I'll try and do the short version. I'll focus on the stuff running down the leg, and ignore upper body stuff. Nerves cells are extremely long, often running the entire length of the nerve. These are encased an protected by sheets of collagen. If the nerves or the sheets become irritated/not used for a while/subjected to poor posture etc., they can shorten and/or adhere to surrounding structures (often muscle fascia). This can result in neural tissue tightness, and are often secondary to acute compression onto the nerve structures (over stretching can cause this), chronic overuse (especially unaccustomed activities), muscle imbalances, joint issues, poor posture, etc. That then causes changes in the nerve structures - swelling, local lack of oxygen, chemical irritation, decreased blood flow, etc. - all causing pain. The pain causes muscle guarding/spasm to protect the inflamed neural tissue. If the issue persists, it may lead to fibrosis around the nerves, leading to deceased elasticity, and adhesion to the surrounding structures. This then causes more load on the nerves, and you become stuck in a downward spiral.
