Membrane Potentials and Nerve Impulse Transmission: Action and Graded Potentials

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Membrane Potentials and Nerve Impulse Transmission

Introduction to Membrane Potentials

Membrane potentials are essential for the transmission of nerve impulses through the nervous system. Neurons and muscle cells utilize changes in membrane potential as signals for communication, integration, and the relay of information. These changes are fundamental to the functioning of the nervous system.

Membrane potential: The electrical potential difference across the cell membrane, primarily due to the movement of ions such as Na+ and K+.
Changes in membrane potential can be produced by:
- Alterations in membrane permeability to specific ions.
- Changes in ion concentrations on either side of the membrane.
Two main types of signals:
- Action potentials: Long-distance signals.
- Graded potentials: Short-distance signals.

Key Terms and Definitions

Potential: Movement of ions (Na+, K+) and the resulting electrical charge.
Action Potential: Large, rapid change in membrane potential that travels over a great distance.
Graded Potential: Small, localized change in membrane potential over a short distance.
Resting Potential: The baseline membrane potential, typically around -70 mV.
Depolarization: Membrane potential becomes less negative (moves in a positive direction).
Repolarization: Membrane potential returns to a more negative value after depolarization.
Hyperpolarization: Membrane potential becomes more negative than the resting potential.

Action Potential: Phases and Mechanisms

Resting Membrane Potential

The resting membrane potential is typically -70 mV. At rest, the cell has high concentrations of Na+ outside and K+ inside. All voltage-gated Na+ and K+ channels are closed.

ECF (extracellular fluid): High Na+, low K+
ICF (intracellular fluid): High K+, low Na+

Depolarization

During depolarization, Na+ channels open, allowing Na+ to enter the cell. This influx causes the membrane potential to become more positive.

Threshold for excitation is typically around -55 mV.
Depolarization is the initial phase of the action potential.

Peak Action Potential

At the peak of the action potential (around +30 mV), the cell's interior is highly positive. Sodium channels close, stopping further Na+ entry.

Repolarization

During repolarization, Na+ channels close and K+ channels open. K+ exits the cell, restoring the negative membrane potential.

Hyperpolarization

Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential due to continued K+ efflux. The Na+/K+ pump restores the resting potential.

Na+/K+ pump exchanges 3 Na+ out for 2 K+ in.

Action Potential Graph

The following table summarizes the phases of the action potential:

Phase	Membrane Potential (mV)	Main Ion Movement	Channel Status
Resting	-70	None (channels closed)	Na+, K+ closed
Depolarization	-70 to +30	Na+ influx	Na+ open
Peak	+30	Na+ channels close	Na+ closed
Repolarization	+30 to -70	K+ efflux	K+ open
Hyperpolarization	Below -70	K+ continues to exit	K+ open, Na+/K+ pump active

Propagation of Action Potentials

Transmission Along the Axon

For an action potential to serve as a signal, it must be propagated along the axon. Each segment of the axon membrane undergoes depolarization and repolarization, restoring the resting potential in that region.

In unmyelinated axons, the action potential moves continuously along the membrane.
In myelinated axons, the action potential jumps between nodes of Ranvier, a process called saltatory conduction.

Threshold and the All-or-None Phenomenon

Threshold for Action Potential Generation

Not all local depolarizations produce action potentials. The membrane must reach a threshold (typically 15-20 mV above resting value) for the axon to "fire".

Subthreshold stimuli produce graded potentials but do not trigger action potentials.

All-or-None Principle

The generation of an action potential is compared to lighting a match. If the stimulus is strong enough to reach threshold, the action potential is generated and propagated regardless of continued stimulus. If not, no action potential occurs.

Action potentials are all-or-none: once threshold is reached, the response is maximal.

Refractory Periods

Absolute Refractory Period

During the absolute refractory period, the neuron cannot respond to another stimulus, regardless of strength. This ensures each action potential is a separate event.

Relative Refractory Period

During the relative refractory period, a stronger-than-usual stimulus can trigger another action potential. This period occurs after the absolute refractory period, when sodium channels are closed and potassium channels are open.

Refractory Periods Table

Period	Channel Status	Response to Stimulus
Absolute	Na+ open	No response possible
Relative	Na+ closed, K+ open	Response only to strong stimulus

Graded Potentials

Characteristics of Graded Potentials

Graded potentials are local changes in membrane potential that signal over short distances. Their magnitude varies directly with the intensity of the stimulus.

More intense stimulus → greater voltage change → farther current flows.
Triggered by changes in the neuron's environment that open ion channels.
Named according to location and function (e.g., receptor potential in sensory neurons).

Comparison: Action Potential vs. Graded Potential

Feature	Action Potential	Graded Potential
Distance	Long	Short
Magnitude	All-or-none	Varies with stimulus
Threshold	Must reach threshold	No threshold required
Propagation	Self-propagating	Decreases with distance

Key Equations

Resting membrane potential is determined by the Nernst equation:
Na+/K+ pump activity:

Example: Receptor Potential

When a sensory neuron is excited by energy (such as light or heat), the resulting graded potential is called a receptor potential.

Additional info: The notes reference videos for further illustration of action and graded potentials, which can be useful for visual learners.