Video: Muscles of the thoracic wall

How am I ever going to remember all this information for tomorrow? Okay, okay… I think I just need to take a few deep breaths. In and out…in and out… Ahh…I already feel calmer.

We'll begin today's tutorial by taking a look at the thoracic wall and exploring its structure and functions. We will then focus on the muscles of the thoracic wall which can be divided into two groups – the intrinsic and extrinsic muscles of the thoracic wall. Within both groups, we will explore each muscle's attachment points, functions, and nerve supply. To finish, we'll take a look at some clinical correlations to help consolidate our knowledge. So without further ado, let's begin with exploring the function and anatomical structure of the thoracic wall.

Before we dive into the muscles of the thoracic wall, let's take some time to define the general structure and function of the thoracic wall. The thoracic wall consists of a skeletal framework, fascia, muscles, and neurovasculature – all connected together to form a strong and protective yet flexible cage which functions to protect the content of the thorax and facilitate ventilation or breathing.

The skeletal framework of the thoracic wall, otherwise known as the thoracic cage, is formed by the 12 thoracic vertebrae, 12 pairs of ribs, and associated costal cartilages, sternum, and interconnecting joints. The muscles of the thoracic wall are defined as muscles attached to the bony framework of the thoracic cage. As we have already identified, the muscles of the thoracic wall can be divided into two groups – the intrinsic muscles and the extrinsic muscles of the thoracic wall. The intrinsic muscles are also known as the true muscles as they primarily act on the thoracic wall. The extrinsic muscles attach to the thoracic wall and have a minor secondary role to play in its movements and are therefore identified as accessory muscles of the thoracic wall.

Now that we have a good understanding of the structure and function of the thoracic wall, it's time to zone in on the muscles, beginning with the intrinsic muscles of the thoracic wall.

The intrinsic muscles originate and insert onto the thoracic wall and contributes to its structure. This group of muscles generally work together to facilitate breathing movements through the elevation and depression of the ribcage, subsequently facilitating the expansion and deflation of the lungs. The true muscles of the thoracic wall can be identified along the posterior, lateral, and anterior surfaces of the thoracic wall and include the serratus posterior, levatores costarum, intercostal, subcostal, and transversus thoracis muscles. We'll explore each of these muscles now.

Let's begin by taking a look at some of the muscles of the posterior thoracic wall, beginning with the serratus posterior muscles. The serratus posterior muscles are made up of the serratus posterior superior muscle and the serratus posterior inferior muscle. Both of these muscles originate from the spinous processes of associated vertebrae and extends to insert onto the ribcage. Let's take a look at each of these muscles separately.

The serratus posterior superior muscle originates from the spinous processes of vertebrae C7 to T3 and extends laterally to insert onto the superior border of ribs 2 to 5. This muscle receives innervation from the second to fifth intercostal nerves. The serratus posterior inferior muscle arises from the spinous processes of vertebrae T11 to L2 and extends laterally to insert onto the inferior border of ribs 9 to 12 near their angles. The anterior rami of thoracic spinal nerves T9 to T12 innervate the serratus posterior inferior muscle.

According to their attachment points, the serratus posterior superior muscle has been described to elevate ribs 2 to 5, thus increasing the anteroposterior diameter of the thorax, while the serratus posterior inferior muscle was said to depress ribs 9 to 12. These movements would facilitate the mechanical action of breathing. However, there has been no evidence to support a respiratory role for these muscles, and in fact, a recent study suggests that these muscles might not be primarily motor in nature but rather function primarily in proprioception. All in all, the precise function of these muscles remains unconfirmed.

Still on the posterior thoracic wall and just deep to the serratus posterior muscles, we meet a series of muscles which run along either side of the vertebral column known as the levatores costarum muscles. The levatores costarum are strong, small triangular muscles – 12 on each side – that arise from the tips of the transverse processes of C7 to T11. They extend in an inferolateral direction to insert onto the external aspect of the superior border of the rib immediately below the vertebra from which it takes its origin, between the costal angle and tubercle of the rib.

Similar to the serratus posterior muscles, the action of the levatores costarum muscles is not fully understood. This group of muscles are thought to function primarily in rib elevation but their role in normal inspiration is disputed. They may also function in rotation and lateral flexion of the vertebral column. Innervating these muscles are the lateral branches of the dorsal rami of the C8 to T11 spinal nerves.

Located within the costal spaces are the 11 pairs of intercostal muscles. The intercostal muscles are formed from three muscular layers. Their names are derived from the spatial relationship; so from superficial to deep, they are the external, internal, and innermost intercostal muscles. All three groups of muscles support the ribcage. Moreover, they are all accessory respiratory muscles that participate in the process of forced breathing.

Let's take a look at the 11 external intercostal muscles first.

As its name suggests, the external intercostal muscles are the most superficial of the intercostal muscles. As we can see from this posterolateral view of a section of the thoracic wall, they occupy the space between the tubercles of the ribs posteriorly to the costochondral junctions anteriorly. The muscle fibers are replaced anteriorly by the external intercostal membranes. Each external intercostal muscle arises from the inferior border of the rib above, courses inferomedially, and inserts onto the superior border of the rib below. The external intercostal muscles work with the internal intercostal muscles to elevate the ribs during forced inspiration and are innervated by adjacent intercostal nerves which travel deep to the inner surface of the internal intercostal muscles within the intercostal space.

Next, we meet the internal intercostal muscles. These flat muscles run deep to and at right angles to the external intercostal muscles forming the middle layer of the intercostal musculature. Similar to the external intercostal muscles, the internal intercostal muscles run from the rib above to the rib below but in the opposite direction. Specifically, the internal intercostal muscles originate from the costal groove of the rib above and course posteroinferiorly and medially to insert onto the superior border of the immediate rib below.

An easy way to remember the direction of the muscle fibers of the external and internal intercostal muscles is to think of putting your hands into your pocket. The inferomedial position of your hand sliding into your pocket matches the inferomedial direction of the fibers of the external intercostal muscle. The internal intercostal muscles are similarly innervated by the intercostal nerves and function to depress the ribs during forced expiration. The interchondral part of the internal intercostal muscles may also contribute to the elevation of the ribs and therefore works together with the external intercostal muscles.

The final group of intercostal muscles are the innermost intercostal muscles. These muscles are the most internally located intercostal muscles and are very similar to their internal counterparts in that they follow the same muscle fiber direction. The innermost intercostals are separated from the internal intercostals by the intercostal neurovascular bundle. Similar to its external and internal counterparts, the innermost intercostal muscles extend from the internal surfaces of the rib above to the rib below and are thought to function in the same way as the internal intercostal muscles. They receive their nerve supply from the intercostal nerves which lie on the lateral surface of this muscle group.

Located on the inner surface of the posterior thoracic wall are the subcostal muscles. The subcostal muscles consist of muscle and aponeurotic fasciculi and lie on the deep surface of the innermost intercostal muscle near the angles of the ribs, usually running over two to three intercostal spaces. Subcostal muscles originate from the inner surface of one rib near its posterior angle. Their fibers descend inferomedially in the same fashion as those of the innermost and internal intercostals.

Each subcostal muscle inserts onto the inner surface of the second or third rib below its origin. The subcostal muscles are usually more developed in the lower portion of the thoracic cage and sometimes fused with the innermost intercostal muscles in their origin. On contraction, this group of muscles depress the ribs and therefore contribute and facilitate forced expiration. The subcostal muscles receive their nerve supply from adjacent intercostal nerves.

The next intrinsic muscle group of the thoracic wall are the transversus thoracic muscles. Transversus thoracis spreads over the deep surface of the anterior thoracic wall and arises from the inferior third of the posterior surface of the body of the sternum, posterior surface of the xiphoid process, and the sternal ends of the costal cartilages of ribs 4 to 7. The muscle fibers diverge and descend laterally as slips, which insert into the inner surfaces of the costal cartilages of ribs 2 to 6, respectively. The transversus thoracis is supplied by adjacent intercostal nerves and is a weak depressor of the costal cartilages thereby facilitating forced expiration.

Now let's take a look at the extrinsic muscles of the thoracic wall.

The extrinsic muscles of the thoracic wall attach to the thoracic cage but are functionally related with the neck, abdomen, back, and all upper limbs. These muscles also act at the thoracic wall and may facilitate its movements during breathing. Extrinsic muscles of the thoracic wall include the subclavius, the pectoralis major and minor muscles, and the inferior aspect of the serratus anterior muscle. We'll take a quick look at each of these muscles now.

The subclavius muscle is a small triangular muscle located anterior to the first rib and inferior to the clavicle. It arises from the superior surface of the costal end of the first rib and its adjacent first costal cartilage. The fibers of the subclavius muscle extend superolaterally and insert via a broad tendon into the lower surface of the middle third of the body of the clavicle. The primary function of the subclavius muscle is to stabilize the clavicle during movements of the shoulder and arm. On contraction, it also depresses the sternal end of the clavicle and subsequently elevates the first rib to facilitate forced inspiration. The subclavius is innervated by the subclavian nerve, a small branch that arises from the superior trunk of the brachial plexus.

The pectoralis major muscle is a large fan-shaped muscle on the superficial surface of the pectoral region of the thoracic wall. It consists of three parts which originate from different sites – a clavicular part which originates from the anterior surface of the medial half of the clavicle, a sternocostal part which arises from the anterior surface of the sternum and the anterior aspects of the costal cartilages of ribs 1 to 6, and finally, an abdominal part which originates from the anterior layer of the rectus sheath. The muscle fibers from all three parts converge to form a tendon which inserts onto the lateral lip of the intertubercular sulcus of the humerus.

The pectoralis minor muscle is a thin triangular shaped muscle which sits just deep to the pectoralis major muscle. This muscle arises from the upper margins and outer surface of ribs 3 to 5 and from the fascia overlying the external intercostal muscles of associated ribs. It tapers as it extends laterally and inserts onto the coracoid process of the scapula. Similar to its major counterpart, the pectoralis minor muscle plays a secondary role as an accessory muscle of inspiration. It primarily acts on stabilizing the scapula. Due to its attachment on the chest wall, it contributes to deep active inspiration. Innervating this muscle are the medial and lateral pectoral nerves.

The serratus anterior muscle is a fan-shaped muscular sheet which hugs the thorax. Its muscular slips arise anteriorly from the outer surfaces and superior borders of the upper 8 to 10 ribs. It curves around the thorax and inserts onto the anterior surface of the medial border of the scapula. The serratus anterior muscle is divided into three parts – a superior, middle, and inferior portion. The superior portion extends from the first and second ribs to the superior angle of the scapula, the middle portion from ribs 2 to 3 to the medial border of the scapula, and the inferior portion from the fourth to eighth or ninth rib to the medial border and inferior angle of the scapula.

The inferior part of the muscle is the most prominent and powerful. It is this part that plays a minor role as an accessory muscle of inspiration as it elevates the ribs on contraction to facilitate forced inspiration. The serratus anterior muscle is innervated by the long thoracic nerve.

Now that we have a good understanding of the intrinsic and extrinsic muscles of the thoracic wall, it's time to take a look at a clinical note on the subject.

Injury to the intercostal muscles of the thoracic wall does not typically occur during daily activities but rather due to overexertion or direct trauma in response to physical activity, exercise, or contact sports. Symptoms of intercostal muscle strains vary depending on severity of injury; however, generalized symptoms include sudden severe pain to the ribcage and back, difficulty breathing, tenderness to the affected area, muscle tension and stiffness, and localized inflammation. Intercostal muscle strains are typically diagnosed through patient history and clinical examination.

An x-ray may be requested to rule out the possibility of a rib fracture. The first line of treatment is rest. If inflammation persists, nonsteroidal antiinflammatory medication is advised. Healing is dependent on the level of severity of the injury and may range from two to three days all the way up to eight weeks. If pain persists, physiotherapy is recommended in order to strengthen and rehabilitate the affected area.

And that's a wrap on the muscles of the thoracic wall. Before you go, let's just take a quick look at what we learned today.

We began today's tutorial with exploring the intrinsic or true muscles of the thoracic wall which include the serratus posterior superior and inferior muscles; the levatores costarum muscles; the external, internal, and innermost intercostal muscles; the subcostal muscles; and finally, the transversus thoracic muscles. These muscles originate and insert onto the thoracic wall and are directly involved in facilitating breathing movements. Extrinsic muscles of the thoracic wall attach to the thoracic cage but are functionally related to the neck, abdomen, back, and all upper limbs. This group of muscles have secondary functions at the thoracic wall and are typically involved in forced inspiration. Extrinsic muscles of the thoracic wall include the subclavius, pectoralis major and minor muscles, and the inferior aspect of the serratus anterior muscle.

That brings us to the end of this tutorial. I hope you enjoyed learning about the muscles of the thoracic wall, and happy studying.