Basic and Clinical Sciences (BCSE) Practice Exam

Which of the following is an accessory muscle of respiration involved in inspiration?

1. Serratus posterior inferior

2. Pectoralis major

3. Internal intercostals

4. Abdominal muscles

The correct answer is: Pectoralis major

The pectoralis major is indeed considered an accessory muscle of respiration, particularly during forced inspiration. This muscle is primarily known for its role in movements of the shoulder and arm, but when the upper limbs are fixed, such as during certain physical activities, it can assist in elevating the ribcage, thereby increasing thoracic volume and enabling deeper inhalation. During normal resting respiration, the primary muscles involved are the diaphragm and the intercostal muscles. However, during increased physical exertion or respiratory distress, accessory muscles come into play to augment the respiratory effort. The pectoralis major achieves this by drawing the ribcage upwards and outwards when the arms are fixed, enhancing lung capacity and improving airflow. The other choices listed do not serve this specific function of assisting with inspiration in the same manner. The serratus posterior inferior is more involved in expiration, while the internal intercostals primarily facilitate forced expiration rather than aiding in inspiration. The abdominal muscles play a crucial role in expiration, particularly when forcefully expelling air, rather than serving as an accessory muscle for inspiration.