Speaking up in meetings is hard enough, but speaking up when you're surrounded by senior colleagues can be a very intimidating feat. Even people who think of themselves as extroverts can find they've lost their voice, afraid of saying the 'wrong thing' in front of their seniors. In truth, you can and should speak up, because research has shown that people who speak up early and often are perceived as 'leaders'.
To make sure you do speak up, try to prepare in advance. Get hold of the meeting agenda, if you can, and highlight two or three items where you think you have a valid contribution to make. Bullet point a few things you might say, because having a few notes to hand means you'll know when to chip in and exactly what you want to express.
If there's no agenda available, try to get a few words in early on. It doesn't even have to be your area of expertise – it could be something as simple as seconding an opinion or asking how a decision is going to impact XYZ. If you get in there early, your brain has less time to talk you out of speaking up.
If you're really nervous and think you might talk yourself out of speaking, it might be worth having a word with the person chairing the meeting beforehand. If you let them know you have a contribution to make, they'll be on the lookout for your signal and will give you the floor. That means you can choose your moment and ensure your voice is heard.
Get hold of the agenda in advance
Prepare a few ideas you can contribute
If there is no agenda, speak up early on
Have a word with the chair about contributing