Comprehensive Notes on the Thoracic Wall and Autonomic Nervous System

Introduction and Structural Boundaries of the Thoracic Wall

The thoracic wall is defined by a segmental design and is primarily composed of skeletal elements and associated muscles. It functions as a protective and functional cage that extends between two major openings: the superior thoracic aperture and the inferior thoracic aperture. The superior thoracic aperture, often referred to as the thoracic inlet, serves as a passage for various structures moving between the thorax, the neck, and the upper limbs. Its anatomical boundaries consist of the body of the $T_{1}$ vertebra, the first pair of ribs and their costal cartilages, and the superior border of the manubrium of the sternum.

The inferior thoracic aperture, or the thoracic outlet, facilitates the passage of structures between the thoracic cavity and the abdominal cavity. The boundaries of this aperture include the body of the $T_{12}$ vertebra, the $11\text{th}$ and $12\text{th}$ pairs of ribs, the costal margins (formed by the costal cartilages of ribs $7$ through $10$), and the xiphisternal joint. This region is structurally closed off by the diaphragm, which is the primary muscle of respiration.

The Thoracic Skeleton and Rib Classification

The thoracic skeleton is comprised of the thoracic vertebrae, ribs, and the sternum. Thoracic vertebrae feature several distinct landmarks including the vertebral body, pedicles, laminae, spinous processes, transverse processes, and the vertebral canal. They specifically possess costal facets for articulation with the ribs, including superior and inferior costal facets on the bodies for the heads of the ribs and transverse costal facets on the transverse processes for the rib tubercles.

Ribs are classified based on their connection to the sternum and their bony features. By connection, they are categorized into three groups: True Ribs (pairs $1$-$7$), which connect directly to the sternum via their own costal cartilages; False Ribs (pairs $8$-$10$), which connect indirectly to the sternum via the cartilage of the ribs immediately above them; and Floating Ribs (pairs $11$ and $12$), which have no anterior connection. Morphologically, ribs are divided into Typical and Atypical varieties.

Typical ribs (the $3\text{rd}$ through the $9\text{th}$) consist of a wedge-shaped head with two articular facets separated by a crest, a neck connecting the head and the body at the tubercle, a tubercle containing both articular and non-articular parts, and a thin, flat, curved body. The body features a costal angle, representing the point of maximum curvature, and a costal groove on the internal surface of the inferior border. This groove is a critical anatomical landmark that provides protection for the intercostal nerves and vessels. The internal surface of the rib also contains bone marrow, which serves as hematopoietic tissue.

Atypical Ribs and Their Specialized Features

Atypical ribs include the $1\text{st}$, $2\text{nd}$, $10\text{th}$, $11\text{th}$, and $12\text{th}$ ribs. The $1\text{st}$ rib is uniquely characterized by a single facet on its head, articulating only with the $T_{1}$ vertebra. It possesses grooves on its superior surface for the subclavian vein and subclavian artery, separated by the scalene tubercle, which serves as the attachment point for the anterior scalene muscle. The $2\text{nd}$ rib is distinguished by a rough area on its upper surface known as the tuberosity for the serratus anterior muscle.

Ribs $10$ through $12$ are atypical because they possess only a single facet on their heads, articulating with a single corresponding vertebra. Furthermore, the $11\text{th}$ and $12\text{th}$ ribs are notably short and lack both a neck and a tubercle, reflecting their "floating" status and minimal involvement in complex joint articulations.

Anatomy of the Sternum and Thoracic Joints

The sternum consists of three primary parts: the manubrium, the body (composed of fused segments called sternebrae), and the xiphoid process. Key landmarks include the jugular (suprasternal) notch, clavicular notches for articulation with the collarbones, and the sternal angle (angle of Louis), which marks the manubriosternal joint. The sternal angle is a clinical landmark situated at the level where the $2\text{nd}$ costal cartilage articulates with the sternum. The xiphoid process is the smallest part, connecting to the body at the xiphisternal joint.

The thoracic cage features several types of joints. Sternocostal joints involve the articulation between costal cartilages and the sternum; the $1\text{st}$ pair is a primary cartilaginous joint (synchondrosis), while pairs $2$ through $7$ are synovial plane joints. Interchondral joints occur between the costal cartilages of the $6\text{th}$-$7\text{th}$, $7\text{th}$-$8\text{th}$, and $8\text{th}$-$9\text{th}$ ribs and are synovial plane joints, while the joint between the $9\text{th}$ and $10\text{th}$ is fibrous. Intervertebral joints between vertebral bodies are secondary cartilaginous joints (symphyses involving the IV disc), while the zygapophysial joints between articular facets are synovial plane joints.

Costovertebral joints are responsible for the attachment of ribs to the spinal column. The head of a rib articulates with the superior costal facet of its own vertebra, the inferior costal facet of the vertebra above it, and the intervening intervertebral disc. The tubercle of a rib articulates with the transverse costal facet of the vertebra of the same number, forming a costotransverse joint. These joints are essential for allowing the movements required for respiration.

Respiratory Movements and Dimensions

Respiration involves changes in the thoracic dimensions in three planes: anterior-posterior, lateral, and vertical. The vertical dimension increases as the diaphragm—the primary muscle of inspiration at rest—contracts and descends. The anterior-posterior dimension increases via the "Pump Handle Movement," primarily involving ribs $1$ through $6$. In this movement, the sternum moves anteriorly and superiorly, rotating around an axis that passes through the neck of the ribs. The lateral dimension increases via the "Bucket Handle Movement," primarily involving ribs $7$ through $12$. The axis of movement for these lower ribs is more posterior, causing the rib cage to expand laterally.

Muscles of the Thoracic Wall and Accessory Respiratory Muscles

The muscles of the thoracic wall are organized into several layers and groups. The primary intercostal muscles consist of three layers: the external intercostals, internal intercostals, and innermost intercostals. External intercostal fibers pass downwards and forwards from the tubercles of the ribs to the costochondral junction, where they are replaced by the external intercostal membrane; they function to raise the ribs during inspiration. Internal intercostal fibers pass downwards and backwards from the sternum to the angle of the ribs, replaced posteriorly by the internal intercostal membrane. Their interosseous part depresses the ribs, while their interchondral part raises them. Innermost intercostal muscles are a deep, often incomplete layer separated from the internal intercostals by the neurovascular bundle.

Additional thoracic muscles include the subcostals, located near the angles of the ribs and spanning one or two intercostal spaces, and the transversus thoracis, which radiates from the posterior side of the sternum to the costal cartilages of ribs $2$ through $6$. Secondary muscles of respiration include the scalene muscles (anterior, middle, and posterior), which are innervated by the anterior rami of $C_{3}$-$C_{7}$, and the serratus posterior muscles. The serratus posterior superior (innervated by the $2\text{nd}$-$5\text{th}$ intercostal nerves) and the serratus posterior inferior (innervated by the anterior rami of $T_{9}$-$T_{12}$) also assist in movement.

Superficial muscles associated with the pectoral girdle, upper limb, or abdominal wall that assist in respiration include:

• Pectoralis Major: Innervated by Medial ($C_{8}$, $T_{1}$) and Lateral ($C_{5}$-$C_{7}$) pectoral nerves.
• Pectoralis Minor: Innervated by the Medial pectoral nerve ($C_{8}$, $T_{1}$).
• Serratus Anterior: Innervated by the Long thoracic nerve ($C_{5}$-$C_{7}$).
• External Oblique and Rectus Abdominis: Both innervated by the anterior rami of $T_{7}$-$T_{12}$.

Neurovascular Structures of the Thoracic Wall

The intercostal space between adjacent ribs contains the intercostal muscles and a neurovascular plane located between the internal and innermost intercostal muscles. The intercostal vein, artery, and nerve (V-A-N) run within the costal groove of the superior rib in each space, with the vein being the most superior and the nerve being the most inferior. Collateral branches of these structures often run along the superior border of the rib below.

Blood supply to the thoracic wall is provided by the anterior and posterior intercostal arteries. The first six anterior intercostal arteries arise directly from the internal thoracic artery, while those for spaces $7$ through $9$ arise from the musculophrenic artery. The internal thoracic artery eventually divides into the musculophrenic and superior epigastric arteries. The posterior intercostal arteries arise primarily from the aorta. Venous drainage involves the intercostal veins and the azygos system. The nerves of the thoracic wall are the anterior rami of thoracic spinal nerves, which give off lateral and anterior cutaneous branches to supply the skin. Dermatomes follow a segmental pattern, with a notable clinical landmark being the nipple level, which corresponds to the $T_{4}$ dermatome.