Lecture 3B. Action Potential

Question 1

Action potential

Answer 1

• neurons undergo brief, but large, rapid changes in membrane potential that is propagated down the length of the axon

Question 2

Voltage-gated ion channels

Answer 2

• protein pores embedded in cell membrane
• channel opens in response to specific voltage
• permits ions of specific size & charge to pass through
• think toll booths

Question 3

Ion channel gating: What are the two discrete states?

Answer 3

• Open: conducting/passing ions
• Closed: non-conducting
• Switching states requires conformational change: change in voltage and binding of ligand (chemical)

Question 4

________ ________ changes the membrane potential.

Answer 4

Ion movement

Question 5

At rest, the membrane is ________.

Answer 5

POLARIZED; charge difference, inside neuron more negative

Question 6

At rest, the membrane is ________.

Answer 6

POLARIZED

Question 7

________ is a decrease in membrane potential.

Answer 7

Depolarization; reduction in membrane potential (charge difference), voltage going up (> -70 mV)

Question 8

________ & ________ increase in membrane potential.

Answer 8

Repolarization & Hyperpolarization; increase in charge difference, more negatively charged (< -70 mV) ex. - 80 mV

Question 9

What are the steps of the action potential?

Answer 9

1. Axon membrane potential
2. Threshold voltage reached
3. Depolarization
4. Repolarization
5. Hyperpolarization

Question 10

Axon membrane potential

Answer 10

• membrane potential-70 mV at rest
• Na+ ion channels control the membrane potential at rest

Question 11

Threshold voltage is reached

Answer 11

• if summation of local potentials reaches the threshold (-55 mV), large numbers of Na+ channels open and Na+ rushes into the cell very quickly, because of voltage gated ion channels

Question 12

In step 2, what happens to the dendrites?

Answer 12

receives a signal; when neurotransmitters bind to receptors on postsynaptic neuronal membranes, it produces local potential
- voltage change that spreads passively across POSTSYNAPTIC neuron’s cell membrane

Question 13

What are the different types of local potentials?

Answer 13

• Excitatory postsynaptic potential (EPSP): produces a small local depolarization
  
  • “You should turn on”
  • increase voltage
• Inhibitory postsynaptic potential (IPSP): produces a small hyperpolarization
  
  • “Don’t do anything”
  • negatively charged

Question 14

Neuron receptors receive information from thousands of synaptic connections at any given instant. What determines whether an action potential is fired?

Answer 14

Balance between the number of excitatory and inhibitory signals; threshold (-55 mV); all or none phenomenon

Question 15

Where does the integration of EPSPs and IPSPs occur?

Answer 15

axon hillock

Question 16

Depolarization

Answer 16

“rising phase”
- if summation of local potentials reaches the threshold, large numbers of Na+ channels open up and Na+ rushes out very quickly (threshold voltage reached)
- causes rapid change in membrane potential from -50 mV to +40 mV (rising phase of action potential)
- when membrane potential = +40 mV, voltage-gated Na+ channels close and cannot be opened for a fixed period — the absolute refractory period, cannot create new action potential

Question 17

Repolarization

Answer 17

“falling phase”
- at +40 mV, voltage-gated Na+ channels close & voltage-gated K+ channels open
- K+ ions flow out of cell

Question 18

Hyperpolarization

Answer 18

• too much K+, leaves neuron
• refractory

Question 19

Now there’s too much Na+ in the cell and too much K+ outside. How do you think the cell “resets” itself?

Answer 19

sodium potassium pump; restore normality

Question 20

The ins and outs of an action potential

Answer 20

• rising phase: inward sodium current (depolarization)
• falling phase: outward potassium current (repolarization/hyperpolarization)
• generated at the axon hillock
• move along the axon because Na+ ions spread passively to near by regions, which changes the membrane potential to threshold, which opens more Na+ channels

Question 21

What factors influence conduction velocity?

Answer 21

spread of action potential along membrane
- dependent upon axon structure
- temperature (cold = longer for signals to move)
- diameter of axon (wider = faster electronic signal
- myelination (more myelin = faster)

Question 22

Action potential propagation

Answer 22

• in myelinated axons, regeneration of the action potential occurs only at the nodes of ranvier
  
  • conduction seems to jump along the axon — saltatory conduction
  • less energy is needed because ion channels are only at the nodes

Question 23

Maximum speed of conduction in myelinated vertebrae axons ________.

Answer 23

120 m/s (270 mph); expressway

Question 24

Small, unmyelinated axon of human autonomic nervous system ________.

Answer 24

1 m/s (2.25 mph); under construction, need ion channels