Chapter 05: Membrane Potentials and Action Potentials

 

Membrane and action potential diagram

Previouse chapter:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-04-transport-of-substances.html

Membrane Potentials 

Membrane potentials represent electrical voltage differences across cell membranes, created by uneven distribution of ions and selective membrane permeability. Resting membrane potentials exist in almost all cells, but excitable tissues like nerve and muscle rely on them to generate and propagate action potentials.

At rest, a typical nerve fiber maintains a resting potential of about -90\{ mV} inside relative to the outside. This negative baseline is driven by three main factors: the diffusion of potassium ({K}^+) out through potassium leak channels, a minor influx of sodium ({Na}^+) through sodium leak channels, and the continuous electrogenic action of the {Na}^+/\{K}^+ ATPase pump. Because the resting membrane is far more permeable to potassium than to sodium, the diffusion potential of potassium dominates, pulling the internal charge close to potassium's Nernst equilibrium potential. The electrogenic in pump adds an extra negative contribution by pumping three \{Na}^+ ions out for every two {K}^+ ions brought in.

Basic Concept:

  • Electrical potentials exist across all cell membranes.
  • Nerve and muscle cells generate rapid impulses.
  • Impulses transmit signals along nerve or muscle membranes.
  • Local potential changes activate various cell functions.

Basic Physics of Membrane Potentials

Potassium Diffusion Potential:

  • Potassium concentration is high inside nerve fibers.
  • Potassium concentration is very low outside.
  • Membrane is permeable only to potassium ions.
  • Potassium diffuses outward carrying positive charges.
  • Outward diffusion creates positivity outside, negativity inside.
  • Un-diffused negative anions remain inside the membrane.
  • Diffusion potential stops further net potassium diffusion.
  • Mammalian nerve potassium diffusion potential is -94mV.

Sodium Diffusion Potential

  • Sodium concentration is high outside nerve fibers.
  • Sodium concentration is low inside nerve fibers.
  • Membrane is permeable only to sodium ions.
  • Positive sodium ions diffuse into the fiber.
  • Inward diffusion creates positivity inside, negativity outside.
  • Potential stops further net sodium diffusion within milliseconds.
  • Mammalian nerve sodium diffusion potential is +61mV.

The Nernst Potential

Definition:

  • Opposes net diffusion of a specific ion.
  • Calculated based on ion concentration ratio across membranes.
  • Greater concentration ratio requires higher Nernst potential.

Nernst Equation:

Formula: 

  • EMF mV = \pm 61 \time\log\left(\fraConcentration inside}}Concentration outside}}\right).
  • Calculates potential for univalent ions.
  • Assumes extracellular fluid potential equals zero.
  • The sign is negative for diffusing positive ions.
  • The sign is positive for diffusing negative ions.
  • Tenfold potassium concentration ratio equals -61mV inside.

Measuring the Membrane Potential

Measurement Technique:

  • Uses small electrolyte-filled pipette micropipette electrodes.
  • Impales cell membrane to enter fiber interior.
  • An indifferent electrode sits in extracellular fluid.
  • A highly sophisticated voltmeter measures voltage difference.
  • Micropipette lumen diameter is under 1 micrometer.
  • Micropipette tip resistance exceeds 1 million ohms.
  • Microelectrode connects to an oscilloscope for rapid changes.

Recorded Potential Levels:

  • Extracellular fluid potential is recorded as zero.
  • The membrane boundary area is the electrical dipole layer.
  • Potential drops abruptly to -90mV inside fiber.
  • Potential stays steady -90mV across the fiber center.
  • Potential reverses to zero at the opposite membrane.

Ion Transfer Requirements

  • Only surface ions form an electrical dipole layer.
  • Few ions transfer to establish -90mV potential.
  • Requires transferring 1/3,000,000 to 1/100,000,000 positive charges.
  • Inward charge movement shifts potential to +35mV.
  • Voltage shift occurs in 1/10,000 of a second.

Resting Membrane Potential of Nerves

Basic Resting Value

  • Large nerve resting membrane potential is -90mV.
  • The interior is 90mV more negative than the exterior.
  • Sodium-Potassium Na^+-K^+ Pump
  • Continually pumps sodium out, potassium in.
  • Pumping ratio is 3 Na^+ out for 2 K^+ in.
  • An electrogenic pump leaves a net positive charge deficit inside.
  • Creates negative electrical potential inside the cell membrane.

Ion Concentrations and Ratios

  • Na^+ outside: 142mEq/L.
  • Na^+ inside: 14mEq/L
  • Na^+ inside/outside ratio: 0.1.
  • K^+ outside: 4mEq/L.
  • K^+ inside: 140mEq/L.
  • K^+ inside/outside ratio: 35.0.

Potassium Leak Channels:

  • Tandem pore domain channels allow continuous leakage.
  • Channels leak potassium even in resting cells.
  • Channels may leak sodium ions slightly.
  • Potassium permeability is 100 times higher than sodium.
  • Key factor determining resting membrane potential level.


Comments

Post a Comment

Popular posts from this blog

Ameboid Movement and Chemotaxis

Image
  📌 previous Notes:  https://mednotesbyrazaullah.blogspot.com/2026/08/functions-of-endoplasmic-reticulum-and.html Cell Locomotion Cell locomotion represents one of the most remarkable physical capabilities of specialized animal cells, allowing them to crawl through tissues to execute essential physiological tasks. Unlike the passive movement seen in blood flow, true cellular crawling termed ameboid movement relies on a dynamic internal framework that constantly reshapes the cell. Initiated when the leading edge of the cell extends a thin , fan-like projection called a pseudopodium, this process is driven by the rapid polymerization of actin filaments beneath the cell membrane. As the pseudopodium reaches forward, it anchors itself to surrounding tissue structures through specialized receptor proteins, establishing a firm mechanical footing from which the rest of the cell can pull itself ahead. Once the leading edge is anchored , the trailing body of the cell must contract to...
Read more

Structure of Skeletal Muscle Fiber - Guyton Notes

Image
  Muscle Fiber Muscle fibers range from 10 to 80 micrometers . Most fibers run the full length of muscle. About 98% are innervated by one nerve ending. Nerve endings are usually near the fiber middle. Sarcolemma : The sarcolemma encloses each skeletal muscle fiber. It consists of a plasma membrane and polysaccharide coat . The outer coat contains collagen fibrils. Sarcolemma fuses with tendon fibers at each end. Tendon fibers form bundles that attach to bone. Filaments and Myofibrils: Each fiber contains hundreds to thousands of myofibrils. Each myofibril has about 1,500 thick myosin filaments. Each myofibril has about 3,000 thin actin filaments. Actin and myosin proteins drive muscle contraction. Sarcomere and Z discs: Actin filament ends attach to the Z disc. Z discs connect adjacent myofibrils across the fiber. Filament alignment gives skeletal muscle its striated appearance. A sarcomere lies between two successive Z discs. The contracted sarcomere length is about 2 micro...
Read more

Functions of Endoplasmic Reticulum and Golgi Apparatus

Image
  📌 Previous Notes  (Very Important) https://mednotesbyrazaullah.blogspot.com/2026/08/precellular-life-forms-and-cellular.html 2: https://mednotesbyrazaullah.blogspot.com/2026/08/smooth-er-organelles-and-mitochondria.html Endoplasmic Reticulum and Golgi Apparatus Endoplasmic reticulum and the Golgi apparatus form a closely integrated membranous network that drives the synthesis, processing, and intracellular transport of cellular products. The endoplasmic reticulum, composed of an extensive system of interconnected tubules and flattened sacs, serves as the primary manufacturing site within the cell. Proteins synthesized on the ribosomes of the rough endoplasmic reticulum enter its internal lumen, where they undergo initial folding, while the smooth endoplasmic reticulum synthesizes lipids and phospholipids. These newly formed molecules are then encapsulated into small transport vesicles that bud off from the endoplasmic reticulum, physically bridging the gap to the nearby Go...
Read more