Lecture 7: Excitable Cells: Muscle and Nerve Flashcards
Electrical signals are change in membrane potential
Neurons/Muscles have a separation of charge at the membrane which results in a membrane potential known as Vm
Ion channels allow ions to cross the cell membrane rapidly
Current (movement of ions) change the membrane potential
Membrane potential changes are the basis of neuronal/ muscle signaling
Lipid bilayer
Impermeable to ions
Cell membrane does not allow ions to cross
What are two key contributions to Vm?
1) ions are not distributed equally across the cell membrane
2) membranes are unequally permeable to various ions.
Unequal ion concentrations
Distributions vary by animal species and neuron type, but relative distributions similar
Distribution depends on ATP driven pumps
Case 1:
Permeable to all ions equally
Ions move down concentration gradients
No electrical potential
Unequal permeability to various ions
At rest: more permeable to K+ than Na+
Permeable- a surface that allows another substance to pass through it
Case 2:
Membrane permeable to k+ only
K+ moves down concentration gradient
Electrical potential develops
K+ moves back inside cell due to electrostatic attraction
A simple cell model: two opposing forces
Concentration gradient and electrical potential
What is the equilibrium potential (Ek)?
The equilibrium potential is the membrane potential at which the concentration gradient and electrical potential forces are equal and opposite.
At Ek k+ is at equilibrium (no net movement of k across the membrane)
A common misconception
Despite movement of ions, concentration of ions does not significantly change!
A very small percent of ions move to generate a sufficient Vm to balance the diffusion force.
What is the Nernst equation?
Ex= (58/z) log (Xo/Xi)
Ex is the mV
X is mM
Z is the charge in the ion (valence)
Na+ and K+ z=1
Cl- z=-1
Electrical signals: Nernst equation
Describes the membrane potential that a single ion would produce if the membrane were permeable to only that ion:
Eion= (61/z) log(ion out)/ion in)
Membrane potential is influenced by:
Concentration gradient of ions
Membrane permeability to those ions
Nernst equation
Ex= (58/z) log(Xo/Xi)
The concentration gradient and charge of an ion determines the membrane potential at which it is at equilibrium
If no concentration gradient then Xo=Xi and Ex=0
Membrane permeable to multiple ions
Vm is the cell membrane at which the net flux of ALL ions together equals zero.
Vm is a steady state, but not an equilibrium.
Vm depends on Ex and permeability of ions
If a cell is permeable to multiple ions Vm is a weighted average of the Ex for each permeable ion
Given by Goldman-Hodgkin-Katz equation
Goldman-Hodgkin-Katz equation
Predicts membrane potential that results from the contribution of all ions that can cross the membrane.
Vm= 58 log (PkKo + PnaNao + PclCli)/PkKi + PnaNai + PclClo)
Vm is somewhere between the largest Ex and smallest Ex.
Like a mathematical average
Goldman-Hodgkin-Katz equation
Ions with greater permeability have greater influence on Vm. The large the influence of an ion X, the closer Vm will be to Ex.
At rest mostly permeable to k+ so Vm close to Ek.
If only permeable to 1 ion (for example K) P for all others is 0 and the equation reduces down to the Nernst equation.
Key concept:
The GHK equation shows us that you can change the membrane potential of a neuron Vm by changing the permeability of different ions.
Resting membrane potential
Vm depends upon Ex of permeable ions
K+ tends to leave the cell at Vm
Na+ tends to enter the cell at Vm
The concentration gradient AND the membrane potential are pushing Na+ ions into the cell
The Na-K pump
At Vm only the NET movement of all ions considered together is 0 ( ie steady state).
The concentration gradients of ions will run down over a period of hours to days
The Na-K pump uses ATP to maintain the ion concentrations at constant levels
The ions are pumped against their concentration gradient:
Move Na out of the cell
Changes in membrane potential
Terminology associated with changes in membrane potential
Depolarization
Repolarization
Hyper polarization
Synaptic potential
Binding of neurotransmitter causes receptors to open which increases the permeability of the ions that the receptor prefers
GHK: if a neurotransmitter receptor opens and increases permeability to Na+ Vm will increase towards Ena
Synaptic Potentials
Depolarization of the membrane potential is more positive than Vm ( excitatory EPSP)
Hyper polarization if membrane potential more negative than Vm
(Inhibitory IPSP)
Passive properties-Grades potentials
Graded potentials decrease in strength as they spread out from the point of origin. Like synaptic potentials
Propagation of synaptic potentials
Synaptic potentials travel from synapse in all directions
Synaptic potentials get smaller and slower over distance
Due to passive membrane properties (like an electrical cable).
Integration of synaptic potentials
Soma receives lots of EPSPs and IPSPs
The neurons decision maker is at the base of the axon = axon hillock
If membrane potential depolarizes beyond a certain level (threshold) an action potential is triggered
Neuron constantly receiving IPSPs and EPSPs (has thousands of synapses).
These changes are localized in some and time. When two signals run into each other they are linearly added together.
Temporal summation
A synapse receives 2 EPSPs separated in time. No summation
2 EPSPs at nearly the same time. Summation.
