the membrane in myelinated regions of the axon), but increases the distance between the conducting surfaces, so decreases membrane capacitance. These factors lead to more rapid conduction of action potentials down the axon and is responsible for the phenomenon known as saltatory conduction.
To review, increased capacitance corresponds to a decreased impulse velocity (speed), and decreased capacitance means a faster impulse velocity. Action potential velocity is slower in unmyelinated axons due to membrane capacitance, short distances between action potential sites (the shorter the distance between action potentials, the more action potentials that are required to travel a given distance of axon), and the presence of passive ion leak channels (these cause a slight decrease in depolarization magnitude due to the loss of some ions). Myelinated axons have much faster action potentials because of three main properties. First, in the internodal regions of myelinated axons the myelin eliminates the loss of ions, and subsequent decrease in depolarization magnitude, by drastically increasing membrane resistance (there is basically no diffusion through passive leak channels under myelin). Second, myelin greatly increases the distances between the extra and intra cellular solutions (the conducting surfaces if the axon where a capacitor), thus decreasing membrane capacitance. Finally, due to myelination, there is a greater distance between the sites of action potentials in myelinated vs. unmyelinated cells. This corresponds to a smaller number of action potentials needed to travel a given axonal distance, and thus a shorter amount of time is required. These properties allow myelinated axons to conduct impulses up to 150 times faster than unmyelinated axons. Many axons however remain unmyelinated. This may seem to be detrimental, but these axons are generally transmitting signals significantly shorter distances, where the increase in speed is not necessary.
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