Pain in the arm often originates in the nerves and sometimes is from a pinch in the cervical or neck region in the spine.
The nerve gets pinched in the neck and can cause numbness, pins and needles, dullness in the arm and hand.
Bruised nerve
If a nerve is bruised from a knock or fall it can also cause pain in the above region. If a person lays on the side of the body in an awkward way for too long for example when a mum is breast feeding it can effect the nerve in the deltoid region. See Radial nerve.
Repair
Nerves take time to repair sometimes longer than most muscles. They are very hard to feel so not as user friendly like a muscle or bone to fix.
Mostly good posture, massage and stretching around the area of the pain and the originating cervical or thoracic spinal region is recommended.
Warmth and heat can also speed up the recovery as energy is supplied to the area and the cells start to repair. Also a positive mind helps as the patient focuses on the region mentally and brings their life force into play for the cells health.
Anatomy
Radial Nerve
The radial, ulnar, and median nerves, having already supplied connections to the arm and forearm, continue into the hand where they form a branching network of nerve fibers.
Median Nerve
The median nerve supplies the muscles and sensory receptors of the skin in the lateral (thumb side) palm, first, second, and third digits (thumb, index, and middle fingers), and lateral half of the fourth digit (ring finger). Along the dorsum (back) of the hand, the radial nerve supplies the muscles and sensory receptors in the lateral dorsum, and the first, second, and third digits.
Ulnal Nerve
On the medial side of the hand, the ulnar nerve supplies the sensory receptors and muscles in the medial palm, medial dorsum, medial half of the fourth digit, and the fifth digit (pinky finger).
Sources
https://www.google.com/search?q=muscles+of+upper+arm+and+shoulder&oq=muscles+of+upper+arm&aqs=chrome.1.69i57j0l7.15148j0j7&sourceid=chrome&ie=UTF-8
https://www.innerbody.com/anatomy/nervous/arm-hand
