Trapezius

From Ganfyd

The trapezius is a very large, triangular muscle, with three parts, which covers and acts upon the neck, thorax, spine, posterior abdominal wall and shoulder girdle.

Surface Anatomy

The muscle bulk of trapezius is easily palpable. The superior fibres are more obvious when the shoulder is fully abducted or in scapular elevation.

Trapezius
System:	Muscle
Function:	Lateral rotation, elevation and retraction of the scapula If scapula fixed, extension and lateral flexion of the neck
Origin:	Middle third of the superior nuchal line of the occipital bone, nuchal ligament, spinous processes and supraspinous ligaments of C7 to T12
Branches:
Insertion:	Lateral third of clavicle, spine of scapula, and acromion
Arterial supply:
Venous drainage:
Lymphatic drainage:
Innervation:	Spinal accessory nerve, lateral roots of C1 to C5
Vertebral levels:
Search for Trapezius in Gray's.

Shape

The trapezius is a flat triangular muscle. It arises from a large area between the occipital bone and the T12 vertebra. However its fibres converge; the superior fibres moving posterolaterally, the middle fibres transversely and the lower fibres superolaterally. It originates at the occipital bone through a thin fibrous lamina, to the spinous processes via an aponeurosis between C5 and T3, and through short tendinous fibres elsewhere. Its insertions depend on the fibres involved - the superior fibres connect to the posterolateral clavicle directly, the middle fibres to the acromion and spine of scapula directly, and the inferior fibres to a tubercle on the scapular spine via an aponeurosis.

Actions

• Natural tone of fibres helps to keep the shoulder girdle in its anatomical position
• Elevation of the scapula
• Retraction of the scapula
• Lateral rotation of the inferior angle of the scapula
• Scapula fixed:
  • Extends the neck
  • Laterally flexes the neck
• Superior fibres alone:
  • Elevation of the scapula
• Middle fibres alone:
  • Retraction of the scapula
• Inferior fibres alone:
  • Depression of the scapula
• Superior and inferior fibres together:
  • Lateral rotation of the inferior angle of the scapula

Joints Affected

• Shoulder girdle
• Neck

Origin

• Superior fibres:
  • Superior nuchal line of the occipital bone
  • Nuchal ligament
• Middle fibres:
  • Spinous processes and supraspinal ligaments of C7 to T5
• Inferior fibres:
  • Spinous processes and supraspinal ligaments of T6 to T12

Insertion

• Superior fibres:
  • Posterior border of lateral third of clavicle
• Middle fibres:
  • Medial margin of the acromion
  • Superior part of posterior border of spine of scapula
• Inferior fibres:
  • Form an aponeurosis which glides over the triangular medial spine of scapula to insert at a tubercle towards the apex of this triangle

Nerve Supply

• Spinal root of the accessory nerve
• Lateral root of C1 - motor only
• Lateral root of C2 - sensory and motor
• Lateral root of C3 - sensory and motor
• Lateral root of C4 - motor only
• Lateral root of C5 - motor only

Clinical Relevance

• Loss of the accessory nerve may lead to severe drooping of the arm, depending on how much is innervated by the spinal nerve roots

Variations

• Attachments to the lower thoracic vertebrae are often absent or attenuated
• The occipital attachment is often attenuated or absent
• There may be separation between the cervical and thoracic portions
• Extensive deficiencies and even complete absence can occur
• The clavicular insertion may be extensive, and/or blend with or overlap the posterior edge of sternocleidomastoid.