Most EDs in the US will also employ physician assistants or physician associates (PAs). PAs are utilized differently in different environments and states, so I'm speaking from my experience and location.
PAs are able to see patients independently, under a written supervision agreement with a physician (although some states are removing that requirement.) We assess, diagnose, test, and treat in a variety of settings, including the ED, ICU, general medical floors, surgery, and outpatient offices. Pretty much anywhere a physician works, PAs can work, too. Some places require that the physician poke their head in at least once during the visit but in my practice, I only need to track down a physician if I have a question for them. They don't even need to be in the building, just reachable.
Like medical school, PAs receive a graduate-level education. Most programs require some level of patient care experience prior to admission--nursing, paramedic, medical assistant, etc. Typically PA programs are 2-3 years long, earning a master's degree, although there are some doctoral level programs out there now. The first year is typically a year of full-time classroom work, and the second year is full-time clinical rotations, about 4-5 weeks each. We sit for a national certifying exam (delightfully called the PANCE, pronounced 'pants') which earns us the letters PA-C, and then get licensed on a state level. We repeat the exam every 10 years, and have to complete a certain amount of continuing education every 2 years to maintain our certifications.
Historically, PAs were military corpsmen (medics) who received additional training in order to practice medicine stateside. This began in the 1960s, when there was a shortage of general medical practitioners as more people moved into specialty care. Nowadays, the profile of PAs has shifted more toward students with a traditional education background.
PAs, by education, generalists; we can work in different fields throughout our career without needing to do extra training. We learn the specifics of our field by doing. At the start, there's usually some shadowing, then taking on a lighter case load, and then building up to a full panel of patients/cases. (There are elective residencies and specialty certifications that people can get, but these are not required and are really just things to make you more attractive to employers.)
PAs are HEAVILY utilized in the ED where I'm from, even in places with EM residents. Where I was, most of the bigger cases, like arrests or critically ill patients, went to the residents because they were there to learn. Other than that, cases were evenly split--whoever was available would take the next patient.
One area of difference, though, was that only PAs staffed the 'fast track' area, which is basically what McKay is doing this season. This is where patients who could be moved through the department quickly were seen: UTIs, musculoskeletal injuries (my hospital was not a trauma center, so we really didn't have any major injuries), lacerations, mild respiratory illnesses, etc.
Lastly, a note about our name: historically, we are called physician assistants (no apostrophe-s, no capitalization) but recently our professional/national title was changed to physician associate. This was done to convey that we work with physicians; we do not just assist them. That being said, while it was adopted on a professional/national level, my state hasn't changed it yet, so I still have to introduce myself/sign things as a physician assistant. But mostly I just say PA :)
If you made it this far, thanks for reading, and I'm always happy to talk more! I've been a PA for 9 years and absolutely in love with it.