Lecture 3 - Action Potential

Question 1

What is an action potential?

Answer 1

An action potential is a rapid and transient change in the membrane potential of a neuron, allowing for the transmission of electrical signals along the axon.

Question 2

Who were Camillo Golgi and Santiago Ramón y Cajal?

Answer 2

Camillo Golgi and Santiago Ramón y Cajal were the researchers awarded the Nobel Prize in 1906 for their research on the nervous system’s structure (they discovered the structure of the nervous system by staining it via crystallization).

Question 3

What does the Golgi stain do?

Answer 3

It helps 2% of brain cells change color by creating silver chromate, which crystallizes inside neurons, making them easier to see.

Question 4

What is crystallization?

Answer 4

Crystallization is when atoms or molecules arrange themselves into a rigid structure, forming a crystal lattice to minimize their energy.

Question 5

What are the key parts of a neuron’s structure?

Answer 5

The key parts of a neuron’s structure are the soma (cell body), axon, axon hillock, dendrites, and synapses.

Question 6

What is the resting membrane potential of a neuron?

Answer 6

The resting membrane potential is between -40 to -90 mV, with the inside of the neuron being more negatively charged than the outside.

Question 7

What is the threshold potential?

Answer 7

The threshold potential is the critical level of depolarization that must be reached for an action potential to occur, typically around -55 mV.

Question 8

What is the role of ion channels in neurons?

Answer 8

Ion channels allow ions to pass in and out of the cell, which is crucial for maintaining membrane potential and transmitting signals.

Question 9

What is the function of the sodium-potassium pump in neurons?

Answer 9

It pumps 3 sodium ions out of the cell and 2 potassium ions in, helping maintain the concentration gradients that contribute to the resting membrane potential.

Question 10

What is electrostatic pressure?

Answer 10

Electrostatic pressure is the force between oppositely charged ions that attract and similarly charged ions that repel each other.

Question 11

What is the force of diffusion?

Answer 11

Diffusion is the movement of molecules from areas of high concentration to areas of low concentration, helping ions like K+ move in and out of the cell.

Question 12

What are the phases of an action potential?

Answer 12

• Resting phase
• Stimulus/threshold reached
• Depolarization (rising phase)
• Overshoot/peak action potential
• Repolarization (falling phase)
• Hyperpolarization/refractory period
• Resting phase

Question 13

What happens when voltage-gated sodium channels open?

Answer 13

Sodium ions rush into the cell, causing depolarization of the membrane, which helps propagate the action potential down the axon.

Question 14

What is the resting phase?

Answer 14

The neuron is at rest, maintaining a resting membrane potential of approximately -70 mV. This potential is established by the differential distribution of ions, primarily sodium (Na⁺) and potassium (K⁺), across the neuronal membrane. The sodium-potassium pump actively transports Na⁺ out of the cell and K⁺ into the cell, while potassium leak channels allow K⁺ to flow out.

• Closed voltage-gated sodium and potassium channels.
• The neuron is polarized and ready to respond to a stimulus.

Question 15

What is the stimulus/threshold reached phase?

Answer 15

A stimulus (e.g., neurotransmitter binding) causes a small depolarization of the membrane potential. If the stimulus is strong enough to depolarize the membrane to a threshold level (around -55 mV), an action potential will be triggered.

• Depolarization occurs as sodium channels begin to open, but not all are activated yet.
• The membrane potential moves closer to zero.

Question 16

What is depolarization?

Answer 16

Once the threshold is reached, voltage-gated sodium channels open rapidly. Na⁺ ions rush into the neuron, causing the membrane potential to become significantly more positive. This phase is characterized by a rapid increase in voltage.

• Membrane potential rises rapidly, often reaching around +30 mV.
• Sodium influx dominates, causing the inside of the cell to become positively charged.

Question 17

What is the overshoot/peak action potential phase?

Answer 17

The membrane potential peaks and surpasses zero, reaching its maximum value (the overshoot phase). At this point, the neuron is fully depolarized.

• Maximum membrane potential is reached (+30 mV or higher).
• Voltage-gated sodium channels begin to close, and the neuron is less responsive to further stimulation (absolute refractory period starts).

Question 18

What is the repolarization/falling phase?

Answer 18

Voltage-gated sodium channels close, and voltage-gated potassium channels open. K⁺ ions flow out of the neuron, leading to a rapid decrease in membrane potential back toward the resting level.

• Membrane potential decreases quickly.
• The neuron becomes more negative as K⁺ exits the cell.

Question 19

What is the hyperpolarization/refractory phase?

Answer 19

The membrane potential becomes more negative than the resting potential (hyperpolarization), often reaching around -80 mV. This occurs due to the prolonged opening of potassium channels. During this phase, the neuron is less likely to fire another action potential (relative refractory period).

• Membrane potential is below resting level.
• Some sodium channels are closed, but a few may be in a state that can be opened with a strong stimulus (relative refractory period).

Question 20

What is the return to resting phase?

Answer 20

Voltage-gated potassium channels close, and the sodium-potassium pump helps restore the ion concentrations to their resting states. The membrane potential stabilizes back at the resting level.

• The neuron returns to its resting membrane potential (-70 mV).
• The neuron is ready to respond to new stimuli again.

Question 21

What role do voltage-gated sodium channels play in action potentials?

Answer 21

Voltage-gated sodium channels open in response to depolarization, allowing sodium ions to flow into the neuron, further depolarizing the membrane and leading to the rising phase of the action potential.

Question 22

What role do voltage-gated potassium channels play in an action potential?

Answer 22

They help restore the resting membrane potential by allowing K+ ions to flow out of the cell after an action potential.

Question 23

What is repolarization in the context of an action potential?

Answer 23

Repolarization is the phase following depolarization, where the membrane potential returns to its resting state due to the efflux of potassium ions through voltage-gated potassium channels.

Question 24

What is the refractory period in neurons?

Answer 24

It’s a brief period after an action potential when the membrane potential dips below its resting state, making it harder to trigger another action potential.

Question 25

How are action potentials recorded in a laboratory setting?

Answer 25

Researchers use glass microelectrodes and an oscilloscope to measure the electrical charge across a squid axon’s membrane.

Question 26

What happens when an action potential reaches the axon terminal?

Answer 26

Voltage-gated calcium channels open, allowing Ca2+ ions to enter, which triggers neurotransmitter release into the synapse.

Question 27

How does myelination affect action potentials?

Answer 27

Myelination increases the speed of action potential propagation by allowing the electrical signal to jump between the nodes of Ranvier, a process known as saltatory conduction.