Voltage Clamp Studies of Action Potentials

Action Potential Currents

• Voltage clamp reveals that during the action potential, there are distinct phases of ionic currents:

  1. Initial inward current (a)

  2. Delayed outward current (b)

• These currents can be observed by voltage clamping above the threshold level:

  • The inward current is primarily carried by Na+ ions:

  1. When clamped to the equilibrium potential of Na+ (E_Na), no inward current is observed.

  2. If the voltage is clamped to a value more positive than E_Na, two outward currents are evident.

  • The outward current is carried by K+ ions:

  1. This current begins after a slight delay in response to the voltage change.

Conductance Calculations

• By measuring the currents carried by ions, along with known values of membrane potential (Vm) and equilibrium potential of ions (Eion), the conductance (g) for the ion can be derived:

  1. Formula:
     $g<em>{ion} = \frac{I</em>{ion}}{(V<em>m - E</em>{ion})}$

  2. This conductance provides insights into the membrane's permeability to that specific ion.

Pharmacological Study of Ion Channels

• Certain drugs serve as poisons that aid in studying the function of ion channels:

  • TTX (Tetrodotoxin): blocks Na+ channels.

  • TEA (Tetraethylammonium): blocks K+ channels.

  • The application of either of these allows for detailed study of the unblocked channels, enabling the investigation of the ionic currents specifically.

Example from Literature: Aconite and Its Effects

• In "Harry Potter and the Sorcerer’s Stone", a character mentions monkshood (aconite) which has historical context as a toxin for wolves.

• Aconitine, a potent neurotoxin, has various effects on neuronal activity:

  1. Duration of the action potential is markedly prolonged.

  2. Threshold for action potential is lowered, allowing it to occur at a more negative membrane potential than normal.

  3. Under voltage clamp conditions:

  • The initial inward current occurs more rapidly than usual.

  • The late current recorded is also an inward current.

  1. When both aconitine and TTX are utilized on a voltage clamped axon, the usual late outward current can still be observed.

  • Visual representation of action potentials before and after application of aconitine would illustrate differences and must be clearly labeled to show:

    • Y-axis: Membrane potential (V_m)

    • X-axis: Time

    • Observations to indicate:

    • The action potential rises sooner and faster.

    • The action potential lasts longer.

Hypothetical Scenarios of Observations

• For the observations related to aconitine's effects on the action potential:

  1. Possibility 1: Na+ channels do not close, or K+ channels are delayed in their opening.

  2. Possibility 2: Na+ channels open more readily, or K+ channels remain closed during the action potential.

  3. Possibility 3: Na+ channels open quickly and do not close, or K+ channels fail to initiate opening appropriately.

  4. Conclusive Finding: This indicates that the K+ channels are not affected by aconitine.

Recording Challenges and Solutions

• It is noted that performing voltage clamp experiments using two electrodes into one cell is challenging due to potential interference and accuracy issues:

  • This challenge can be addressed using the patch clamp technique:

  1. Patch Clamp Method:

     • Forms a high-resistance seal around a specific patch of the cell membrane.

     • This allows for the recording of ion flow exclusively through a limited number of channels situated in that membrane area.

• The patch clamp recording focuses on measuring ion currents similarly to voltage clamp:

  • Multiple records are compiled to obtain a comprehensive measure of the ionic current maneuvers across the membrane.

  • By employing TTX and TEA, one can distinctly study the currents from Na+ and K+ ions isolated from one another.

  • Patch clamp results align with voltage clamp findings:

  1. The initial inward current is indeed carried by Na+ ions.

  2. The delayed outward current is contributed to by K+ ions.