membrane potential and neuronal action potentials Flashcards
electrical signals necessary for neural function are mediated by the flow of ions through:
aqueous pores in the nerve cell membrane
what are one of the principle theraputic targets of drugs
ion channels
what is membrane potential
charge difference across the plasma membrane
the plasma membrane acts as a:
capacitor
what is a capacitor
two conductive surfaces (ions in solution) separated by a non-conductive surface (lipid bilayer)
when ion channels open, current is allowed:
to flow across the plasma mebrane potential
what is the resting membrane potential of a neuron
-70mV
what is depolarization
- any positive change in the membrane potential
- inside of the cell becomes more positive
what is hyperpolarization
- any negative change in the membrane potential
- inside of the cell becomes more negative
what is hyperpolarization
- any negative change in the membrane potential
- inside of the cell becomes more negative
is a chloride channel (Cl- flows into neuron) depolarization or hyperpolarization
hyperpolarization
what is Ohm’s law
- describes the relationship between ion flow (current) and membrane voltage
- voltage (V) = current (I) x resistance (R)
the amount of current is ____ ____ to the voltage across the membrane
directly proportional
what dictates the amplitude of ion flux
Vm (membrane potential)
what is the membrane potential
cell interior is electrically negative relative to the exterior
what are the two forces that determine the asymmetric equilibrium distribution of ions across the plasma membrane
electrical (membrane potential) and chemical (concentration gradient) forces = electrochemical gradient
how is equillibrium achieved
when the tendency of an ion to move down its concentration gradient is exactly counter-balanced by an opposing electrical gradient, resulting in no net ion flux
what is electrical gradient
ion diffusion is affected by electrical charge differences and they tend to move toward opposite charge
what is chemical gradient
ions move by diffusion from high to low concentration
what is the main determinants of the resting membrane potential
the differential distribution of ions across the plasma membrane
Na+/K+ ATPase
contributes only a small amount of the resting membrane potential
K+ leak channels
relatively selective permeability of the plasma membrane to K+ contributes most of the resting membrane potential
what is the equilibrium (reversal) potential , Eion
- defined as the membrane potential at which the direction of ion flow through a particular channel reverses direction
- enables one to predict which direction ions will flow
- determined by the intracellular and extracellular concentration of each ion
what is the nerst equation used for
to calculate the equilibrium potential for a given set of ionic conditions
what is needed to pull K+ into the cell against outward concentration gradient
very negative cell interior
what is needed to push Na+ out of the cell against inward concentration gradient
very positive cell interior
what is an action potential
- rapid change in membrane potential
- occurs when a “stimulation” of the nerve cell membrane depolarizes the membrane enough to allow Na+ channels to to open (threshold ~ -50mV)
what are voltage-gated NA+ channels responsible for
depolarizing membrane and (inward current) conduction of action potential
what are the voltage-gated K+ channels responsible for
repolarization following action (outward currents) potential (returning towards resting potential)
what are Na+/K+ ATPase and K+ leak channels responsible for
sets resting potential and returns membrane potential back to resting during refractory period
when does depolarization occur
when ligand-gated nonselective cation channels at the post-synaptic membrane generate small excitatory depolarizing stimuli
what does the small excitatory depolarization stimuli allow
Na+ to begin to pass into the cell and the depolarization spreads to the axon hillock and initital segment
why does the AP start at the axon hillock and initial segment
the initital segment contains high conc. of voltage-gated sodium channels
what are EPSP (excitatory post-synaptic potentials)
- positive charges into the cell; graded potential
- depolarizations are excitatory
what are IPSP (inhibitory post-synaptic potentials)
- negative charges into the cell; graded potential
- hyperpolarizations are inhibitory
what is the threshold
depolarizations to approx. -55 to -50 mV is required for action potential
what is sodium channel inactivation important for
unidirectional AP conductance
what are the 6 phases of the action potential
- threshold
- rising phase (depolarization)
- peak (Na+ channels inactivate)
- falling phase (hyperpolarization)
- undershoot
- recovery
what is the absolute refractory period
time immediately after action potential which another AP cannot occur (due to Na+v channel inactivation and delay in K+v channel closing)
what is the relative refractory period
time after AP during which greater stimulation is necessary to cause another AP
how does the AP travel down the axon
one spot
- some Na+ ions flow in
- membrane potential rises to ~50 mv
- reaches threshold for voltage-gated Na+ channels
- voltage gated Na+ channels activated
- Na+ flowing in - membrane potential continutes to rise rapidly
- reaches threshold for voltage-gated K+ channels - they open
- Na+ channels inactivate and close
- K+ flowing out rapidly - membrane potential drops (repolarization)
- membrane potential drops below -70mV
- K+ channels inactivate
- Na+ channels reset
- K+ channels reset
what happens along the whole axon during the depolarization phase
as the membrane potential rises in this spot, the membrane potential directly next to it is also affected
what happens along the whole axon along the repolarization phase
as the membrane potential falls in this spot, the membrane potential directly next to it is also affected and follows
what factors will improve axon potential conduction velocity and distance
- increase the density of Na+v channels
- increase the diameter of the axon
- insulation (myelin)
what is myelin
- schwann cells (glia) wrap around the axon and acts as insulation
- conduction jumps from node to node (saltatory conduction)
what is saltatory conduction
- metabollically efficient - ion movements limited to nodal regions
- 100x faster conduction velocities
what are the consequences/benefits of axon myelination
- less “leaky” = more downstream
- lower capacitance = more downstream current
- have highly increased concentration of Na+ channels at the nodes
summation - spatial vs temporal
- spatial - E1 and E1 occur at different spots one right after the other
- temporal - E1 and E2 ontop of each other
