Membrane Potentials and Neural Signaling: Resting, Graded, and Action Potentials

Membrane Potentials in Neurons

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a neuron when it is not actively transmitting a signal. This potential is essential for the excitability of neurons and is primarily established by the distribution of ions across the membrane.

• Definition: The voltage difference between the inside and outside of a neuron at rest, typically around -70 mV.

• Key Ions: Potassium (K+), Sodium (Na+), and Chloride (Cl-) are the main contributors.

• Ion Channels: The membrane is highly permeable to K+ due to leak channels, allowing K+ to diffuse out, making the inside more negative.

• Sodium-Potassium Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining the gradient.

Equation:

$E_{K} = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Example: A typical neuron has a resting membrane potential of approximately -70 mV.

Electrochemical Gradients

Electrochemical gradients drive the movement of ions across the membrane, combining both concentration (chemical) and electrical gradients.

• Chemical Gradient: Difference in ion concentration across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: The sum of chemical and electrical forces acting on an ion.

Example: K+ tends to move out of the cell due to its concentration gradient, but the negative charge inside attracts it back.

Graded Potentials

Characteristics of Graded Potentials

Graded potentials are changes in membrane potential that vary in size and are localized to a specific region of the cell membrane. They are important for initiating action potentials.

• Local Changes: Occur in response to stimuli, such as neurotransmitter binding.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

• Decay with Distance: The effect diminishes as it spreads from the site of stimulation.

• Summation: Multiple graded potentials can combine to produce a larger change.

Example: Opening of ligand-gated Na+ channels causes a local depolarization.

Action Potentials

Generation and Propagation of Action Potentials

An action potential is a rapid, large change in membrane potential that propagates along the axon, allowing neurons to transmit signals over long distances.

• Threshold: If a graded potential depolarizes the membrane to a critical level (threshold), an action potential is triggered.

• Phases:

  • Depolarization: Voltage-gated Na+ channels open, Na+ rushes in, membrane potential becomes positive.

  • Repolarization: Na+ channels close, K+ channels open, K+ exits, membrane potential returns to negative.

  • Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than resting.

• All-or-None Principle: Action potentials occur fully or not at all once threshold is reached.

• Propagation: Action potentials travel along the axon without decreasing in strength.

Equation:

$I = g(V - E)$

Example: Neuronal signaling relies on the propagation of action potentials to communicate with other cells.

Comparison of Graded and Action Potentials

Key Differences

The following table summarizes the main differences between graded potentials and action potentials:

Feature	Graded Potential	Action Potential
Depolarizing/Hyperpolarizing	Either	Always depolarizing
Amplitude	Varies with stimulus strength	All-or-none, fixed amplitude
Propagation	Decreases with distance	Propagates without decrement
Refractory Period	None	Present
Summation	Possible	Not possible

Example: Graded potentials occur in dendrites and cell bodies; action potentials occur in axons.

Summary of Neural Signaling

• Neurons use changes in membrane potential to transmit information.

• Resting membrane potential is maintained by ion gradients and selective permeability.

• Graded potentials allow for local signaling and integration of inputs.

• Action potentials enable rapid, long-distance communication.

Additional info: These notes cover material relevant to Chapter 12: Nervous Tissue, focusing on membrane potentials, graded potentials, and action potentials, which are foundational concepts in neurophysiology.