The action potential is a fundamental process in neurophysiology that allows neurons to communicate by transmitting electrical signals. It consists of several phases, including depolarization, repolarization, and hyperpolarization. One of the lesser-known but crucial phases is the undershoot phase, also called the after-hyperpolarization phase.
Understanding the undershoot phase in action potential is essential for comprehending how neurons regulate electrical signals and maintain proper function. This topic explores what undershoot is, why it happens, its role in neural signaling, and its significance in neurophysiology.
1. What Is Undershoot in Action Potential?
a) Definition of Undershoot
The undershoot phase refers to the period after repolarization when the membrane potential becomes more negative than the resting membrane potential. This occurs due to the continued outflow of potassium (K⁺) ions through voltage-gated potassium channels before they fully close.
In simpler terms, the neuron temporarily overshoots its resting potential, making the inside of the cell more negative than usual. This phase ensures that the neuron does not fire another action potential too soon.
b) Why Does Undershoot Happen?
Undershoot occurs because potassium channels remain open longer than necessary, allowing additional K⁺ ions to leave the cell. Since potassium ions carry a positive charge, their exit makes the inside of the neuron more negative than its typical resting state, leading to the after-hyperpolarization phase.
2. The Phases of Action Potential
To fully understand the undershoot phase, it is essential to review the five key phases of an action potential:
a) Resting Membrane Potential
- Before an action potential starts, the neuron is at its resting state with a membrane potential of around -70 mV.
- This is maintained by the sodium-potassium (Na⁺/K⁺) pump, which actively transports Na⁺ out and K⁺ in to keep the cell stable.
b) Depolarization Phase
- When a neuron receives a stimulus, voltage-gated sodium (Na⁺) channels open, allowing Na⁺ to rush in.
- This causes the membrane potential to become less negative, reaching around +30 mV.
- This phase is crucial for signal transmission.
c) Repolarization Phase
- After reaching its peak, the neuron must return to its resting state.
- Sodium channels close, and potassium channels open, allowing K⁺ ions to exit the cell.
- This outflow of positive ions helps the membrane potential become negative again.
d) Undershoot (After-Hyperpolarization Phase)
- The potassium channels remain open longer than needed, causing the membrane potential to drop below the resting potential (more negative than -70 mV).
- This phase prevents immediate reactivation of the neuron, ensuring controlled neural signaling.
e) Return to Resting Potential
- The sodium-potassium pump restores the balance by moving Na⁺ out and K⁺ in, bringing the membrane potential back to -70 mV.
- The neuron is now ready for the next action potential.
3. The Role of Undershoot in Neural Function
a) Prevents Continuous Firing
One of the most critical roles of the undershoot phase is preventing neurons from firing too frequently. If neurons kept firing without a break, it could lead to uncontrolled electrical activity, as seen in conditions like epilepsy.
b) Contributes to the Refractory Period
The undershoot phase is part of the relative refractory period, during which a neuron requires a stronger-than-normal stimulus to generate another action potential. This ensures that neural signals travel in one direction and do not interfere with previous impulses.
c) Helps Maintain Signal Accuracy
By ensuring a brief delay before the next action potential, the undershoot phase allows for precise timing in neural communication. This is essential for coordinated muscle movements, reflexes, and cognitive functions.
4. The Ion Channels Involved in Undershoot
a) Voltage-Gated Potassium (K⁺) Channels
- These channels open during repolarization and remain open briefly during the undershoot phase.
- They allow K⁺ ions to exit, making the inside of the neuron more negative than usual.
b) Sodium-Potassium (Na⁺/K⁺) Pump
- The pump helps restore the resting membrane potential by actively moving Na⁺ out and K⁺ in.
- This balances the ion concentrations and prepares the neuron for another action potential.
c) Leak Channels
- These are always open and help maintain baseline ion movement.
- They contribute to the final stabilization of the resting potential after undershoot.
5. Disorders Related to Abnormal Action Potentials
a) Epilepsy
- In epilepsy, neurons fire excessively without proper refractory periods.
- This can be due to malfunctioning ion channels, leading to uncontrolled electrical activity.
b) Multiple Sclerosis (MS)
- MS affects the myelin sheath, which insulates neurons and speeds up action potential transmission.
- Damaged myelin can disrupt normal action potential patterns, including the undershoot phase, leading to muscle weakness and cognitive issues.
c) Neuropathic Pain
- When neurons fire inappropriately, they can send pain signals continuously, even without actual injury.
- This is often linked to dysfunction in potassium and sodium channels.
6. Experimental Evidence of the Undershoot Phase
a) Studies on Neuronal Activity
Electrophysiological studies using patch-clamp recordings have demonstrated the undershoot phase in cortical neurons, spinal cord neurons, and motor neurons.
b) Effects of Blocking Potassium Channels
When potassium channels are blocked using drugs like tetraethylammonium (TEA), the undershoot phase is reduced or absent. This confirms that prolonged K⁺ efflux is responsible for the undershoot.
c) Impact of Temperature and Ion Concentration
Changes in temperature and extracellular ion concentrations affect the duration and magnitude of the undershoot phase, highlighting its dependence on physiological conditions.
The undershoot phase in action potential is a critical component of neural signaling. It occurs when potassium ions continue to exit the neuron after repolarization, making the membrane potential more negative than its resting state.
This phase plays an essential role in regulating neuronal activity, preventing excessive firing, and ensuring accurate signal transmission. Dysfunction in this process can contribute to neurological disorders, making it a key area of study in neuroscience and medicine.
By understanding the mechanisms behind the undershoot phase, scientists can develop better treatments for conditions like epilepsy, multiple sclerosis, and neuropathic pain. This highlights the importance of ion channels, refractory periods, and neural regulation in maintaining proper nervous system function.