2.3 - Graded Potentials and Action Potentials

Question 1

Where do graded potentials occur?

Answer 1

dendrites or cell body of neurons

Question 2

How are graded potentials triggered?

Answer 2

by 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

TRUE OR FALSE:

Graded potentials travel long distances through the neuron

Answer 4

FALSE

Short distances only

Question 5

Why do graded potentials lose strength?

Answer 5

• current leak: some +ve charges leak back with the depolarization wave
• cytoplasmic resistance: organelles in cytoplasm restrict the flow of the current

Question 6

How is the strength of the signal determined?

Answer 6

by the number of ions entering the cell

Question 7

TRUE OR FALSE

Graded potentials diminish in strength as distance increases

Answer 7

TRUE

Question 8

How is an action potential different from a graded potential?

Answer 8

• there is no volume control, action potential is either fired or not fired “on” or “off”
• they do not diminish in strength when travelling long distances, stays strong all the way

Question 9

Action potentials start at the _____ , also known as ________

Answer 9

at the axon hillock, also known as the integrating center of the neuron also known as the trigger zone

Question 10

How does the location of the trigger zone differ in various types of neurons?

Answer 10

sensory neurons (afferent) = TZ adjacent to receptor

efferent & interneurons = axon hillock and initial segment of the axon

Question 11

what is the minimum depolarization necessary to trigger an action potential called?

Answer 11

threshold potential

Question 12

What is excitability?

Answer 12

the ability for a neuron to respond to stimuli and fire an action potential

Question 13

How can graded potentials lead to action potentials?

Answer 13

• spatial summation

- temporal summation

Question 14

What are EPSPs and IPSPs?

Answer 14

EPSP: Excitatory Post Synaptic Potentials

• make the membrane potential less -ve
• brings membrane potential closer to threshold potential
• increase chance of exciting the axon to fire

IPSP: Inhibitory Post Synaptic Potential

• hyperpolarize cell membrane
• make more -ve
• decrease chance of firing AP

Question 15

List the phases of the AP

Answer 15

• resting membrane potential: -70 mV
• depolarizing stimulus: reaches threshold potential
• rising phase: increases to max +30 mV (Na+ enters cell)
• repolarization: membrane potential travels down towards resting membrane potential (K+ leaves cell)
• hyperpolarization phase: MP overshoots resting potential
• resting membrane potential: repolarization back to resting potential

Question 16

What is sodium activation?

Answer 16

• the opening of voltage gated Na+ channels which occurs once the threshold potential is exceeded

Question 17

What sets the threshold potential for the initiation of the AP?

Answer 17

• sodium channel properties

- open at -55mV

Question 18

List the steps of the rising phase

Answer 18

1. resting membrane potential
2. depolarizing stimulus
3. voltage gated Na+ channels open at -55mV
4. Na+ enters the cell
5. causes further depolarization
6. more Na+ channels open

Question 19

What is the trigger for the closing of the inactivation gate of Na+?

Answer 19

when the membrane potential reached -55mV, triggered opening of activation channel and closing of inactivation channel

Question 20

How does the membrane potential return to the resting level?

Answer 20

when K+ ions leave the cell: falling phase of the AP

Question 21

The full gating potential for K+ channels happen at

Answer 21

30 mV

Question 22

When does the refractory period happen?

Answer 22

During the hyperpolarizing phase

Question 23

What are the 2 parts of the refractory period? Explain

Answer 23

absolute refractory period:

• last 1msec to 2msec
• no AP can be triggered, no matter how large the stimulus

relative refractory period

• really large stimulus is required to bring forth an AP
• threshold value moved closer to zero
• K+ channels are still open

Question 24

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

Answer 24

• Na+ channels are inactivated

Question 25

Why is the refractory period important?

Answer 25

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

Question 26

What determines how fast an AP can travel along the neuron?

Answer 26

• diameter of the axon

- resistance of the membrane to ion leakages

Question 27

What is saltatory conduction?

Answer 27

• when action potentials jump from one node of Ranvier to the next
• allows rapid AP through small axons