Cell Biology: Resting Membrane Potential

Resting Membrane Potential

• Defined as the voltage difference across a cell membrane when the cell is at rest, typically between -70 to -90 millivolts in neurons.
• Exists in all cell types, not just neurons.

Key Mechanisms Establishing Resting Membrane Potential

• Sodium-Potassium ATPases (Na+/K+ Pump)

  • Pumps three sodium ions out of the cell and two potassium ions into the cell.
  • Creates a slight negative charge inside the cell by excluding more positive ions than it brings in.
  • Establishes concentration gradients: higher sodium outside and higher potassium inside the cell.

• Leaky Potassium Channels

  • These channels allow potassium ions to move freely across the membrane.
  • Potassium moves out of the cell down its concentration gradient, leading to a more negative interior.
  • Unoccupied anions inside the cell (negatively charged ions or large proteins) contribute to increased negativity.

• Leaky Sodium Channels

  • Allow some sodium ions to enter the cell, though the permeability to potassium is significantly greater.
  • Sodium moves into the cell down its concentration gradient, making the inside slightly less negative but not enough to counteract the outflow of potassium.

Concentration Gradients

• The Na+/K+ pump and leak channels work together to maintain relative ion concentrations, crucial for cell function.
• Potassium is much more permeable than sodium in resting neurons, which is why the resting membrane potential is closer to the equilibrium potential of potassium (-90 mV).

Nernst Potential

• Used to calculate the equilibrium potential for ions, reflecting the balance between concentration gradient and electrostatic forces.

• For potassium:

  • Equation: E(K) = 61.5 / z * log([K+]outside/[K+]inside)
  • Typical values yield around -90 mV.

• For sodium:

  • Equation: E(Na) = 61.5 / z * log([Na+]outside/[Na+]inside)
  • Typical values yield around +70 mV.

Summarizing Resting Membrane Potential

• Resting membrane potential results from combined influences of the sodium-potassium pump, leaky potassium channels, and leaky sodium channels.
• Calculating resting membrane potential involves understanding the balance of these ion distributions and their respective equilibrium potentials, with the final resting membrane potential being a weighted average of the potentials of sodium and potassium ions.### Resting Membrane Potential
• Membrane potential of a neuron at rest is approximately negative 70 millivolts (mV).
• This is influenced by ion permeability, particularly potassium (K+) which moves out of the cell more readily than sodium (Na+).

Graded Potentials

• Graded potentials can depolarize (make less negative) or hyperpolarize (make more negative) the neuron.
• To depolarize towards threshold voltage for action potential, which is around negative 55 mV, slight depolarization is needed.
• Hyperpolarization can occur, reducing the membrane potential, for example, to negative 90 mV, thus inhibiting the neuron.

EPSPs and IPSPs

• Excitatory postsynaptic potential (EPSP) refers to the depolarization caused by the influx of positive ions (like Na+ or Ca2+) when a neurotransmitter (e.g., glutamate) binds to ligand-gated ion channels.
• Inhibitory postsynaptic potential (IPSP) occurs when negative ions (like Cl-) flow in or positive ions (like K+) flow out, leading to hyperpolarization.

Neuronal Communication

• Neurons are classified as presynaptic (sending signals) and postsynaptic (receiving signals) based on their position relative to the synapse.
• The interaction between presynaptic and postsynaptic neurons occurs at synapses, where neurotransmitters are released and interact with receptors.

Types of Summation

• Temporal Summation: Multiple signals from a single presynaptic neuron occur in rapid succession, increasing the likelihood of reaching threshold.
• Spatial Summation: Multiple presynaptic neurons fire simultaneously at the postsynaptic neuron, cumulatively raising the membrane potential towards threshold.

Action Potential Initiation

• Upon reaching the threshold voltage (negative 55 mV) at the axon hillock, voltage-gated sodium channels open, allowing Na+ to rush into the cell.
• This influx transforms the membrane potential from negative 55 mV to positive 30 mV, leading to the action potential.

Axon Hillock and Trigger Zones

• The axon hillock is known as the trigger zone where action potentials are initiated.
• It is densely populated with voltage-gated sodium channels that are crucial for action potential generation.

Voltage Changes During Action Potential

• The rapid influx of sodium ions during an action potential shifts the cell's membrane potential dramatically, causing depolarization that swiftly transitions to a positive intracellular environment.### Action Potential Generation
• Resting membrane potential is around negative 70 millivolts.
• Threshold potential is approximately negative 55 millivolts, triggering action potentials.
• At positive 30 millivolts, the inactivation gate of voltage-gated sodium channels closes, halting sodium entry.

Mechanism of Depolarization

• Depolarization occurs when sodium ions rush into the cell after activation of voltage-gated sodium channels.
• The influx of sodium raises the internal voltage from negative 70 to positive 30 millivolts.
• The sequence begins at the axon hillock, propagating a depolarizing wave down the axon.

Role of Calcium in Neurotransmitter Release

• At positive 30 millivolts, voltage-gated calcium channels activate, allowing calcium ions to enter the axon terminal.
• Calcium facilitates the fusion of synaptic vesicles with the cell membrane, releasing neurotransmitters into the synaptic cleft.

Key Definitions

• Depolarization: The process of making the inside of the cell more positive relative to the outside.
• Repolarization: The return of the cell’s interior voltage back to resting potential after depolarization.
• Hyperpolarization: The condition where the inside of the cell becomes more negative than the resting potential.

Repolarization Process

• Voltage-gated potassium channels open following the peak voltage of positive 30 millivolts.
• As potassium exits the cell, it brings the voltage down to negative 90 millivolts, causing hyperpolarization.
• The sodium-potassium pumps and leaky channels eventually restore resting membrane potential to negative 70 millivolts.

Action Potential Trajectory

• Action potentials follow the sequence of depolarization followed by repolarization, with channels activating and inactivating in response to voltage changes.
• The absolute refractory period occurs between the peak of the action potential and return to resting potential, during which the neuron cannot be re-stimulated.

Configuration of Voltage-Gated Sodium Channels

• At resting state, the activation gate is closed and the inactivation gate is open.
• During depolarization, the activation gate opens while the inactivation gate starts to close.
• At the peak (positive 30 millivolts), the activation gate is open and the inactivation gate is shut, preventing further sodium influx.

Refractory Periods

• Absolute Refractory Period: Period during which no stimulus can trigger another action potential, corresponding to when inactivation gates are closed.
• Relative Refractory Period: Follows the absolute refractory period; the neuron can potentially be re-stimulated with a stronger than usual stimulus.

Summary of Resting and Action Potentials

• Transition from resting potential (-70mV) to threshold potential (-55mV) is influenced by excitatory postsynaptic potentials (EPSPs).
• Returning to a stable resting potential involves the inactivation of channels and the action of pumps and channels to balance ion concentrations.

Resting Membrane Potential

• Resting membrane potential is the voltage difference across a cell membrane when at rest, typically between -70 to -90 mV in neurons, and is present in all cell types.

Key Mechanisms Establishing Resting Membrane Potential

• Sodium-Potassium ATPases (Na+/K+ Pump):

  • Transports three sodium ions out and two potassium ions into the cell, establishing a net negative charge inside.
  • Creates concentration gradients with higher sodium outside and higher potassium inside the cell.

• Leaky Potassium Channels:

  • Allow potassium ions to flow out of the cell, making the interior more negative.
  • Unoccupied negatively charged ions inside the cell enhance the membrane's negativity.

• Leaky Sodium Channels:

  • Permit a small influx of sodium ions, contributing to less negativity inside the cell but less than the outflow of potassium.

Concentration Gradients

• The Na+/K+ pump and leak channels maintain vital ion concentrations, essential for cell functionality.
• The resting membrane potential aligns closer to potassium's equilibrium potential around -90 mV due to higher potassium permeability.

Nernst Potential

• Formula used to calculate equilibrium potentials for ions, balancing concentration gradients and electrostatic forces.

• Potassium Equilibrium Potential:

  • Calculated as E(K) = 61.5 / z * log([K+]outside/[K+]inside) yielding approximately -90 mV.

• Sodium Equilibrium Potential:

  • Calculated as E(Na) = 61.5 / z * log([Na+]outside/[Na+]inside) yielding approximately +70 mV.

Summarizing Resting Membrane Potential

• Resultant from the sodium-potassium pump, leaky potassium, and leaky sodium channels.
• Final resting membrane potential is a weighted average of sodium and potassium ion potentials.

Graded Potentials

• Graded potentials can either depolarize (make less negative) or hyperpolarize (make more negative) the neuron.
• Depolarization is needed to reach the action potential threshold of approximately -55 mV.
• Hyperpolarization can push the membrane potential to around -90 mV, inhibiting neuron activity.

EPSPs and IPSPs

• Excitatory Postsynaptic Potential (EPSP):

  • Results from positive ion influx (e.g., Na+ or Ca2+) when a neurotransmitter binds to receptors.

• Inhibitory Postsynaptic Potential (IPSP):

  • Occurs due to the flow of negative ions (like Cl-) or the outflow of positive ions (like K+), resulting in hyperpolarization.

Neuronal Communication

• Neurons are categorized as presynaptic (sending signals) and postsynaptic (receiving signals) regarding their synaptic position.
• Communication happens at synapses where neurotransmitters are released and interact with receptors.

Types of Summation

• Temporal Summation:

  • Involves multiple signals from one presynaptic neuron in quick succession, raising the likelihood of reaching the threshold.

• Spatial Summation:

  • Multiple presynaptic neurons fire simultaneously at the postsynaptic neuron, cumulatively elevating the membrane potential towards the threshold.

Action Potential Initiation

• When threshold voltage (approximately -55 mV) is met at the axon hillock, voltage-gated sodium channels open, leading sodium ions to flood into the cell.
• This influx shifts the membrane potential sharply from -55 mV to approximately +30 mV, resulting in an action potential.

Axon Hillock and Trigger Zones

• The axon hillock acts as the trigger zone for action potentials, containing a high density of voltage-gated sodium channels critical for their generation.

Voltage Changes During Action Potential

• Rapid sodium influx during an action potential dramatically depolarizes the cell, rapidly transitioning towards a positive intracellular environment.

Explore the resting membrane potential essential for cellular function. This quiz covers the key mechanisms like the Sodium-Potassium ATPase and leaky channels that help establish and maintain this electrical state across cell membranes. Ideal for students learning about cellular physiology.