Muscles and Anatomy of the Back and Shoulder

Objectives

• Identify the extrinsic and intrinsic back muscles, their attachments, actions, and innervation.

• Identify the muscles that produce primary movements of the intervertebral joints.

• Describe the suboccipital triangle, including its boundaries, floor, roof, and contents.

• Anticipate the functional impairment resulting from the loss of nerve supply to a back muscle.

• Recognize the ligaments and bony structures associated with the shoulder.

• Explain the nerve supply, attachments, and functions of the shoulder muscles.

• Identify the rotator cuff muscles and understand their clinical anatomy.

• Outline the blood circulation to the shoulder.

• Describe the shoulder joint structure and injuries involving fractures or ligamentous tears.

Introduction

• This presentation is prepared from multiple resources, including:

  • Snell’s Clinical Anatomy

  • Moore’s Clinical Anatomy

  • Gray’s Anatomy

  • Netter’s Anatomy

• Images in this presentation are referenced accordingly. All information is intended for educational purposes.

Muscles of the Back

• Most body weight lies anterior to the vertebral column; many muscles attach to the spinous and transverse processes of the vertebrae to support and move the column.

• Muscle Classification:

  • Extrinsic Muscles (affecting upper limb):

  • Superficial Group:[Trapezius, Latissimus dorsi, Levator scapulae, Rhomboids, Serratus posterior (superior and inferior)]

  • Responsible for limb movements and spinal movements for posture maintenance.

  • Intrinsic Muscles:

  • Superficial, Intermediate, Deep Groups: Primarily act on the vertebral column, producing its movements and stabilizing.

Extrinsic Muscles

• Innervation of Superficial Extrinsic Muscles:

  • Arise from anterior rami of cervical nerves, affecting upper limb movements.

  • The trapezius receives motor fibers from the spinal accessory nerve (CN XI).

  • Intermediate Extrinsic Muscles receive innervation from the intercostal nerves.

Superficial Extrinsic Muscles

1. Trapezius

   • Origin: Skull, ligamentum nuchae, spinous processes of C8-T12.

   • Insertion: Clavicle, acromion, spine of the scapula.

   • Innervation: CN XI, proprioceptor fibers from C3 and C4.

   • Action: Upper fibers elevate and rotate scapula, middle fibers retract, lower fibers depress.

2. Latissimus Dorsi

   • Origin: Spinous processes of T6-T12, thoracolumbar fascia, iliac crest, inferior three ribs.

   • Insertion: Intertubercular sulcus of the humerus.

   • Innervation: Thoracodorsal nerve (C6-C8).

   • Action: Extends, adducts, and medially rotates the upper limb.

3. Rhomboids

   • Minor Origin: Spinous processes of C7-T1. Major Origin: Spinous processes of T2-T5.

   • Insertion: Medial scapula at spine (minor) and below spine (major).

   • Innervation: Dorsal scapular nerve (C4-5).

   • Action: Retracts and rotates the scapula.

4. Levator Scapulae

   • Origin: Transverse processes of C1-C4 vertebrae.

   • Insertion: Medial border of the scapula.

   • Innervation: Dorsal scapular nerve (C5) with fibers from C3/4.

   • Action: Elevates the scapula.

5. Serratus Posterior

   • Superior Origin: Ligamentum nuchae, spinous processes C7-T3.

   • Insertion: Superior aspect of ribs 2-5 (superior); inferior aspect of ribs 9-12 (inferior).

   • Innervation: T1-T4 (superior); T9-T12 (inferior).

   • Action: Elevates (superior) and depresses (inferior) ribs.

Clinical Correlations

• Muscle Injury and Presentation:

  • Trapezius: Injury to the accessory nerve results in drooping shoulder, inability to raise the arm above head, weakness in shoulder elevation.

  • Latissimus Dorsi: Injury to thoracodorsal nerve causes inability to pull the body upright or perform pull-ups.

Intrinsic (Deep) Muscles of the Back

• Develop embryologically, running from sacrum to skull. Innervated by posterior rami of spinal nerves.

• Associated with movements of the vertebral column and posture control, covered in deep fascia.

• Divided into three layers: superficial, intermediate, deep.

Superficial Intrinsic Muscles (Spinotransversalis)

1. Splenius Capitis:

   • Origin: Lower ligament of nuchae, spinous processes of C7-T3/4.

   • Insertion: Mastoid process, occipital bone.

   • Innervation: C3/C4.

   • Action: Rotates head to the same side.

2. Splenius Cervicis:

   • Origin: Spinous processes of T3-T6.

   • Insertion: Transverse process of C1-3/4.

   • Innervation: Lower cervical nerves.

   • Action: Rotates head to the same side.

Intermediate Intrinsic Muscles (Erector Spinae)

• Composed of three muscles – iliocostalis, longissimus, spinalis – that function as:

  • Primary extensors and lateral flexors of the vertebral column.

  • Lie posterolateral to the vertebral column between spinous processes and the angles of the ribs.

  • Arise from broad tendon attached to the sacrum, lumbar spinous processes, and iliac crest.

Deep Intrinsic Muscles (Transversospinales)

1. Semispinalis (most superficial):

   • Origin: Transverse processes of C4-T10.

   • Insertion: Spinous processes of C2-T4, occipital bone.

   • Action: Extends and contralaterally rotates head and vertebral column.

2. Multifidus:

   • Broad origin: Includes sacrum, posterior iliac spine, common tendinous origin of erector spinae, lumbar transverse processes.

   • Action: Stabilizes the vertebral column.

3. Rotatores (deepest; prominent in thoracic region):

   • Origin: Vertebral transverse processes.

   • Insertion: Lamina and spinous processes of the vertebrae above.

   • Action: Extension and rotation of the spine, stabilizes vertebra, proprioception.

Minor Deep Intrinsic Muscles

• Interspinales: Spans between adjacent spinous processes; stabilizes the vertebral column.

• Intertransversarii: Spans between adjacent transverse processes; stabilizes the vertebral column.

• Levatores Costarum: From transverse processes of C7-T11 to the rib below, elevates ribs.

Suboccipital Triangle

• A small, deep triangle located at the base of the skull.

• Contents: Vertebral artery/vein, and suboccipital nerve.

• Boundaries:

  • Medially: Rectus capitis posterior major.

  • Laterally: Obliquus capitis superior.

  • Inferiorly: Obliquus capitis inferior.

The Shoulder

Bones of the Shoulder

• Major bones include:

  • Scapula

  • Clavicle

  • Proximal end of the humerus.

Scapula

• Description: A large, flat triangular bone with:

  • Three Borders: Superior, lateral, medial.

  • Two Surfaces: Costal and posterior.

  • Three Processes: Acromion, spine, coracoid process.

Clavicle

• The only bony attachment between the trunk and upper limb.

• Ends:

  • Acromial (lateral) end: Flat.

  • Sternal (medial) end: Robust, quadrangular.

• Functions: Articulates with the sternum (manubrium) and first costal cartilage; roughened by muscle attachments.

Proximal End of the Humerus

• Greater tubercle: Lateral position important for muscle attachments.

• Facets for muscle attachment:

  • Superior facet: Supraspinatus.

  • Middle facet: Infraspinatus.

  • Inferior facet: Teres minor.

  • Lesser tubercle: Anterior for subscapularis.

Joints of the Shoulder

1. Sternoclavicular Joint:

   • Between the proximal clavicle and the manubrium of the sternum (with first costal cartilage).

   • Synovial joint with surrounding joint capsule and ligaments:

     • Anterior and Posterior sternoclavicular ligaments.

     • Costoclavicular ligament.

     • Interclavicular ligament.

2. Acromioclavicular Joint:

   • A small synovial joint between facets on the acromion and acromial end of the clavicle.

   • Surrounded by a joint capsule and reinforced by:

     • Acromioclavicular ligament (superior).

     • Coracoclavicular ligament (important accessory ligament for weight-bearing support).

3. Glenohumeral Joint:

   • A synovial ball-and-socket joint between the head of the humerus and glenoid cavity of the scapula.

   • Allows a wide range of movements: flexion, extension, abduction, adduction, medial rotation, lateral rotation, and circumduction.