sinoatrial node vs heart muscle

sinoatrial node and heart muscle both are nouns.

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

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

sinoatrial 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 sinoatrial node include cardiac muscle.

sinoatrial 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