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Here is a complete, textbook-accurate breakdown of Resting Membrane Potential (RMP) directly from Guyton and Hall Textbook of Medical Physiology (Chapter 5, pp. 78-79):

Resting Membrane Potential (RMP) - Guyton & Hall

Definition

The RMP of a large nerve fiber at rest (not transmitting signals) is approximately -70 mV (inside of the fiber is 70 mV more negative than the outside extracellular fluid).

Key Factors Responsible for RMP

1. The Na⁺-K⁺ (Sodium-Potassium) Pump

Na⁺-K⁺ pump and K⁺ leak channels - Guyton Fig. 5.4

Continuously pumps 3 Na⁺ out for every 2 K⁺ in (electrogenic pump)
This creates a net loss of positive charges from inside the cell
Maintains concentration gradients:

Ion	Outside	Inside
Na⁺	142 mEq/L	14 mEq/L
K⁺	4 mEq/L	140 mEq/L

Na inside/outside ratio = 0.1
K inside/outside ratio = 35.0

2. K⁺ "Leak" Channels (Tandem Pore Domain Channels)

Always open at rest - allow K⁺ to leak out down its concentration gradient
These channels are about 100 times more permeable to K⁺ than Na⁺
K⁺ leaking out carries positive charge outside, making the inside more negative

Origin of the RMP - Three Contributions

Establishment of RMP under three conditions - Guyton Fig. 5.5

(A) K⁺ Diffusion Alone

If only K⁺ moved, the Nernst potential = -94 mV
(Log of 35 = 1.54; × -61 mV = -94 mV)

(B) Na⁺ + K⁺ Diffusion (Goldman Equation)

Na⁺ Nernst potential = +61 mV (ratio 0.1 inside/outside)
K⁺ Nernst potential = -94 mV
Since membrane is 100x more permeable to K⁺ than Na⁺, K⁺ dominates
Using Goldman equation: resultant potential = -86 mV

(C) Diffusion + Na⁺-K⁺ Pump (Full Picture)

The electrogenic pump adds an additional -4 mV of negativity
Final RMP = -90 mV (theoretical)
Clinically quoted as -70 mV for a resting large nerve fiber

Summary of Contributions

Factor	Contribution
K⁺ diffusion (mainly)	~-86 mV
Electrogenic Na⁺-K⁺ pump	~-4 mV additional
Net theoretical RMP	~-90 mV
Quoted RMP (large nerve fiber)	~-70 mV

Key Points to Remember

K⁺ is the main determinant of RMP because the membrane is far more permeable to K⁺ at rest
The Na⁺-K⁺ pump is both concentrogenic (maintains ion gradients) and electrogenic (directly contributes ~4 mV)
The Goldman-Hodgkin-Katz (GHK) equation accounts for multiple ions and their relative permeabilities to calculate the actual RMP
Chloride ions must also be considered for a truly accurate calculation

Source: Guyton and Hall Textbook of Medical Physiology, Chapter 5 - Membrane Potentials and Action Potentials, pp. 78-79

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