action potential steps Flashcards
What is depolarisation?
Depolarisation occurs when a neurone is stimulated, causing Na⁺ to enter via voltage-dependent channels, making the inside of the cell more positive.
What happens when the potential difference becomes positive during depolarisation?
More Na⁺ voltage-dependent channels open, and once a threshold is reached, it becomes an “all-or-nothing” response, leading to depolarisation reaching +40 mV.
What is repolarisation?
Repolarisation happens after 0.5 ms when Na⁺ channels close, and K⁺ channels open, causing K⁺ to leave the cell, making the inside more negative again.
How does the potential difference become negative during repolarisation?
As K⁺ leaves the cell, the potential difference becomes negative, restoring towards the resting potential.
What is hyperpolarisation?
Hyperpolarisation occurs when the potential difference temporarily becomes more negative than the resting potential due to slow closing of K⁺ channels.
What restores the resting potential after hyperpolarisation?
K⁺ voltage-dependent channels close, and K⁺ diffuses through the axon, returning the potential difference to resting levels.
Why does the action potential only move forward along the neurone?
The hyperpolarisation ensures the action potential moves forward by preventing immediate reactivation of the previous section of the neurone.
How does the sodium-potassium pump work?
The pump actively transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, using ATP, to maintain the resting potential.
What triggers the opening of voltage-gated Na⁺ channels?
A stimulus causes a slight depolarisation of the membrane, and if this depolarisation reaches the threshold potential (-55 mV), voltage-gated Na⁺ channels open.
Why is the action potential described as “all-or-nothing”?
Once the threshold potential is reached, the action potential will occur completely. If the threshold is not reached, no action potential occurs.
What is the refractory period?
The refractory period is the time during which a neurone cannot fire another action potential, ensuring one-way transmission of the impulse.
How does myelination affect the speed of action potential transmission?
Myelination increases transmission speed by allowing action potentials to jump between nodes of Ranvier in a process called saltatory conduction.
What are the nodes of Ranvier?
The nodes of Ranvier are gaps in the myelin sheath where voltage-gated ion channels are concentrated, facilitating rapid conduction of action potentials.
What role does K⁺ play in repolarisation?
K⁺ exits the cell through voltage-gated K⁺ channels, restoring the negative membrane potential during repolarisation.
What ensures unidirectional propagation of an action potential?
The refractory period prevents the action potential from traveling backward, ensuring it moves only in one direction along the neurone.
What factors affect the speed of nerve impulse transmission?
Axon diameter: Larger diameters result in faster transmission.
Temperature: Higher temperatures increase conduction speed.
Myelination: Myelinated fibres transmit impulses faster than unmyelinated ones.
What are voltage-dependent ion channels?
Voltage-dependent ion channels are membrane proteins that open or close in response to changes in membrane potential, allowing specific ions to flow across the membrane.
What is the role of voltage-gated sodium (Na⁺) channels in action potentials?
Voltage-gated Na⁺ channels open when the membrane potential reaches the threshold, allowing Na⁺ to rush into the cell and cause rapid depolarisation.
When do voltage-gated sodium (Na⁺) channels close?
These channels close after about 0.5 ms, at the peak of the action potential, to prevent further Na⁺ influx.
What is the role of voltage-gated potassium (K⁺) channels in action potentials?
Voltage-gated K⁺ channels open during repolarisation, allowing K⁺ to leave the cell, restoring the negative membrane potential.
What happens during hyperpolarisation related to K⁺ channels?
Voltage-gated K⁺ channels close slowly, causing an excess of K⁺ to leave the cell, making the membrane potential temporarily more negative than the resting potential.
How do voltage-gated channels ensure the unidirectional flow of action potentials?
Inactivation of Na⁺ channels during the refractory period prevents the action potential from traveling backward.
What triggers the opening of voltage-gated potassium (K⁺) channels?
K⁺ channels open when the membrane potential becomes positive (around +40 mV) at the peak of the action potential.
What is the threshold potential for voltage-gated Na⁺ channels?
The threshold potential is typically around -55 mV, triggering the opening of voltage-gated Na⁺ channels for an action potential.
What happens during depolarization in the action potential?
Voltage-dependent Na⁺ channels open, allowing Na⁺ to flow into the axon, which depolarizes the membrane.
What triggers repolarization of the membrane?
Voltage-dependent Na⁺ channels close, and voltage-dependent K⁺ channels open, allowing K⁺ to leave the axon.
Why does hyperpolarization occur during an action potential?
The membrane becomes hyperpolarized because voltage-dependent K⁺ channels remain open longer than necessary, causing excess K⁺ to leave the axon.
How is the resting potential restored after hyperpolarization?
Voltage-dependent K⁺ channels close, and K⁺ diffuses back into the axon to recreate the resting potential.
What is the resting potential of a typical neuron?
The resting potential is around -70 mV, with the inside of the axon being negatively charged relative to the outside.
Which channels are responsible for the inflow of Na⁺ during depolarization?
Voltage-dependent Na⁺ channels.
Which channels allow K⁺ to leave the axon during repolarization?
Voltage-dependent K⁺ channels.
What is the role of voltage-dependent ion channels in generating an action potential?
They regulate the flow of Na⁺ and K⁺ ions, which causes depolarization, repolarization, and hyperpolarization of the membrane.
What is the key event that marks the peak of the action potential?
Voltage-dependent Na⁺ channels close, and voltage-dependent K⁺ channels begin to open.
How does the polarity of the axon membrane change during an action potential?
It switches from negative (resting potential) to positive (depolarization) and back to negative (repolarization and hyperpolarization).
