The Significance of Skeletal Muscle vs. Nerve Fibre Membrane Potential

Understanding the resting membrane potential of skeletal muscle and nerve fibres involves delving into the intricacies of ion channel permeability, the Nernst equation, and the specific physiological roles of these cell types. This article will explore why the resting membrane potential of skeletal muscle is more negative, reaching -90 mV, compared to nerve fibres, which reach -70 mV.

Ion Channel Distribution

The resting membrane potential of skeletal muscle fibers is more negative due to their higher permeability to potassium ions (K ) compared to sodium ions (Na ). This difference is primarily because skeletal muscle fibers have a greater density of leak channels, allowing more K to flow out of the cell, making the inside of the cell more negative. This higher permeability to K results in a greater driving force, pushing the membrane potential towards the K equilibrium potential, typically around -90 mV.

The Nernst Equation

The resting potential is influenced by the concentration gradients of ions across the membrane, primarily K and Na . The Nernst equation can be used to calculate the equilibrium potential for these ions. For skeletal muscle, the equilibrium potential for K is around -90 mV, directly contributing to the more negative resting potential. This high K permeability and the associated Nernst potential are critical in determining the membrane potential of skeletal muscle.

The Sodium-Potassium Pump (Na /K ATPase)

The Na /K pump actively transports 3 Na ions out of the cell and 2 K ions into the cell, maintaining the concentration gradients. This pump is more active in skeletal muscle, helping to sustain a more negative resting membrane potential. The pump ensures that the cell maintains a stable negative potential, which is crucial for rapid depolarization during muscle contraction.

Cell Type and Function

The physiological roles of skeletal muscle and nerve cells differ. Skeletal muscle cells are designed to maintain a stable negative resting potential to facilitate rapid depolarization during contraction. In contrast, nerve cells are optimized for signal transmission, which requires a slightly less negative resting potential to facilitate action potential generation. This difference in function influences the resting membrane potential, with nerve cells having a slightly more positive potential, around -70 mV.

Additional Insights into Membrane Potential

Interestingly, in colloquial terms, we might say that the membrane potential of muscle is more negative because it is more 'polarized.' However, the resting potential of all biological cells is set by a proportional summation of the electrical forces exerted by all the ions that can cross the membrane. The permeability of a given ion determines its contribution to the membrane potential. In skeletal muscle, the resting permeability is almost entirely for potassium ions, which have a very negative Nernst potential, typically around -80 to -90 mV. Nerve cells, on the other hand, have more permeability to chloride and sodium, which have less negative and positive Nernst potentials, respectively.

From a functional perspective, muscles need to be 'fail safe' in both directions. You don’t want your muscles twitching when you don't want them to, and you want them to move definitely when you want them to. Therefore, the resting membrane potential of skeletal muscle is placed at a very negative value to prevent 'accidental' contractions. Nerve cells, involved in stochastic firing and computational properties, require 'noise' in neural network models. Thus, they are designed to activate more easily from a more negative potential.