Origin and Insertion
The anatomical bed of musculus trapezius covers a significant area of the back and neck. The muscle has an extensive line of origin that descends from the cranium down the vertebral column:
- Begins at the external occipital protuberance on the occipital bone.
- The attachment line continues along the superior nuchal line and the dense nuchal ligament.
- Distally, the muscle originates from the spinous processes of all cervical vertebrae (starting from the seventh, C7) and all thoracic vertebrae (down to Th12).
The muscle fibers converge toward the shoulder girdle to form the insertion zone. The superior, middle, and inferior fibers attach to various bony structures:
- Acromial end of the clavicle (predominantly superior fibers).
- Acromion of the scapula.
- Spine of the scapula (middle and ascending inferior fibers).
This broad area of attachment accounts for the variety of movements performed and the muscle's ability to stabilize the scapula in various positions.
Biomechanics and Functions
Due to the fan-like orientation of its muscle fibers, the functions of the trapezius muscle depend on which specific fibers contract and which point is fixed (punctum fixum).
- Scapular Movements: When specific parts contract, the muscle can elevate the scapula (superior fibers) or depress it (inferior fibers). Simultaneous contraction of all fibers retracts the scapula toward the midline (spine).
- Neck and Head Movements: If the scapulae are firmly fixed by other muscles, bilateral contraction of the trapezius muscles results in neck extension (tilting the head backward). Unilateral contraction with a fixed scapula causes lateral flexion of the head to the corresponding side.
Blood Supply and Innervation
Sustaining such a large muscle requires a rich vascular network and reliable neural regulation.
- Innervation: The primary motor nerve is the accessory nerve (n. accessorius, cranial nerve XI). Additional innervation (predominantly proprioceptive) is provided by branches of the cervical spinal nerves (segments C3–C4).
- Blood Supply: Arterial blood is supplied from multiple sources. The primary roles are played by the transverse cervical artery (a. transversa colli), occipital artery (a. occipitalis), and suprascapular artery (a. suprascapularis). Branches of the posterior intercostal arteries also participate in supplying the lower portions of the muscle.
Adjacent Muscle Groups: Multifidus and Levatores Costarum
When studying superficial muscles, it is important to remember the deeper structures. Literature frequently discusses muscles responsible for spinal statics and respiratory mechanics together.
Multifidus Muscles (mm. multifidi) These deep muscles extend in a continuous band from the sacrum to the cervical region. Their muscle fascicles have a characteristic feature: they span 2–4 vertebrae. Their main function is spinal stabilization and participation in extension of the vertebral column. They are innervated by the dorsal rami of spinal nerves (C3 to L5).
Levator Costarum Muscles (mm. levatores costarum) These are short thoracic muscles divided into two groups:
- Long (longi) — span across one rib, originating from transverse processes (C7–Th11) and inserting into the angles of the ribs (II–XII).
- Short (breves) — attach to the adjacent immediately inferior ribs without skipping a segment.
Their primary function, as the name implies, is elevation of the ribs, which facilitates inspiration. Additionally, they assist in the rotation of the thoracic vertebrae. They are supplied by posterior intercostal arteries (aa. intercostales posteriores) and innervated by dorsal rami of spinal nerves and intercostal nerves (C8–Th11).