2.4 Peripheral Nerve Plexuses and Autonomic Nervous System Pathways

2.4 Peripheral Nerve Plexuses and Autonomic Nervous System Pathways

Regional anaesthesia and perioperative autonomic homeostasis require an in-depth understanding of the somatic nerve plexuses and the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS).

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1. Brachial Plexus Architecture: Roots to Terminal Branches

The brachial plexus provides motor and sensory innervation to the entire upper limb, shoulder girdle, and chest wall. It is formed by the anterior rami of C5, C6, C7, C8, and T1 (with occasional minor contributions from C4 [pre-fixed plexus] or T2 [post-fixed plexus]).

                      [ BRACHIAL PLEXUS ARCHITECTURE ]

  Roots (5)     Trunks (3)       Divisions (6)      Cords (3)       Terminal Nerves
(Interscalene) (Interscalene)  (Retroclavicular)    (Axillary)         (Branches)

    C5 -----\ 
             +-- Superior (C5-6) --+- Anterior -----+-- Lateral --+- Musculocutaneous (C5-7)
    C6 -----/                      \               /              \
                                    +- Posterior -+                +- Median (C5-T1)
    C7 --------- Middle (C7) ------+- Anterior --/ \              /
                                   \                +-- Posterior -+- Axillary (C5-6)
    C8 -----\                       +- Posterior -+ |              \
             +-- Inferior (C8-T1) -+- Anterior ---|-+-- Medial ---+- Radial (C5-T1)
    T1 -----/                      \              | |             |
                                    +- Posterior -+ |             +- Ulnar (C8-T1)

The Five Architectural Stages

To remember the sequence from medial to lateral: Roots, Trunks, Divisions, Cords, Branches ("Remember To Drink Cold Beer").

1. Roots (C5-T1): Emerge between the anterior scalene and middle scalene muscles in the neck alongside the subclavian artery.
2. Trunks (3): Formed within the posterior triangle of the neck in the interscalene groove:
   • Superior Trunk: Fusion of C5 and C6 rami. Gives off the suprascapular nerve and nerve to subclavius.
   • Middle Trunk: Direct continuation of C7 ramus.
   • Inferior Trunk: Fusion of C8 and T1 rami. Rests directly on the superior surface of the first rib behind the subclavian artery.
3. Divisions (6): Behind the clavicle, each trunk splits into an anterior division (innervating anterior/flexor compartments) and a posterior division (innervating posterior/extensor compartments), yielding 3 anterior and 3 posterior divisions.
4. Cords (3): Emerge beneath the clavicle and enter the axilla behind the pectoralis minor muscle. They are named strictly according to their anatomical relationship to the second part of the axillary artery:
   • Lateral Cord: Formed by the union of anterior divisions of the superior and middle trunks (C5, C6, C7).
   • Posterior Cord: Formed by the union of posterior divisions of all three trunks (C5, C6, C7, C8, T1).
   • Medial Cord: Formed by the direct continuation of the anterior division of the inferior trunk (C8, T1).
5. Terminal Branches:
   • Musculocutaneous Nerve (C5-C7): Terminal branch of the lateral cord; pierces the coracobrachialis muscle; supplies the anterior compartment of the arm (biceps brachii, brachialis, coracobrachialis) and terminates as the lateral antebrachial cutaneous nerve.
   • Axillary Nerve (C5-C6): Branch of the posterior cord; traverses the quadrangular space with the posterior circumflex humeral artery; supplies the deltoid and teres minor muscles and provides sensory innervation over the "military patch" area of the lateral shoulder.
   • Radial Nerve (C5-T1): Continuation of the posterior cord; runs in the radial spiral groove of the humerus; supplies all extensors of the arm and forearm and provides sensation over the posterior arm, posterior forearm, and dorsal web space of the thumb.
   • Median Nerve (C5-T1): Formed by the union of the lateral root (from lateral cord) and medial root (from medial cord), straddling the axillary artery; supplies the anterior forearm flexors/pronators and thenar muscles, with sensation over the radial palmar surface and radial 3.5 digits.
   • Ulnar Nerve (C8-T1): Continuation of the medial cord; descends medial to the brachial artery, passes behind the medial epicondyle of the humerus, and enters the forearm; supplies flexor carpi ulnaris, medial half of flexor digitorum profundus, intrinsic hand muscles (hypothenar, interossei, adductor pollicis), and sensation over the ulnar 1.5 digits.

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2. Ultrasound-Guided Brachial Plexus Blocks and Sparing Patterns

Clinical Pearl: The interscalene block must be avoided in patients with severe baseline pulmonary disease (e.g., severe COPD or contralateral diaphragmatic paralysis) because the near-universal 100% phrenic nerve blockade decreases forced vital capacity (FVC) by 25% to 30%.

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3. Lumbar Plexus Anatomy and Fascial Compartments

The lumbar plexus provides motor and sensory innervation to the lower abdominal wall, anterior and medial thigh, and medial leg.

• Origin: Formed within the substance of the psoas major muscle by the anterior rami of L1, L2, L3, and L4 (with occasional T12 contribution).
• Topography: Nerve roots emerge from the intervertebral foramina, divide inside the psoas compartment, and emerge from the lateral, anterior, and medial borders of the muscle.

                        [ LUMBAR PLEXUS BRANCHES (L1-L4) ]

       L1 -------------------> Iliohypogastric & Ilioinguinal Nerves
       L1, L2 ---------------> Genitofemoral Nerve (pierces anterior psoas)
       L2, L3 ---------------> Lateral Femoral Cutaneous Nerve (exits lateral psoas)
       L2, L3, L4 (Post) ----> Femoral Nerve (exits lateral psoas, under inguinal lig.)
       L2, L3, L4 (Ant) -----> Obturator Nerve (exits medial psoas, obturator canal)
       L4, L5 ---------------> Lumbosacral Trunk (descends to Sacral Plexus)

Major Branches of the Lumbar Plexus

1. Iliohypogastric & Ilioinguinal (L1): Emerge from the lateral border of psoas, cross the quadratus lumborum; innervate the transverse abdominis, internal oblique, and suprapubic/inguinal/scrotal/labial skin.
2. Genitofemoral Nerve (L1-L2): Pierces the anterior surface of the psoas major; divides into genital branch (cremasteric reflex, scrotum/labium) and femoral branch (sub-inguinal skin).
3. Lateral Femoral Cutaneous Nerve of the Thigh (LFCN, L2-L3): Emerges from the lateral psoas, traverses the iliacus, and passes beneath the inguinal ligament approximately 1 cm medial to the anterior superior iliac spine (ASIS). Purely sensory to the anterolateral thigh. Entrapment here causes meralgia paraesthetica.
4. Femoral Nerve (L2-L4, posterior divisions): The largest branch of the lumbar plexus. Emerges from the lower lateral border of psoas, descends in the groove between psoas and iliacus beneath the fascia iliaca, and enters the thigh lateral to the femoral artery beneath the inguinal ligament. Supplies the quadriceps femoris (knee extension), sartorius, and pectineus. Terminates as the saphenous nerve (purely sensory to the medial leg, medial malleolus, and arch of the foot).
5. Obturator Nerve (L2-L4, anterior divisions): Emerges from the medial border of the psoas major, descends into the true pelvis across the pelvic brim, and exits through the obturator foramen. Innervates the thigh adductors (adductor longus, brevis, magnus, gracilis, obturator externus) and provides sensory innervation to an oval area on the medial thigh and articular branches to the hip and knee joints.

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4. Sacral Plexus and Sciatic Nerve Arborization

The sacral plexus supplies the posterior thigh, the entire lower leg and foot (except the medial strip supplied by the saphenous nerve), and the perineum.

• Origin: Formed by the lumbosacral trunk (L4-L5) uniting with the anterior rami of S1, S2, S3, and S4 on the anterior surface of the piriformis muscle on the posterior pelvic wall.

The Sciatic Nerve (L4-S3)

The sciatic nerve is the largest and thickest nerve in the human body (approximately 2 cm in width).

• Pelvic Exit: Leaves the pelvis through the greater sciatic foramen immediately inferior to the piriformis muscle (in 85% of individuals; anatomic variations involve passage through or above the piriformis, contributing to piriformis syndrome).
• Course: Descends deep to the gluteus maximus, midway between the greater trochanter of the femur and the ischial tuberosity. Descends through the posterior thigh deep to the biceps femoris.
• Bifurcation: Typically bifurcates into its two distinct components at the superior angle of the popliteal fossa (approximately 5 to 7 cm above the popliteal crease):
  1. Tibial Nerve (L4-S3, anterior divisions): Continues straight through the popliteal fossa with the popliteal artery and vein; innervates posterior compartment calf muscles (gastrocnemius, soleus, tibialis posterior: plantarflexion) and divides at the medial malleolus into medial and lateral plantar nerves (supplying the sole of the foot).
  2. Common Peroneal (Fibular) Nerve (L4-S2, posterior divisions): Tracks laterally along the biceps femoris tendon, winds around the neck of the fibula (where it is vulnerable to compression from surgical lithotomy stirrups, causing foot drop), and divides into:
     • Deep Peroneal Nerve: Supplies anterior compartment muscles (tibialis anterior, extensor hallucis longus: dorsiflexion and toe extension) and sensory web space between 1st and 2nd toes.
     • Superficial Peroneal Nerve: Supplies lateral compartment muscles (peroneus longus and brevis: eversion) and sensation over the dorsum of the foot.

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5. Autonomic Nervous System: Sympathetic vs Parasympathetic Outflow

The autonomic nervous system (ANS) maintains visceral organ homeostasis and autonomic cardiovascular tone.

                      [ AUTONOMIC OUTFLOW COMPARISON ]

          SYMPATHETIC SYSTEM                   PARASYMPATHETIC SYSTEM
      "Thoracolumbar Outflow"                   "Craniosacral Outflow"
             (T1 to L2)                       (CN III, VII, IX, X + S2-S4)
                 |                                         |
     Intermediolateral Horn of                 Brainstem Motor Nuclei &
        Spinal Cord (T1-L2)                     Sacral Lateral Horn (S2-S4)
                 |                                         |
    Short Preganglionic Axons                  Long Preganglionic Axons
  (White Rami Communicantes, B fibers)           (Reach close to target organ)
                 |                                         |
     Paravertebral Ganglia                     Terminal / Intramural Ganglia
      (Sympathetic Trunk)                       (In or on visceral wall)
                 |                                         |
     Long Postganglionic Axons                 Short Postganglionic Axons
  (Grey Rami Communicantes, C fibers)                    |
                 |                             Releases Acetylcholine (ACh)
      Releases Noradrenaline (NA)                 onto Muscarinic Receptors
        onto Adrenergic Receptors

Detailed Structural Comparison

Cranial Parasympathetic Components:

• CN III (Oculomotor): Edinger-Westphal nucleus $\rightarrow$ ciliary ganglion $\rightarrow$ pupillary sphincter (miosis) and ciliary muscle (accommodation).
• CN VII (Facial): Superior salivatory nucleus $\rightarrow$ pterygopalatine ganglion (lacrimal gland, nasal mucosa) and submandibular ganglion (submandibular and sublingual salivary glands).
• CN IX (Glossopharyngeal): Inferior salivatory nucleus $\rightarrow$ otic ganglion $\rightarrow$ parotid gland.
• CN X (Vagus): Dorsal motor nucleus and nucleus ambiguus $\rightarrow$ cardiac plexus, pulmonary plexus, and gastrointestinal tract from oesophagus through the mid-transverse colon.

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6. Stellate Ganglion Anatomy and Horner's Syndrome

The Stellate Ganglion (Cervicothoracic Ganglion)

The sympathetic chain in the neck features three cervical ganglia: superior (C2-C3), middle (C6), and inferior (C7). In 80% of individuals, the inferior cervical ganglion fuses with the first thoracic sympathetic ganglion to form the stellate ganglion (cervicothoracic ganglion).

Anatomical Relations of the Stellate Ganglion:

• Location: Situated anterior to the neck of the first rib and the transverse process of the C7 vertebra, extending to the superior border of the T1 vertebra.
• Anterior Relations: Carotid sheath (common carotid artery, IJV, vagus nerve) and the vertebral artery and vein (the vertebral artery originates from the subclavian artery and enters the transverse foramen at C6, lying directly anterior to the ganglion at C7).
• Posterior Relations: Longus colli muscle, transverse process of C7, ventral ramus of T1 nerve root.
• Inferior Relations: Dome of the cervical pleura (cupula) and apex of the lung.

Clinical Trap during Stellate Ganglion Block (at C6 Chassaignac's Tubercle): To avoid puncturing the vertebral artery or cupula of the lung, the injection is performed at the level of the C6 transverse process (Chassaignac's tubercle), where the vertebral artery is protected inside the bony foramen transversarium and the pleura is far inferior. The local anaesthetic then tracks caudally along the prevertebral fascia to bathe the stellate ganglion at C7-T1.

Horner's Syndrome Pathophysiology

Interruption or pharmacological blockade of the ascending sympathetic pathway to the head and neck at or above the stellate ganglion produces ipsilateral Horner's syndrome.

The Classic Clinical Tetrad:

1. Ptosis (Partial): Drooping of the superior eyelid caused by paralysis of the superior tarsal muscle (Müller's muscle), a smooth muscle innervated by sympathetic fibers (in contrast to the complete ptosis seen in CN III palsy due to levator palpebrae superioris paralysis).
2. Miosis: Pupillary constriction resulting from paralysis of the sympathetically innervated pupillodilator muscle, leaving the parasympathetically innervated pupilloconstrictor (sphincter pupillae) unopposed.
3. Anhidrosis: Absence of facial sweating on the ipsilateral side due to loss of sympathetic sudomotor innervation to sweat glands.
4. Enophthalmos (Apparent): The eye appears sunken into the orbit, predominantly an optical illusion secondary to narrowing of the palpebral fissure from ptosis and elevation of the lower lid (upside-down ptosis).

Associated Vasomotor Signs: Loss of sympathetic vasoconstrictor tone causes ipsilateral facial skin flushing, elevated facial skin temperature, conjunctival injection, and nasal mucosal congestion (Guttmann's sign).