Video: Nerves of the thoracic wall
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People think it's all about the strength of those pectoral muscles, but actually, the power of your pecs stems from a pair of little tiny nerves of the thoracic wall – the medial and lateral pector... Read more
People think it's all about the strength of those pectoral muscles, but actually, the power of your pecs stems from a pair of little tiny nerves of the thoracic wall – the medial and lateral pectoral nerves. Every time he flexes his pecs, those little nerves are firing to initiate those bouncy movements. Anyway, let's give this guy a little privacy, while we take a closer look at the nerves of the thoracic wall.
Before we begin, let's take a quick look at what exactly we're going to learn about today. We'll begin this tutorial with a quick look at the structural components and functions of the thoracic wall. We will then move on to the main topic of this tutorial and explore the nerves of the thoracic wall. Here we will describe the two main groups of nerves which supplied the muscles and bony structures of this region – the thoracic spinal nerves and their branches and the thoracic branches of the brachial plexus. To finish off, we'll take a look at some clinical notes relevant to the innervation of the thoracic wall to help consolidate our knowledge. So without further ado, let's begin with the anatomical structure of the thoracic wall.
The thoracic wall consists of skeletal framework, fascia, muscles, and neurovasculature – all connected together to form a strong and protective yet flexible cage which functions to protect the contents 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, and the sternum.
The muscles of the thoracic wall include the intercostal and transversus thoracis muscles anteriorly, the subcostal on the inner surface of the posterior thoracic wall, and the serratus posterior and levatores costarum which we can see from a posterior view on the outer surface of the posterior thoracic wall. These muscles play an active role during breathing.
Other muscles which attach to the thoracic wall but act on the upper limb include the pectoralis major and minor, the subclavius, and the serratus anterior muscles. But we're not going to explore the bones or muscles of the thoracic wall today, we're going to focus on the nerves which we'll take a look at now.
Let's begin with taking a look at the first group of nerves – the thoracic spinal nerves and their branches. There are 12 pairs of thoracic spinal nerves. These nerves originate directly from the thoracic spinal cord and emerge between each thoracic vertebra. They are numbered from T1 to T12 according to each thoracic vertebral level. The thoracic spinal nerves are mixed nerves, meaning that they carry both motor and sensory innervation.
If we look at this image of an isolated intercostal space, we can see that as each spinal nerve extends from the spinal cord, it splits into a posterior ramus and an anterior ramus. As we can see from this posterior view of the thoracic wall, the posterior rami of spinal nerves T1 to T12 travel in a posterior direction innervating structures of the back and posterior thoracic wall. The anterior rami of spinal nerves T1 to T11 continue anteriorly traveling within the intercostal spaces between adjacent ribs and are subsequently known as the intercostal nerves. These nerves can also be viewed from this anterior perspective. The anterior rami of spinal nerve T12 travels beneath rib 12 and is known as the subcostal nerve. We'll take a closer look at the posterior and anterior rami of the spinal nerves next.
As we have just mentioned, the posterior rami of the thoracic spinal nerves are one of the two terminal branches of these nerves. From this superior view of an isolated vertebra and surrounding structures, we can see that each posterior ramus branches from the spinal nerve and travels in a posterior direction. It then splits into a medial and lateral branch which supply motor innervation to the transversospinal, deep segmental, and erector spinae muscles of the back as well as cutaneous innervation to the skin of the back and posterior thoracic region.
Now let's turn our attention to the anterior rami of the thoracic spinal nerves. At the thoracic level, the anterior rami of the spinal nerves are also known as the intercostal nerves. Each intercostal nerve travels within an intercostal space between adjacent ribs along with the corresponding posterior intercostal artery and vein forming the intercostal neurovascular bundle which occupies the costal groove of the rib above. The vein is the most superior structure, the artery occupies an intermediate position, and the nerve is the most inferior structure of this bundle.
The intercostal nerves travel anteriorly to reach the sternum on the anterior aspect of the thoracic wall. Each intercostal nerve gives off a lateral cutaneous branch, an anterior cutaneous branch, a small collateral branch that runs along the superior border of the rib below, and numerous slender muscular branches that leave the nerve along its course. Through these branches, the intercostal nerves supply the skin on the anterior and lateral aspects of the thoracic wall, pleura, the intercostal and transversus thoracis muscles anteriorly as well as the subcostal, levatores costarum and serratus posterior muscles posteriorly.
The anterior ramus of the 12th thoracic spinal nerve runs below the 12th rib and is known as the subcostal nerve. While the subcostal nerve is initially located within the thoracic region, it travels along the inferior border of the twelfth rib descending to enter and supply structures of the abdominal region.
Now that we have met the nerves of the thoracic wall derived from the thoracic spinal nerves, it is time to focus on the branches of the brachial plexus that supply the thoracic wall.
The brachial plexus is a network of nerves formed by the anterior rami of C5 to T1. This plexus gives off a number of nerves which supply motor and sensory innervation to the upper limb and also to some structures of the thorax. The end branches of the brachial plexus will not be mentioned in this tutorial as they mainly supply regions of the upper limb. The branches of the brachial plexus which supply the thoracic wall include the long thoracic nerve, the lateral and medial pectoral nerves, the subclavian nerve, and the accessory phrenic nerve. Let's explore each of these branches in a bit more detail.
The long thoracic nerve originates directly from the C5 to C7 nerve roots of the brachial plexus. It courses superficially in an inferolateral direction along the lateral margin of the ribs, roughly, at the midaxillary line. It travels along the superficial surface of the serratus anterior muscle providing motor innervation to this structure. Due to its long and relatively superficial course, the long thoracic nerve is frequently subject to damage during surgical procedures or injury through direct trauma. Stay tuned to find out what might happen if the long thoracic nerve is injured.
The next branches of the brachial plexus that innervate structures of the chest wall are the medial and lateral pectoral nerves. The medial pectoral nerve arises from the medial cord of the brachial plexus. It carries nerve fibers which originate from the C8 to T1 nerve roots. The medial pectoral nerve courses anteriorly between the axillary artery and vein. It then enters the deep surface of the pectoralis minor muscle which it supplies. Some branches of the medial pectoral nerve pierce and pass through the pectoralis minor muscle to reach and supply the overlying pectoralis major muscle. Other branches of the nerve may pass around the borders of the pectoralis minor muscle to end in the pectoralis major muscle.
The lateral pectoral nerve arises from the lateral cord of the brachial plexus and carries fibers from the C5 to C7 nerve roots. It extends just anterior to the axillary artery and vein and pierces the clavipectoral fascia to reach the pectoralis major muscle. Just distal to its origin, the lateral pectoral nerve gives off a communicating branch to the medial pectoral nerve known as the ansa pectoralis. The lateral pectoral nerve supplies the pectoralis major muscle and through its communicating branch also provides motor innervation to the pectoralis minor muscle.
Next, we come to the subclavian nerve, which is also known as the nerve to the subclavius. The subclavian nerve originates from the superior trunk of the brachial plexus and carries fibers from C5 to C6 nerve roots. It courses anterior to the subclavian artery and vein to reach and supply the subclavius muscle.
The final nerve of the thoracic wall is the accessory phrenic nerve. This highly variable nerve is only present in approximately 36 percent of the population. When present, it most commonly arises from the subclavian nerve; however, it has also been identified to arise from roots C5 and C6. The accessory phrenic nerve carries fibers mostly from the C5 nerve root. It courses along the thoracic wall and into the thoracic cavity where it joins with the phrenic nerve. The accessory phrenic nerve contributes to the innervation of the diaphragm and has been variably reported to also contribute to the innervation of the subclavius muscle.
Now that we have met the nerves of the thoracic wall, it's time to get clinical.
The long thoracic nerve is susceptible to damage due to its long and relatively superficial course. Damage to this nerve can occur during certain surgical procedures in the thoracic region such as radical mastectomy and chest tube placements or through direct trauma or stretch. There are also reports of non-traumatic causes of long thoracic nerve damage such as viral infections like poliomyelitis and allergic drug reactions.
As we know, the long thoracic nerve provides motor innervation to the serratus anterior muscle. The serratus anterior muscle originates from ribs 1 to 8 and extends posteriorly to insert onto the medial border of the scapula. As well as facilitating breathing, this muscle is largely responsible for protraction of the scapula and for keeping the medial border and inferior angle of the scapula opposed to the thoracic wall.
Injury to the long thoracic nerve results in interrupted motor innervation to the serratus anterior muscle, therefore, resulting in a phenomenon known as winging of the scapula. The serratus anterior is unable to stabilize the scapula and as a result, it protrudes from the posterior thoracic wall like wings. Upon physical examination, the scapular winging is usually evident at rest but it can also be accentuated when pushing the outstretched arm against a wall. Treatment involves a combination of conservative and surgical methods. Physical therapy is recommended to maintain mobility of the shoulder girdle while surgical intervention may be needed to repair the damaged nerve.
And that's it! Before you run off, let's take a quick look at what we learnt today.
In today's tutorial, we began with taking a look at the components of the thoracic wall. Here we explored the skeletal framework of the chest wall which is comprised of the 12 thoracic vertebrae, 12 pairs of ribs, and associated costal cartilages, and the sternum. Next, we explored the muscles of the thoracic wall which include the intercostal, transversus thoracis, subcostal, serratus posterior, and levatores costarum. Other muscles which attach to the thoracic wall but act on the upper limb include the pectoralis major and minor muscles, the subclavius, and the serratus anterior muscles.
Onto the bulk of this tutorial, we met the nerves of the thoracic wall. Here we explored two groups of nerves – the thoracic spinal nerves and their branches and the thoracic branches of the brachial plexus. We identified 12 pairs of thoracic spinal nerves which can each be divided into anterior and posterior rami. The posterior rami of spinal nerves T1 to T12 travel posteriorly to supply structures of the back. The anterior rami of spinal nerves T1 to T11 are also known as the intercostal nerves as they extend anteriorly within the intercostal spaces between adjacent ribs. Next, we met the anterior rami of spinal nerve T12, also known as the subcostal nerve.
Moving on, we explored the branches of the brachial plexus which supply structures of the thoracic wall. We began with the long thoracic nerve which supplies the serratus anterior muscle. Next, we met the medial and lateral pectoral nerves which supply the pectoralis major and minor muscles. Moving on to the next thoracic branch of the brachial plexus, we met the subclavian nerve. This small nerve provides innervation to the subclavius muscle. The final nerve of the thoracic wall is the highly variable accessory phrenic nerve which also contributes to the innervation of the subclavius muscle.
To finish off today's tutorial, we had a look at the clinical pathology known as winged scapula which is caused by an injured or damaged long thoracic nerve resulting in a weak serratus anterior muscle.
And that's a wrap! I hope you enjoyed learning about the nerves of the thoracic wall. Happy studying and see you next time!