Intercostal Muscles and Neurovascular Supply

Intercostal Muscles

• External intercostals extend from the tubercles of ribs to the costochondral junctions, running inferoanteriorly from a superior rib to an inferior rib.
• Internal intercostals occur deep to the external intercostals at a 90° angle, spanning ribs and costal cartilages to the sternum.
• Innermost intercostals are separated from internal intercostals by the intercostal neurovascular bundle and span the internal surfaces of adjacent ribs.
• Subcostalis muscles span 2-3 levels and originate near the angles of the lower ribs, positioned deep to the innermost intercostals.
• Transverse thoracis is continuous inferiorly with the transverse abdominis muscle.
• Levator costarum is generally categorized with intrinsic back muscles.

Neurovascular Supply to Intercostals

• Neurovascular structures run in the costal groove between internal and innermost intercostal layers, arranged from superior to inferior as vein, artery, nerve (VAN).
• Innervation is provided by segmental anterior primary rami (APRs).
• Arterial supply varies by intercostal space:
  • Spaces 1-2 receive blood from the superior/supreme intercostal arteries (from axillary artery).
  • Spaces 3-7 are supplied by intercostal arteries.
  • Below rib 8, blood supply is via the musculophrenic artery (off the internal thoracic artery).
• Venus drainage is achieved through intercostal veins.

Intrinsic Back Muscles

Superficial Intrinsics

• Splenius Capitis

  • Originates at the ligamentum nuchae and spinous processes of C7-T4.
  • Inserts at the mastoid process and the lateral third of the superior nuchal line.
  • Functions include lateral flexion, ipsilateral rotation, and extension (bilaterally).

• Splenius Cervicis

  • Originates at ligamentum nuchae and spinous processes of T3-T6.
  • Inserts at the posterior tubercles of transverse processes of C1-C4.
  • Similar functions as splenius capitis.

Intermediate Intrinsics: Erector Spinae

• A common aponeurotic origin from the thoracolumbar fascia attaches at the iliac crest, sacrum, and lumbar processes.

• Iliocostalis

  • Divided into lumborum, thoracis, and cervicis sections.

• Longissimus

  • Divided into thoracis, cervicis, and capitis sections.

• Spinalis

  • Divided into thoracis and cervicis, with the added semispinalis spanning 4-6 vertebral segments.

• Multifidus and Rotatores

  • Multifidus spans 2-4 vertebral segments.
  • Rotatores span 1-2 vertebral segments.

Minor Deep Layer

• Interspinales: Located between the spinous processes of adjacent segments.
• Intertransversarii: Positioned between the transverse processes of adjacent vertebrae, aiding in lateral flexion.
• Levator Costarum: Elevates the rib between the tubercle and angle during respiration and lateral flexion.

Thoracolumbar Fascia

• Composed of superficial and deep layers.
• The superficial layer contains subcutaneous tissue, possibly with adipose tissue.
• The deep layer blends with gluteal fascia and contains multiple laminae enveloping intrinsic back muscles, stabilizing the lumbar spine during muscle contraction.

Innervation to the Back

• Extrinsic Muscles: Innervated by branches of the brachial plexus; serratus muscles innervated by ventral rami (T2-5, T9-12).
• Intrinsic Muscles: Innervated by segmental posterior primary rami (PPRs), with the lateral portion of intertransversarii receiving APRs.
• Most connective tissues are also innervated by segmental PPRs, including outer portions of intervertebral discs via sinuvertebral nerves.