Neurons are specialized cells that transmit information within the nervous system through electrical impulses known as action potentials. The generation of these signals requires a delicate balance of ions across the neuronal membrane. This page explores the processes of resting and action potentials in detail.
Resting Potential
Sodium-Potassium Pump
- Function: Maintains the resting membrane potential, crucial for neuron functionality.
- Mechanism: Utilizes ATP to move three sodium ions (Na+) out and two potassium ions (K+) in.
- Result: More Na+ outside and more K+ inside the cell, leading to a negative internal charge around -70 mV.
Ion Channels
- Potassium Leak Channels: Freely allow K+ to move out, contributing to the negative internal charge.
- Importance: Ensures readiness for action potential generation.
Action Potentials
Depolarization
Initiation
- Stimulus Reception: Sufficient stimulus opens voltage-gated Na+ channels.
- Threshold Potential: The critical level that must be reached, usually about -55 mV.
Sodium Influx
- Mechanism: Open Na+ channels allow Na+ to rush in, making the internal charge more positive.
- Peak Potential: At about +40 mV, Na+ channels close, ending depolarization.
Repolarization
Opening of Potassium Channels
- Mechanism: Voltage-gated K+ channels open, allowing K+ to leave the cell.
Restoring Negative Charge
- Process: Efflux of K+ restores the negative internal charge.
- Hyperpolarization: Brief state where the inside becomes even more negative.
Restoration to Resting Potential
- Role of Sodium-Potassium Pump: Restores the ionic balance.
Refractory Period
Absolute Refractory Period
- Importance: Prevents the backward propagation of the action potential.
Relative Refractory Period
- Explanation: The neuron can only be reactivated by a much stronger stimulus; ensures the unidirectional flow.
Significance of Action Potentials
All-or-None Response
- Explanation: An action potential either happens in full or not at all, ensuring consistent signal strength.
Directionality
- Mechanism: Moves from the axon hillock to the synaptic terminals; refractory period ensures directionality.
Speed Factors
- Myelination: Myelin sheath acts as an insulator, speeding up transmission through saltatory conduction.
- Axon Diameter: Larger diameters reduce resistance, increasing speed.
Integration and Frequency Coding
- Frequency Coding: The frequency of action potentials encodes stimulus strength.
- Spatial and Temporal Summation: The combined effects of different neurons or the same neuron over time can lead to action potential generation.
Additional Considerations
Diseases and Disorders
- Multiple Sclerosis: Deterioration of myelin affects action potential propagation.
- Hyperkalemia and Hypokalemia: Abnormal K+ levels disrupt resting potentials.
Pharmacology and Toxins
- Local Anaesthetics: Block Na+ channels, inhibiting action potential propagation.
- Tetrodotoxin: A toxin that blocks Na+ channels, affecting signal transmission.
Technological Applications
- Electroencephalogram (EEG): Measures electrical activity in the brain.
- Deep Brain Stimulation: Manipulates action potentials for therapeutic purposes.
FAQ
Local anaesthetics work by blocking the voltage-gated sodium channels in the neuronal membrane. By inhibiting these channels, they prevent the influx of Na+ ions, stopping depolarization, and thereby blocking the generation of action potentials. This interruption in nerve transmission results in a temporary loss of sensation in the targeted area.
The resting potential doesn't reach the equilibrium potential for potassium because the neuron's membrane is not only permeable to potassium ions. There are also leaky sodium channels and other active mechanisms like the sodium-potassium pump, which prevent the membrane potential from reaching the equilibrium value for potassium alone. Thus, the resting potential is a balance between influences from sodium and potassium.
Hyperpolarization is a state where the membrane potential becomes more negative than the resting potential. It can occur during the refractory period of an action potential or by inhibitory neurotransmitters. The opening of additional potassium channels or chloride channels may lead to more K+ ions leaving or Cl- ions entering the neuron, respectively, causing the membrane potential to drop further, resulting in hyperpolarization.
A stimulus below the threshold potential cannot trigger an action potential because it is not strong enough to cause the voltage-gated sodium channels to open. The threshold potential is a critical value that must be reached for these channels to open, allowing Na+ ions to enter the neuron and initiate depolarization. If the stimulus is below this value, it will not cause an action potential, leading to the "all-or-none" response of neurons.
If the sodium-potassium pump is inhibited, the active transport of Na+ and K+ ions across the membrane would cease. This would lead to an imbalance in the concentrations of these ions, disrupting the resting potential. Over time, the membrane would become depolarized, and the neuron might become unable to generate action potentials, significantly affecting nerve impulse transmission.
Practice Questions
The sodium-potassium pump maintains the resting potential by actively transporting three sodium ions (Na+) out of the neuron and two potassium ions (K+) into the neuron using ATP. This leads to a higher concentration of Na+ outside and K+ inside the cell, resulting in a negative resting membrane potential of approximately -70 mV. When a stimulus reaches the threshold potential, usually about -55 mV, it triggers depolarization by opening voltage-gated Na+ channels. Na+ rushes into the cell, increasing the internal charge towards +40 mV, leading to an action potential.
The all-or-none response ensures that an action potential either occurs in full or not at all. It guarantees that once the threshold potential is reached, the signal strength remains consistent throughout propagation. The refractory period consists of absolute and relative phases. The absolute refractory period prevents backward propagation by making it impossible to generate another action potential, ensuring unidirectional flow. The relative refractory period requires a stronger stimulus to reactivate the neuron, further reinforcing directionality. Together, these mechanisms contribute to consistent and directional signal transmission in neuronal communication.
