Topic: Neuromuscular transmission.
Question: Describe the different stages in nerve action potential. Add a note on sodium and potassium voltage gated channels involved in this process.
Click here for Reference Material-This material is informational alone and is not specifically prepared as an answer for any question. Readers may do their own research before finalising diagnoses according to the characteristics unique to each question. Readers should not proceed without cross-referencing with relevant textbooks as well as standard guidelines available.
A nerve action potential involves several stages:
1. Resting state: When a nerve is not conducting an impulse, it is in the resting state. The axon membrane maintains a negative voltage relative to the outside due to higher concentrations of Na+ and Cl- ions outside the membrane and K+ ions inside.
2. Depolarization: If the membrane potential reaches a threshold level, voltage-gated Na+ channels open, allowing Na+ to flow into the axon. This reverses the membrane voltage to positive, marking the start of depolarization. Depolarization opens more Na+ channels, fully depolarizing the membrane.
3. Repolarization: Na+ channels close and voltage-gated K+ channels open, allowing K+ to flow out of the axon. This causes the membrane potential to return to negative, repolarizing the axon. Repolarization restores the resting state voltage.
4. Hyperpolarization: During repolarization, more K+ flows out than is necessary to return the membrane to the resting state potential. This causes the inside of the axon to become slightly more negative than the resting state for a brief period—known as hyperpolarization.
5. Refractory period: Na+ and K+ voltage-gated channels become inactivated and unresponsive for a brief time (1-2 milliseconds) after depolarization—this is the refractory period. It prevents re-depolarization of the axon and ensures the impulse can only travel in one direction.
The stages of an action potential are made possible through the orchestrated opening and closing of voltage-gated Na+ and K+ channels in the axon membrane:
– Na+ channels normally remain closed but open transiently in response to depolarization from a threshold stimulus, allowing Na+ influx that completely depolarizes the membrane. They have a short refractory period when they will not reopen, then return to the closed state.
– K+ channels also remain closed at rest but open in response to depolarization, then stay open a little longer than Na+ channels as they restore the negative membrane voltage. They gradually close as the membrane repolarizes, then remain closed until the next action potential.
– Differential opening and closing of these channel types creates the characteristic upstroke (Na+), downstroke (K+) and overshoot/undershoot (K+) of shape of nerve action potentials measured on an oscilloscope.
– Ion concentrations and pump activity maintain correct gradients for impulse conduction. The sodium-potassium ATPase pump restores ion balance after each impulse.
Nerve action potentials are an elegant means of rapidly transmitting information with millisecond timing and precision using electricity and voltage-sensitive ion channels. They allow nerves to sense the environment, convey signals between the brain and body, and control nearly every bodily function with a high degree of fidelity and coordination.
