Why do these Nitrogens experience steric hindrance?

[instantresults]

Wikipedia says that the steric hindrance makes this a weak nucleophile, but what exactly is it about the N's that is sterically hindered?
I realize that the N's are bonded covalently 3 times so each N has a lone pair e-, but is it the location on the ring that makes for steric effects for this molecule? thanks for any help

 

[TeethWhitener]

The nitrogens are tertiary amines: the methyl groups bonded to each of the nitrogens take up a lot of volume around the nitrogens' lone pairs. This makes it very difficult for the lone pairs to participate in a nucleophilic addition or substitution: there's just not much room for a decent-sized molecule to hang onto one of those tertiary amines. However, this compound is a very strong base (this particular compound is known as "proton sponge"). It's a member of a class of compounds known as non-nucleophilic bases. Other examples include LDA and LHDMS. The main feature is a tertiary amine nitrogen with bulky (sterically hindering) electron donating groups. The idea is that steric hindrance prevents the amine from adding in a nucleophilic addition or substitution, but a proton is small enough to access the lone electron pair on nitrogen. This means you can use these compounds as strong bases and not have to worry about them participating in unwanted nucleophilic side reactions.