Lecture 3 - Anterior Abdominal Wall - ASYNCH

**Lecture 3 - Anterior Abdominal Wall - ASYNCH**\ \ In this mini lecture, I would like to walk you through the muscles, the vessels, and the nerves of the thoracic wall, as well as the anterior abdominal wall. While we are in the thoracic region, we'll also cover the breast in this region. So as a...

**Lecture 3 - Anterior Abdominal Wall - ASYNCH**\ \ In this mini lecture, I would like to walk you through the muscles, the vessels, and the nerves of the thoracic wall, as well as the anterior abdominal wall. While we are in the thoracic region, we'll also cover the breast in this region. So as a summary, we have 12 pairs of ribs along with the sternum and the thoracic vertebrae; they form the thoracic cage. In this thoracic cage, there are spaces between the ribs referred to as intercostal spaces, and at the bottom part of the chest (the inferior thoracic aperture), we have the costal cartilage of ribs 7, 8, 9, and 10. The costal cartilages of ribs 8, 9, and 10 articulate with the sternum via the costal cartilage of rib 7, forming a line or costal border. MUSCLES OF THE CHEST WALL: These are the muscles sitting in the intercostal spaces, known as the intercostal muscles.\ The muscle that seals off the inferior thoracic aperture (costal border) is the diaphragm. The intercostal spaces are 11 in number and are located between the ribs.\ They contain not only the intercostal muscles but also the intercostal nerves and intercostal vessels. When it comes to the muscles located between the ribs (in the intercostal spaces), we have external and internal intercostal muscles as the important ones, plus innermost intercostal muscles (clinically less important), which share the same function as the internal intercostals. Regarding these muscles, you already know we need to discuss origin, insertion, function, and innervation. - The external intercostal muscles attach to the inferior border of the rib above and insert on the superior border of the rib below. - The internal intercostal muscles attach to the superior border of the rib below and insert on the inferior border of the rib above. A good point to know is that the external intercostal muscle fibers run from lateral to medial and from superior to inferior (like putting hands in side pants pockets). In terms of function, when a muscle contracts, the insertion moves toward the origin. So for external intercostal muscles, they elevate the ribs, while internal intercostal muscles depress the ribs. Innervation is by the intercostal nerves. External intercostal muscles are active during inhalation (elevating the ribs), and although internal intercostals depress the ribs, exhalation under normal conditions does not necessarily require active muscle contraction. Another muscle to discuss is the diaphragm, a muscular sheath that closes off the inferior sternal aperture (also called the costal border).\ The diaphragm is dome-shaped with very broad attachments: around the inside of the costal margin, anteriorly at the xiphoid process, along the inner aspect of ribs 11 and 12, and posteriorly at the lumbar vertebrae (L1, L2, and L3).\ The muscle fibers merge toward a central tendon in the middle of the diaphragm, a whitish central part. During contraction, the domes of the diaphragm move downward, which increases the vertical dimension of the thoracic cavity; during relaxation, they move upward. Inside the diaphragm, there are three main openings (hiatuses): 1. The esophageal hiatus (for the esophagus) 2. The aortic opening (for the aorta) 3. An opening in the central tendon for the inferior vena cava (IVC) The inferior vena cava is a major vein in the abdominal cavity that carries venous blood from the abdomen, pelvis, and lower limbs to the right side of the heart. Its opening is at the level of the T8 vertebra. The esophageal hiatus is at T10, and the aortic opening is at T12. The diaphragm is innervated by the phrenic nerves, which come from the C3, C4, and C5 spinal nerves (mnemonic: "C3, C4, and C5 keep the diaphragm alive"). During inspiration, the domes of the diaphragm contract and move downward, increasing the volume of the thoracic cavity (and the pleural cavity), which decreases pressure and allows air to flow into the lungs. About 75% of incoming air is due to contraction of the diaphragm, and about 25% due to contraction of the external intercostals. For a simple demonstration, imagine a water bottle as the chest cavity and a balloon inside as the lung, while the bottom piece represents the diaphragm. Pull the diaphragm downward, and air flows into the balloon (the lung). Relax and push it upward, and air flows out. Regarding movements of the thoracic wall, when external intercostals contract, the ribs elevate and move outward like a "bucket handle" movement (increasing the transverse diameter) and a "pump handle" movement (raising the sternum and increasing the anterior-posterior diameter). Meanwhile, the diaphragm moving downward increases the superior-inferior diameter. All these dimensional increases cause a drop in pressure, and air flows in. BREAST: The breast is located in the pectoral region, external to the anterior thoracic wall. The breast includes the mammary gland (the glandular part), plus associated structures like skin, fascia, the nipple, and the areola. Surface anatomy: - Medial border is near the parasternal line - Lateral border is near the mid-axillary line - Superiorly, its booundary is around rib 2 - Inferiorly around rib 6 There is a pigmented area around the nipple called the areola, which contains smooth muscle that can cause nipple erection. The mammary gland is actually a modified sweat gland in the superficial fascia (subcutaneous layer). It sits atop the deep fascia (the pectoral fascia). Extensions of the deep fascia (called suspensory ligaments of the breast) pass through the mammary gland and attach to the skin, helping support the breast. Clinically, if a tumor pulls on these suspensory ligaments, it can create a "dimpling" or "orange peel" (peau d'orange) appearance on the skin. Blood supply to the breast comes from branches of the intercostal arteries (via perforating branches), the axillary artery (particularly the lateral thoracic artery), and the internal thoracic artery ITA (also called the internal mammary artery), which sends perforating branches medially. Venous drainage generally parallels these arteries. Innervation is via the 2nd to 6th intercostal nerves. The nipple region is innervated mainly by the 4th intercostal nerve (a key landmark in the dermatomes). These nerves primarily carry sensory fibers and sympathetic fibers to blood vessels and smooth muscle in the areola and nipple. Lymphatic drainage is critical clinically. About 75% of breast lymph drains to axillary lymph nodes, especially the lateral side of the breast. The medial side drains to the parasternal nodes, which can connect to the opposite side, meaning metastases can cross to the other breast's lymph nodes. BLOOD SUPPLY OF THE CHEST WALL: The main artery in the chest cavity is the aorta. There is the aortic arch and then the descending thoracic aorta. From the thoracic aorta, there are "visceral" branches (to organs) and "parietal" branches (to the chest wall including Diaphragm). The important parietal branches here are the posterior intercostal arteries, which run along the ribs in the intercostal spaces. Meanwhile, the anterior intercostal arteries arise from the internal thoracic (internal mammary) artery, which is a branch of the subclavian artery. These anterior and posterior intercostal arteries meet within each intercostal space. Venous drainage mirrors the arteries: - Anterior intercostal veins eventually drain into the internal thoracic vein, then into the brachiocephalic vein -- one on the right, one on the left. - Posterior intercostal veins on the right side drain into the azygos vein, which empties into the superior vena cava. On the left side, they drain into the hemiazygos and accessory hemiazygos veins, which then connect to the azygos system on the right. The diaphragm also receives blood from "superior phrenic arteries" (parietal branches of the thoracic aorta). INNERVATION OF THE CHEST WALL: We have 12 thoracic spinal nerves. Each nerve exits the vertebral canal via an intervertebral foramen and then divides into a posterior (dorsal) ramus and an anterior (ventral) ramus. The anterior rami of T1 to T11 run in the intercostal spaces and are called intercostal nerves. (T12 is called the subcostal nerve.) These spinal nerves are somatic nerves, carrying both sensory (to the skin) and motor (to muscles). The lower intercostal nerves also continue into the anterior abdominal wall. The diaphragm's primary motor innervation is the phrenic nerve (C3--C5), but lower intercostal nerves can carry sensory (proprioceptive) fibers from the diaphragm. Remember the concept of dermatomes. Important landmarks include: - T4 at the nipple (approximately) - T6 at the xiphoid process (approximately) - T10 at the umbilicus If something compresses or irritates a lower intercostal nerve near the spine, symptoms (like tingling) may appear in the corresponding area of the anterior abdominal wall. INNERVATION AND BLOOD SUPPLY OF THE ANTERIOR ABDOMINAL WALL: The skin and muscles of the anterior abdominal wall are supplied by the lower intercostal nerves (T7 to T11), the subcostal nerve (T12), and branches from L1 (the iliohypogastric and ilioinguinal nerves). Arterial supply of the anterior abdominal wall includes: - The superior epigastric artery (terminal branch of the internal thoracic artery) - The inferior epigastric artery (branch of the external iliac artery) - Intercostal arteries continuing anteriorly - The superficial epigastric artery (branch of the femoral artery) Superficial Veins of the Anterolateral Wall:\ \ All of these can supply the skin and muscles in that region. Venous drainage follows similar-named veins. Around the umbilicus, superficial veins are called the peri-umbilical veins; clinically, these can connect with deeper veins (para-umbilical veins in the round ligament region), forming an important link to the portal system. A final important note on intercostal nerve location: the main nerve, artery, and vein typically run along the inferior border of each rib (in the costal groove). When placing a chest tube, practitioners usually aim for the superior border of the rib below to avoid damaging the neurovascular bundle. Conversely, for an intercostal nerve block, the injection is targeted nearer to the inferior border of the rib to anesthetize the nerve.