Fundamentals of the Nervous System & Nervous Tissue 11C: Membrane Potentials and Action Potentials

Chapter 11 Part C: Fundamentals of the Nervous System & Nervous Tissue

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a resting neuron. This potential is essential for neural signaling and is maintained by ion gradients and selective membrane permeability.

• Typical value: Approximately -70 mV (inside of the neuron is negative relative to the outside).

• Polarization: The membrane is polarized due to unequal distribution of ions (Na+, K+, Cl-, and protein anions).

• Ion permeability: The plasma membrane is slightly permeable to Na+ and much more permeable to K+ due to leak channels.

• Na+-K+ pump: Maintains concentration gradients by actively transporting Na+ out and K+ into the cell.

Equation for Resting Membrane Potential:

• The Nernst equation can be used to calculate equilibrium potential for a single ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• For multiple ions, the Goldman-Hodgkin-Katz equation is used:

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Membrane Potential Signals

Neurons use changes in their membrane potentials as communication signals to receive, integrate, and send information. These changes are produced by alterations in ion concentrations or membrane permeability.

• Graded potentials: Incoming signals operating over short distances, typically on dendrites and cell bodies.

• Action potentials: Outgoing signals operating over long distances, typically on axons.

Membrane Potential Changes

• Depolarization: Decrease in membrane potential; inside becomes less negative (e.g., from -70 mV to -65 mV). Moves toward zero and can reverse polarity.

• Hyperpolarization: Increase in membrane potential; inside becomes more negative (e.g., from -70 mV to -75 mV). Moves away from zero.

• Threshold: The critical level of depolarization required to trigger an action potential (typically -55 to -50 mV).

Graded Potentials: Overview

Graded potentials are short-lived, localized changes in membrane potential that can be either depolarizations or hyperpolarizations.

• Triggered by a stimulus that opens gated ion channels.

• Magnitude varies directly with stimulus strength.

• Decreases in intensity with distance due to leaky plasma membrane.

• Occur along dendrites and cell bodies, traveling toward the axon hillock.

• Sufficiently strong graded potentials can initiate an action potential.

Action Potentials: Overview

An action potential (AP) is a brief reversal of membrane potential with a total amplitude of about 100 mV (from -70 mV to +30 mV). It is the principal means of neural communication.

• Occurs only in cells with excitable membranes (neurons and muscle cells).

• Does not decrease in strength over distance; propagated along the axon.

• Generated only in axons where voltage-gated channels are present.

Action Potential Generation: Steps

The generation of an action potential involves four main steps:

1. Resting state: All gated Na+ and K+ channels are closed; leak channels are open. Na+ channels have activation and inactivation gates.

2. Depolarization: Graded potentials open fast activation gates of Na+ channels; Na+ rushes in, depolarizing the cell. When threshold is reached, depolarization becomes self-generating.

3. Repolarization: Na+ channels inactivate; K+ channels open, K+ flows out, restoring internal negativity.

4. Hyperpolarization: Some K+ channels remain open, causing excessive K+ efflux and a slight dip below resting potential. Na+ channels reset.

Action Potential Generation: Overview Table

Step	Voltage-gated ion channels	Ion permeability	Action potential curve
Resting state	all channels closed	no ion movement	flat
Depolarization	Na+ channels open (activation gates)	Na+ flows into cell	sharp upward spike
Repolarization	Na+ channels inactivating (inactivation gates), K+ channels open	K+ flows out of cell	downward curve
Hyperpolarization	some K+ channels remain open, Na+ channels reset	some K+ continues to flow out of cell	slight dip below resting membrane potential

Action Potential Propagation

Action potential propagation is the transmission of an action potential along the axon's entire length, away from its point of origin (usually the axon hillock) toward the axon terminals.

• Self-propagates at a constant velocity.

• Repolarization wave follows the depolarization wave down the axon.

• Not all local depolarizations produce APs; stimulus must reach threshold.

• All-or-none phenomenon: APs either happen completely or not at all.

Stimulus Intensity

The central nervous system (CNS) determines stimulus strength by the frequency of action potentials, not their amplitude.

• All APs are alike once generated.

• Strong stimuli: Produce more frequent action potentials.

• Weak stimuli: Produce less frequent action potentials.

Refractory Periods

Refractory periods ensure the one-way transmission of nerve impulses and limit the frequency of action potentials.

• Absolute refractory period: Time from opening of Na+ activation gates until closing of Na+ inactivation gates. No new AP can be generated.

• Relative refractory period: Interval following the absolute refractory period; most Na+ channels have reset, but a strong stimulus can generate a new AP.

Conduction Velocity

The speed at which an action potential travels along an axon depends on two main factors:

• Axon diameter: Larger axons conduct impulses faster due to less resistance to current flow.

• Degree of myelination: Myelinated axons conduct impulses faster via saltatory conduction (APs jump from node to node), while unmyelinated axons conduct more slowly via continuous conduction.

Conduction Types Table

Type	Speed	Mechanism
Unmyelinated axons	Slow	Continuous conduction
Myelinated axons	Fast	Saltatory conduction (APs jump between nodes)

Summary of Action Potential Propagation

• Graded potentials decay with distance due to leaky membranes.

• Action potentials are regenerated at each segment of the axon, allowing long-distance signaling.

• Saltatory conduction in myelinated axons allows rapid transmission by jumping between nodes of Ranvier.

Key Terms and Definitions

• Resting membrane potential: The voltage difference across the membrane of a resting neuron.

• Depolarization: Reduction in membrane potential; inside becomes less negative.

• Hyperpolarization: Increase in membrane potential; inside becomes more negative.

• Graded potential: Localized, short-lived change in membrane potential.

• Action potential: Brief, long-distance reversal of membrane potential.

• Threshold: Critical level of depolarization required to trigger an AP.

• Absolute refractory period: Period during which no new AP can be generated.

• Relative refractory period: Period during which a new AP can be generated only by a strong stimulus.

• Saltatory conduction: Rapid AP propagation in myelinated axons.

Example: In myelinated axons, the action potential jumps from one node of Ranvier to the next, greatly increasing conduction velocity compared to unmyelinated axons.

Additional info: The notes cover sections 11.4–11.6 of a standard Anatomy & Physiology textbook, focusing on the biophysical basis of neural signaling, which is foundational for understanding nervous system function.