Axial Musculature Overview

The muscles that move the torso are collectively known as the axial musculature. These muscles are essential for posture, movement, and stability of the vertebral column and trunk. Axial muscles are divided into two main groups based on their embryological origin and anatomical position: epaxial and hypaxial muscles.

Development and Organization of Axial Musculature

• Epaxial muscles are located dorsal (posterior) to the transverse processes of the vertebrae and are innervated by the dorsal rami of spinal nerves.

• Hypaxial muscles are located ventral (anterior) to the transverse processes and are innervated by the ventral rami of spinal nerves.

• Both groups are derived from the myotome region of somites during embryonic development.

Epaxial Muscles

Epaxial muscles are found along the dorsal aspect of the vertebral column and are primarily responsible for extending and stabilizing the spine.

Layered Structure of Epaxial Muscles

• Epaxial muscles are organized into three major layers: superficial, middle, and deep.

• Each layer contains specific muscle groups with distinct functions and attachments.

Superficial Layer: Erector Spinae Group

The erector spinae is the primary muscle group of the superficial epaxial layer, located deep to the scapular muscles.

• Iliocostalis (lateral most group)

• Longissimus dorsi (middle group)

• Spinalis (medial most group)

Actions:

• Bilateral contraction: Extension of the vertebral column

• Unilateral contraction: Lateral flexion of the vertebral column to the same side

Innervation: Dorsal rami of spinal nerves

Spinalis (Spinalis cervicis and thoracis)

• Origin: Spinous processes of C5-T2

• Insertion: Spinous processes of C2-T5

Longissimus Dorsi

• Longissimus capitis: Originates from transverse processes of C4-T3, inserts on the mastoid process of the temporal bone

• Longissimus cervicis: Originates from transverse processes of T1-T6, inserts on transverse processes of C2-C5

• Longissimus thoracis: Originates from sacrum, iliac crest, and lumbar vertebrae, inserts on ribs 2-12 and thoracic vertebrae

• Additional actions: Bilateral extension and unilateral rotation of the head to the same side

Iliocostalis

• Iliocostalis cervicis: Originates from ribs 3-7, inserts on transverse processes of C4-C6

• Iliocostalis thoracis: Originates from ribs 7-12, inserts on ribs 1-6

• Iliocostalis lumborum: Originates from sacrum, iliac crest, and thoracolumbar fascia, inserts on ribs 6-12 and upper lumbar vertebrae

Functional Summary Table: Erector Spinae Muscles

Key Terms and Definitions

• Epaxial muscles: Muscles located posterior to the vertebral column, involved in extension and stabilization of the spine.

• Hypaxial muscles: Muscles located anterior to the vertebral column, involved in flexion and lateral movements of the trunk.

• Transverse process: Lateral projections on each vertebra, serving as attachment points for muscles and ligaments.

• Dorsal rami: Posterior branches of spinal nerves that innervate epaxial muscles.

• Ventral rami: Anterior branches of spinal nerves that innervate hypaxial muscles.

Example: Functional Application

• During activities such as standing upright or bending backward, the erector spinae muscles contract bilaterally to extend the vertebral column.

• When bending to the side, unilateral contraction of these muscles produces lateral flexion.

Additional info: The notes above are based on the provided slides and standard anatomical knowledge. Further details on hypaxial muscles, abdominal wall, thoracic, and perineal muscles would be included in subsequent sections of the lecture.