Intercostal Nerves

• Mixed spinal nerve

  • ⇒ Emerges from Intervertebral Foramen and gives off its posterior ramus (NB: Thus, it is the anterior rami that becomes the remainder of the intercostal nerve)
  • ⇒ Passes around neurovascular plane (between Internal Intercostals and Transversus Thoracis group)
  • ⇒ Gives off a Collateral Branch and a Lateral Cutaneous Branch before it reaches costal angle
    • NB: They are initially anterior to the Internal Intercostal Muscles
    • Collateral Branch – runs along inferior border of its space and supplies:
      • Muscles of the space
      • Parietal pleural
      • Periosteum of the ribs

• Lateral Cutaneous Branch – pierces intercostal muscles and overlying muscles of body wall along the Midaxillary line and divides into Anterior and Posterior Branches to supply skin over the space

• Muscles of thoracic wall innervated segmentally by the anterior rami

• Intercostal nerve ends as an Anterior Cutaneous Nerve

• In upper 6 spaces: Pass anterior to Internal Thoracic Artery and pierces intercostal muscles to reach the skin

• In its course around the body wall, the main nerve lies in a wider circle that embraces the narrower circle of vessels

• Thus: at the back of the Intercostal Space, the nerve lies behind the artery; at the front of the space, the nerve crosses in front of the Internal Thoracic Artery

• Lower 5 Intercostal Nerves (T8 – T12) (and their Collateral Branches) slope downwards behind costal margin into the neurovascular plane of the abdominal wall which they supply

  • NB: These nerves do NOT supply the parietal and visceral peritoneum

• Anterior ramus of T1 ascends across the neck of the 1st Rib to enter the Brachial Plexus

  • Gives off small 1st Intercostal nerve before entering Brachial Plexus

• Lateral Cutaneous Branch of the 2nd Intercostal Nerve: Crosses axilla to the medial side of the arm as the Intercostobrachial Nerve

  • NB: Anterior rami have some fibres of the first 3 thoracic nerves that goes to the upper limb

• Subcostal Nerve (12th Thoracic)

• Although arises in thorax, quickly leaves by passing behind Lateral Arcuate Ligament of the Diaphragm into the abdomen, below the Subcostal Artery and Vein

• Posterior Rami of the trunk:

  • Supplies all muscles of the Erector Spinae and Transversospinalis groups that lie deep to the thoracolumbar fascia
  • Supplies Levator Costae muscles
  • Supplies splenius and all muscles deep to it

• Each ramus divides into medial and lateral branch

  • Both branches supply muscle
  • Only one branch supplies the skin
    • In upper half of Thorax: Medial branches supply skin
    • In rest of the body: Lateral branches supply skin
    • Supply skin almost to the posterior axillary lines

• C1 has no cutaneous branch

• Posterior rami of the lower 2 nerves in the cervical and lumbar regions do not supply skin

• No posterior ramus ever supplies skin or muscle of a limb