Graded potentials are short lived and travel only short distances. Action potentials a long lasting and travel long distances. Where do graded potentials occur? Where do action potentials occur? What is the difference in membrane channels in these two locations and how does this relate to the differences between graded potentials and action potentials? 2. If the resting membrane potential of a neuron is -70mV, give an example of the membrane potential for a depolarized cell and explain what is happening. If the resting membrane potential of a neuron is -70mV, give an example of the membrane potential for a hyperpolarized cell and explain what is happening. 3. Once AP has begun, second AP can’t trigger for about 2 msec, Why? 4. What are the two factors that affect the speed of action potentials? 5. Describe the difference between continuous conduction vs saltatory conduction down the length of a neuron and how this affects the speed of action potentials.
The Correct Answer and Explanation is :
- Graded potentials and action potentials:
- Graded potentials occur primarily in the dendrites and cell body of a neuron. These are small changes in membrane potential that can vary in size depending on the strength of the stimulus. They occur because of the activation of ligand-gated ion channels and are localized, meaning they don’t travel long distances.
- Action potentials occur at the axon hillock and propagate along the axon to the axon terminals. These are large, rapid depolarizations of the membrane caused by the opening of voltage-gated ion channels, specifically Na⁺ channels for depolarization and K⁺ channels for repolarization. Action potentials are all-or-nothing events that travel long distances without diminishing in size. The difference in membrane channels relates to the nature of these two potentials:
- Graded potentials involve chemically gated channels and can be summed up, either spatially (from different locations) or temporally (from repeated stimuli at the same location). This explains why graded potentials are short-lived and localized.
- Action potentials depend on voltage-gated channels, which trigger the large-scale and rapid depolarization and repolarization of the neuron. This causes action potentials to propagate over long distances without fading out.
- Resting membrane potential and changes:
- Depolarized cell: If the resting membrane potential is -70mV, a depolarized membrane might have a potential of -55mV or less. This occurs when Na⁺ channels open and Na⁺ ions rush into the cell, making the inside of the cell more positive, moving the potential toward the threshold for triggering an action potential.
- Hyperpolarized cell: A hyperpolarized membrane might have a potential of -90mV. This happens when K⁺ channels remain open longer than usual or when Cl⁻ ions enter the cell, making the inside of the cell more negative than the resting potential, moving further away from the threshold for an action potential.
- Refractory period: Once an action potential has begun, a second action potential cannot trigger for about 2 milliseconds because of the absolute refractory period. During this time, Na⁺ channels are inactivated and cannot open again until they return to their resting state, preventing the propagation of another action potential immediately after the first.
- Factors affecting the speed of action potentials:
- Myelination: Myelinated axons conduct action potentials faster because myelin acts as insulation, allowing the action potential to jump between nodes of Ranvier, a process known as saltatory conduction.
- Axon diameter: Larger axon diameters allow for faster conduction due to lower internal resistance to ion flow.
- Continuous vs. Saltatory conduction:
- Continuous conduction occurs in unmyelinated axons, where the action potential travels continuously along the entire axon, triggering the opening of voltage-gated ion channels along the way. This results in slower conduction.
- Saltatory conduction occurs in myelinated axons, where the action potential jumps from one node of Ranvier to the next, where voltage-gated ion channels are concentrated. This significantly speeds up the conduction of the action potential compared to continuous conduction.
In summary, graded potentials occur in dendrites and cell bodies and are small, localized, and variable, while action potentials occur in axons and are large, all-or-nothing events that propagate long distances. The differences in membrane channel types—ligand-gated for graded potentials and voltage-gated for action potentials—account for these characteristics. Additionally, factors like axon diameter and myelination affect the speed of action potential conduction.