Neurophysiology 2019

NEUROPHYSIOLOGY OF LOCAL ANESTHESIA

  • Instructor: Darlene O'Brien RDH, MSDH

  • Institution: Springfield Technical Community College

  • Year: 2019

THE NERVOUS SYSTEM

  • Central Nervous System (CNS): Comprises the brain and spinal cord.

  • Peripheral Nervous System (PNS):

    • Autonomic Nervous System: Involuntary control (e.g., heart rate, digestion).

    • Somatic Nervous System: Voluntary control (e.g., muscle movement).

  • Sensory Information:

    • Cranial Nerves transmit information.

    • Afferent Nerves: Carry sensory information to the CNS.

    • Efferent Nerves: Relay motor commands from the CNS.

SENSORY NERVE ANATOMY

  • Neuron: Transmits messages between the CNS and body parts.

  • Dendrites:

    • Free nerve endings located on the distal segment.

    • Respond to sensory stimuli.

  • Axon:

    • Transmits nerve impulses.

    • Encased in a cylinder of neural cytoplasm known as axoplasm.

  • Cell Body:

    • Provides metabolic support and does not transmit impulses.

SENSORY NERVE ANATOMY CONTINUED

  • Myelin Sheath:

    • Layer of lipoprotein covering myelinated nerve fibers.

    • Formed by Schwann cells.

  • Nodes of Ranvier:

    • Constrictions located between Schwann cells.

    • Allow for faster impulse conduction in myelinated fibers.

IMPULSE SPREAD

  • Unmyelinated Nerves: Slow conduction method.

  • Myelinated Nerves:

    • Conduct impulses via saltatory conduction (bypassing nodes).

    • Requires 8-10mm coverage for local anesthesia to block properly.

NERVE ANATOMY

  • Fasciculi: Bundles of neurons surrounded by connective tissue.

  • Mantle Fibers:

    • Outer nerve bundles.

  • Core Fibers:

    • Center nerve bundles.

NERVE CONDUCTION

  • Resting Potential: -70mV.

  • Stimulus: Begins the initial phase of depolarization.

  • Firing Threshold: Requires a decrease of 15mV to activate.

  • Phases:

    • Depolarization:

      • Changes from -70mV to +40mV.

      • Occurs within 0.3 ms.

    • Repolarization: Returns to -70mV in 0.7 ms.

  • The entire cycle occurs in 1 ms.

DEPOLARIZATION

  • Increased permeability to Na+ ions.

  • Rapid influx of Na+: Passive movement requiring no energy.

  • Results in depolarization from -70mV to +40mV.

REPOLARIZATION

  • Na+ channels close and Na+ ions exit: Active transport using ATP via the sodium pump.

  • K+ channels open; K+ ions flow into axoplasm.

  • The nerve fiber then enters a resting state.

ACTION OF LOCAL ANESTHETICS

  • Definition: Chemicals that block nerve impulse generation and conduction.

  • Mechanism:

    • Must diffuse through the nerve cell membrane to bind with Na+ channels.

ACTION OF LOCAL ANESTHETICS CONTINUED

  • Specific Receptor Site Theory:

    • Binds to Na+ channel, decreasing Na+ permeability.

    • Leads to little or no depolarization, effectively blocking nerve conduction.

  • Site of Action: Primarily at the Nodes of Ranvier.

  • Diffusion Process: Through the cell membrane to bind to Na+ channels.

PROPERTIES OF LOCAL ANESTHETICS

  • Majority are tertiary amides.

  • Structure includes:

    • Aromatic lipophilic region.

    • Intermediate chain: Ester or Amide.

    • Hydrophilic amine structure, which is water-soluble.

    • Few are secondary amides (e.g., Prilocaine).

LOCAL ANESTHETICS IN STERILE SOLUTION

  • Exists in two forms: uncharged (weak base) and charged (salt).

  • pH influences the predominant form.

  • Local anesthetic action relies on:

    • Diffusion through nerve sheath.

    • Binding at the receptor site.

FACTORS AFFECTING LOCAL ANESTHETIC DURATION

  • pH: Acidic tissues decrease effectiveness.

  • pKa: Higher pKa slows onset due to less uncharged form (RN).

  • Onset of Action for Common Agents:

    • Mepivacaine: pKa 7.7, onset 2-4 mins.

    • Lidocaine: pKa 7.7, onset 2-4 mins.

    • Articaine: pKa 7.8, onset 2-4 mins.

    • Bupivacaine: pKa 8.1, onset 5-8 mins.

FACTORS INFLUENCING LOCAL ANESTHETIC ACTION

  • Absorption into non-neural tissue.

  • Dilution by interstitial fluids.

  • Local vascularity affects distribution.

  • Lipid Solubility: Facilitates membrane penetration.

  • Protein Binding: Influences duration of action.

  • Size/type of nerve.

INDUCTION OF LOCAL ANESTHESIA

  • Diffusion: Movement from extra-neural site to the nerve.

  • Greater initial concentration of LA leads to better diffusion and rapid onset.

  • Mantle Bundles: Innervate proximal regions sooner due to higher LA concentration.

  • Core Bundles: Innervate distal regions more slowly and need adequate volume/concentration of LA.

RECOVERY FROM LOCAL ANESTHESIA

  • Reverse Diffusion:

    • Mantle bundles lose anesthesia first (proximally innervated areas).

    • Core bundles lose anesthesia more gradually, resulting in slower recovery.

REINJECTING LOCAL ANESTHETICS

  • Profound Anesthesia:

    • Mantle fibers have lower LA concentration compared to core fibers.

  • Reinjecting LA increases overall concentration for faster profound anesthesia.

  • Tachyphylaxis: Increased tolerance due to repeated administration, more likely if nerve function returns temporally.

  • Factors affecting include edema, hemorrhage, clot, transduction, hypernatremia, and decreased tissue pH.

CONCLUSION

  • Presentation designed by Stephanie Zarse, RDH, MSDH.

  • Revised by Darlene O'Brien RDH, MSDH.

  • Source: Springfield Technical Community College

  • Year: 2019.