The muscles situated between the ribs are collectively known as the intercostal muscles. These thin sheets of muscle and fascia occupy the eleven spaces between the twelve pairs of bony ribs that form the thoracic cage. They serve a dual purpose, providing structural support to the chest wall while simultaneously acting as primary agents of respiration. Their layered structure allows the rib cage to expand and contract with every breath.
Anatomical Breakdown: The Three Layers
The intercostal muscles are organized into three distinct layers, defined by their depth and the orientation of their muscle fibers. The most superficial layer is the external intercostals, whose fibers run obliquely, angled downward and forward. Contraction of these muscles pulls the ribs upward and outward, making them the main contributors to inhalation.
Deep to this layer are the internal intercostals, where the fiber direction runs downward and backward, almost perpendicular to the externals. This opposing orientation means that when they contract, they pull the ribs downward and inward, compressing the chest cavity. The deepest layer is the innermost intercostals, which are structurally similar to the internal layer and separated by the neurovascular bundle containing the intercostal nerve, artery, and vein. Both the internal and innermost layers primarily support the forceful expulsion of air from the lungs.
Primary Role in Breathing
The primary function of the intercostal muscles is to facilitate breathing by altering the volume of the thoracic cavity. During quiet inhalation, the external intercostals contract to elevate the rib cage, working with the diaphragm. This elevation involves two distinct movements of the ribs that increase the chest volume.
The upper ribs exhibit a “pump handle” movement, pushing the sternum forward and increasing the chest’s front-to-back diameter. The lower ribs primarily demonstrate a “bucket handle” movement, swinging outward to increase the side-to-side diameter of the thoracic cavity. This combined action lowers the pressure inside the chest, drawing air into the lungs.
Quiet exhalation is largely a passive process, relying on the elastic recoil of the lungs and the relaxation of the diaphragm and external intercostals. However, during strenuous activity, coughing, or sneezing, the internal and innermost intercostals contract forcefully. This active contraction pulls the ribs down and inward, rapidly decreasing the volume of the chest cavity to expel air.
Clinical Relevance and Common Issues
The intercostal muscles are susceptible to injury, with muscle strain being a common and painful ailment. This injury often results from sudden, forceful movements such as aggressive coughing, excessive twisting of the torso, or overexertion. Symptoms include sharp, localized pain between the ribs that worsens with deep breathing, coughing, or laughing.
Another source of pain in this region is intercostal neuralgia, which is neuropathic pain affecting the intercostal nerves. This condition manifests as a sharp, burning, or aching pain that often follows a band-like pattern around the chest or abdomen. Causes range from trauma, such as rib fractures, to infections like shingles. Intercostal nerve blocks, involving injecting a local anesthetic near the affected nerve, are sometimes used to manage the pain.