From giraffes to elephants, to mice to humans, all mammals have an average of 26 bony segments in their spines. The spinal cord leaves the brain and travels in the bone all the way down the low back. Between each vertebra there are discs for padding and at the level of the disc, two nerve roots exit from either side of the spinal cord. Many things can cause the nerve roots or the spinal cord to become pinched, causing pain in the neck and shoulder.
Local Pain
Nerve impingement can be caused by a herniated disc, inflamed joints in the spine, or stenosis in which there's a narrowing of the spinal cord passageway. All of these problems will cause swelling and pain in the area as well as deep achy pain in the back or side of the neck. The pain levels can range from mild to intense, depending on the cause of the nerve impingement.
Referred Pain
Pain in the neck can also travel down the arm, to the shoulder or all the way down into the fingers. The website Spine Universe states that a pinched nerve in the spine is usually perceived as traveling down the arm or leg. The pain will feel as though it is going down into the shoulders or finger when in fact it is not. This is called referred pain. Neck Solutions, a website providing information on neck pain, states that the C5 and C8 nerves in particular can cause pain in or around the shoulder.
Sensation Alteration
Nerves provide sensation to the arms and fingers. Each nerve root has an area of skin, or dermatome, that it provides the sensation to. The specific nerve in the neck that is compressed will determine what dermatome will be affected. Images from Netter Anatomy indicate that shoulder pain or altered sensation in and around the shoulder can be felt if the nerve from C5 to T1 is compressed or irritated.
Muscle Weakness
Just as the nerves control sensation, they also control muscle contractions. The nerve that controls and affects the shoulder muscles is the C5 nerve, according to Neck Solutions. If there is impingement or irritation on those nerves, muscle testing of the deltoids will be weaker than the other upper extremity muscles.
