intercostals

The intercostal muscles are a group of skeletal muscles located between the ribs. They are arranged in three layers: external intercostal muscles, internal intercostal muscles, and innermost intercostal muscles. The external intercostals run obliquely downward and forward; the internal intercostals run obliquely downward and backward; innermost lie deep to internal and are often separated by the neurovascular bundle. The subcostal muscles and transversus thoracis are sometimes described as accessory intercostal muscles.

Function and innervation: The external intercostals elevate the ribs during inspiration, expanding the thoracic cavity. The internal intercostals primarily depress the ribs during forced expiration, aiding expiration; the innermost muscles and subcostals also assist with expiration and rib stabilization. Innervation is by the intercostal nerves (ventral rami of thoracic spinal nerves). Blood supply is via the posterior intercostal arteries and anterior intercostal arteries from the internal thoracic arteries, with corresponding veins.

Clinical relevance: Intercostal muscles can be strained or injured during heavy lifting or coughing, causing chest wall pain that can mimic cardiac or pulmonary conditions. They are important landmarks in thoracic procedures, including chest tube placement and regional anesthesia. The intercostal spaces, occupied by these muscles and a neurovascular bundle, are clinically significant in diagnostics and surgery.