15.Conduction

15.1 Threshold

• Definition of Threshold:

  • The critical level of depolarization needed to initiate an all-or-none action potential.

  • An action potential will not occur if the potential remains below this threshold.

• Voltage-Gated Sodium (Na+) and Potassium (K+) Channels:

  • These channels determine the threshold potential.

  • Dennis Noble (1966) interpreted Hodgkin-Huxley’s model, plotting leak potassium current (IK) and voltage-dependent sodium current (INa) against voltage.

  • Assumed voltage-dependent delayed rectifier K+ channel influence was minimal.

• Mechanics of Threshold:

  • Starting at resting potential (Vr), a small depolarization can result in an outward current due to the change in driving force.

  • If the inward current from INa equals IK, the net current (I) becomes zero, marking a "tipping point".

  • Thermal noise can cause further depolarization, pushing the potential into a region where net inward current occurs, creating a positive feedback loop of depolarization.

  • Spike Threshold:

    • When total inward current equals total outward current, defining spike threshold.

15.2 Refractory Period

• Definition of Refractory Period:

  • Time after neuron fires when a stimulus cannot evoke a response due to Na+ channel inactivation.

  • Divided into absolute refractory period (no spike response) and relative refractory period (reduced spike amplitude possible).

• Mechanics of Refractory Period:

  • Absolute refractory period lasts for about 5 ms; no action potential can be triggered regardless of stimulus strength.

  • Relative refractory period follows, where a stronger stimulus can trigger a weaker spike due to the de-inactivation of Na+ channels.

  • During these periods, spike threshold is dynamically elevated.

• Anode Break Excitation:

  • Observed when an inhibitory postsynaptic potential (IPSP) raises the Na+ channel state, effectively increasing inward current during the decay of the IPSP.

• Firing Types:

  • Class 1 (Type 1) Firing:

    • Exhibits a continuous relationship between frequency and stimulus current.

  • Class 2 (Type 2) Firing:

    • Not continuous, characterized by a fixed attractor state preventing firing despite reaching threshold.

  • Type 1 relies on a continuous range of input versus Type 2 featuring a distinct preferred firing frequency.

15.3 Spike Propagation/Conduction

• Action Potential Propagation:

  • Action potentials move rapidly along axons from the soma to synaptic terminals.

  • Use of the wave equation optimizes understanding of conduction velocity.

• Conduction Velocity Dependencies:

  • Conduction velocity is proportional to the radius of the axon.

  • Dependency factors include intracellular resistivity and membrane capacitance.

15.4 Qualitative Description of Action Potential Propagation

• Current Flow Mechanism:

  • Inward current generated at an action potential site spreads bidirectionally but is limited by inactivation of Na+ channels.

  • Net inward current precedes the spike, leading to action potential propagation as a wave.

• Factors Affecting Propagation:

  • Rate of propagation relies on longitudinal current flow and membrane capacitance.

15.5 Myelinated Axon

• Myelination:

  • Axons are wrapped in myelin by oligodendrocytes (CNS) or Schwann cells (PNS); nodes of Ranvier are regions where the axon is exposed.

  • Myelin sheath increases conduction velocity by enhancing membrane resistance and reducing capacitance.

  • Resulting node-to-node transmission is termed saltatory conduction.

• Conduction Velocity in Myelinated Axons:

  • Demonstrated reduction in capacitance and increased resistance enhance speed of action potential propagation.

  • Different fiber types exhibit varying conduction speeds based on diameter and myelination.

  • Examples include:

    • Squid Giant axon: 350-500 µm, 20-25 m/s.

    • Cockroach Giant Interneuron: 60 µm, 7 m/s.

    • Sensory Nerve Fiber: 1-2 µm, 1 m/s.