Intercostal muscles are located between the ribs within the chest wall. They connect each rib to its neighbors, contributing to ribcage mechanics and changes in chest cavity size.
Understanding Their Position
Named for their location “between ribs,” intercostal muscles are situated in the 11 intercostal spaces, the gaps between the 12 ribs. Each muscle attaches to two ribs, one above and one below, spanning the space. This helps stabilize the chest wall. The intercostal spaces also contain nerves and blood vessels.
Different Muscle Layers
Within each intercostal space, these muscles are organized into three distinct layers.
The outermost layer consists of the external intercostal muscles. Their fibers generally run downwards, forwards, and medially. These muscles extend from the vertebral column at the back almost to the sternum at the front.
Beneath the external layer are the internal intercostal muscles, forming the middle layer. The fibers of the internal intercostals run in an oblique direction opposite to those of the external intercostals, downwards, backwards, and laterally. These muscles begin anteriorly at the sternum and extend backward as far as the angles of the ribs. They are separated from the innermost layer by a neurovascular bundle.
The deepest layer is composed of the innermost intercostal muscles. These muscles are often considered deep parts of the internal intercostals due to their similar fiber direction, which is also downwards, backwards, and laterally. However, they are distinct muscles, separated from the internal intercostals by the neurovascular bundle. While present in all intercostal spaces, they are typically more developed in the lower intercostal spaces.
Their Function in Respiration
The intercostal muscles play a significant role in the mechanical process of breathing, working to expand and shrink the chest cavity. During inhalation, particularly during quiet or forced inspiration, the external intercostal muscles contract. This contraction pulls the ribs upwards and outwards, which expands the overall volume of the ribcage. The increase in thoracic volume creates a pressure difference, drawing air into the lungs.
Conversely, the internal intercostal muscles contribute primarily to forced exhalation. When these muscles contract, they pull the ribs downwards and inwards. This action helps to decrease the volume of the thoracic cavity, which then pushes air out of the lungs. While quiet exhalation is a passive process, the internal intercostals become active during forceful actions such as coughing, sneezing, or exercising when more rapid expulsion of air is necessary.
The innermost intercostal muscles also assist in forced exhalation, similar to the internal intercostals. Their contraction helps to further reduce the volume of the thoracic cavity, expelling air from the lungs. All three groups of intercostal muscles collectively provide mechanical support to the thoracic cage, ensuring its integrity during respiratory movements. They work in coordination with the diaphragm to facilitate the continuous process of breathing.