Muscles of the Thorax, Abdominal Wall, and Shoulder: Structure and Function Chapter 10 part 2

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Muscles of Respiration

Deep Muscles of the Thorax: Breathing

The deep muscles of the thorax are essential for the process of breathing, which consists of two phases: inspiration (inhaling) and expiration (exhaling). These muscles alter the size of the thoracic cavity, enabling air movement into and out of the lungs.

Inspiratory Muscles: The diaphragm and external intercostals contract to enlarge the rib cage, increasing thoracic volume and drawing air into the lungs.
Expiratory Muscles: Expiration is primarily passive, resulting from relaxation of inspiratory muscles. However, contraction of the internal intercostals and abdominal muscles can actively decrease thoracic volume, forcing air out.
Diaphragm: This dome-shaped muscle separates the thoracic and abdominal cavities and is the most important muscle for inspiration.
Intercostal Muscles: Located between the ribs, these muscles assist in elevating or depressing the rib cage during breathing.

Example: During vigorous exercise, the internal intercostals and abdominal muscles contract forcefully to enhance expiration.

Table of deep muscles of the thorax: breathing Diagram of external and internal intercostal muscles Diagram of the diaphragm and its anatomical features

Table: Deep Muscles of the Thorax

Muscle	Description	Origin & Insertion	Action	Nerve Supply
External intercostals	11 pairs between ribs; fibers run obliquely downward and forward	Origin: inferior border of rib above; Insertion: superior border of rib below	Pull ribs toward one another to elevate rib cage; aids in inspiration	Intercostal nerves
Internal intercostals	11 pairs between ribs; fibers run deep and at right angles to external intercostals	Origin: superior border of rib below; Insertion: inferior border of rib above	Draw ribs together and depress rib cage; aids in forced expiration	Intercostal nerves
Diaphragm	Broad muscle; forms floor of thoracic cavity	Origin: inferior surface of rib cage, sternum, lumbar vertebrae; Insertion: central tendon	Prime mover of inspiration; flattens on contraction	Phrenic nerves

Table of deep muscles of the thorax: breathing

Muscles of the Abdominal Wall

Trunk Movements and Compression of Abdominal Viscera

The abdominal wall is formed by four paired muscles and their connective tissue structures. These muscles are crucial for trunk movement, stabilization, and compression of abdominal contents.

Rectus abdominis: Runs vertically along the midline; responsible for flexing the lumbar region and stabilizing the pelvis.
External obliques: Located on the lateral sides; fibers run diagonally downward.
Internal obliques: Located beneath the external obliques; fibers run diagonally upward.
Transversus abdominis: Deepest muscle; fibers run horizontally.
Fascicle Arrangement: The muscles' fascicles run at different angles, providing strength and flexibility.
Innervation: All are innervated by intercostal nerves.
Actions: Lateral flexion, rotation of the trunk, and compression of abdominal viscera. These actions assist in urination, defecation, childbirth, vomiting, coughing, and screaming.

Table of muscles of the abdominal wall Diagram of abdominal wall muscles Side view of abdominal wall muscles Cross-section of abdominal wall muscles

Table: Muscles of the Abdominal Wall

Muscle	Description	Origin & Insertion	Action	Nerve Supply
Rectus abdominis	Medial muscle; segmented by tendinous intersections	Origin: pubic crest and symphysis; Insertion: xiphoid process and costal cartilages	Flexes and rotates lumbar region; stabilizes pelvis	Intercostal nerves
External oblique	Largest and most superficial; fibers run downward and medially	Origin: lower eight ribs; Insertion: linea alba, pubic crest, iliac crest	Flexes and rotates vertebral column; compresses abdominal wall	Intercostal nerves
Internal oblique	Fibers run upward and medially	Origin: lumbar fascia, iliac crest; Insertion: linea alba, pubic crest, last three ribs	Same as external oblique	Intercostal nerves
Transversus abdominis	Deepest muscle; fibers run horizontally	Origin: inguinal ligament, lumbar fascia, cartilages of last six ribs; Insertion: linea alba, pubic crest	Compresses abdominal contents	Intercostal nerves

Superficial Muscles of the Anterior and Posterior Thorax

Movements of the Scapula and Arm

These muscles are primarily extrinsic shoulder muscles that act in combination to fix the shoulder girdle and move the scapula, increasing the range of arm movements. They are divided into anterior and posterior groups, each with specific actions.

Actions: Elevation, depression, rotation, lateral and medial movements, protraction, and retraction of the scapula.
Anterior Thorax Muscles: Include pectoralis major, pectoralis minor, serratus anterior, and subclavius.
Posterior Thorax Muscles: Include trapezius, levator scapulae, rhomboids, and latissimus dorsi.

Table of superficial muscles of the thorax and shoulder Diagram of superficial muscles of the thorax and shoulder Diagram of posterior thorax muscles

Table: Superficial Muscles of the Anterior and Posterior Thorax

Muscle	Description	Origin & Insertion	Action	Nerve Supply
Pectoralis major	Large, fan-shaped; covers upper chest	Origin: clavicle, sternum, ribs; Insertion: humerus	Flexes, adducts, and medially rotates arm	Lateral and medial pectoral nerves
Serratus anterior	Fan-shaped; lies deep to scapula	Origin: ribs; Insertion: scapula	Protracts and holds scapula against thoracic wall	Long thoracic nerve
Trapezius	Large, triangular; covers back and neck	Origin: occipital bone, vertebrae; Insertion: scapula, clavicle	Elevates, retracts, and rotates scapula	Accessory nerve (CN XI)
Rhomboids	Rectangular; deep to trapezius	Origin: vertebrae; Insertion: scapula	Stabilizes scapula	Dorsal scapular nerve

Summary Table: Muscle Actions and Functions

Muscle Group	Main Actions	Key Functions
Thorax (Respiration)	Inspiration, expiration	Ventilation, gas exchange
Abdominal Wall	Flexion, rotation, compression	Trunk movement, protection, visceral compression
Shoulder/Thorax	Scapular movement	Arm mobility, shoulder stabilization

Additional info: The tables and diagrams provided reinforce the anatomical structure, origin, insertion, action, and innervation of each muscle group, which are essential for understanding their physiological roles in movement and stability.