Lecture 8 + 9 Flashcards
Patch Clamp Technique
permits recording of a single ion channel’s current
Created by Neher and Sakman (won Nobel Prize)
Macroscopic Currents
the sum of all the single channel currents of all Na and K channels expressed in the neuron/axon
Macroscopic Na+ Currents
The inward Nav current shows fast activation followed by fast inactivation
Macroscopic K+ Currents
The outward K current has slow activation and no inactivation
What determines how soon action potentials can follow each other?
Refractory period
Axon Initial Segment (AIS)
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
Threshold
tipping point, where enough axonal Nav channels have been activated to overcome any outward K+ currents (i.e. through leak and K channels)
Rising Phase of the Action Potential
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
Falling Phase of the Action Potential
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)
Undershoot aka. Afterhyperpolarization
Activated K channels temporarily drive Vm below RMP to generate the AP undershoot
Is the AP threshold static?
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)
Fast Nav channel inactivation
occurs via “ball and chain” structure
Slow Nav channel inactivation
occurs CONCURRENTLY via conformation changes in various regions, in particular S6 helices
Refractory Period
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
Absolute refractory period
- Due Nav channel inactivation
- No amount of stimulation can generate a new action potential
determines the maximum AP frequency
Relative refractory period
• 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
AP frequency and the Human Nervous System
information is encoded depending on AP frequency
• Stronger stimulus = higher frequency
• e.g.retinal ganglion cells
Given the strongest stimulus possible, how fast can APs fire in succession?
Depends on Absolute Refractory Period
e.g. for a 1 ms absolute refractory period, max. freq. = 1000Hz
___ Na+ leak conductance depolarizes RMP
↑ Na+ leak conductance depolarizes RMP
Increases AP frequency in a “tonically” firing neuron
___ Na+ leak conductance hyperpolarizes RMP
↓ Na+ leak conductance hyperpolarizes RMP
___ K+ conductance hyperpolarizes RMP
↑ K+ conductance hyper polarizes RMP
• Decreases AP frequency
___ K+ conductance depolarizes RMP
↓ K+ conductance depolarizes RMP
2-pore K+ channels
provide the major K+ leak current at rest (gK)
15 genes in mammals
NALCN (sodium leak channel)
POTENTIAL candidate for Na+ leak currents
however, the source of Leak Na+ currents is still unknown
Voltage-gated Ca2+ (Cav) channels
modulate AP shape and frequency
- P-loop channels with similar structure to Nav channels
- Slower gating than Nav channels
“Low voltage activated” (LVA) Cav channels
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
“High voltage activated” (HVA) Cav channels
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
LVA channels and the developing human heart
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
HVA channels and the adult heart
- 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
Small conductance K+ (SK) channels
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+
Big conductance K+ (BK) channels
directly bind cytoplasmic Ca2+ via C-terminal “calcium bowl” motif
• Also activated by depolarization and Ca2+ influx
How do Cav channel activation of BK and SK channels modulate afterhyperoliarization
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
HVA Cav channels drive spike frequency adaptation
- Sequential AP activate more and more Cav channels
- This activates more and more Ca2+ activated K+ channels
• Leads to increased afterhyperpolarization - AP frequency thus slows down until APs eventually stop
Like in the heart, in the absence of SK and BK channels, ______ can contribute to neuronal afterdepolarization
Answer: Cav channels
Like in the heart, in the absence of SK and BK channels, Cav channels can contribute to neuronal afterdepolarization
