NRS 250 Final - Nerve Signaling

Compare and contrast passive and active signaling 

• Passive signaling involves small, local voltage changes (e.g., at synapses) that decay as they spread; no ion channel amplification is involved. 

• Active signaling (action potentials) involves voltage-gated ion channels that regenerate the signal, allowing it to travel long distances without decrement. 

• Passive signals are essential for neural computation in dendrites and cell bodies. 

• Active signals are essential for long-distance communication, especially in large or long neurons. 

• Passive signaling decreases with distance like ripples in water, while active signals are all-or-none and self-propagating. 

• Action potentials require a threshold to trigger; passive potentials do not. 

What happens during an action potential? What are its key features? 

• Depolarization Phase: Triggered by threshold voltage, Na⁺ channels open rapidly, causing inward Na⁺ flow. 

• Overshoot: Membrane potential becomes positive inside the cell. 

• Repolarization: Na⁺ channels inactivate; K⁺ channels open, K⁺ flows out. 

• After-hyperpolarization: Membrane becomes more negative than resting due to extra K⁺ outflow. 

• Refractory Periods: 

• Absolute: Na⁺ channels inactivated; no new AP can occur. 

• Relative: Some Na⁺ channels reset; stronger stimulus needed for AP.

How are these features accounted for by Na⁺ and K⁺ channel properties? 

• Na⁺ Channels: Two gates (activation and inactivation). 

• Activation is fast, allowing Na⁺ in. 

• Inactivation is slower, stopping Na⁺ flow—key to brief depolarization. 

• K⁺ Channels: One slower gate; open as Na⁺ channels inactivate. 

• Stay open longer, leading to repolarization and after-hyperpolarization. 

• Timing differences between Na⁺ and K⁺ channel gating explain the shape and sequence of the action potential phases. 

How does the action potential propagate? 

• Initiated at the axon hillock; local depolarization spreads to adjacent areas. 

• Voltage-gated Na⁺ and K⁺ channels open in the next segment, regenerating the signal. 

• This continues like dominoes falling down the axon until it reaches the terminal. 

• Action potentials only move forward due to the refractory state behind the signal—prevents backward activation. 

What influences the speed of propagation? 

• Not affected much by ion channel gating speed or current speed. 

• Main factor: Distance the depolarization spreads passively before needing to be regenerated. 

• Two strategies to increase speed: 

• Larger axon diameter: Less internal resistance, allows further passive spread. 

• Myelination (noted elsewhere in general neuroscience): Increases passive spread and speeds conduction via saltatory conduction.