2 Nervous System: Transmembrane Potentials and Action Potentials

Nervous System Chapter 7 Study Notes

Transmembrane Potentials

• Definition: Neurons can generate, maintain, and transmit electrical impulses (action potentials) by changing the concentration of ions across the cell membrane.

• Voltage: The voltage of a neuron indicates the difference in ion concentration across the membrane, which varies based on the neuron's activity.

• Resting Membrane Potential (RMP):

  • Definition: The voltage difference when the neuron is not sending a signal.

  • Typical value: $-70 ext{ mV}$

Changes in Transmembrane Potential During Action Potential

1. Voltage Changes: When sending an action potential, the membrane potential changes as follows:

   • Increases

   • Decreases

   • Returns to RMP

2. Causes of Changes in Membrane Potential:

   • Increase in Voltage: Influx of positive ions.

   • Decrease in Voltage: Stopping the influx of positive ions and releasing positive ions.

   • Return to RMP: Restoration of original ion levels.

Phases of Action Potential

• 1. Depolarization:

  • Process: Influx of Na+ ions, leading the membrane potential to become more positive and approach $0 ext{ mV}$.

• 2. Overshoot:

  • Note: Constant Na+ influx continues, pushing the membrane potential further away from $0 ext{ mV}$.

• 3. Repolarization:

  • Action: Efflux of K+ ions occurs, becoming more negative and moving closer to the RMP of $-70 ext{ mV}$.

• 4. Hyperpolarization:

  • Condition: Excess K+ is released, resulting in a membrane potential more negative than RMP (approximately $-80 ext{ mV}$).

• 5. Slight Depolarization:

  • Definition: A small influx of K+ ions occurs to return the membrane to RMP.

Action Potentials and Protein Channels

• Neurons utilize three primary types of intermembrane protein channels to regulate changes in membrane permeability:

  • 1. Voltage-Gated Na+ Channels

  • 2. Voltage-Gated K+ Channels

  • 3. Leakage or “Leaky” K+ Channels

• Importance: These channels can physically change their configurations between open and closed states without denaturing.

Structural Components of Neurons

1. Dendrites: Receive action potentials or respond to specialized stimuli.

2. Soma: Generates graded potentials to promote or inhibit depolarization.

3. Axon Hillock: Origin point for all-or-none action potentials.

4. Axon: Responsible for the propagation of all-or-none action potentials.

5. Axon Terminals and Synapses: Relay action potentials to another neuron or an effector.

Resting Membrane Potential (RMP)

• At RMP of $-70 ext{ mV}$:

  • Voltage-Gated Na+ and K+ Channels: Remain closed/active.

  • Leaky K+ Channels: Always open, help stabilize RMP, and expedite repolarization post-depolarization.

Triggering Depolarization (The "Initial Spark")

1. An excitatory action potential arriving from the soma reaches the axon hillock and achieves threshold potential.

2. Voltage-gated Na+ channels open.

3. Na+ ions influx due to concentration gradient, causing membrane depolarization:

   • Before: $-70 ext{ mV}$

   • After: $+30 ext{ mV}$

Triggering Repolarization

• As the Na+ ions influx slows down due to reaching equilibrium potential:

1. Voltage-gated Na+ channels close and become inactive.

2. Voltage-gated K+ channels open, allowing K+ to leave the cell.

3. Membrane repolarizes, moving closer to RMP.

Returning to Resting Membrane Potential

• After repolarization, K+ diffusion through leaky channels continues. The membrane potential eventually returns to RMP:

1. Voltage-gated K+ channels close beyond RMP, resulting in hyperpolarization.

2. RMP is restored as K+ exits and Na+ channels reset to a closed/active state.

Propagation of an Action Potential

• The membrane potential is maintained across myelinated regions of the neuron. No ion channels are present at the Nodes of Ranvier, leading to potential changes only in those areas for efficient propagation.

Mechanism of Action Potential Propagation (Saltatory Conduction)

• Action potentials are regenerated at Nodes of Ranvier, allowing for rapid transmission along the axon. This ensures signal strength and directionality.

• Ensures movement in one direction due to the inactivation of previously activated Na+ channels during the refractory phase.

Absolute & Relative Refractory Periods

• Absolute Refractory Period:

  • Condition: Voltage-gated Na+ channels are closed/inactive; a new stimulus has no effect.

• Relative Refractory Period:

  • Condition: Voltage-gated Na+ channels transition to closed/active; a new stimulus can cause them to open but must be strong.

Refractory Periods and Frequency

• Frequency of Action Potential: Increases with the magnitude of the initial stimulus, yet cannot increase indefinitely.

  • Action potential response to varying stimulus magnitude:

  • Low stimulus magnitude → Low action potential frequency.

  • High stimulus magnitude → High action potential frequency.

Summary of Action Potentials and Stimuli:

• As the stimulus strength varies over time, the neuron’s action potential frequency reflects that change. Sustained stimuli can lead to repeated activation of action potentials.