Study Notes on Action Potential Propagation and Synaptic Transmission

Action Potential Propagation

• Propagation Mechanism

  • Action potential moves away from the initiation site due to:

  • Refractory period of Na⁺ channels:

    • This period prevents immediate re-firing of the neuron, ensuring unidirectional signaling.

  • Diffusion of incoming Na⁺ ions:

    • The influx of Na⁺ during the action potential induces a local depolarization that triggers subsequent depolarization in neighboring membrane patches.

• Speed of Propagation

  • Propagation of action potentials is faster with lower resistance in the axon:

  • How Membrane Potential (Vm) Declines:

    • Vm decreases as it travels down the axon due to various resistive and capacitive properties.

  • Factors affecting propagation speed:

    1. Internal Resistance (rI):

    • Refers to how easily ions can flow down the axon.

    1. Membrane Resistance (rm):

    • Associated with leakiness of the axonal membrane.

    1. Membrane Capacitance (Cm):

    • Reflects the ability of the membrane to store charge and how many ions get stuck while charging the capacitors in the membrane.

Length and Time Constants

• Length Constant (λ):

  • Indicates how far a change in Vm spreads before its amplitude declines to 1/e.

  • The equation for length constant is given by:
    $\lambda = \sqrt{\frac{r<em>m}{r</em>i}}$

• Time Constant (τ):

  • Describes how long the Vm change will last in a given patch of membrane.

  • The time constant is computed as:
    $\tau = C<em>m r</em>m$

  • Note: Neurons generally have similar values for Cm but may vary considerably in rm.

Role of Myelin

• Myelin increases the speed at which action potentials can travel by:

  • Decreasing Capacitance:

  • The myelin sheath reduces the capacity for charge accumulation on the membrane.

  • Absence of Ion Channels Underneath Myelin:

  • This effectively increases rm, providing greater resistance to current leak.

  • Concentration of Voltage-Activated Na⁺ Channels:

  • These channels are specially concentrated at nodes of Ranvier, optimizing the propagation of action potentials through saltatory conduction.

Synaptic Transmission

• At the axon terminal, action potentials must transmit to the next neuron, a process referred to as synaptic transmission.

Electrical Synapses

• Electrical synapses utilize gap junctions:

  • Ions pass through pores in connexons:

  • Connexons consist of six protein subunits arranged in a circle, creating a central pore.

  • Characteristics of electrical synapses:

  • Fast:

    • Enables rapid signal transmission compared to chemical synapses.

  • Reliable:

    • Provides consistent signaling with minimal interruption.

  • Bidirectional Current Passage:

    • Many can transmit signals in either direction.

  • Hyperpolarizing Signals:

    • They are capable of transmitting hyperpolarizing signals, which allows for more complex signaling patterns.

Exceptions to Electrical Synapses

• There are notable exceptions with certain electrical synapses:

  • Rectifying Electrical Synapses:

  • These can preferentially allow current to pass in one direction only.

  • Conditional Closure:

  • Some synapses may close in response to changing internal conditions, affecting their connectivity and signaling capability.

Utility of Electrical Synapses

• Electrical synapses serve critical functions:

  • Coordinating Groups of Neurons:

  • Facilitates synchronized activity across networks of neurons.

  • Providing Fast and Reliable Signaling:

  • Essential for functions such as reflex and escape responses, where speed is crucial to survival.