pacemaker vs heart muscle

pacemaker and heart muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
pacemaker	Yes	No	No	No
heart muscle	Yes	No	No	No

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

pacemaker: 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 pacemaker include cardiac muscle.

pacemaker (noun)	heart muscle (noun)
a horse used to set the pace in racing	the muscle tissue of the heart; adapted to continued rhythmic contraction
an implanted electronic device that takes over the function of the natural cardiac pacemaker
a specialized bit of heart tissue that controls the heartbeat
a leading instance in its field