The Backbone of Motion: Epaxial and Thoracic Hypaxial Systems

Axial muscles are defined as the muscles belonging specifically to the trunk and the neck. They are distinct from appendicular muscles, which are the muscles of the limbs and are not included in the axial classification.
The division between axial muscle groups is defined by their relationship to the transverse processes of the vertebrae, separated conceptually by a horizontal line identifying the approximate location of the vertebral column.
Epaxial Muscles:
- Location: These lie dorsal to the transverse processes of the vertebrae.
- Primary Function: They act as extensors of the vertebral column.
- Innervation: They are innervated by the dorsal branches of spinal nerves.
- Secondary Functions: They produce lateral movements when contracting on only one side and serve to stabilize the vertebral column overall.
Hypaxial Muscles:
- Location: These lie ventral to the transverse processes of the vertebrae.
- Primary Function: They act as flexors of the vertebral column.
- Scope: This group includes the muscles of the neck, thoracic wall, and abdominal wall.

Specific systems are identified based on their lateral-to-medial positioning relative to one another.
Iliocostalis System (The Most Lateral System):
- Extent: Courses between the ilium and the $C_7$ vertebra.
- Note on Location: This system is not present in the neck.
- Components:
  - Iliocostalis lumborum m.
  - Iliocostalis thoracis m.
- Action:
  - Fixation of the vertebral column.
  - Lateral movement when one side contracts.
  - Aids in inspiration by pulling the ribs caudally (inhaling).
Longissimus System (The Intermediate System):
- Location: Medial to the iliocostalis system.
- Extent: Extends from the ilium to the head (specifically the mastoid process of the skull).
- Components:
  - Longissimus thoracis et lumborum m.
  - Longissimus cervicis m.
  - Longissimus capitis m.
- Action: Responsible for the extension and fixation of the vertebral column.
Transversospinalis System (The Most Medial System):
- Extent: Courses between the sacrum and the head.
- Key muscles within this system to focus on:
  - Splenius m.
  - Semispinalis capitis m. (which is further divided into the Biventer cervicis and Complexus muscles).

Splenius m.:
- Appearance: A flat, triangular-shaped muscle.
- Origin and Insertion: Courses from the spinous processes of the $T_2-T_3$ vertebrae and the median raphe of the neck to the temporal bone of the skull.
- Action: Extends the neck and raises the head; unilateral contraction draws the head and neck laterally.
Semispinalis capitis biventer cervicis m.:
- Origin and Insertion: Courses from the transverse processes of the $T_4-T_2$ vertebrae to the caudal aspect of the skull.
- Action: Extends the neck and raises the head; unilateral contraction draws the head and neck laterally.
Semispinalis capitis complexus m.:
- Origin and Insertion: Courses from the caudal articular processes of the $T_1-C_3$ vertebrae to the caudal aspect of the skull.
- Action: Extends the neck and raises the head; unilateral contraction draws the head and neck laterally.

Neck Muscles:
- Longus capitis, Longus colli.
- Also includes Sternocephalicus, Sternothyroideus, and Sternohyoideus mm.
Thoracic Wall Muscles:
- Scalenus.
- Serratus ventralis cervicis and thoracis.
- Serratus dorsalis cranialis and caudalis.
- External intercostal and Internal intercostal mm.
- Rectus thoracis mm.
Abdominal Wall Muscles:
- External abdominal oblique.
- Internal abdominal oblique.
- Transversus abdominis.
- Rectus abdominis mm.
Other Hypaxial Groups (Not detailed in this session):
- Sublumbar muscles (covered in the pelvic limb unit).
- Ventral tail muscles (not studied in this course).
- Muscles of the pelvic outlet (covered in the pelvic cavity unit).

Innervation: These are innervated by the ventral branches of the cervical spinal nerves.
General Action: They flex the atlanto-occipital joint and draw the neck downward.
Longus capitis m.:
- Position: Located along the ventral vertebral bodies from $C_6$ to the skull.
- Relation: Found lateral to the longus colli m.
Longus colli m.:
- Position: Located along the ventral vertebral bodies from $T_6$ to the $C_1$ (atlas).
- Relation: Found medial to the longus capitis m.

Boundaries:
- Dorsal Boundary: Formed by the Longus capitis m. and the Longus colli m.
- Ventral Boundary: Formed by the Sternocephalicus m., Sternohyoideus m., and Sternothyroideus m.
Major Organs and Structures:
- Trachea.
- Esophagus: Positioned dorsal to the trachea and shifts towards the left.
- Thyroid and Parathyroid glands.
- Lymph nodes.
- Recurrent laryngeal nerves.
- Carotid Sheath: A connective tissue structure containing the Common carotid artery, the Vagosympathetic trunk, and the Internal jugular vein.
The Jugular Groove (Jugular Furrow):
- Contains the External jugular vein.
- Dorsal Boundary: Brachiocephalicus m. (specifically the cervical part of the cleidocephalicus m.).
- Ventral Boundary: Sternocephalicus m.

Accessory Nerve (CN XI):
- Type: This is a cranial nerve, not a cervical spinal nerve.
- Motor Innervation: It provides motor supply to the Cleidocephalicus m., Omotransversarius m., Sternocephalicus m., and Trapezius m.
Second Cervical Spinal Nerve ( $C_2$ ), Ventral Branch:
- Great auricular n.: Provides sensory innervation from the neck, ear, and the caudal part of the head.
- Transverse cervical (colli) n.: Provides sensory innervation from the cranioventral part of the neck.
Third, Fourth, and Fifth Cervical Spinal Nerves ( $C_3-C_5$ ), Ventral Branches:
- Function: Provide motor innervation to the muscles of the neck and sensory innervation from the skin of the neck.
- Pathway: These nerves penetrate through the omotransversarius m.
- Phrenic Nerve Contribution: The ventral branch of the $C_5$ spinal nerve gives rise to a branch that contributes to the phrenic nerve.

Muscles of Inspiration (Inhaling):
1. Diaphragm.
2. Serratus dorsalis cranialis m.
3. Scalenus m.
4. Rectus thoracis m.
5. External intercostals.
6. Iliocostalis (an epaxial muscle that aids inspiration by pulling ribs caudally).
Muscles of Expiration (Exhaling):
1. Serratus dorsalis caudalis m.
2. Transversus thoracis.
3. Internal intercostals.
4. Abdominal wall muscles.
Inspirational Note: "Don't stop succeeding" - Ron Hugo.