Answer
The "all-or-none" law is a fundamental principle that applies to action potentials in neurons. This law states that an action potential either occurs fully or does not occur at all in response to a stimulus. In other words, there is a threshold that must be reached for an action potential to be generated, and once that threshold is crossed, the action potential is always of the same magnitude and duration.
Here's how the all-or-none law applies to an action potential:
1. **Threshold:** To initiate an action potential, a neuron must reach a certain threshold membrane potential, typically around -55 to -50 millivolts (mV). If the membrane potential does not reach this threshold, no action potential will be generated.
2. **All-or-None Response:** Once the threshold is reached, the neuron will generate an action potential that is consistent in its characteristics, regardless of the strength of the stimulus that initially triggered it. This means that a stronger stimulus does not result in a larger action potential; it merely increases the frequency of action potentials generated.
3. **Magnitude and Duration:** The action potential that occurs when the threshold is reached has a specific magnitude (amplitude) and duration (typically about 1 millisecond). It does not matter whether the stimulus is just strong enough to reach the threshold or significantly stronger; the action potential's magnitude and duration remain the same.
Now, let's contrast action potentials with local potentials:
**Properties of Action Potentials in Contrast to Local Potentials:**
1. **All-or-None vs. Graded:** Action potentials follow the all-or-none law, while local potentials (also known as graded potentials) do not. Local potentials vary in magnitude depending on the strength of the stimulus. Stronger stimuli result in larger local potentials, whereas weaker stimuli produce smaller ones.
2. **Propagation:** Action potentials are self-propagating and can travel long distances along the axon without a decrease in magnitude. In contrast, local potentials are decremental, meaning they diminish in strength as they spread away from their source and do not propagate over long distances.
3. **Threshold:** Action potentials have a distinct threshold that must be reached to be initiated, typically around -55 to -50 mV. Local potentials do not have a fixed threshold; their magnitude depends on the strength of the stimulus.
4. **All-or-None vs. Summation:** Action potentials do not summate; they occur as discrete events. Local potentials, on the other hand, can summate (both temporally and spatially). Temporal summation occurs when multiple local potentials arrive at the trigger zone in rapid succession, while spatial summation involves the integration of local potentials from different synapses on the same neuron.
5. **Duration:** Action potentials have a consistent duration of about 1 millisecond. In contrast, the duration of local potentials can vary and is influenced by the duration of the stimulus.
6. **Role:** Action potentials are the means by which neurons transmit electrical signals over long distances. They are the "all-or-none" events that ensure reliable long-distance communication. Local potentials are subthreshold changes in membrane potential that serve as the initial signals that may or may not lead to the generation of an action potential.
In summary, the all-or-none law is a fundamental property of action potentials, ensuring that they are consistent in magnitude and duration once the threshold is reached. This property distinguishes action potentials from local potentials, which are graded, variable in magnitude, and subject to summation. Action potentials are specialized for long-distance signal transmission in neurons, while local potentials are involved in signal initiation and integration.
Work Step by Step
The "all-or-none" law is a fundamental principle that applies to action potentials in neurons. This law states that an action potential either occurs fully or does not occur at all in response to a stimulus. In other words, there is a threshold that must be reached for an action potential to be generated, and once that threshold is crossed, the action potential is always of the same magnitude and duration.
Here's how the all-or-none law applies to an action potential:
1. **Threshold:** To initiate an action potential, a neuron must reach a certain threshold membrane potential, typically around -55 to -50 millivolts (mV). If the membrane potential does not reach this threshold, no action potential will be generated.
2. **All-or-None Response:** Once the threshold is reached, the neuron will generate an action potential that is consistent in its characteristics, regardless of the strength of the stimulus that initially triggered it. This means that a stronger stimulus does not result in a larger action potential; it merely increases the frequency of action potentials generated.
3. **Magnitude and Duration:** The action potential that occurs when the threshold is reached has a specific magnitude (amplitude) and duration (typically about 1 millisecond). It does not matter whether the stimulus is just strong enough to reach the threshold or significantly stronger; the action potential's magnitude and duration remain the same.
Now, let's contrast action potentials with local potentials:
**Properties of Action Potentials in Contrast to Local Potentials:**
1. **All-or-None vs. Graded:** Action potentials follow the all-or-none law, while local potentials (also known as graded potentials) do not. Local potentials vary in magnitude depending on the strength of the stimulus. Stronger stimuli result in larger local potentials, whereas weaker stimuli produce smaller ones.
2. **Propagation:** Action potentials are self-propagating and can travel long distances along the axon without a decrease in magnitude. In contrast, local potentials are decremental, meaning they diminish in strength as they spread away from their source and do not propagate over long distances.
3. **Threshold:** Action potentials have a distinct threshold that must be reached to be initiated, typically around -55 to -50 mV. Local potentials do not have a fixed threshold; their magnitude depends on the strength of the stimulus.
4. **All-or-None vs. Summation:** Action potentials do not summate; they occur as discrete events. Local potentials, on the other hand, can summate (both temporally and spatially). Temporal summation occurs when multiple local potentials arrive at the trigger zone in rapid succession, while spatial summation involves the integration of local potentials from different synapses on the same neuron.
5. **Duration:** Action potentials have a consistent duration of about 1 millisecond. In contrast, the duration of local potentials can vary and is influenced by the duration of the stimulus.
6. **Role:** Action potentials are the means by which neurons transmit electrical signals over long distances. They are the "all-or-none" events that ensure reliable long-distance communication. Local potentials are subthreshold changes in membrane potential that serve as the initial signals that may or may not lead to the generation of an action potential.
In summary, the all-or-none law is a fundamental property of action potentials, ensuring that they are consistent in magnitude and duration once the threshold is reached. This property distinguishes action potentials from local potentials, which are graded, variable in magnitude, and subject to summation. Action potentials are specialized for long-distance signal transmission in neurons, while local potentials are involved in signal initiation and integration.
