Trapezius
The trapezius is a large, flat, triangular muscle extending from the skull to the mid-back. Its three parts (upper, middle, lower) work together to control scapular movement and support the shoulder.
Origin, Insertion, Action, Innervation
OOrigin
Superior nuchal line, external occipital protuberance, nuchal ligament, and spinous processes of C7-T12 vertebrae.
IInsertion
Lateral third of clavicle, acromion, and spine of scapula
AAction
- • Elevation of the scapula (upper fibers)
- • Retraction of the scapula (middle fibers)
- • Depression of the scapula (lower fibers)
- • Rotation of the scapula (upper and lower fibers working together)
NInnervation
Accessory nerve (CN XI) for motor; C3, C4 for proprioception
Blood Supply
Transverse cervical artery
Clinical Relevance
Accessory nerve damage (e.g., from lymph node biopsy in posterior triangle) causes weakness in shoulder elevation and scapular rotation. The patient cannot shrug the shoulder or fully abduct the arm above 90°. Trapezius strain is common with poor posture.
Palpation
Easily palpated from the neck to the mid-back. Have the patient shrug the shoulders (upper fibers), retract the scapulae (middle fibers), or depress the shoulders (lower fibers) to feel different parts contract.
Study Tips
- ✓Three parts = three actions (elevation, retraction, depression)
- ✓Only muscle innervated by accessory nerve (CN XI)
- ✓Works with serratus anterior for scapular upward rotation
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Common questions about the trapezius
The trapezius has three functional parts: upper (descending) fibers that elevate the scapula, middle (transverse) fibers that retract the scapula, and lower (ascending) fibers that depress the scapula.
The trapezius is innervated by the accessory nerve (CN XI) for motor function. Proprioceptive fibers come from C3 and C4. This is the only skeletal muscle innervated by a cranial nerve.
Accessory nerve damage causes weakness in shoulder shrugging (upper trapezius) and difficulty fully abducting the arm above 90° (due to impaired scapular rotation). The shoulder may droop on the affected side.
The upper and lower trapezius fibers work together with serratus anterior to rotate the scapula upward. This rotation is necessary for full arm abduction above 90°, as it repositions the glenoid fossa to face upward.