Unit 2.3: Graded Potentials & Action Potentials

Question 1

Where do graded potentials occur?

Answer 1

In the dendrites or cell body of the neurons`

Question 2

What are graded potentials triggered by?

Answer 2

The opening or closing of ion channels

Question 3

Why are graded potentials called “graded”?

Answer 3

Because the amplitude of the potential is proportional to the strength of the triggering event

Question 4

Why do graded potentials lose strength?

Answer 4

1. Current leak

2. Cytoplasmic resistance

Question 5

What causes ions to enter the cell?

Answer 5

Initiated by neurotransmitters binding to the membrane receptors and opening ion channels

Question 6

What happens after the neurotransmitter binds?

Answer 6

1. ion channels open
2. Ions move in (Na+) or out (K+) of the neuron along their electrochemical gradient
3. A wave of depolarization spreads through the cell

Question 7

What is signal strength determined by?

Answer 7

Strength is determined by number of ions entering the cell

Question 8

How do action potentials differ from graded potentials?

Answer 8

1. Action potentials are all identical (no volume control, they are on or off)
2. Action potentials do not diminish in strength as they travel long distance through the neuron, signal is strong all the way

Question 9

Where do action potentials start?

Answer 9

At the trigger zone, also called axon hillock

Question 10

What is the axon hillock known to the neuron?

Answer 10

The integrating center of the neuron

Question 11

Where is the trigger zone in afferent and efferent neurons/interneurons?

Answer 11

Afferent (sensory) neurons: adjacent to the receptor

Efferent (motor) neurons/interneurons: axon hillock and initial segment (very first part of the axon)

Question 12

What is the minimum depolarization necessary to trigger an action potential called? What is its value?

Answer 12

Threshold potential, -55 mV

Question 13

What are the 2 ways graded potentials can sum over?

Answer 13

1. Spatial summation

2. Temporal summation

Question 14

What are depolarizing graded potentials?

Answer 14

Make the membrane potential less negative

Brings the membrane potential closer to the threshold potential

Question 15

What are depolarizing graded potentials also called? Why?

Answer 15

Excitatory Post Synaptic Potentials (EPSP)

Because they increase the chance of exciting the axon to “fire”

Question 16

What are hyperpolarizing graded potentials?

Answer 16

Make the membrane potential more negative

Brings the membrane potential away to the threshold potential

Question 17

What are hyperpolarizing graded potentials also called? Why?

Answer 17

Inhibitory Post Synaptic Potentials (EPSP)

Because they decrease the chance of exciting the axon to “fire”

Question 18

What are the phases of the action potential?

Answer 18

1. Resting membrane potential (-70 mV)
2. Depolarizing stimulus -> reaches threshold potential
3. Rising (depolarization) phase -> max +30 mV
4. Repolarization phase -> membrane potential travels towards resting potential
5. Hyperpolarization phease -> membrane potential overshoots resting potential
6. resting membrane potential -> repolarization back to resting

Question 19

What is sodium activation?

Answer 19

Action potentials cannot fire unless the voltage gated Na+ channels open

Question 20

What sets the threshold potential?

Answer 20

Sodium channels properties

They open at -55 mV

Question 21

What are the steps to the rising phase?

Answer 21

1. Resting membrane potential
2. Depolarizing stimulus
3. Voltage-gated Na+ channels open
4. Na+ enters the cell
5. Causes further depolarization
6. More voltage-gated Na+ channels opens

Question 22

What stops the positive feedback loops of the Na+?

Answer 22

After a short delay, the inactivation gate closes, preventing further Na+ entry to the cell

Question 23

How does the membrane potential return to the resting level?

Answer 23

K+ leave the cells -> falling phase of AP

Question 24

What is the gating potential of K+?

Answer 24

+30 mV

Question 25

What are the 2 parts of the refractory period?

Answer 25

1. Absolute refractory period (NO action potentials can be triggered at all)
2. Relative refractory period (really large suprethreshold stimulus is required)

Question 26

Why can’t an action potential be generated during the absolute refractory period?

Answer 26

Because the Na+ channel is in the inactive state

Question 27

Why does it take a suprathreshold stimulus to generate an AP during the relative refractory period?

Answer 27

Because the K+ channels are still open to more Na+ is needed to reach threshold potential

Question 28

Why is the refractory period important?

Answer 28

It sets the direction of the current flow, prevents temporal summation and it prevents the AP from going backwards

Question 29

How can action potentials travel long distances along neurons without decreasing in strength?

Answer 29

Because of the Na+ channel positive feedback loop

Question 30

What determines how fast an action potential can travel along the neuron?

Answer 30

The resistance of the membrane

Question 31

How can membrane resistance be altered?

Answer 31

By changing the diameter of the neuron or by reducing the amount of leakiness

Question 32

How are axons in invertebrates?

Answer 32

They have GIANT axons
This lowers the surface area to volume ratio
Reduces leakiness and increases conduction

Question 33

How are axons in vertebrates?

Answer 33

Vertebrates have insulated nerves

Reduces resistance and increases conduction

Question 34

What is a break in the myelin sheath called?

Answer 34

Node of Ranvier

Question 35

What is saltatory conduction>

Answer 35

Na+ channels are concentrated in nodes of Ranvier
Action potentials jump from one node to the other
Conduction is faster