Lecture 8 + 9

Question 1

Patch Clamp Technique

Answer 1

permits recording of a single ion channel’s current

Created by Neher and Sakman (won Nobel Prize)

Question 2

Macroscopic Currents

Answer 2

the sum of all the single channel currents of all Na and K channels expressed in the neuron/axon

Question 3

Macroscopic Na+ Currents

Answer 3

The inward Nav current shows fast activation followed by fast inactivation

Question 4

Macroscopic K+ Currents

Answer 4

The outward K current has slow activation and no inactivation

Question 5

What determines how soon action potentials can follow each other?

Answer 5

Refractory period

Question 6

Axon Initial Segment (AIS)

Answer 6

region of the axon where APs initiate

The AIS can be variable in its location, even in the same neurons where it can be modulated by neurohormones

Question 7

Threshold

Answer 7

tipping point, where enough axonal Nav channels have been activated to overcome any outward K+ currents (i.e. through leak and K channels)

Question 8

Rising Phase of the Action Potential

Answer 8

Occurs after threshold has been reached

Once over the tipping point, depolarizing Nav currents activate even more Nav channels, creating an all or none response

Question 9

Falling Phase of the Action Potential

Answer 9

Occurs after the rising phase

K channels are like sleeping giants…
• They are slower to respond to depolarization from EPSPs and Nav channels
• However once activated, they conduct massive outward K+ currents that quickly repolarize the membrane (i.e. falling phase of the action potential)

Question 10

Undershoot aka. Afterhyperpolarization

Answer 10

Activated K channels temporarily drive Vm below RMP to generate the AP undershoot

Question 11

Is the AP threshold static?

Answer 11

NO! AP threshold is not static

It can change significantly depending on RMP, and the expression levels and
gating properties of Na , K and other channels expressed in the axonal membrane (AIS)

Question 12

Fast Nav channel inactivation

Answer 12

occurs via “ball and chain” structure

Question 13

Slow Nav channel inactivation

Answer 13

occurs CONCURRENTLY via conformation changes in various regions, in particular S6 helices

Question 14

Refractory Period

Answer 14

After an AP, the membrane is hyperpolarized, and lots of Kv channels are activated

The hyperpolarized Vm removes inactivation of Nav channels (i.e. the Nav channels recover from inactivation… a.k.a deinactivate)

The time required to deinactivate enough Nav channels so that we can have a new AP sets the refractory period

Question 15

Absolute refractory period

Answer 15

• Due Nav channel inactivation
• No amount of stimulation can generate a new action potential

determines the maximum AP frequency

Question 16

Relative refractory period

Answer 16

• Involves the de-inactivation of Nav
channels
• Also involves the closure of Kv channels which hyperpolarize the membrane

determines the relationship between stimulation strength and AP frequency

A neuron with a faster (shorter) relative refractory period will have a higher AP frequency for a given stimulus than one with a slower relative refractory period

Question 17

AP frequency and the Human Nervous System

Answer 17

information is encoded depending on AP frequency
• Stronger stimulus = higher frequency
• e.g.retinal ganglion cells

Question 18

Given the strongest stimulus possible, how fast can APs fire in succession?

Answer 18

Depends on Absolute Refractory Period

e.g. for a 1 ms absolute refractory period, max. freq. = 1000Hz

Question 19

___ Na+ leak conductance depolarizes RMP

Answer 19

↑ Na+ leak conductance depolarizes RMP

Increases AP frequency in a “tonically” firing neuron

Question 20

___ Na+ leak conductance hyperpolarizes RMP

Answer 20

↓ Na+ leak conductance hyperpolarizes RMP

Question 21

___ K+ conductance hyperpolarizes RMP

Answer 21

↑ K+ conductance hyper polarizes RMP

• Decreases AP frequency

Question 22

___ K+ conductance depolarizes RMP

Answer 22

↓ K+ conductance depolarizes RMP

Question 23

2-pore K+ channels

Answer 23

provide the major K+ leak current at rest (gK)

15 genes in mammals

Question 24

NALCN (sodium leak channel)

Answer 24

POTENTIAL candidate for Na+ leak currents

however, the source of Leak Na+ currents is still unknown

Question 25

Voltage-gated Ca2+ (Cav) channels

Answer 25

modulate AP shape and frequency

• P-loop channels with similar structure to Nav channels
• Slower gating than Nav channels

Question 26

“Low voltage activated” (LVA) Cav channels

Answer 26

require only slight depolarization for activation

• Activated by graded and action potentials, and other modalities for membrane depolarization
• Start activating before Nav channels, so they can help regulate when APs take place

Question 27

“High voltage activated” (HVA) Cav channels

Answer 27

equire strong depolarization for activation

• Activate only after Vm is considerably depolarized, hence after an AP has been initiated
• Thus they tend to play important roles in coupling AP events with cellular events…
  * Pre-synaptic Cav drive NT secretion
  * Somatic Cav channels regulate gene expression
  * Muscle Cav channels drive contraction

Question 28

LVA channels and the developing human heart

Answer 28

LVA channels help set the heart rate in the developing human heart
• Play a more prominent role in hearts of small adult mammals and invertebrates

Question 29

HVA channels and the adult heart

Answer 29

• In the adult mammalian heart, high numbers of HVA channels widen the cardiac action potential for more Ca2+ influx and stronger contraction
• These are called afterdepolarizations

Question 30

Small conductance K+ (SK) channels

Answer 30

bind calmodulin (a Ca2+ sensor) at the C-terminus

• Activated by both depolarization and Ca2+ influx
• Associate closely or directly couple with Cav channels to be close to the source of Ca2+

Question 31

Big conductance K+ (BK) channels

Answer 31

directly bind cytoplasmic Ca2+ via C-terminal “calcium bowl” motif
• Also activated by depolarization and Ca2+ influx

Question 32

How do Cav channel activation of BK and SK channels modulate afterhyperoliarization

Answer 32

In neurons, Cav channel activation of BK and SK channels prolongs action potential afterhyperpolarization

• BK channels activate very quickly and contribute to fast AHP
• SK channels are slower and last longer, leading to slow AHP
• When expressed in neurons, more Ca2+ = more AHP

Question 33

HVA Cav channels drive spike frequency adaptation

Answer 33

1. Sequential AP activate more and more Cav channels
2. This activates more and more Ca2+ activated K+ channels
   • Leads to increased afterhyperpolarization
3. AP frequency thus slows down until APs eventually stop

Question 34

Like in the heart, in the absence of SK and BK channels, ______ can contribute to neuronal afterdepolarization

Answer 34

Answer: Cav channels

Like in the heart, in the absence of SK and BK channels, Cav channels can contribute to neuronal afterdepolarization