Cellular Physiology: Graded Potentials and Action Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest. It is essential for the excitability of neurons and muscle cells.

• Definition: The voltage difference between the inside and outside of a cell when the cell is not transmitting signals, typically around -70 mV in neurons.

• Ion Distribution: The intracellular fluid has high concentrations of K+ and low concentrations of Na+, while the extracellular fluid has high Na+ and low K+.

• Na+/K+ ATPase Pump: Maintains the gradient by pumping 3 Na+ ions out and 2 K+ ions into the cell, using ATP.

• Membrane Permeability: The membrane is more permeable to K+ than Na+ due to more K+ leak channels.

Equation:

Additional info: This is the simplified Nernst equation for potassium, which largely determines the resting membrane potential.

Types of Gated Ion Channels in Neurons

Ion channels are proteins that allow specific ions to pass through the membrane, and their opening or closing changes membrane permeability.

• Chemically-gated (Ligand-gated) Channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

• Voltage-gated Channels: Open or close in response to changes in membrane potential.

• Mechanically-gated Channels: Open in response to mechanical deformation (e.g., stretch, pressure).

Location:

• Chemically-gated channels are typically found on dendrites and cell bodies.

• Voltage-gated channels are concentrated at the axon hillock and along the axon.

• Mechanically-gated channels are found in sensory receptors.

Role of Channels in Membrane Permeability

Channels determine the resting and dynamic permeability of the membrane to ions, which is crucial for generating electrical signals.

• Leak Channels: Always open; more K+ leak channels than Na+, contributing to the negative resting potential.

• Gated Channels: Open in response to specific stimuli, allowing rapid changes in permeability during signaling.

Graded (Local) Potentials

Graded potentials are small, localized changes in membrane potential that vary in size and direction, and occur primarily in dendrites and cell bodies.

• Produced by: Opening of chemically or mechanically gated channels.

• Characteristics:

  • Vary in amplitude (not all-or-none).

  • Can be depolarizing (more positive) or hyperpolarizing (more negative).

  • Decremental: decrease in strength as they spread from the point of origin.

  • Can undergo summation (add together) over time (temporal) and space (spatial).

• Function: Serve as signals that, if strong enough, can trigger an action potential at the axon hillock.

Example: A neurotransmitter binding to a receptor on a dendrite opens ligand-gated Na+ channels, causing a local depolarization.

Action Potentials

An action potential is a rapid, large change in membrane potential that propagates along the axon and is the primary means of long-distance neural signaling.

• All-or-None: Once threshold is reached, the action potential always occurs with the same amplitude.

• Produced by: Opening of voltage-gated Na+ and K+ channels in response to summation of graded potentials reaching threshold.

• Phases:

  • Depolarization: Rapid Na+ entry through voltage-gated channels.

  • Repolarization: K+ exits the cell as voltage-gated K+ channels open.

  • Hyperpolarization: Membrane potential temporarily becomes more negative than resting potential.

• Threshold: The critical level to which the membrane potential must be depolarized to initiate an action potential (typically around -55 mV).

Graphical Representation: The action potential is characterized by a rapid upstroke (depolarization), a downstroke (repolarization), and a brief undershoot (hyperpolarization).

Properties of Voltage-Gated Ion Channels

Voltage-gated Na+ and K+ channels are essential for the generation and propagation of action potentials.

• Voltage-Gated Na+ Channels:

  • Have two gates: activation (opens rapidly with depolarization) and inactivation (closes shortly after opening).

  • Responsible for the rapid depolarization phase.

  • Inactivation sets the refractory period, during which a new action potential cannot be initiated.

• Voltage-Gated K+ Channels:

  • Open more slowly in response to depolarization.

  • Responsible for repolarization and hyperpolarization phases.

Propagation of Action Potentials

Action potentials are conducted along the axon by the sequential opening of voltage-gated channels.

• Axon Hillock (Trigger Zone): The site where action potentials are typically initiated.

• Local Current Flow: Depolarization at one segment of the axon causes adjacent segments to depolarize, propagating the action potential.

• Unidirectional Propagation: The refractory period prevents the action potential from traveling backward.

Saltatory Conduction and Myelination

In myelinated axons, action potentials "jump" from one node of Ranvier to the next, greatly increasing conduction speed.

• Saltatory Conduction: Regeneration of the action potential occurs only at nodes of Ranvier, where voltage-gated channels are concentrated.

• Myelination: Myelin sheaths insulate the axon, preventing ion leakage and speeding up signal transmission.

• Effect of Myelin Damage: Demyelination slows or blocks action potential conduction, as seen in diseases like multiple sclerosis.

• Conduction Velocity: Faster in myelinated neurons and in neurons with larger diameters.

Summary Table: Types of Ion Channels and Their Functions

Key Concepts and Applications

• Graded potentials are variable, local changes in membrane potential that can summate to trigger an action potential.

• Action potentials are all-or-none, long-distance signals that depend on voltage-gated ion channels.

• Myelination and axon diameter are critical for rapid signal conduction in the nervous system.

• Demyelination impairs neural communication, leading to neurological deficits.