Excitable Membranes Flashcards
what is an excitable cell or membrane?
any membrane that can hold a charge or propagate and electrical signal
2 types of excitable tissues and what they do?
- neurons: transmit electrical impulses
2. muscles: contract to produce a pulling force
diffusion
net movement of molecules from an area of high concentration to an area of low concentration
osmotic gradient
the difference in concentration b/w 2 solutions on either side of a semi-permeable membrane and is used to tell the difference in percentages of the concentration of a specific particle dissolved in a solution
what is a gradient?
difference in concentration of any parameter across a distance
what are the 4 types of gradients?
- ion gradient (osmotic gradient)
- energy gradient
- temperature gradient
- pressure gradient
what is net flux and what equation do we use?
(Jnet) occurs from high concentration to low concentration until the gradient disappears; Fick’s law of diffusion
when does diffusion work best?
over small areas
what do we mean by resting membrane potential?
all excitable cells maintain a non-zero membrane potential; the cell is inactive but ions are still diffusing/maintaining a negative voltage
what is the resting membrane potential for neurons?
-70 mV
what is the resting membrane potential for muscles?
-85 mV
what kinds of ions are inside/outside the cell during resting membrane potential?
- high concentration of K+ and proteins inside (low concentrations outside cell)
- high concentration of Cl- and Na+ outside (low concentrations inside cell)
what is the Nernst Equation used for?
calculate electrical potential from concentration gradients but dose NOT consider permeability of the membrane
what affects membrane permeability?
membrane channels
what 3 things are happening at resting membrane potential?
- lots of K+ leaking out of cell through leak channels
- little Na+ leaking in cell through membrane
- Cl- leaking through leak channels
membrane potential only exists because?
ions are flowing through the membrane
What does the Goldman-Hodgkin-Katz Equation tell us?
it determines the reversal potential across a cell’s membrane, taking into account all of the ions that are permeant through that membrane
what prevents the concentration gradient from approaching zero?
maintenance of gradients requires an active process which require ATP to pump ions against their concentration gradient (Na+K+ATPase pump)
for every 1 ATP hydrolyed–>
3 Na+ move OUT
2 K+ move in
is the Na+K+ pump working while the cell is at rest?/why?
YES; the pump maintains the resting membrane potential
3 parts of an action potential
- depolarization
- repolarization
- hyperpolarization
what happens during depolarization?
there is a positive change in voltage due to activation of voltage-gated channels
voltage-gated channels control permeability of what ion?
Na+ and K+
when does Na+ flow into the cell?
only when both the activation gate and inactivation gates are open
when the cell is at rest, the activation/inactivation gates are open/closed?
activation gate is closed and inactivation gate is open
at depolarization, which gates open/close? what happens?
activation gate opens, inactivation gate is open, Na+ flows into cell
what occurs when the cell is at rest?
threshold has not been met. RMP is still at -70 mV. K+ flows out of cell through leak channels, activation gate of Na+ channel is closed so Na+ doesn’t flow in, voltage-gated K+ channels are closed
what occurs during depolarization?
threshold stimulus is met and cell becomes more positive and approaches zero. K+ leaking out of cell through leak channels, activation gate for Na+ channels opens and Na+ flows in to cell, activation gate of K+ channels opens slowly
what occurs at the peak of an AP?
the AP reaches its highest point and becomes the most positive. K+ leaking out of cell through leak channels, inactivation gate of Na+ channels is closing, activation gate of K+ channels continues to open and more K+ flows out of cell
what occurs during repolarization?
cell becomes more negative again.K+ leaking out of cell through leak channels, inactivation gate is closed for Na+ channels, activation gate of K+ channels is fully open and more K+ flows out of cell
what occurs during hyperpolarization?
cell undershoots -70 mV. K+ still leaking out through leak channels, inactivation gate reopens and activation gate closes of Na+ channels. activation gate of K+ closes slowly
what is the refractory phase?
period of time where we cannot re-stimulate the membrane
what is the absolute refractory period?
activation gate of Na+ channels is open and insensitive to any additional voltage, inactivation gate is closed and insensitive to any voltage altogether
what is the relative refractory period?
voltage-gated Na+ channels activation gate closes and inactivation gate reopens and both gates can be re-stimulated, need a supra-threshold stimulus
what happens when 1 voltage-gated Na+ channel opens?
it creates a cascade of continuous conduction, voltage-gated Na+ channels open in sequences along the length of the cell
structure of neurons?
- dendrites- receiving end
- axon- AP travels down it
- cell body w/ soma- houses nucleus
- terminal- sends out signal to postsynaptic cell
what is continuous conduction?
voltage-gated Na+ channels open in sequences along the length of the cell, propagates AP at ~2 m/s; very slow
what is saltatory conduction?
requires myelin sheath that insulates the axon: oligodendrocytes in CNS and schwann cells in PNS, rich in cholesterol, insulates axon from ion leakage, voltage-gated Na+ and K+ channels are located in nodes b/w myelinated regions of axon, propagates AP at ~ 100 m/s; very fast
what are the steps that occur at the end of an A.P.?
- AP propagates down axon
- AP stimulates voltage-gated Ca2+ channels to open
- Influx of Ca2+
- Ca2+ signals vesicles to fuse with membrane and exocytose their neurotransmitter contents into synapse (transduce electrical signal into a chemical signal
- neurotransmitters bind to ligand-gated channels on postsynaptic cell
what are ligand-gated channels?
chemically-gated channels on the postsynaptic cell that receive the neurotransmitter from the presynaptic cell
what is the effect of Na+ ligand-gated channels?
make the cell more positive
what is the effect of K+ ligand-gated channels?
make the cell more negative
what is the effect of Cl- ligand-gated channels?
make the cell more negative
