A&P notes 3-6-25

Cell Membrane Potential Changes

Resting Membrane Potential

• The resting membrane potential of a neuron is typically around -70 mV, indicating that the cell is polarized due to the uneven distribution of ions across the membrane. This potential is essential for neuronal function as it prepares the neuron for action potentials.

• Ionic Contributions: The primary ions involved in maintaining this potential are sodium (Na+) and potassium (K+). The imbalance is due to the selective permeability of the membrane and the actions of the sodium-potassium pump.

  • Sodium-Potassium Pump: This pump is a critical active transport mechanism that moves 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP to maintain a negative internal environment necessary for cellular function. This process helps in keeping the inside of the neuron negatively charged in relation to the outside.

Depolarization

• Definition of Depolarization: Depolarization refers to the transition of the membrane potential from -70 mV to -50 mV, making the inside of the neuron less negative. This shift is crucial for initiating an action potential.

• Initiation: The depolarization phase is initiated by an action potential or sensory stimulus, which leads to the opening of voltage-gated sodium channels in the neuron's membrane. This is a crucial step, as it begins the process of signal propagation.

• Ion Movement During Depolarization: When these channels open, Na+ ions rapidly enter the cell, driven by both the concentration gradient (more Na+ outside than inside) and the electrical gradient (the inside of the cell is more negative).

  • As Na+ ions flood into the cell, they alter the internal charge, reducing the overall negative charge, increasing the membrane potential, and moving closer to the threshold needed to trigger an action potential.

Molecular Events

• The depolarization process involves significant changes in the permeability of the cell membrane to Na+ ions. When the membrane potential reaches about -55 mV (the threshold), voltage-gated sodium channels begin to activate in a cascading manner.

• Chain Reaction: The activation of one voltage-gated sodium channel causes adjacent channels to activate, leading to a rapid increase in Na+ influx and a sharp rise in the membrane potential towards positive values (up to +30 mV).

• Action Potential Generation: If the membrane potential exceeds the critical threshold, an action potential is generated. This depolarization and subsequent restoration of the membrane potential is fundamental in nerve impulse conduction down the axon.

Importance of the Mechanism

• This intricate mechanism of depolarization ensures that cells can respond accurately to stimuli, which is fundamental in processes such as muscle contraction and neural signaling. The ability to undergo rapid changes in membrane potential enables complex signaling processes necessary for communication within the nervous system and between the nervous system and muscles.