Video Transcript
In nerve cell A, the speed of the nervous impulse is 12 meters per second. In nerve cell B, the speed of the nervous impulse is 140 meters per second. Which nerve cell is myelinated? (A) A, (B) B.
To answer this question, we need to recall what myelin is and how it changes the speed of the nervous impulse. Let’s start by reviewing what we know about myelin.
Myelin is a lipid-rich substance, found surrounding the axons of most nerve cells. Its main function is to help conduct the nerve impulse down the axon by ensuring the nerve impulse does not degrade or fizzle out as it travels towards the next neuron or effector organ.
A main property of myelin is that it acts as an insulator. Like a plastic insulator around an electrical cable, the myelin keeps electrons within the path of the conducting axon. So, myelin itself is not very good at conducting the electrical nervous impulse. But it does prevent the electric signal from leaking sideways out of the nerve cell. Thus, the parts of the axon that are covered by myelin are able to transmit the electrical energy of the nerve impulse faster inside the axon. This is because the action potential doesn’t need to depolarize the membrane at every point along the axon.
However, the myelin sheath is not continuous. Instead, there are small unmyelinated gaps between each myelin sheath. These gaps, which are called the nodes of Ranvier, enable the electrical impulse to be regenerated regularly as it travels along the axon. In myelinated neurons, the nerve impulse appears to jump from one unmyelinated node of Ranvier to the next unmyelinated node of Ranvier. It is this jumping action of the nervous impulse which helps to speed the impulse down the axon no matter its length as well as preserve its integrity as it travels to its final destination.
In unmyelinated neurons, nerve impulses are conducted down the axons at a speed of 0.5 to two meters per second, which is about as fast as you can walk or jog. This slower speed is due to the lack of myelin. Without myelin, there are no nodes of Ranvier. This means the action potential would propagate very slowly because it would need to be regenerated at every point along the axon. This can make the nerve impulse lose integrity and degrades before it reaches the axon terminals. When axons are myelinated, the nerve impulse can travel at speeds of 70 to 120 meters per second, which is almost as fast as a speeding race car. This means nerve cells that are able to conduct nervous impulses at high speeds can do so because they’re myelinated.
Now that we’ve reviewed the function of myelin and how it increases the speed of the nervous impulse as it travels down the axon, we can answer the question. The nerve cell that is myelinated is nerve cell B.
