The shoulder is actually a rather sophisticated joint. It has the largest range of motion of any joint in the human body. There are many muscles that help to keep this joint moving. Depending on the type of motion one is attempting, some or all of these muscles will come into play.
The Rotator Cuff Muscles
First, and deepest in the shoulder, are the muscles of the rotator cuff. These include the supraspinatus that runs along the top of the shoulder blade, the infraspinatus that runs just beneath the spine of the shoulder blade, the teres minor that is located beneath the infraspinatus and the subscapularis that is on the front of the shoulder blade. These muscles help to stabilize the shoulder-joint and rotate the arm or lift it to the side. These muscles all originate at the shoulder blade, the scapula, or at the upper arm, the bone called the humerus.
Other Shoulder Muscles
The deltoid muscle also acts to move the shoulder. It is the largest of the muscles that work on that joint. This muscle has a very large range of motion. It is generally broken down into its three regions. The anterior deltoid runs from the collar bone to the upper arm and is involved in motions that are like a push or a bench press type motion. The middle deltoid runs from the point of the shoulder blade to the upper arm region and is involved in raising the arm away from the body. The posterior deltoid runs from the back of the shoulder blade into the upper arm area and is used when the arms are pulled backward. The deltoid is one large muscle that has three regions with various responsibilities.
The Nerves that Supply the Muscles of the ShoulderCredit: http://www.thecountymedicalexaminers.com/shoulder-anatomy/
The main nerves that travel down into the arm actually run through the armpit which is under the shoulder. The three nerves that begin together at the shoulder are the radial nerve, the ulnar nerve, and the median nerve. Messages to and from the brain will travel through the shoulder and to its muscles as well as those of the whole arm along these neural pathways. These messages can tell a muscle to move a nerve or can alert the brain to an unusual sensation, a change in temperature or other sensation that is important to the body. Another nerve that travels around the back of the shoulder-joint supplies signals to the deltoid muscle so that the brain can activate the shoulder to move in the ways that the deltoid muscle is designed to move it. That nerve is called the axillary nerve. Collectively, these nerves are vital to one’s ability to be able to properly move the muscles of the shoulder and, therefore, the arm.
While there are other important structures in the area of the shoulder, such as tendons, ligaments and blood vessels, it is indeed, these muscles of the shoulder and the nerves that help the brain to control them that move the shoulder joint in the varied ways it is designed to work.