Nerve Plexuses and Reflex Arcs

Exam Information

  • Final Exam Information for Section 04 (Wednesday Section)

    • Date: December 22nd, 2025 (Monday)

    • Time: 12:00 - 3:00 PM

    • Location: Usual classroom

    • Duration: Up to 2 hours

    • Content Coverage: Majority of questions from topics after Exam #2

  • Final Exam Information for Section 03 (Monday Section)

    • Date: December 22nd, 2025 (Monday)

    • Time: 12:00 - 3:00 PM

    • Location: Usual classroom

    • Duration: Up to 2 hours

    • Content Coverage: Majority of questions from topics after Exam #2

Nerve Plexuses

Overview

  • Plexuses: Networks of nerves originating from ventral rami of the spinal nerves.

  • Significance: They provide innervation to specific body regions.

Types of Plexuses

  1. Cervical Plexus

    • Spinal Nerves: C1 - C4

  2. Brachial Plexus

    • Spinal Nerves: C5 - T1

  3. Lumbar Plexus

    • Spinal Nerves: T12 - L4

  4. Sacral Plexus

    • Spinal Nerves: L4 - S4

  5. Lumbosacral Plexus

    • Combination of lumbar and sacral plexuses.

Gross Anatomy of the Spinal Cord

Anatomy Highlights

  • Cervical Enlargement: A region of enlargement in the cervical region.

  • Lumbosacral Enlargement: A region of enlargement that corresponds to lumbar and sacral nerves.

  • Conus Medullaris: The tapered end of the spinal cord, marking the end of the spinal canal.

  • Cauda Equina: A collection of nerve roots that continue from the conus medullaris.

  • Filum Terminale: A fibrous extension that anchors the spinal cord to the coccyx.

Spinal Nerves

  • Cervical Nerves: C1, C2, C3, C4, C5, C6, C7, C8

  • Thoracic Nerves: T1 through T12

  • Lumbar Nerves: L1 through L5

  • Sacral Nerves: S1 through S5

  • Coccygeal Nerve: Co1

Nerve Structures and Fiber Types

Nerve Structures

  • Dorsal Root: Contains sensory fibers only.

  • Ventral Root: Contains motor fibers only.

  • Spinal Nerve: Contains both motor and sensory fibers.

  • Dorsal Ramus of Spinal Nerve: Contains both fibers (motor and sensory).

  • Ventral Ramus of Spinal Nerve: Contains both fibers.

Fiber Types

  • You should know the type of fibers associated with each nerve structure as it relates to motor versus sensory function.

Major Nerves of the Plexuses

Cervical Plexus (C1-C5)

  • Innervation: Muscles of the neck and diaphragm.

  • Key Nerves:

    • Lesser Occipital Nerve: C2; Sensory input from neck and scalp.

    • Great Auricular Nerve: C2-C3; Sensory input from neck and ear.

    • Phrenic Nerve: C3-C5; Motor for diaphragm.

Brachial Plexus (C5-T1)

  • Innervation: Muscles of the pectoral girdle and upper limbs.

  • Key Nerves:

    • Axillary Nerve: Innervates deltoid and teres minor; sensory from shoulder skin.

    • Musculocutaneous Nerve: Flexor muscles of the arm; sensory from lateral forearm skin.

    • Median Nerve: Flexor muscles of forearm; sensory from anterolateral hand skin.

    • Ulnar Nerve: Flexor carpi ulnaris; sensory from medial hand skin.

    • Radial Nerve: Extensor muscles of arm and forearm; sensory from posterior limb skin.

Lumbar Plexus (T12-L4)

  • Major Nerves:

    • Iliohypogastric Nerve: Motor for abdominal muscles; sensory for lower abdomen and buttocks.

    • Femoral Nerve: Motor for quadriceps, sensory for anterior thigh and medial leg.

    • Obturator Nerve: Motor for inner thigh muscles; sensory from medial thigh skin.

Sacral Plexus (L4-S4)

  • Major Nerves:

    • Sciatic Nerve: Largest nerve, motor for posterior thigh and all of the lower leg.

    • Pudendal Nerve: Motor to perineal muscles, sensory from the external genitalia.

    • Superior/Inferior Gluteal Nerves: Innervation for gluteal muscles.

Clinical Applications

Common Nerve Injuries

  1. Radial Nerve Damage: Can cause "wrist drop" due to loss of extensor function.

  2. Median Nerve Issues: Results in wasting of thumb side muscles.

Dermatomes

  • Definition: Areas of skin supplied by specific spinal nerves.

  • Significance: Understanding dermatomes is vital for diagnosing nerve injuries and conditions such as shingles.

  • Diagram: Illustrates dermatome maps for spinal cord regions including cervical, thoracic, lumbar, and sacral areas.

Reflexes

Reflex Fundamentals

  • Reflex: An immediate, involuntary motor response to a stimulus.

  • Reflex Arc: The neural circuitry that connects the sensory stimulus to the effector response.

Steps of a Reflex Arc

  1. Activation of a sensory receptor.

  2. Transmission of the signal via a sensory neuron.

  3. Information processing in the CNS.

  4. Activation of a motor neuron.

  5. Response by the effector.

Examples of Reflexes

  • Stretch Reflex: Elicited by muscle spindles; includes pathways such as the patellar reflex.