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Regional somatic nerve anatomy

Each spinal nerve (31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) exits the vertebral column and divides into:

  • Anterior Rami   - Supplies the anterolateral body wall, limbs, and forms major nerve plexuses
  • Posterior Rami
    • Innervates true (intrinsic) back muscles, facet joints, zygapophyseal joints, and skin overlying

Brocken down by region

  • Upper Cervical
    • Anterior Rami:
      • C1–C4: Form the Cervical Plexus
      • Sensory: skin of scalp (lesser occipital), neck (transverse cervical), mandible (greater auricular), supraclavicular area (supraclavicular nerve)
      • Motor: infrahyoid muscles, geniohyoid, thyrohoid, prevertebral muscles
      • Phrenic nerve (C3–C5): diaphragm
    • Posterior Rami:
      • Supply intrinsic back muscles (e.g., splenius, semispinalis, suboccipital muscles)
      • C1 (suboccipital nerve): motor only, to suboccipital muscles
      • C2 (greater occipital): sensory to posterior scalp
      • C3-C8
        • Run posteriorly around their respective articular pillars and provide motor innervation to various muscles including semispinalis capitis, multifidus, interspinalis, longissimus colli, splenius colli and iliocostalis colli muscles.
        • In addition, sensory innervation is provided to the facet joints above and below the nerve and the skin around the medial back region.
  • Lower cervical
    • Anterior Rami
    • C5-C8
      • Run posteriorly around their respective articular pillars and provide motor innervation to various muscles including semispinalis capitis, multifidus, interspinalis, longissimus colli, splenius colli and iliocostalis colli muscles.
      • In addition, sensory innervation is provided to the facet joints above and below the nerve and the skin around the medial back region
  • Thoracic
    • Anterior Rami:
       - T1–T11: Form Intercostal Nerves (segmental, not plexiform)
        - Motor: intercostal muscles, abdominal wall muscles (lower T6–T11)
        - Sensory: overlying skin, pleura, peritoneum
       - T12: Subcostal nerve (below 12th rib)
       - T1: also contributes to brachial plexus (inferior trunk)
    • Posterior Rami:
       - Supply erector spinae, multifidus, rotatores
       - Segmental cutaneous branches supply midline back skin
  • Lumbar
    • Anterior Rami:
       - Form the Lumbar plexus (L1–L4)
        - Motor: abdominal wall (iliohypogastric, ilioinguinal), anterior and medial thigh (femoral, obturator)
        - Sensory: groin, upper thigh, medial leg (genitalfemoral nerve, lateral femoral cutaneous and femoral nerve)  - L4–L5: contribute to lumbosacral trunk, joining sacral plexus
    • Posterior Rami:
       - Supply deep back muscles (erector spinae, multifidus, quadratus lumborum)
       - Medial branches: innervate zygapophyseal (facet) joints
       - Lateral branches: supply posterior skin over lumbar region and iliac crest
  • Sacral Region
    • Anterior Rami:
       - L4–S4: form the Sacral plexus
        - Motor: posterior thigh, most of leg and foot (via sciatic nerve)
        - Sensory: posterior thigh, leg, foot, perineum
       - S2–S4: Pudendal nerve → perineum (motor and sensory)
       - Pelvic splanchnic nerves (S2–S4): parasympathetic to pelvic organs, hindgut
    • Posterior Rami:
       - Supply multifidus, erector spinae, gluteal skin
       - Superior cluneal nerves (L1–L3 posterior rami) and medial cluneal nerves (S1–S3 posterior rami) → sensory to buttocks

Cranial nerves

The cranial nerves that carry somatic components are:

Cranial Nerves with Somatic Motor Fibres

  • CN III – Oculomotor
     - Somatic motor to most extraocular muscles (SR, IR, MR, IO, levator palpebrae superioris)
  • CN IV – Trochlear
     - Somatic motor to superior oblique muscle
  • CN VI – Abducens
     - Somatic motor to lateral rectus muscle
  • CN XII – Hypoglossal
     - Somatic motor to intrinsic and extrinsic tongue muscles (except palatoglossus)

Cranial Nerves with Somatic Sensory Fibres

  • CN V – Trigeminal
     - Major somatic sensory nerve of face, scalp, cornea, oral and nasal mucosa, teeth, anterior 2/3 of tongue, and dura
  • CN VII – Facial
     - Somatic sensory from skin of external ear and external auditory canal
  • CN IX – Glossopharyngeal
     - Somatic sensory from posterior 1/3 of tongue, oropharynx, middle ear, and Eustachian tube
  • CN X – Vagus
     - Somatic sensory from external ear, external auditory canal, part of tympanic membrane, and dura of posterior cranial fossa

Graph View

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