Neurophysiology 2019

NEUROPHYSIOLOGY OF LOCAL ANESTHESIA

THE NERVOUS SYSTEM

• Components:

  • CNS (Central Nervous System): Brain, spinal cord

  • PNS (Peripheral Nervous System):

    • Autonomic: Involuntary control

    • Somatic: Voluntary control

• Sensory Pathways:

  • Cranial Nerves: Transmit sensory information

  • Afferent Nerves: Carry sensory information to CNS

  • Efferent Nerves: Carry motor information from CNS

SENSORY NERVE ANATOMY

• Neuron:

  • Transmits messages between CNS and body parts

  • Dendrite:

    • The distal segment with free nerve endings that respond to stimulation

  • Axon:

    • Transmits impulses; encased in neural cytoplasm (axoplasm)

  • Cell Body:

    • Provides metabolic support, not involved in impulse transmission

MYELIN SHEATH AND NERVE CONDUCTION

• Myelin Sheath:

  • Lipoprotein layer covering myelinated nerve fibers

  • Specialized Schwann cell form

• Nodes of Ranvier:

  • Constrictions between Schwann cells; 0.5 to 3 mm apart

  • Exposed nerves; enables rapid impulse conduction

IMPULSE SPREAD

• Unmyelinated Nerves:

  • Slow conduction method

• Myelinated Nerves:

  • Utilize saltatory conduction (leaps from node to node) for faster impulses

  • If conduction blocked at one node, can skip to the next

  • Local anesthesia must cover 8 to 10 mm for effective blockage

NERVE CONDUCTION MECHANISM

• Voltage Changes:

  • Resting potential: -70 mV

  • Initial depolarization phase: Need 15 mV decrease

  • Firing threshold: Rapid depolarization to +40 mV

  • Process timeline:

    • Depolarization: 0.3 msec

    • Repolarization: 0.7 msec

DEPOLARIZATION AND REPOLARIZATION

• Depolarization Process:

  • Increased Na+ permeability

  • Rapid influx causing potential increase from resting (-70 mV) to +40 mV

• Repolarization Process:

  • Na+ channels close; active transfer of Na+ out

  • K+ channels open; K+ influx

  • Returns nerve to resting state

ACTIONS OF LOCAL ANESTHETICS

• Definition:

  • Chemicals that prevent generation and conduction of nerve impulses

• Mechanism:

  • Must diffuse through nerve cell membrane, binding to Na+ channel

  • Decreases Na+ permeability, leading to little or no depolarization

PROPERTIES OF LOCAL ANESTHETICS

• Typical Structure:

  • Composed of aromatic lipophilic part, intermediate chain (ester/amide), and amine hydrophilic part

• Main Types:

  • Most are tertiary amides; some are secondary amides like Prilocaine

LOCAL ANESTHETIC STATES

• When Added to Solution:

  • Exists in both uncharged (weak base) and charged (salt) forms

• pH Influence:

  • Tissues' pH affects predominant form of LA

• Action Factors:

  • Include diffusion through nerve sheath and binding at receptor sites

FACTORS AFFECTING LOCAL ANESTHESIA DURATION

• pH Levels:

  • Acidification decreases effectiveness

• pKa:

  • Higher pKa means slower onset due to fewer RN molecules available to diffuse through nerves

• Examples:

  • Mepivicaine (7.7, 2-4 min), Lidocaine (7.7, 2-4 min), Bupivacaine (8.1, 5-8 min)

INFLUENCING FACTORS OF LA ACTION

• Absorption: Into non-neural tissues

• Lipid Solubility: Affects membrane penetration

• Vascularity: Affects dilution and distribution

• Protein Binding: Determines duration of action based on type and size of nerve

INDUCTION OF LOCAL ANESTHESIA

• Diffusion Process:

  • From extra neural site to nerve

  • Higher initial concentration leads to faster onset

  • Mantle vs. Core Bundles:

    • Mantle fibers innervate proximal regions first, while core fibers innervate distal points

RECOVERY FROM LOCAL ANESTHESIA

• Reverse Diffusion:

  • Mantle bundles lose anesthesia faster than core bundles

  • Disparity in recovery time between proximal and distal innervations

REINJECTING LOCAL ANESTHETICS

• Profound Anesthesia:

  • Reinjection can increase concentration, leading to rapid onset

  • Tachyphylaxis: Tolerance increases with repeated doses; more likely if pain sensation returns