Lecture 11

Question 1

Are mature neurons capable of cell division?

Answer 1

No, this is why damage to nervous tissue can be permanent. PNS has a little capacity to heal itself, but CNS does not.

Question 2

Can nerve fibres be repaired?

Answer 2

Yes, if the damage is not extensive and the neurilemma (Schwann cell cytoplasm around the nerve fibre) are intact, and scarring has not occurred.

Question 3

Can spinal nerve fibres with long axons repair?

Answer 3

No, this is why a spinal injury can result in paralysis.

Question 4

Neurons can exhibit…

Answer 4

Conductivity and excitability; they can conduct electrical impulses, as well as generate them.

Question 5

Describe a nerve impulse.

Answer 5

A wave of electrical fluctuation that travels along the plasma membrane.

Question 6

Resting membrane potential.

Answer 6

Difference in ion concentration across the membrane; -70 mV.

Question 7

Ion distribution inside and outside of the cell.

Answer 7

More potassium inside the cell; more sodium outside the ell.

Question 8

Movement across a membrane os regulated by…

Answer 8

Permeability of the cell membrane.

Question 9

Is a cell at resting membrane potential polarized?

Answer 9

Yes, as the inside is more negative than the outside.

Question 10

2 principle ions that drive membrane potential.

Answer 10

Sodium and potassium.

Question 11

Potassium is the primary contributor to resting membrane potential.

Answer 11

Active transport of K+ outside of the cell creates a gradient that makes the inside of the cell more negative. Changing the permeability of an ion causes a change in resting membrane potential.

Question 12

Which pump contributes to the membrane potential?

Answer 12

Na+K+ pump” 3 Na+ pout for every 2 K+ in; this maintains the electrochemical gradient.

Question 13

Depolarization.

Answer 13

Membrane potential becomes less negative.

Question 14

Repolarization.

Answer 14

A return to the resting membrane potential.

Question 15

Hyperpolarization.

Answer 15

Resting membrane potential becomes even more negative than -70 mV. This makes it difficult for a neuron to fire, it would need a stronger signal.

Question 16

Mechanically gated channels.

Answer 16

Sensory neurons, physical trigger.

Question 17

Chemically gated channels.

Answer 17

Respond to ligands.

Question 18

Voltage gated channels.

Answer 18

Respond to changes inc ell membrane potential.

Question 19

Channelopathies.

Answer 19

Mutations in a channel. Example: cystic fibrosis.

Question 20

How is resting membrane potential maintained?

Answer 20

Transport channels are selective in what they allow across.

Question 21

Can anionic proteins exit the cell through a channel?

Answer 21

No, there are no channels for anionic proteins, which dominate the intracellular fluid. They are trapped inside, which helps maintain the membrane potential.

Question 22

Membrane channels dictate…

Answer 22

Permeability to ions.

Question 23

Active transport also helps maintain resting membrane potential.

Answer 23

Maintains the imbalance of ions (example: sodium and potassium).

Question 24

Sodium potassium ATPase dependent pump is also responsible for maintaining the resting membrane potential.

Answer 24

Maintains the difference in ionic balance. selective permeability.

Question 25

Stimulating a neuron: local potentials.

Answer 25

A slight shift away from the resting membrane potential in response to certain stimuli. Example: excitation of a neuron occurs when a stimulus triggers the opening of Na+ channels on the dendrites, allowing the potential to move to 0 (depolarization); this is also an action potentials.

Question 26

Summation zone.

Answer 26

Sum must be enough to reach the threshold value of -59 mV for the neuron to fire.

Question 27

Inhibition of a neuron.

Answer 27

Occurs when a stimulus triggers the opening of additional K+ channels at the synapse, increasing the membrane potential by allowing K+ to diffuse out of the cell.

Question 28

What happens to a neuron when potassium channels are opened?

Answer 28

K+ leaks out, preventing the membrane potential from reaching the threshold value (moves below -70 mV), thereby inhibiting the neuron.

Question 29

Action potential (nerve impulse).

Answer 29

it is the membrane potential of an active neuron. Adequate stimulus triggers Na+ channels to open, allowing Na+ oto diffuse into the cell, thereby causing a local depolarization. It is the nerve impulse that conducts an electrical signal down the axon and that causes the release of a neurotransmitter.

Question 30

Where are Na+ and K+ channels found in the neuron?

Answer 30

There are many in the membrane of the neurons summation zone and conduction zone.

Question 31

Where do local depolarizations occur?

Answer 31

On the dendrites.

Question 32

What charge does the neuron local depolarization reach when Na+ is at its maximum entry into the cell?

Answer 32

+30 mV.

Question 33

How long do Na+ channels stay open?

Answer 33

1 millisecond; this allows for the same magnitude of the action potential to always be reached.

Question 34

Na+ channels have 2 gates.

Answer 34

Activation gates and inactivation gates. At the resting membrane potential, the inactivation gate closes the channel. Depolarization stimulus: activation gates open and sodium goes in; after 1 millisecond, the inactivation gate is put in, so Na+ entry stops. During the depolarization caused by K+ leaving the cell, the 2 gates reset to their original positions.

Question 35

Depolarization can be triggered by 3 types of signals.

Answer 35

Chemical, mechanical, and electrical.

Question 36

Refractory period.

Answer 36

Cell is hyper polarized because the potassium channels are still open.

Question 37

Absolute refractory period.

Answer 37

An area of the neurones membrane resists restimulation; will not respond to stimulus, no matter how strong. Time is required before a neuron can fire again (usually about half of a millisecond).

Question 38

Relative refractory period.

Answer 38

The membrane is repolarizing; will only respond to a very strong stimulus.

Question 39

At what charge is the neuron when it is firing?

Answer 39

0 mV.

Question 40

Conduction of the action potentials.

Answer 40

Depolarization during the action potential causes electrical current to flow between the site of the action potential and the adjacent regions of the membrane, triggering voltage-gated Na+ channels to open in that next segment; this area now exhibits the action potential.

Question 41

Positive feedback loop.

Answer 41

Positive charge allows more sodium ions to enter until the signal is strong enough for an action potential.

Question 42

Why does an action potential never move backwards?

Answer 42

The refractory period prevents restimulation.

Question 43

In myelinated fibres, where fo action potentials occur?

Answer 43

Only at the Nodes of Ranvier; this is called saltatory conduction. Impulse regeneration leaps from node to node, as only the nodes have Na+ voltage gated channels.

Question 44

Myelinated fibres insulate the axon and allow for what?

Answer 44

Optimal conductance of the electrical signal.

Question 45

Advantages of the myelin sheath.

Answer 45

You need less sodium to propagate the action potential because the sodium and electrical signal is conserved. Results in an increase in conduction velocity.

Question 46

Damage to myelinated axons.

Answer 46

Conduction slows and leaks out; the signal may never get to where it needs to go. Example: multiple sclerosis; sodium moves down the axon, but leaks out due to damaged myelin sheath, so you lose the strength of the signal.