What is the approximate threshold value for mammalian neurons?
A.
-55 mV.
B.
-80 mV.
C.
+35 mV.
D.
0 mV.
The Correct answer and Explanation is:
Correct Answer: A. -55 mV
Explanation:
The threshold value for a mammalian neuron is typically around -55 mV. This value is crucial in the generation of an action potential, which is the electrical signal that neurons use to communicate.
Understanding the Resting Membrane Potential:
Neurons maintain a resting membrane potential of approximately -70 mV. This potential is a result of the distribution of ions (mainly sodium (Na⁺) and potassium (K⁺)) across the neuron’s membrane, maintained by the sodium-potassium pump (Na⁺/K⁺ ATPase). At rest, the inside of the neuron is more negatively charged relative to the outside.
Threshold and Action Potential:
An action potential is an all-or-nothing event that occurs when the membrane potential reaches a certain critical level, known as the threshold. For most mammalian neurons, this threshold is about -55 mV. If the membrane potential depolarizes to this value (becomes less negative), voltage-gated sodium channels open, leading to a rapid influx of sodium ions. This causes the membrane potential to rise sharply, generating an action potential.
Sequence of Events:
- Depolarization: A stimulus causes the membrane potential to become less negative, moving toward the threshold.
- Threshold Reached: If the membrane potential reaches approximately -55 mV, an action potential is triggered.
- Sodium Influx: Voltage-gated sodium channels open, allowing Na⁺ ions to enter the neuron, causing the membrane potential to rise rapidly.
- Peak Potential: The action potential peaks around +35 mV, at which point the sodium channels close, and potassium channels open.
- Repolarization: K⁺ ions leave the neuron, returning the membrane potential toward the resting level.
- Hyperpolarization and Return to Resting State: The membrane potential briefly becomes more negative than the resting potential before stabilizing back to -70 mV.
Significance of the -55 mV Threshold:
The -55 mV threshold is a critical point. If the membrane does not reach this value, an action potential will not occur, which means the neuron will not send a signal. This all-or-nothing characteristic is essential for the reliable transmission of information in the nervous system.
In summary, the -55 mV threshold is vital for the proper functioning of neurons, ensuring that signals are generated and transmitted only when necessary.