what is the resting membrane potential (RMP)
the electrical charge difference across a cell membrane
what contributes to the RMP
- Na⁺/K⁺ ATPase pump
- non-gated K⁺ channels
what does the Na⁺/K⁺ ATPase pump do for the RMP
- uses ATP to move ions against their gradients
- 3 Na⁺ out
- 2 K⁺ in
- this creates and maintains high Na⁺ outside and high K⁺ inside
what does the K⁺ leak channels do for the RMP
- because of the Na/K ATPase, there’s lots of K⁺ inside
- K⁺ moves out through these channels down its concentration gradient
- when positive K⁺ leaves, the inside becomes more negative relative to the outside
what is the plasma membrane permeable to at rest
- K+ (K+ leak channels)
- few Cl-
- few Na+
- few Ca2+
what were some questions regarding the K+ leak channel’s structure
K+ is bigger than Na+, so how is Na+ not able to leak through if K+ is
what is special about the K+ leak channel that only lets K+ through
- it has a selectivity filter, where there is a backbone of carbonyl oxygens
- these oxygen atoms are arranged at just the right distances to mimic the water molecules that normally surround K⁺
- so when K⁺ sheds its waters, the filter stabilises it almost perfectly
why can’t Na+ get through the K+ leak channel
- Na⁺ is smaller so it does not sit at the right distance from those carbonyl oxygens in the selectively filter
- the filter is too wide for sodium to be optimally coordinated
- so Na⁺ cannot get the same stabilising interactions that K⁺ gets
what did scientists do with a squid’s axons
1) inserted electrodes inside and outside the squid axon
2) measured the membrane potential during a nerve impulse
3) used voltage clamp methods to hold the membrane at certain voltages
4) recorded the currents flowing across the membrane
what did scientists discover with a squid’s axon
- during an action potential, there is first a rapid inward current caused by Na⁺ entering the axon
- then a delayed outward current caused by K⁺ leaving the axon
- this is why the membrane depolarises and then repolarises
describe the phases observed during an action potential
1) RMP is about -70 mV
2) depolarisation: due to a voltage change, voltage-gated Na⁺ channels open and Na⁺ rushes into the cell, making the membrane potential +40 mV
3) the concentration and electrical gradient pulling Na+ eventually dissipates, and Na+ stops entering
3) repolarisation: depolarisation triggers voltage-gated K⁺ channels , and K⁺ leaves the cell, bringing the membrane potential back down
4) hyperpolarisation: voltage gated K+ channels are a bit slow to close, so the membrane potential becomes more negative than resting around -80 or -90 mV for a short time before reaching RMP (-70mV)
how do voltage gated sodium channels propagate signals
- signals only travel in one direction
- the inactivating segment is a part of voltage-gated Na+ channels that physically blocks the pore from the inside shortly after it opens
- this stops Na⁺ from flowing in even if the membrane is still depolarised
- this makes the action potential propagate only forward, not backward
what happens to the transmembrane domains in relation to propagation of signals
when the reversal of charge occurs during an action potential, the charged transmembrane regions of the voltage-gated Na+ channels changed conformation, and this played a role in movement of the inactivating segment
what are the Na+ channels found in algae
blue light-activated Na+ channels called channelrhodopsins
what is the significance of channelrhodopsins
- the gene from algae could be extracted then inserted into a different organism’s cell, like a mouse
- you could then shine blue light on certain parts of the brain and only activate specific populations of the mouse’s brain
- this helped to learn about what different population’s of brain cells were responsible for
- this also helped to think about genetic engineering that could help activate parts of the brain when, in disease states like Parkinsons, they are inactivated
how were channels that detect and regulate body temperature
1) researchers took sensory neurons that responded to heat
2) they scanned the DNA to find what might be a gene for a temperature sensitive channel
3) the DNA was extracted and put into cells in vitro
4) capsaicin was added to the cells to stimulate heat
5) they found channels that opened only in response to capsaicin
- this explains the action potential triggered that enables us to feel heat when we eat chilli
how were channels that detect and regulate pressure
1) cells were grown in culture then “poked”
2) the poking stimulated action potentials
where would pressure channels be abundant
- our stomach to stimulate feeling of fullness
- our bladder to stimulate feeling of needing to urinate
what is the size of the pressure regulating channels
- massive protein
- 38 transmembrane helix topology
