How is a nerve impulse initiated, and what is the role of the threshold potential in this process?

Sourav PanNovember 9, 2024

Answered

A nerve impulse is initiated through a series of electrochemical changes in a neuron, primarily involving the movement of ions across the neuronal membrane. The threshold potential plays a critical role in this process by determining whether an action potential will be generated.

Initiation of a Nerve Impulse

1. Resting Membrane Potential:
   • Neurons maintain a resting membrane potential of approximately -70 mV, where the inside of the cell is negatively charged relative to the outside. This state is primarily maintained by the sodium-potassium pump, which actively transports sodium (Na++) out of the cell and potassium (K++) into the cell.
2. Stimulus Application:
   • When a neuron receives a stimulus (which could be electrical, chemical, or mechanical), it causes a change in membrane potential. If this change is sufficient to depolarize the membrane to a certain level, known as the threshold potential (typically around -55 mV), an action potential is triggered.
3. Threshold Potential:
   • The threshold potential is crucial because it represents the critical level of depolarization needed to open voltage-gated sodium channels. If the stimulus is subthreshold (not strong enough), it will not result in an action potential. Conversely, if the stimulus reaches or exceeds this threshold, it initiates a rapid depolarization phase.

Role of Threshold Potential

1. Opening of Voltage-Gated Sodium Channels:
   • Upon reaching threshold, voltage-gated Na++ channels open rapidly, allowing Na++ ions to flood into the neuron. This influx of positive ions further depolarizes the membrane, creating a positive feedback loop that accelerates depolarization.
2. Action Potential Generation:
   • As more Na++ channels open and more sodium enters, the membrane potential can rise sharply to around +30 mV. This rapid change in polarity constitutes an action potential, which propagates along the axon as a wave of depolarization.
3. Propagation Mechanism:
   • The action potential travels along the axon by sequentially opening adjacent voltage-gated Na++ channels through local currents generated by ion movements. Each segment of the axon undergoes depolarization and then repolarization as Na++ channels close and voltage-gated K++ channels open, allowing K++ to exit the neuron.
4. All-or-None Response:
   • The action potential follows an all-or-none principle; once threshold is reached, an action potential will occur with a consistent amplitude and duration regardless of the strength of the stimulus beyond that point.