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The muscles of respiration are also called the ‘breathing pump muscles’, they form a complex arrangement in the form of semi-rigid bellows around the lungs.
- Muscles that are helpful in expanding the thoracic cavity are called the inspiratory muscles because they help in inhalation.
- Those that compress the thoracic cavity are called expiratory muscles and they induce exhalation.
The speciality of these muscles are that they are composed of fatigue resistant muscle fibers, they are controlled by both voluntary and involuntary mechanisms (if we want to take a breath we can, even if we do not think about breathing the body automatically does it)
Image: Overview of the respiratory system
The primary inspiratory muscles are the diaphragm and external intercostals. Relaxed normal expiration is a passive process, happens because of the elastic recoil of the lungs and surface tension. However, there are a few muscles that help in forceful expiration and include the internal intercostals, intercostalis intimi, subcostals and the abdominal muscles.
The accessory inspiratory muscles are the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the inferior fibres of serratus anterior and latissimus dorsi, the serratus posterior superior may help in inspiration also the iliocostalis cervicis. Technically any muscle attached to the upper limb and the thoracic cage can act as an accessory muscle of inspiration through reverse muscle action (muscle work from distal to proximal)
And in the thoracolumbar region the lowest fibres of iliocostalis and longissimus, the serratus posterior inferior and quadratus lumborum. The accessory muscles are recruited during times of exercising because of the increased metabolic need and also during dysfunction in the respiratory system
It’s a double-domed musculotendinous sheet of internal skeletal muscle located at the inferior-most aspect of the rib cage that separates the thoracic cavity from the abdominal cavity.
It serves two main functions:
-Separates the thoracic cavity from the abdominal cavity
-Undergoes contraction and relaxation, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration.
- Origin: Xiphoid process (posterior surface), lower six ribs and their costal cartilage (inner surface) and upper three lumbar vertebrae as right crus and upper two lumbar vertebrae as left crus.
- Insertion: central tendon
- Nerve Supply: Motor nerve supply by Phrenic nerve (C3 C4 C5) and sensory supply by phrenic nerve to central tendon and lower 6 or 7 intercostal nerve to peripheral parts.
- Blood supply: inferior phrenic arteries deliver the majority of blood supply and the remaining supply is delivered via superior phrenic, musculophrenic and pericardiacophrenic arteries.
- Action: diaphragm is the main inspiratory muscle, during inspiration it contracts and moves in an inferior direction that increases the vertical diameter of the thoracic cavity and produces lung expansion, in turn, the air is drawn in.
They are three types: External intercostal muscles (the most superficial muscle of intercostal muscles), internal intercostal muscles, and innermost intercostal muscles.
External intercostal muscles[edit | edit source]
- Origin: inferior border of rib above and
- Insertion: superior border of rib below
Internal intercostal muscles[edit | edit source]
- Origin: from the costal groove (lower part of inner surface of rib near the inferior border) of the rib above and
- Insertion: upper border of rib below
Innermost intercostal muscles:[edit | edit source]
It is an incomplete muscle layer and crosses more than one intercostal space. These muscles assist in the function of external and internal intercostal muscles.
- Origin: from the costal groove of the rib above and
- Insertion: the superior border of rib below
- Nerve supply: all the intercostal muscles are supplied by their respective intercostal nerves.
- Blood supply: all three muscles receive blood supply from anterior and posterior intercostal arteries, in addition to internal thoracic and musculophrenic arteries; costocervical trunk for internal and innermost intercostal muscles.