Final answer:
During practice, the myelin sheath grows, enhancing the speed and efficiency of neural communication by improving the conduction of nerve impulses along the axon. Option d.
Step-by-step explanation:
When you practice, the myelin sheath that insulates nerve fibers can be strengthened, leading to improved efficiency and speed of nerve signal conduction. This biological process is known as myelination, where myelinating glia (such as Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system) wrap several layers of cell membrane around parts of an axon.
Therefore, the correct answer to what happens to myelin when you practice is that it grows (Option C). This growth is crucial for the normal operation of neurons, enabling more rapid and efficient neural communication. It's important to note that the myelin sheath is not the same as gray matter, which consists primarily of neuron cell bodies, and both are integral parts of the nervous system.