Skeletal Muscle Fibers as Electrically Excitable Cells

Skeletal Muscle Fibers as Electrically Excitable Cells

Introduction

Skeletal muscle fibers are specialized cells capable of generating and conducting electrical impulses. This property is essential for muscle contraction and is governed by the movement of ions across the cell membrane, leading to changes in membrane potential.

Relative Concentration of Sodium and Potassium Ions

The distribution of sodium (Na+) and potassium (K+) ions across the muscle cell membrane is crucial for electrical excitability.

• Inside the cell: High concentration of K+, low concentration of Na+

• Outside the cell: High concentration of Na+, low concentration of K+

• This difference is maintained by the sodium-potassium pump (Na+/K+-ATPase), which actively transports 3 Na+ ions out and 2 K+ ions into the cell.

Example: In a typical skeletal muscle fiber, the intracellular K+ concentration is about 140 mM, while Na+ is about 10-15 mM. Extracellularly, Na+ is about 145 mM and K+ is about 4 mM.

Concentration Gradient vs. Electrochemical Gradient

A concentration gradient refers to the difference in the concentration of a substance across a membrane. An electrochemical gradient combines both the concentration gradient and the electrical potential difference across the membrane.

• Concentration gradient: Drives ions from areas of high to low concentration.

• Electrochemical gradient: Considers both the concentration difference and the charge difference (membrane potential).

Example: K+ tends to move out of the cell due to its concentration gradient, but the negative charge inside the cell attracts K+ back in, creating an electrochemical balance.

Generation of the Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane when the cell is not actively sending a signal.

• Typically, the inside of the cell is negative relative to the outside (about -70 mV in muscle fibers).

• This is mainly due to the movement of K+ out of the cell through leak channels and the activity of the sodium-potassium pump.

• The membrane is more permeable to K+ than Na+, so K+ efflux dominates.

Equation: The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

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

Additional info: R is the gas constant, T is temperature in Kelvin, z is the charge of the ion, and F is Faraday's constant.

Sequence of Events in a Skeletal Muscle Fiber Action Potential

An action potential is a rapid change in membrane potential that travels along the muscle fiber, triggering contraction.

1. Depolarization: Na+ channels open, Na+ rushes into the cell, making the inside more positive.

2. Repolarization: Na+ channels close, K+ channels open, K+ exits the cell, restoring negativity inside.

3. Return to Resting State: Ion gradients are restored by the sodium-potassium pump.

Example: The action potential travels along the sarcolemma and into the T-tubules, initiating muscle contraction.