Action Potential Characteristics

• Changes occur with action potential, which can be broken down into types of membrane potential, characters of action potential, changes in the nerve during action potential and excitability.

Types of Membrane Potential

• Resting Membrane Potential (RMP).
• Action Potential (AP).
• Local Excitatory State (graded potential).

Characters of Action Potential in the Nerve Fiber

• Action potential is a wave that has a strength, duration, and velocity of conduction.
• Its conduction is nondecremental, meaning it propagates from one site to the next without a decrease in intensity.
• It obeys the "All-or-none" rule: an action potential in a single nerve fiber either occurs or does not occur at all.

Local Excitatory State (Graded Potential)

• It is a localized area of depolarization due to stimulation of the nerve fiber by ineffective stimuli (subthreshold) that are not strong enough to reach the firing level and spread along the nerve fiber.
• It is a type of graded potential with a short duration.
• It is localized to the area of stimulation.
• Its strength depends on the strength of the stimulus and does not obey the all-or-none law and has no threshold.
• It can be summated.

Changes in the Nerve During Action Potential & Conduction

• Electrical changes: Occur in the form of spike potential and after potentials.
• Metabolic changes:
  • At rest: Metabolic reactions occur at a low rate, producing enough ATP to maintain the polarized state.
  • During conduction of a nerve impulse: More energy is needed, which is derived from the breakdown of ATP and creatine phosphate (CP).
  • Thermal changes:
    • During Rest: Heat production at rest is very low due to a low resting metabolic rate.
    • During Activity: Heat production is markedly increased and released in 2 stages.
    • Initial Heat: Coincides with the generation and propagation of the action potential; it is due to anaerobic breakdown of ATP & Creatine phosphate (CP).
    • Delayed Heat: Is 30 times the initial heat and remains for 30 minutes; it is due to metabolic reactions needed to reform ATP utilized during action potential.
• Excitability changes:
  • During the transmission of a nerve impulse, the excitability of the nerve fibers passes through the following phases: Absolute Refractory Period (ARP); Relative Refractory Period (RRP); Supernormal phase of excitability; Subnormal phase of excitability.

Excitability Changes

• During transmission of nerve impulse the excitability of the nerve fibers passes in the following phases:

Absolute Refractory Period (ARP)

• The period during which the excitability is completely abolished (zero).
• No stimulus can excite a nerve fiber, whatever its strength.
• It corresponds to the ascending limb of a spike potential (depolarization) and the early part of the descending limb (early 1/3 of repolarization).
• During depolarization, all Na+ channels are opened by the first stimulus.
• At the start of repolarization, the Na+ channels are closed.

Relative Refractory Period (RRP)

• Period during which the excitability is partially recovered.
• A stronger-than-normal stimulus is needed to excite the nerve.
• It corresponds to the remaining part of repolarization until the start of the negative afterpotential (later 2/3 of repolarization).
• Continuing K efflux makes it harder to depolarize the membrane.

Supernormal Phase of Excitability

• The period during which excitability is higher than normal.
• A weaker stimulus than normal is needed to excite the nerve.
• Corresponds to the negative afterpotential.
• Slow K efflux.

Subnormal Phase of Excitability

• Period during which the excitability of the nerve is below normal.
• A stronger stimulus than normal is needed to excite the nerve.
• Corresponds to the positive afterpotential.
• Due to prolonged K+ efflux due to the delayed closure of its channels (hyperpolarization).

Factors Affecting Excitability and Conductivity of Nerve Fibers

• These factors can be physical, chemical, or electrical in nature.

Physical Factors

• Thermal:
  • Cooling reduces excitability and conductivity, as well as metabolic reactions needed for the Na-K pump.
  • Warming increases excitability and conductivity.
• Mechanical: Deep pressure decreases the excitability and conductivity of nerve fibers.

Chemical Factors

• Local anesthetic drugs:
  • Examples include cocaine and novocaine.
  • Block voltage-gated Na+ channels, reducing membrane permeability to Na+ ions, leading to decreased depolarization.
• Ca++ ions:
  • Increased Ca++ ions block Na channels, decreasing membrane permeability to Na+ ions, thus reducing nerve fiber excitability.
  • Lowered Ca++ ions open Na channels, increasing membrane permeability to Na+ ions, thus increasing nerve fiber excitability.
• Na+ ions:
  • Increased Na+ ions increase excitability by facilitating the process of depolarization.
  • Decreased Na+ ions reduce excitability by delaying the process of depolarization.
• K+ ions:
  • Increased K+ ions in the extracellular fluid decrease K+ efflux, decreasing RMP and increasing excitability.
  • Decreased K+ ions in the extracellular fluid increase the efflux of K+ ions to the outside, producing hyperpolarization and decreasing excitability.
• Other chemical factors:
  • O2 lack and CO2 excess decrease excitability.
  • Alkalinity decreases free Ca++ and increases excitability.
  • Acidity increases free Ca++ and decreases excitability.

Electrical Factors

• Electrotonus: Means the electrical changes induced in the nerve membrane by stimulation with a constant galvanic current with subthreshold intensity.
• Anelectrotonus:
  • Refers to the changes occurring at the region of the anode.
  • RMP increases through increased positive charge on the membrane's outer surface, resulting in localized hyperpolarization.
  • It is associated with decreased excitability.
  • Stronger stimuli (more than the threshold) are required to excite nerve fibers.
  • A strong anelectrotonus can abolish excitability completely and cause nerve block.
• Catelectrotonus:
  • The changes that occur at the region of the cathode.
  • RMP decreases via negative charges added the membrane's outer surface, leading to localized depolarization.
  • It is associated with increased excitability.
  • Weaker stimuli (subthreshold) that can now excite fibers.

Nerve Block

• Failure of nerve impulse conduction along a nerve fiber.
• Methods include:
  • Physical factors: application of cold.
  • Mechanical factors: application of pressure or damaging a part of the nerve.
  • Chemical factors (membrane stabilizers, ionic changes): The ionic changes that reduce the nerve excitability, along with local anesthetic drugs (e.g., cocaine and novocaine), which block Na+ channels.
  • Electrical causes: strong anelectrotonus.