Respiratory Muscles: Intercoastal Muscle and Diaphragm

The muscles involved in breathing in and breathing out are known as the muscles of respiration. Inhalation is induced by muscles that expand the thoracic cavity, known as inspiratory muscles, whereas exhalation is triggered by muscles that constrict the thoracic cavity, known as expiratory muscles. The fundamental structure of these muscles is the same as that of all other skeletal muscles. Respiratory muscles are categorized as:

According to mechanism of respiration:

• Muscles of inspiration: Diaphragm, external intercostals muscles are chief muscles.
• Muscles of expiration: Internal intercostals muscles.

According to involvement:

• Primary or major respiratory muscles: Diaphragm and intercostals muscles.
• Accessory muscles: Sternocleidomastoid, scalene group of muscles, serratus anterior, pectoralis and abdominal muscles.

The respiratory muscle consists mainly of diaphragm, intercostals muscles.

The Diaphragm:

A skeletal muscle with a dome shape called the diaphragm creates the lower limit of the thoracic cavity. It divides the abdominal cavity from the thoracic cavity. It extends outward from the sternum's xiphoid process, the thoracic wall's costal margin, the ends of ribs XI and XII, ligaments that cross the structures of the posterior abdominal wall, and the lumbar vertebrae. Muscle fibers converge from these periphery attachments to join the core tendon. The middle section of the central tendon is where the pericardium is connected. It is the most significant inhaling muscle. The diaphragm flattens when it contracts, lowering its dome and widening the thoracic cavity. When it relaxes, the dome resumes its upward position and the size of the thoracic cavity is decreased. It is supplied by phrenic nerve.

The Intercostals Muscles:

The ribs are connected to the intercostals muscles. The external and internal intercostals muscles are arranged in two layers. The intercostals muscles come in 11 pairs. During inspiration, these muscles are responsible for around one-fourth of the increase in intrathoracic volume.

• External intercostal muscles:
  • These muscles run from the upper border of the below rib to the lower border of the above rib in a downward and forward orientation. As a result, there is more room for the lungs to expand during inspiration.
• Internal intercostal muscles:
  • These cross the external intercostal muscles at a straight angle as they extend downward and backward from the lower border of the above ribs to the upper border of the below rib. As a result, it aids in expiration.