Chapter 13: The Peripheral Nervous System and Reflex Activity

Chapter 13: The Peripheral Nervous System and Reflex Activity

Innervation of Specific Body Regions

• Spinal Nerve Rami: Supply the somatic region (skeletal muscles and skin) from the neck down.
  • Dorsal Rami: Supply the posterior body trunk.
  • Ventral Rami: Supply the rest of the trunk and the limbs.
• Roots vs. Rami:
  • Roots: Medial to spinal nerves; strictly sensory or motor.
  • Rami: Distal to spinal nerves; carry both sensory and motor fibers.
• Ventral Rami and Nerve Plexuses:
  • Except for T2-T12, ventral rami branch and join to form nerve plexuses.
  • Nerve plexuses occur in the cervical, brachial, lumbar, and sacral regions.
  • Primarily serve the limbs.
  • Only ventral rami form plexuses.
• Plexus Fiber Redistribution:
  • Fibers from various ventral rami criss-cross.
  • Each branch contains fibers from multiple spinal nerves.
  • Fibers from each ventral ramus travel via multiple routes.
  • Each limb muscle receives nerve supply from more than one spinal nerve.
• Advantage of Plexus Rearrangement:
  • Damage to one spinal segment or root cannot completely paralyze any limb muscle.

Cervical Plexus and the Neck

• Location: Deep in the neck, under the sternocleidomastoid muscle.
• Formation: Ventral rami of the first four cervical nerves (C1-C4), with a minor contribution from C5.
• Branches:
  • Cutaneous Nerves: Supply the skin of the neck, ear area, back of the head, and shoulder; transmit sensory impulses.
  • Motor Branches: Innervate muscles of the anterior neck.
• Phrenic Nerve:
  • Most important nerve from the cervical plexus.
  • Receives fibers from C3, C4, and C5.
  • Runs inferiorly through the thorax.
  • Supplies motor and sensory fibers to the diaphragm (chief muscle for breathing movements).
• Clinical Significance - Phrenic Nerve Irritation:
  • Irritation causes spasms of the diaphragm, or hiccups.
  • Severed phrenic nerves or crushed/destroyed C3-C5 region of spinal cord leads to diaphragm paralysis and respiratory arrest.
  • Mnemonic: "C3, 4, and 5 keep the diaphragm alive."
  • Mechanical ventilators are required to force air into the lungs.

Brachial Plexus and Upper Limb

• Location: Partly in the neck and partly in the axilla.
• Innervation: Gives rise to virtually all the nerves that innervate the upper limb.
• Palpation: Can be felt superior to the clavicle at the lateral border of the sternocleidomastoid muscle.
• Formation: Ventral rami of C5-T1, often receiving fibers from C4 or T2 or both.
• Structure:
  • Roots: Ventral rami (C5-T1).
  • Trunks: Upper, middle, and lower trunks formed by the roots uniting.
  • Divisions: Anterior and posterior divisions from each trunk.
  • Cords: Lateral, medial, and posterior cords formed by the divisions in the axilla.
• Mnemonic for Branching Sequence: "Really Tired? Drink Coffee" (Roots, Trunks, Divisions, Cords).
• Small Nerves Branching Off: Supply the muscles and skin of the shoulder and superior thorax.
• Clinical Significance - Injuries:
  • Injuries to the brachial plexus are common and can weaken or paralyze the entire upper limb.
  • Occur when the upper limb is pulled hard, stretching the plexus (e.g., football tackle) or by blows to the top of the shoulder.

Major Nerves of the Brachial Plexus

• Axillary Nerve:
  • Branches off the posterior cord.
  • Innervates the deltoid and teres minor muscles, and the skin and joint capsule of the shoulder.
• Musculocutaneous Nerve:
  • Major end branch of the lateral cord.
  • Supplies motor fibers to the biceps brachii, brachialis, and coracobrachialis muscles in the anterior arm.
  • Provides cutaneous sensation in the lateral forearm.
• Median Nerve:
  • Descends through the arm to the anterior forearm.
  • Innervates skin and most flexor muscles in the anterior forearm.
  • Innervates five intrinsic muscles of the lateral palm.
  • Activates muscles that pronate the forearm, flex the hand and fingers, and oppose the thumb.
  • Clinical Significance:
    • Median nerve injury makes it difficult to use the pincer grasp.
    • Frequent casualty of lacerations or compression (carpal tunnel syndrome).
• Ulnar Nerve:
  • Branches off the medial cord.
  • Descends along the medial aspect of the arm, swings behind the medial epicondyle, and follows the ulna along the medial forearm.
  • Supplies the flexor carpi ulnaris and the medial part of the flexor digitorum profundus.
  • Innervates most intrinsic hand muscles and the skin of the medial aspect of the hand.
  • Clinical Significance:
    • Vulnerable to injury due to its superficial course.
    • Striking the "funny bone" can cause the little finger to tingle.
    • Severe damage can lead to sensory loss, paralysis, muscle atrophy, and claw hand.
• Radial Nerve:
  • Largest branch of the brachial plexus; continuation of the posterior cord.
  • Wraps around the humerus and divides into a superficial branch and a deep branch.
  • Supplies the posterior skin of the limb along its entire course.
  • Innervates essentially all extensor muscles of the upper limb.
  • Controls muscles that extend the elbow, supinate the forearm, extend the hand and fingers, and abduct the thumb.
  • Clinical Significance:
    • Trauma to the radial nerve results in wrist drop.
    • Improper use of a crutch or "Saturday night paralysis" can compress the radial nerve.

Lumbosacral Plexus and Lower Limb

• Overlap: Sacral and lumbar plexuses overlap substantially; fibers of the lumbar plexus contribute to the sacral plexus via the lumbosacral trunk.
• Function: Serves mainly the lower limb, but also sends branches to the abdomen, pelvis, and buttock.

Lumbar Plexus

• Formation: Arises from spinal nerves L1-L4.
• Location: Lies within the psoas major muscle.
• Branches:
  • Proximal branches innervate parts of the abdominal wall muscles and the psoas muscle.
  • Major branches descend to innervate the anterior and medial thigh.
• Femoral Nerve:
  • Largest terminal nerve of this plexus.
  • Runs deep to the inguinal ligament to enter the thigh and divides into several large branches.
  • Motor branches innervate anterior thigh muscles (quadriceps), which are the principal thigh flexors and leg extensors.
  • Cutaneous branches serve the skin of the anterior thigh and the medial surface of the leg from knee to foot.
• Obturator Nerve:
  • Enters the medial thigh via the obturator foramen.
  • Innervates the adductor muscles.
• Clinical Significance:
  • Compression of the spinal roots of the lumbar plexus (e.g., by a herniated disc) can cause gait problems, pain, or numbness of the anterior and medial thigh.

Sacral Plexus

• Formation: Arises from spinal nerves L4-S4.
• Location: Lies immediately caudal to the lumbar plexus.
• Branches:
  • About a dozen named branches; half serve the buttock and lower limb, the others innervate pelvic structures and the perineum.
• Sciatic Nerve:
  • Largest branch of the sacral plexus; thickest and longest nerve in the body.
  • Supplies the entire lower limb, except the anteromedial thigh.
  • Composed of two nerves—the tibial and common fibular—wrapped in a common sheath.
  • Leaves the pelvis via the greater sciatic notch, courses deep to the gluteus maximus muscle, and enters the posterior thigh just medial to the hip joint.
  • Gives off motor branches to the hamstring muscles and the adductor magnus.
  • Divides into the tibial and common fibular nerves above the knee.
• Tibial Nerve:
  • Continues through the popliteal fossa and supplies the posterior compartment muscles of the leg and the skin of the posterior calf and sole of the foot.
  • Gives off the sural nerve, which serves the skin of the posterolateral leg.
  • Divides into the medial and lateral plantar nerves at the ankle, which serve most of the foot.
• Common Fibular Nerve (Common Peroneal Nerve):
  • Descends from its point of origin, wraps around the neck of the fibula, and divides into superficial and deep branches.
  • Innervates the knee joint, skin of the anterior and lateral leg and dorsum of the foot, and muscles of the anterolateral leg (the extensors that dorsiflex the foot).
• Gluteal Nerves (Superior and Inferior):
  • Innervate the buttock (gluteal) and tensor fascia lata muscles.
• Pudendal Nerve:
  • Innervates the muscles and skin of the perineum, and helps stimulate erection and control urination.
• Clinical Significance:
  • Injury to the proximal part of the sciatic nerve can impair the lower limbs in various ways depending on the nerve roots injured.
  • Sciatica, characterized by stabbing pain radiating over the course of the sciatic nerve, is common.
  • Transection of the sciatic nerve makes the leg nearly useless, leading to footdrop.
  • Injury to the tibial nerve results in a shuffling gait.
  • Compression of the common fibular nerve can cause footdrop.

Anterolateral Thorax and Abdominal Wall

• Arrangement: Only in the thorax are the ventral rami arranged in a simple segmental pattern corresponding to that of the dorsal rami.
• Intercostal Nerves: Ventral rami of T1-T12 course anteriorly, deep to each rib, as the intercostal nerves.
• Innervation: Supply the intercostal muscles, the muscle and skin of the anterolateral thorax, and most of the abdominal wall.
  • Give off cutaneous branches to the skin.
• Unusual Thoracic Nerves:
  • T1: Most fibers enter the brachial plexus.
  • T12: Lies inferior to the twelfth rib, making it a subcostal nerve.

Back

• Innervation: Dorsal rami innervate the posterior body trunk in a neat, segmented pattern.
• Each dorsal ramus innervates a strip of muscle and skin in line with its emergence point from the spinal cord.