Final answer:
The structure that relays an electrical impulse from the SA node to the left atrium is the Bachmann's bundle or the interatrial band.
Step-by-step explanation:
The structure that relays an electrical impulse from the SA node to the left atrium in a normal heart is the Bachmann's bundle or the interatrial band. It consists of specialized conducting cells that transmit the impulse directly from the SA node in the right atrium to the left atrium.