system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does muscle relaxers not stop muscles needed to live. IE: your heart

Happened a long time ago, but I remember being prescribed some muscle relaxers for a pinched nerve in my back. The things they gave me hit me so hard, I could literally feel my heart slowing down. I thought to myself "Well, this is how I die I guess". I woke up about 18 hours later, still alive but with that drugged feeling. How do muscle relaxers not relax yourself into a slow death?
gpt
Imagine any drug you take is like a key and that key can only fit into certain keyholes (receptors) on certain doors (target organ/tissue/nerve etc).  There are different types of muscle relaxers (keys) that open certain doors and causes a reaction to occur behind that door (this is the drug action).  This is how the drug works.  

Now, there are three types of muscles (smooth - ex intestine, cardiac - heart, and skeletal).  A muscle relaxer like Zanaflex fits in one type of keyhole on a certain door (brain) and behind that door a reaction occurs to only cause the skeletal muscle to relax.   In Anesthesia (my profession) We give a paralytic (key) during surgery that will only fit a certain keyhole on skeletal muscle that the heart and intestines do not have (they have different keyholes).  This is how we paralyze the body and not the heart/intestines.