Myology of the Vertebral Column & Trunk: Axial Musculature, Back, Thorax, and Abdomen

Overview of the Muscular System

Main Divisions of Skeletal Muscles

The muscular system is divided into two primary groups based on anatomical location and function: axial musculature and appendicular musculature.

• Axial Musculature: Includes muscles of the head, neck, and torso. These muscles arise from the axial skeleton, position the head and spinal column, assist in respiration, and comprise approximately 60% of the body's skeletal muscle mass.

• Appendicular Musculature: Includes muscles of the limbs. These muscles stabilize or move the appendicular skeleton and make up the remaining 40% of skeletal muscle mass.

Revision: The Axial Skeleton

Features Relevant to Axial Muscles

The axial skeleton provides the structural framework for the body and serves as the attachment site for many axial muscles.

• Cranial Axial Skeleton: Skull

• Post-cranial Axial Skeleton: Hyoid bone, vertebrae, ribcage (ribs and sternum)

These structures are essential for muscle attachment and movement of the trunk and head.

Etymology: Useful Word Roots for Key Terms

Technical/Specific English and Latin/Greek Roots

Understanding word roots helps in learning anatomical terminology.

• Intrinsic (E): within, part of

• Extrinsic (E): outside, external but related

• Intra (L): within, inside

• Extra (L): outside, more, beyond, besides

• Inter (L): between, among

Common Word Roots:

• Abdomen (L): belly region

• Thorax (G): breastplate, chest region

• Pelvis (L): basin

• Oblique: angled/diagonal

• Rect- (L): straight

• Cost- (L): rib

• Serrat- (L): saw, sawlike

• Manubri- (L): handle

• Stern- (G): breast/breastbone

• Xiph- (G): sword

Back Muscles

Intrinsic vs Extrinsic Back Muscles

Back muscles are classified based on their location and function relative to the vertebral column.

• Intrinsic Back Muscles: Located deep and primarily act on the vertebral column itself. Responsible for movements and stabilization of the spine.

• Extrinsic Back Muscles: More superficial, connecting the vertebrae to the shoulder girdle and upper limb. Involved in movements of the upper limb.

Intrinsic Back Muscles: Groups and Functions

Intrinsic back muscles are organized into three main groups, each with specific roles in spinal movement and stability.

• A. Spinotransversales (Splenius Group): Includes splenius capitis and splenius cervicis. Bilaterally, these muscles extend the neck; unilaterally, they rotate and laterally flex the neck.

• B. Erector Spinae: Composed of three columns (from lateral to medial): iliocostalis, longissimus, and spinalis. These muscles extend the vertebral column and are key in maintaining posture. They also contribute to lateral flexion, especially the more lateral parts.

• C. Transversospinales: Includes semispinalis, multifidus, and rotatores. These muscles interconnect and stabilize the vertebrae, contributing to extension, rotation, and lateral flexion of the spine.

Nomenclature: Muscle names often reflect their origin and insertion (e.g., iliocostalis cervicis originates on ribs and inserts on cervical vertebrae).

Table: Intrinsic Back Muscle Groups

Group	Main Muscles	Function
Spinotransversales	Splenius capitis, Splenius cervicis	Neck extension, rotation, lateral flexion
Erector Spinae	Iliocostalis, Longissimus, Spinalis	Spine extension, lateral flexion, posture
Transversospinales	Semispinalis, Multifidus, Rotatores	Spine extension, rotation, stabilization

Cavities & Diaphragms

Major Cavities and Diaphragms of the Trunk

The trunk contains several cavities separated by muscular diaphragms, which play key roles in respiration and pelvic support.

• Thoracic Diaphragm: The primary muscle of respiration, separating the thoracic and abdominal cavities.

• Pelvic Diaphragm: Composed of muscles such as levator ani and coccygeus, supporting pelvic organs and elevating the pelvic floor.

When the thoracic diaphragm contracts, its muscle fibers shorten, causing the diaphragm to flatten and increase the volume of the thoracic cavity, which facilitates inhalation.

Muscles of the Thoracic Wall

Intercostal Muscles

Intercostal muscles are located between the ribs and are essential for breathing movements.

• External Intercostals: Fibers run inferoanteriorly (down and forward); elevate ribs during inspiration.

• Internal Intercostals: Fibers run inferoposteriorly (down and backward); depress ribs during forced expiration.

• Innermost Intercostals: Deepest layer, assist internal intercostals.

Other Thoracic Muscles

• Levatores Costarum: Elevate ribs, assist in respiration.

• Serratus Posterior Superior: Elevates ribs.

• Serratus Posterior Inferior: Depresses ribs.

Pleurae

The pleurae are serous membranes forming an airtight seal around the lungs, consisting of parietal and visceral layers. The pleural cavity contains fluid that keeps the layers together and facilitates lung movement.

Muscles of the Abdominal Wall

Major Abdominal Muscles

The abdominal wall is formed by several layers of muscles, each with distinct fiber orientations and functions.

• External Oblique: Most superficial; fibers run inferomedially (down and in). Responsible for trunk rotation and lateral flexion.

• Internal Oblique: Middle layer; fibers run superomedially (up and in). Also involved in trunk rotation and lateral flexion.

• Transversus Abdominis: Deepest layer; fibers run horizontally. Compresses abdominal contents.

• Rectus Abdominis: Runs vertically along the midline; responsible for trunk flexion and compression of abdominal cavity.

Tendinous Structures

• Abdominal Aponeurosis: Broad, flat tendon covering the abdominal muscles.

• Linea Alba: Midline tendinous seam where aponeuroses meet.

• Tendinous Inscriptions: Transverse bands within the rectus abdominis.

Table: Abdominal Wall Muscles and Fiber Orientation

Muscle	Location	Fiber Orientation	Main Function
External Oblique	Lateral	Inferomedial	Trunk rotation, lateral flexion
Internal Oblique	Lateral	Superomedial	Trunk rotation, lateral flexion
Transversus Abdominis	Deep lateral	Horizontal	Compresses abdominal contents
Rectus Abdominis	Anterior (medial)	Vertical	Trunk flexion, compression

Functions of Abdominal Muscles

• Flexion and rotation of the vertebral column

• Compression of the abdominal cavity (important for forced expiration, defecation, and childbirth)

• Stabilization of the trunk

Summary Table: Intrinsic Back Muscle Actions

Muscle Group	Bilateral Action	Unilateral Action
Spinotransversales	Extend neck	Rotate & lateral flex neck
Erector Spinae	Extend vertebral column	Lateral flexion of vertebral column
Transversospinales	Extend vertebral column	Rotation & stabilization

Key Equations

• Muscle Contraction and Volume Change (Diaphragm):

$\text{Change in Volume} = \text{Area} \times \text{Displacement}$

$\text{Pressure} \propto \frac{1}{\text{Volume}}$

As the diaphragm contracts and flattens, thoracic volume increases and pressure decreases, allowing air to flow into the lungs.

Additional info:

• Some details on muscle fiber orientation and actions were expanded for clarity.

• Tables were inferred and organized for comparison and classification.