Muscles of the thoracic wall

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Introduction0:00–1:04

The thoracic wall surrounds the thoracic cavity, which is the anatomical region where viscera like the heart and lungs can be found.
The thoracic wall contains a variety of muscles where many of the muscles that cover and attach to the thoracic wall have a primary action elsewhere such as the arms, neck and abdomen.
And it's their secondary actions that affect the movements of the thoracic wall. For example, the pectoralis major and pectoralis minor muscles overlay the thoracic wall.
But they are primarily involved with movements of the upper limb. And their secondary action is to act on the thoracic wall as accessory muscles of respiration.
However, the thoracic wall also does have muscles that have their primary action in the thoracic wall. And these are called the true muscles.
They are the serratus posterior levatorius, erum, subcostal transversus thoracis and the innermost internal and external intercostal.
Let's start with the two serratus posterior muscles. There's a serratus posterior superior muscle and serratus posterior inferior muscle.

Serratus posterior muscles1:04–2:18

On each side, the serratus posterior superior muscles have their proximal attachment at the nuchal ligament and the spinus processes of C seven through T three vertebrae and their distal attachment at the superior borders of the second through fifth ribs.
Its main action is the elevation of the superior four ribs. During inspiration, the serratus posterior inferior muscles have their proximal attachment at the spinus processes of T 11 through L2 vertebrae and their distal attachment at the inferior borders of ninth through 12th ribs near their angles.
Its main action is depressing the inferior ribs in order to prevent them from being pulled upwards by the diaphragm. Both of these muscles have been shown to have a role in proprioception, meaning that they might make you aware of the position of the body.
For example, when you're lying in bed with your eyes closed, these muscles among others could send signals to the brain regarding where you are and what's your position in relation to the bed.
Next, the levator costera muscles are 12 triangle shaped muscles with a proximal attachment to the transverse processes of C seven through T 11 vertebrae and distal attachments to subadjacent ribs between the tubercle and the angle of that rib.

Levatores costarum muscles2:18–2:38

Their main action is to elevate the ribs. Then there are the intercostal muscles which are a group of muscles that occupy the space between the ribs.

Intercostal muscles2:38–5:59

The external intercostal muscles form the superficial layer while the internal intercostal muscles form the inner layer.
Now, the innermost intercostal muscles are fibers of the internal intercostal muscles that are the deepest fiber layer and lie internal to the intercostal nerve and vessels.
All three intercostal muscles have the same origin and insertion. So their proximal attachment is at the inferior border of the ribs above and their distal attachment at the superior border of the rib below.
For example, the intercostals at the third intercostal space, all attached to the inferior border of the third rib and the superior border of the fourth rib and so on.
In addition, they are all innervated by the intercostal nerves traveling in their respective intercostal spaces. The main differences between the intercostals are their fiber orientation and how far they span anteriorly to posteriorly.
So let's look at the 11 pairs of external intercostal muscles that occupy the intercostal spaces from the tubercles of the rib posteriorly to the costochondral junctions anteriorly where the most anterior muscle fibers are actually replaced by fascia called the external intercostal membranes.
These muscles run inro anteriorly from the rib above to the rib below. To visualize this, the fibers run in the same way the fingers of your hand would if you placed your hands in the pockets of your jeans, the lower external intercostals are continuous anteriorly with the external oblique muscles in the abdominal wall.
The external intercostal muscles elevate the ribs during forced inspiration. Then there are the 11 pairs of internal intercostal muscles which run deep to and at right angles to the external intercostals.
This means that their muscle fibers run infra posteriorly from their origin compared to infra anteriorly like the external intercostals to visualize this, the fibers run in the same way your fingers would if you were to cross your arms to hug yourself and place your hands on your hips.
The internal intercostal attaches to the bodies of the ribs and their costal cartilages all the way to the sternum anteriorly and posteriorly.
As far as the angles of the ribs between the ribs and medial to the angles. The internal intercostal muscles are replaced by fascia called the internal intercostal membranes.
The inferior internal intercostal muscles are continuous with the internal oblique muscles in the anterior abdominal wall.
The internal intercostal muscles mostly play a role during expiration. Finally, the innermost intercostal muscles are basically the deeper parts of the internal intercostal muscles and are separated from them by intercostal nerves and vessels.
These muscles pass between the internal surfaces of adjacent ribs and occupy the most lateral parts of the intercostal spaces and just like the internal intercostal muscles.
They likely play a role during expiration. Now, take a moment and try and identify some of the structures we have just talked about on these images.

Quiz5:59–6:08

Respiration6:08–8:52

Now, let's have a quick look at the anatomy and physiology of breathing. The diaphragm is the main muscle in inspiration while expiration is usually passive, except when a person is trying to exhale against resistance, like when blowing air into a balloon or when one's trying to expel air more rapidly.
Like when coughing or sneezing, both external and internal intercostals are active during respiration. But their action is mostly isometric, meaning their contraction does not actually cause movement of the thoracic cage.
This isometric contraction is to support the intercostal space, especially during inspiration when internal thoracic pressures are lowest.
However, during forced respiration, these muscles become more active and will produce movement of the ribs. Let's take a look at this image.
Now take a really deep breath literally do it and see what happens when undergoing forced inspiration. The external intercostals and part of the internal intercostals called the interchondral portion contract and rotate the ribs superiorly at their posterior axis.
The result is the elevation of the ribs and an increase of the anteroposterior diameter. This occurs because the ribs are hinged posteriorly by a fixed vertebral column and anteriorly by a movable sternum during forced expiration.
The interosseous part of the internal intercostal muscles contract and rotate the ribs inferiorly at their posterior axis.
The result is the depression of the ribs and sternum and the decrease in the anteroposterior diameter. So this is to say that the interosseous and the interchondral parts of the internal intercostal muscles have two different actions during forced respiration.
Even though these muscle fibers have the same orientation. This all occurs because as the ribs move postero anteriorly, they descend as they run towards the sternum.
Their lowest point is approximately at the junction with the costal cartilage and then ascending toward the sternum, the interchondral fibers of the internal intercostals are medial to this point.
Therefore, their fibers are parallel to the slope of the ribs. And their contraction causes the hinged ribs to rotate superiorly based on their biomechanical pull.
The interosseous part of the internal intercostal is lateral to this low point, causing its fibers to be perpendicular to the slope of the rib.
And contraction of these fibers rotates the ribs inferiorly pulling them down. Other muscles in the thoracic wall include the subcostal muscles.

Subcostal muscles8:52–9:58

They are usually well developed only in the lower thoracic wall and they vary in size and shape proximately. They attach to the internal surface of lower ribs near their angles and distally, they attach at the superior border of the second or third ribs below.
This means that these muscles actually cross one or two intercostal spaces. The subcostal muscles run in the same direction as the internal intercostals and are thought to act in the same way as the internal intercostals.
Now, the transversus thoracis muscles have their proximal attachment at the posterior surface of the lower sternum and their distal attachment at the internal surface of the costal cartilages.
Two through six. They are continuous with the transversus abdominis muscles in the anterolateral abdominal wall, they may slightly depress the ribs during expiration and might provide proprioceptive information.
Both of these muscles are innervated by the intercostal nerves. Finally, let's move on to the endothoracic fascia which is a thin layer that lies between the internal aspect of the thoracic cage and the lining of the pulmonary cavities and attaches the deeper situated costal parietal pleura to the thoracic wall.

Endothoracic fascia9:58–10:23

The endothoracic fascia continues over the apices of the lungs where it becomes more fibrous and is called the suprapleural membrane.

Review10:23–11:18

All right. It's a quick recap.
The true muscles of the thoracic wall are serratus, posterior levatoris, costarum, subcostal transversus, thoracis and the intercostal.
The role of the intercostal muscles in respiration is to support the intercostal space, especially during inspiration. When the internal thoracic pressures are the lowest during forced inspiration, the external intercostals and the interchondral portion of the internal intercostal muscles contract and rotate the ribs superiorly at their posterior axis, elevating the ribs during forced expiration.
The interosseous part of the internal intercostal muscles contract and rotate the ribs inferiorly at their posterior axis, depressing the ribs.
The endothoracic fascia is a thin layer that lies between the internal aspect of the thoracic cage and the lining of the pulmonary cavities.