involuntary muscle vs musculus

involuntary muscle and musculus both are nouns.

Word	Noun	Adjective	Verb	Adverb
involuntary muscle	Yes	No	No	No
musculus	Yes	No	No	No

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

involuntary muscle: a muscle that contracts without conscious control and found in walls of internal organs such as stomach and intestine and bladder and blood vessels (excluding the heart)
musculus: one of the contractile organs of the body

Other hypernyms of involuntary muscle include muscle.

involuntary muscle (noun)	musculus (noun)
a muscle that contracts without conscious control and found in walls of internal organs such as stomach and intestine and bladder and blood vessels (excluding the heart)	one of the contractile organs of the body