electrophysiology Flashcards
RMP
resting membrane potential
charge of cytoplasm of typical cells
-70mV
potassium leak
○ Membrane is 50-75x more permeable to K+ that to Na+
§ K+ leak
Gibbs-donnan effect
§ Macromolecules assembled inside cells
§ Small building blocks diffuse in, and combine to produce larger molecules (proeteins, RNA) that can diffuse out
§ Most proteins ionise as anions (-ve) plus small ion H+ that can leave the cell
§ Traps negatively charged macromolecules in cytoplasm
§ Significant contribution to RMP
leak channels
always open
ligand-gated channels
open or shut when bound boy a specific ligand
voltage gated channels
open at a specific membrane potential, close at a specific membrane potential
influx of Na causes
depolarisation
makes the inside of the cell more positive
efflux of K causes
hyperpolarisation
makes the inside of the cell less positive
need for positive feedback
- Passive local potentials diminish over distance so are no good for transmitting messages
- Depolarising stimuli wont transmit long distances without positive feedback
action potential
□ 'all or nothing response' □ Always the same size (amplitude) □ Positive feedback after threshold value is exceeded □ Des not diminish over distance □ Open voltage gated sodium channels
ligand gated Na channels
open when bound by specific ligands
provide initial stimulus
voltage gated sodium channels
○ Open when membrane potential voltage exceeds threshold
○ Close a few ms later
○ Inactivated for a few ms afterwards (cant be activated again)
- Voltage gated channels K+
○ Open due to membrane potential voltage exceeding threshold
○ Slower to open that Na channels (open later)
○ Close a few ms later
○ K channels don’t lock
positive feedback in AP depolarisation
- Sub-threshold stimuli cause some Na+ channels to open, some Na+ enters the cell causing further depolarisation
- Once the threshold membrane potential is exceeded all voltage gated Na channels open
- Na floods into cell down its electrochemical gradient, causing rapid rise in membrane potential
what causes initial stimulus of depolarisation
ligand gated sodium channel
depolarisation
- At peak membrane potential, the cell is highly positively charges compared to ECF
- Once the cell membrane potential peaks, voltage gates Na+ channels close and voltage gated K channels open
- K+ floods out of the cell down electrochemical gradient causing repolarisation
Hyperpolarisation
- After repolarisation, low K+ in cell and Na is being pumped out
- K+ channels are slower to close so K+ leaks back out of the cell
- Causes ICF to be more negative than at rest (until all the K+ channels close)
Absolute refractory period
Neurone cannot be made to reach AP
intraceullar calcium
maintained at low levels
- Relative refractory period
○ Neurone can generate AP, but requires greater stimulus
○ Some Na+ cannels are reactivated
After AP, neurones need to recover
○ Re-establish polarisation
○ Na+/K+ - ATPase pumps Na out of the cell and K in
3 reasons to have a negative RMP
- Asymmetric ion distribution maintained by active transport
- Membrane is 50-75x more permeable to K+ than Na+ causing K+ leak
- Gibbs donnan effect
a greater stimulus generates
- Greater stimulus does not generate more intense Aps, just more APs