myelin sheath

The myelin sheath is a fatty covering that wraps some axons along their length. It does not carry the signal and it does not receive one: it insulates the fiber so the impulse can jump from gap to gap instead of traveling along every millimeter of membrane, which makes the signal faster in a myelinated axon than in a bare one. The exam separates the sheath from the parts it wraps in a single sentence — the axon carries the signal away from the cell body, the dendrites bring signals in, and the sheath only changes how fast the trip is — so an item that credits the sheath with receiving or releasing a signal has given it another part's job. Myelin is a speed feature, not a direction feature: a myelinated axon still runs dendrite to terminal, one way.