does method invocation involve a lock?

[Tom Sheffler]

This question is for my own research: I’m curious about whether invoking [obj 
method] can block.  If obj is allocated, and method is simple, does method 
invocation involve a lock.

I think the answer is YES.  But I looked at the source code for objc_msgSend 
and didn’t see anything that looked like it acquired a lock.

Thanks for any insights.
-Tom