Heart attack pain can radiate to the arm because visceral fibers from the heart converge in the spinal cord with regions that subserve sensation in the arm.

“So the visceral fibers that subserve the heart, typically the first through the fourth thoracic region, well, those converge in the spinal cord and similar regions that subserve sensation under the arm up here. That's why people will often say they've got pain with a heart attack radiating down into their arm.”