The Backbone of Motion: Epaxial and Thoracic Hypaxial Systems

Introduction to Axial Musculature

• Definition of Axial Muscles: Axial muscles consist of the muscles of the trunk and neck. They are distinguished from appendicular muscles, which are the muscles of the limbs (not included in the axial group).

• Classification by Position relative to the Vertebral Column: Axial muscles are divided into two primary groups based on their relationship to the transverse processes of the vertebrae.

  • Epaxial Muscles: These muscles lie dorsal to the transverse processes of the vertebrae. They function primarily as extensors of the vertebral column.
  • Hypaxial Muscles: These muscles lie ventral to the transverse processes of the vertebrae. They function primarily as flexors of the vertebral column. This group includes muscles of the neck, thoracic wall, and abdominal wall.

The Epaxial Muscle Systems

• General Characteristics:

  • Function: Extensors of the vertebral column (neck, trunk, and tail). When acting unilaterally (on one side), they produce lateral movements. They also serve to stabilize the vertebral column.
  • Innervation: Primarily innervated by the dorsal branches of spinal nerves.

• Iliocostalis System:

  • Position: The most lateral of the three epaxial muscle systems.
  • Course: Courses between the ilium and the $C7$ vertebra.
  • Components:
    • Iliocostalis lumborum m.
    • Iliocostalis thoracis m.
  • Note: This system is not present in the neck.
  • Action: Fixation of the vertebral column and lateral movement during unilateral contraction. It specifically aids in inspiration by pulling the ribs caudally.

• Longissimus System:

  • Position: Located medial to the iliocostalis system.
  • Course: Extends from the ilium all the way to the head, specifically attaching to the mastoid process of the skull.
  • Components:
    • Longissimus thoracis et lumborum m.
    • Longissimus cervicis m.
    • Longissimus capitis m.
  • Action: Extension and fixation of the vertebral column.

• Transversospinalis System:

  • Position: The most medial of the epaxial muscle systems.
  • Course: Courses between the sacrum and the head.
  • Specific Muscles of Focus:
    • Splenius m.: A flat, triangular-shaped muscle. It courses from the spinous processes of the $T2-T3$ vertebrae and the median raphe of the neck to the temporal bone of the skull. Its action is to extend the neck and raise the head; unilateral contraction draws the head and neck laterally.
    • Semispinalis capitis m.: This muscle is divided into two distinct parts:
      1. Biventer cervicis m.: Courses from the transverse processes of the $T4-T2$ vertebrae to the caudal aspect of the skull.
      2. Complexus m.: Courses from the caudal articular processes of the $T1-C3$ vertebrae to the caudal aspect of the skull.
    • Action of Semispinalis capitis: Extend the neck and raise the head; unilaterally draws the head and neck laterally.

The Hypaxial Muscle Systems

• Neck Muscles:

  • Longus capitis m.: Located along the ventral vertebral bodies from $C6$ to the skull, positioned lateral to the longus colli m.
  • Longus colli m.: Located along the ventral vertebral bodies from the $T6$ vertebra to the $C1$ (atlas), positioned medial to the longus capitis m.
  • Infrahyoid / Other Ventral Neck Muscles: Includes the sternocephalicus, sternothyroideus, and sternohyoideus mm.
  • Action: Flexion of the atlanto-occipital joint and drawing the neck downward.
  • Innervation: Ventral branches of cervical spinal nerves.

• Thoracic Wall Muscles:

  • Scalenus m.
  • Serratus ventralis cervicis and thoracis mm.
  • Serratus dorsalis cranialis and caudalis mm.
  • External intercostal and internal intercostal mm.
  • Rectus thoracis mm.

• Abdominal Wall Muscles:

  • External abdominal oblique m.
  • Internal abdominal oblique m.
  • Transversus abdominis m.
  • Rectus abdominis m.

• Other Categories:

  • Sublumbar muscles: Studied within the pelvic limb unit.
  • Ventral tail muscles: Not covered in this scope.
  • Muscles of the pelvic outlet: Studied within the pelvic cavity unit.

The Cervical Visceral Space and Neck Anatomy

• Boundaries of the Cervical Visceral Space:

  • Ventral Boundary: Composed of the Sternocephalicus m., Sternohyoideus m., and Sternothyroideus m.
  • Dorsal Boundary: Composed of the Longus capitis m. and Longus colli m.

• Major Structures (Cervical Viscera):

  • Trachea
  • Esophagus: Positioned dorsal to the trachea and typically shifts towards the left side.
  • Common Carotid Artery: Contained within the carotid sheath.
  • Carotid Sheath Contents: Includes the common carotid a., the vagosympathetic trunk, and the internal jugular vein.
  • Recurrent Laryngeal Nerves
  • Thyroid and Parathyroid Glands

• The Jugular Groove (Furrow):

  • External Jugular Vein: Lies within this groove.
  • Dorsal Boundary of the Groove: Brachiocephalicus m. (specifically the cervical part of the cleidocephalicus m.).
  • Ventral Boundary of the Groove: Sternocephalicus m.

Major Nerves of the Cervical Region

• Accessory Nerve (CN XI):

  • Classification: A cranial nerve, not a cervical spinal nerve.
  • Motor Innervation: Provides motor supply to the Cleidocephalicus m., Omotransversarius m., Sternocephalicus m., and Trapezius m.

• Second Cervical Spinal Nerve ($C2$):

  • Great auricular n. (Ventral branch): Provides sensory innervation from the neck, ear, and the caudal part of the head.
  • Transverse cervical (colli) n. (Ventral branch): Provides sensory innervation from the cranioventral part of the neck.

• Third, Fourth, and Fifth Cervical Spinal Nerves ($C3-C5$):

  • Ventral Branches: Provide motor innervation to the muscles of the neck and sensory innervation from the skin of the neck.
  • Path: These nerves penetrate through the omotransversarius m.
  • Phrenic Nerve Contribution: The ventral branch of the $C5$ spinal nerve gives rise to a branch that contributes to the formation of the phrenic nerve.

Muscles of Respiration

• Muscles of Inspiration (Inhale):

  1. Diaphragm: The primary muscle of inspiration.
  2. Serratus dorsalis cranialis m.
  3. Scalenus m.
  4. Rectus thoracis m.
  5. External intercostals

• Muscles of Expiration (Exhale):

  1. Serratus dorsalis caudalis m.
  2. Transversus thoracis m.
  3. Abdominal muscles (External/Internal oblique, Transversus abdominis, Rectus abdominis).
  4. Iliocostalis (epaxial): Specifically aids expiration / pulling ribs caudally.
  5. Internal intercostals