Resting Membrane Potential and Ion Flux in Neurons

Resting Membrane Potential (RMP)

Definition and Significance

The resting membrane potential (RMP) is the voltage difference across the neuronal membrane when the cell is not actively sending signals. It is typically around -65 millivolts (mV), but can vary among neuron types. The RMP provides a baseline against which electrical signals are produced.

• RMP Value: Usually given as -65 mV (average and convenient number).

• Function: Serves as a baseline for neuronal signaling.

Depolarization and Hyperpolarization

• Depolarization: Membrane voltage becomes more positive.

• Hyperpolarization: Membrane voltage becomes more negative.

• Ion Movement Effects:

  • Positive ion influx → depolarization

  • Positive ion efflux → hyperpolarization

  • Negative ion influx → hyperpolarization

  • Negative ion efflux → depolarization

Example: RMP Changes with Ion Injection

Injecting positive ions into a neuron causes depolarization, while injecting negative ions causes hyperpolarization. Even small ion fluxes can produce large changes in membrane potential.

Mechanisms Producing the RMP

Charge Separation

The RMP is maintained by the separation of charges across the neuronal membrane.

• Lipid Bilayer: Composed of phospholipids; polar heads are hydrophilic, nonpolar tails are hydrophobic.

• Hydration of Ions: Ions are surrounded by water molecules, attracted to hydrophilic regions, and repelled by hydrophobic regions. This maintains charge separation.

Proteins and Ion Channels

• Ion Channels: Proteins with pores allowing ion flow.

• Ion Selectivity: Channels are permeable to specific ions (e.g., sodium, potassium, chloride, calcium).

• Types of Channels:

  • Ligand-gated channels: Open in response to neurotransmitter binding.

  • Voltage-gated channels: Open/close in response to changes in membrane voltage.

  • Leak channels: Not gated by voltage or ligand; default state is open (e.g., K+ leak channel).

• Ion Pumps: e.g., Na+/K+ ATPase, which maintains ion gradients.

Potassium Leak Channels and Diffusion Force

K+ Leak Channels

• High resting permeability to K+ is a major factor in determining RMP.

• K+ leak channels are open and present throughout the neuronal membrane.

Diffusion Force

The concentration gradient of K+ creates an outward diffusion force, driving K+ out of the neuron.

K+ outside neuron	K+ inside neuron
~5 mM	~100 mM

Membrane Voltage Effects on K+ Flux

• At 0 mV: Outward flux, hyperpolarization.

• At -40 mV: Outward flux, hyperpolarization.

• At -80 mV: No net flux, no effect.

• At -120 mV: Inward flux, depolarization.

Driving Force and Equilibrium Potential

Driving Force

The driving force determines the rate of ion flux and is the sum of diffusion and electrostatic forces.

• Calculated as: $V_m - E_{ion}$

• Direction of flux: Outward is positive, inward is negative.

Equilibrium Potential (Eion)

The equilibrium potential is the voltage at which the diffusion and electrostatic forces for an ion are balanced. For K+, $E_K = -80$ mV.

• Calculated using the Nernst Equation:

$E_{ion} = \frac{2.303RT}{zF} \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• At body temperature (37°C), for K+:

$E_K = 61.54 \text{ mV} \log \frac{5}{100} = -80 \text{ mV}$

Ion Concentrations and the RMP

Key Ion Concentrations

	K+	Na+	Cl-	Ca2+
EC (mM)	5	150	150	2
IC (mM)	100	15	13	0.0002
Ratio [out]/[in]	1:20	10:1	11.5:1	10,000:1
Eion (mV)	-80	+62	-65	+123
Permeability at rest (relative to K+)	1	1/40	1/10	0

Additional info: These values are critical for understanding how the RMP is established and maintained.

Goldman Equation

To calculate the actual RMP, the Goldman equation is used, which accounts for the permeability of multiple ions. For exam purposes, understanding the Nernst equation and relative permeabilities is essential.

Establishment of Ion Gradients

Role of Ion Pumps

Ion gradients are established by ion pumps, such as the Na+/K+ ATPase, which actively transports 3 Na+ ions out and 2 K+ ions into the neuron, maintaining the concentration differences necessary for the RMP.

Summary Table: Key Terms and Concepts

Term	Definition	Example/Application
RMP	Resting membrane potential; voltage across membrane at rest	-65 mV in neurons
Depolarization	Membrane voltage becomes more positive	Na+ influx
Hyperpolarization	Membrane voltage becomes more negative	K+ efflux
Equilibrium Potential	Voltage at which net ion flux is zero	$E_K = -80$ mV
Driving Force	Sum of diffusion and electrostatic forces	$V_m - E_{ion}$
Na+/K+ ATPase	Ion pump maintaining gradients	3 Na+ out, 2 K+ in

Additional info:

• The permeability of the membrane to different ions is a key determinant of the RMP.

• Small changes in ion concentrations can have significant effects on membrane potential.

• Understanding the Nernst and Goldman equations is essential for predicting changes in RMP.