@slw18 What comes to my mind is thyroid function. If your thyroid is overactive, or if you take thyroid meds and you are on an overdose, that produces anxiety type symptoms and a racing heart beat. Hopefully your doctors have checked this, if not, it would be something to ask about. Another thing to think about would be a deficiency like Magnesium. Most people are deficient and don't know it. I have been deficient myself and have felt like I had bugs crawling on my skin and little tingles and zaps. Taking magnesium supplements and soaking in epsom salt baths does help. Another cause could be symptoms of menopause which also causes anxiety type symptoms for some women. I don't know if that applies to you at all, but I mention it for those who may benefit. I know my heart rate can be affected and that taking bio-identical hormone replacement helps smooth things out and I have less aches and pains and sleep better. If you don't get good sleep regularly, that can certainly add to anxiety type symptoms. How about breathing issues? I also have asthma and I do have a faster heart beat when I'm getting clogged with phlegm. I check this along with my oxygen levels when I don't feel that great and it helps me know when to use my inhaler. I also know when I may have a chest infection when my heart races just with walking around. I have some physical issues that cause trapped phlegm that can easily become bronchitis, so I try to be aware of changes and treat them right away and stretch when I feel a chest spasm coming on. Keeping allergies under control goes a long way toward good breathing and a lower heart rate and I just feel better.

As for your question of if a neck muscle can cause anxiety and a racing heart, the answer is yes. That has also happened to me. If you have tight neck muscles, it can compress the nerves next to the spine that go to the heart and lungs. I have thoracic outlet syndrome which causes tightness between my neck and chest and a few times, it has caused a spasms in a chest muscle and sped up my heart, or when I tucked my head bending my neck it could bring on the spasm. I am also a spine surgery patient and a time that this happened was right after my surgery to fuse C5/C6 that caused throat swelling from the surgery. I knew it was a neck spasm that started it because the pain started in my throat (which hurts right after ACDF surgery) and it migrated into my chest. I was able to take my hands and massage the chest and stop the pain. It happened a few other times too, and I have been working with a physical therapist on this for several years treating the TOS, and there have also been times that I went to the ER with a racing heart rate because of the trapped phlegm causing a chest infection. (This was all prior to COVID) My dad was a heart patient, and that was all ruled out for me. Extreme stress and harassment can also cause chest pain which I experienced several years ago and consulted a cardiologist. He found no issues whatsoever, so changing my diet when my dad because a heart patient many years ago made a big difference for me since I was in my early twenties at the time.

On MRIs, usually a spasm is noted because of where the shortened muscle will move the bones. For example on a cervical spine MRI, they may note the loss of the lordotic curve (or lordosis) which straightens the spine by taking away the normal curve. My spine was like that before my spine surgery due to spasms. You may also see that vertebrae may be twisted or tilted and not aligned properly. You may want a physical therapy evaluation, and I would recommend looking for one who also does myofascial release. You can have tight tissue anywhere in the body that can compress nerves and we develop patterns of tightness from injuries, infections, inflammation, surgeries, and bad habits. Nerves pass through some pretty small spaces. Here is the link to our MFR discussion.
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/
There is a provider search on the MFR website for therapists. There is a lot to read with most of the information links on the first pages of the discussion. MFR therapy help a a lot of issues and a good therapist can feel the tightness wit there hands and where that might wrap around in your body, for example around the heart or lungs. The tightness of the respiratory diaphragm also affects breathing. One side of my body is to tight, so I work on this all the time too.