The diaphragm and scalene muscles play crucial roles in increasing the vertical diameter of the thoracic cavity during inspiration, contributing to the process of breathing. Let’s discuss their roles individually:
1. Diaphragm:
- Location:
- The diaphragm is a large, dome-shaped muscle that separates the thoracic and abdominal cavities.
- Action during Inspiration:
- During inspiration (inhalation), the diaphragm contracts and moves downward toward the abdominal cavity.
- This contraction results in an increase in the vertical diameter of the thoracic cavity.
- Effect on Thoracic Volume:
- The descent of the diaphragm creates a larger space in the thoracic cavity, reducing intrathoracic pressure.
- The decrease in pressure allows air to flow into the lungs, expanding them and facilitating inhalation.
- Mechanism:
- Contraction of the diaphragm causes the central tendon to move downward, and the muscular rim (peripheral muscle fibers) pulls the lower ribs outward.
2. Scalene Muscles:
- Location:
- The scalene muscles are a group of three paired muscles (anterior, middle, and posterior) located in the lateral aspect of the neck.
- Action during Inspiration:
- The scalene muscles assist in elevating the first and second ribs during forced inspiration.
- They also contribute to increasing the vertical diameter of the thoracic cavity.
- Effect on Thoracic Volume:
- Contraction of the scalene muscles lifts the first and second ribs, enhancing the thoracic cavity’s vertical dimension.
- Mechanism:
- The anterior and middle scalene muscles elevate the first rib, while the posterior scalene assists in elevating the second rib.
- These actions further contribute to expanding the thoracic cavity and aiding in inhalation.
Integration of Diaphragm and Scalene Muscles:
- During quiet breathing, the diaphragm is the primary muscle of inspiration. However, during forced or deep inhalation, the scalene muscles actively contribute to elevating the upper ribs, augmenting the overall expansion of the thoracic cavity.
Clinical Implications:
- In conditions where the diaphragm is compromised, such as in paralysis or neuromuscular diseases affecting the phrenic nerves, accessory muscles like the scalene muscles may play a more significant role in supporting inspiration.
Understanding the coordinated action of the diaphragm and scalene muscles is essential for clinicians, especially in the evaluation of respiratory function and the management of conditions affecting the thoracic cavity. Both muscles contribute synergistically to create the necessary changes in thoracic volume required for effective breathing.