Movement in the human body relies on a complex network of nerves that relay signals from the brain to the muscles, a process known as innervation. In the upper torso and shoulder region, several nerves coordinate the powerful movements of the arm and chest. This article focuses on the Medial Pectoral Nerve, which controls muscles essential for pushing and stabilizing motions.
Defining the Medial Pectoral Nerve
The Medial Pectoral Nerve is a purely motor nerve originating deep within the shoulder area. It arises as a branch from the medial cord of the brachial plexus, a complex web of nerves formed by the C8 and T1 spinal nerve roots.
The nerve’s path takes it behind the first part of the axillary artery before curving forward between the axillary artery and vein. It often receives a communicating branch from the Lateral Pectoral Nerve, forming an anatomical loop called the ansa pectoralis. This nerve travels inferomedially, eventually reaching the muscles it supplies.
The Muscles Under its Command
The Medial Pectoral Nerve provides motor innervation to two primary muscles in the anterior chest wall: the Pectoralis Minor and the Pectoralis Major. The Pectoralis Minor, situated beneath its larger counterpart, receives its entire nerve supply from this source. The nerve typically enters the Pectoralis Minor on its deep surface, providing branches throughout the muscle.
The nerve continues its path past the Pectoralis Minor, either piercing through the muscle or traveling around its inferior border. It then provides innervation to the Pectoralis Major muscle. Specifically, the Medial Pectoral Nerve supplies the lower portion, known as the sternocostal head. This arrangement means the Pectoralis Major has a dual nerve supply, with the medial nerve controlling the sternal fibers and the lateral pectoral nerve controlling the clavicular fibers.
Primary Actions of the Pectoral Muscles
The functional output of the Medial Pectoral Nerve’s control is centered on forceful pushing movements and shoulder stabilization. By innervating the Pectoralis Minor, the nerve enables the muscle to draw the scapula, or shoulder blade, forward and downward against the ribcage. This action stabilizes the scapula during movements of the arm.
The supply to the sternocostal head of the Pectoralis Major facilitates several powerful movements at the shoulder joint. These actions include adduction (drawing the arm toward the midline of the body) and internal rotation (turning the arm inward). This sternocostal portion also assists in the extension of the humerus when the arm is raised above the shoulder. Together, these movements are important for activities that involve pushing, such as a push-up or a chest press.
Causes and Effects of Nerve Damage
Injury to the Medial Pectoral Nerve can occur due to direct trauma or, more commonly, as a complication during certain surgical procedures. The nerve’s close proximity to the axillary artery and vein makes it vulnerable during lymph node dissection in the armpit or during mastectomy procedures. Damage can also result from severe muscle tears or chronic compression, such as from intense, repetitive athletic training.
The primary effect of damage is a loss of function and strength in the muscles it supplies. Impairment can lead to weakness in arm adduction and internal rotation, making forceful pushing motions difficult. Over time, denervation can cause atrophy (wasting) of the sternocostal head of the Pectoralis Major muscle, often resulting in asymmetry of the chest wall. Injury to the nerve also disrupts the Pectoralis Minor’s ability to stabilize the scapula, contributing to altered shoulder mechanics.