Nerve/synapse II

Question 1

How is the resting membrane potential of the neuron maintained?

Answer 1

Na+ and K+ gradients are maintained by the Na+/K+ pump, which uses ATP to pump Na+ out and K+ in against their concentration gradients.

Question 2

What are action potentials?

Answer 2

They are brief electrical impulses that axons propagate from one region of the nervous system to another.

Question 3

Action potentials travel from the […] to the […] of a neuron

Answer 3

Initial segment, presynaptic terminals

Question 4

What is the difference between depolarization and hyperpolarization?

Answer 4

Depolarization is when the membrane potential gets more positive compared to where it started. Hyperpolarization is when the membrane potential gets more negative.

Question 5

The action potential involves a [depolarization/hyperpolarization] from […] mV to […] mV

Answer 5

Depolarization, -70 mV, +30 mV

Question 6

Explain the threshold potential and how it relates to the action potential.

Answer 6

The action potential is initiated when the membrane potential depolarizes to the threshold level, -50 mV. If it depolarizes to below that, it’ll relax back to -70 mV and not send out a signal. If it reaches that threshold, an action potential is sent out, as it is an all-or-nothing event.

Question 7

The depolarizing phase of the action potential is caused by […]. Explain their critical 3 properties.

Answer 7

Sodium ions flowing into the cell through voltage-gated sodium channels. Sodium channels are (1) closed at resting membrane potential but open when the membrane depolarizes, (2) selective for Na+, (3) able to rapidly inactivate after opening, stopping the flow of Na+ ions.

Question 8

Explain the process by which reaching the threshold of depolarization releases the action potential.

Answer 8

When the membrane depolarizes to -50 mV, a small number of Na+ channels will open and allow Na+ to flow in, further depolarizing the membrane potential. This will cause more Na+ channels to open, allowing more Na+ in and further depolarizing the membrane potential. This leads to a positive feedback loop at results in the membrane potential reaching +30 mV.

Question 9

Why do action potentials top out at +30 mV?

Answer 9

The Na+ channels that open as a result of membrane depolarization quickly deactivate. Also, some Na+ can still leak out. When the channels deactivate, there’s no more Na+ permeability, and the membrane potential returns back to -70 mV.

Question 10

Explain why Na+ is able to become the dominant ion when the membrane is depolarized.

Answer 10

The density of voltage-gated sodium channels in the axon membrane is much higher than the density of leak potassium channels, so at the peak action potential, Na+ permeability becomes much higher than the resting permeability for K+.

Question 11

Explain the role of voltage-gated potassium channels in the action potential.

Answer 11

In addition to sodium channel inactivation, a second factor that contributes to the falling phase of the action potential is the delayed activation of voltage-gated potassium channels. When they are activated, they allow for K+ ions to flow out faster than at resting phase, which balances out the inflow of Na+ and speeds up the repolarization phase.

Question 12

What is the advantage of a short action potential?

Answer 12

This allows for many signals to be fired from a neuron in rapid succession.

Question 13

Explain how action potential propagation down the axon occurs.

Answer 13

Say we start with depolarizing the membrane potential in the initial segment. Recall that at rest, the potential is -70 mV everywhere. The initial segment will go up to +30 mV, and the positive charge will be attracted to the adjacent negative charge. This will then create an action potential in the next segment. The cycle will continue until you get to the presynaptic terminal.

Question 14

Why does propagation occur in one direction down the axon?

Answer 14

It does to a small extent, but sodium channels deactivate after a small amount of time, so the previous segments behind the action potential quickly become inactive. This forces the potential overall forward.

Question 15

What are the absolute and relative refractory periods?

Answer 15

The absolute refractory period is when the sodium channels are inactivated and the membrane is unexcitable. It is the time it takes for the membrane to come back down to -70 mV from +30 mV.

The relative refractory period is longer; it is when the voltage-gated potassium channels are open and the membrane potential overshoots its resting level to -90 mV before settling back at -70 mV. During this period, it is very unlikely that the axon will fire another signal.

Question 16

How do neurons transmit specific information using action potentials?

Answer 16

They cannot change the intensity of the action potentials, as they are all-or-nothing. So they send information by means of the frequency and pattern of action potentials.

Question 17

Neurotoxins are usually target […] in victims, because […]

Answer 17

Sodium channels, they are essential in allowing action potentials to be sent out.

Question 18

Puffer fish make the neurotoxin […]. Describe the effect that it has.

Answer 18

Tetrodotoxin. It is a potent inhibitory of sodium channels, which blocks action potentials from being released from neurons.

Question 19

Phyllobates frogs secrete the neurotoxin […]. Describe the effect that it has.

Answer 19

Batrachotoxin. It is a powerful sodium channel activator, which forces them open. This can cause seizures.

Question 20

What types of drugs can block sodium channels?

Answer 20

Local anaesthetics and antiepileptics

Question 21

Give 4 examples of local anaesthetics that can block sodium channels.

Answer 21

Lidococaine, benzocaine, tetracaine, cocaine

Question 22

Give 3 examples of antiepileptics that can block sodium channels.

Answer 22

Phenytoin (dilantin), carbamazepine (tegretol), lamotrigine

Question 23

What is the purpose of drugs that block sodium channels?

Answer 23

They can be taken to prevent seizures by suppressing brain action potential, thus keeping them under control.

Question 24

How do the axons of squids differ from those of mammals? Explain the effect that this has on their function.

Answer 24

The axons of squids have 1000X larger diameter than those of mammals. This causes the propagation of action potentials to go much faster than mammals, which is essential for squids in order to survive situations that require rapid, reflexive responses. The propagation rate of the action potential is proportion of axon diameter.

Question 25

How do mammals make up for the small diameter of the axons of the neurons?

Answer 25

They create a higher conduction velocity by wrapping the axon in myelin

Question 26

Describe the structure and makeup of the myelin that surrounds vertebrate axons.

Answer 26

It is discontinuous; the myelin is broken up by empty spots called nodes of Ranvier. In the PNS, the myelin is formed by Schwann cells, while in the CNS, it is formed by oligodendrocytes.

Question 27

What is the function of the myelin and nodes of Ranvier?

Answer 27

The myelin serves as an electrical insulator, allowing charge to travel farther and faster down the axon. The nodes of Ranvier contain high concentrations of voltage-gated sodium channels, which allow the signal to be regenerated at periodic intervals.

Question 28

Where are Na+ channels found along the axon?

Answer 28

They are found only at the initial segment and at the nodes of Ranvier.

Question 29

What is the cause of multiple sclerosis?

Answer 29

It is caused by the loss of myelin, leading to demyelinated regions on the axon.

Question 30

What is the difference between white and gray matter in the brain?

Answer 30

White matter corresponds to the regions of the brain and spinal cord that contain mostly myelinated axons. Gray matter comprises cell bodies, dendrites, and synapses.