SA node vs heart muscle

SA node and heart muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
SA node	Yes	No	No	No
heart muscle	Yes	No	No	No

As nouns, heart muscle is a hypernym of SA node; that is, heart muscle is a word with a broader meaning than SA node:

SA node: a specialized bit of heart tissue that controls the heartbeat
heart muscle: the muscle tissue of the heart; adapted to continued rhythmic contraction

Other hypernyms of SA node include cardiac muscle.

SA node (noun)	heart muscle (noun)
a specialized bit of heart tissue that controls the heartbeat	the muscle tissue of the heart; adapted to continued rhythmic contraction