test 3: lecture 3 Flashcards
Electrical signals necessary for neural function are
mediated by the flow ___ through aqueous pores
in the nerve cell membrane
of ions (electrical activity)
Vm
membrane potential
charge difference across the plasma membrane
membrane potential
resting Vm is ___
-70 mV
Any positive change in the membrane potential
depolarization (inside of cell becomes more positive)
Any negative change in the membrane potential
hyperpolarization
(inside of the cell becomes more negative)
Na moving in or out would cause depolarization
Na moving in, the inside of the cell would become more positive
(depolarization = more + )
K moving in or out would cause hyperpolarization
K moving out
hyperpolarization= more negative
V= Current x resistance
ohms law
driving force is by membrane potential
the amount of ___(ion flux/flow through an open channel) is directly proportional to the voltage across the membrane, i.e. the electrical “gradient” or electrical driving force
current
___ dictates the amplitude of ion flux
membrane potential
electrochemical gradient is a combo of ___
electrical (membrane potential)
chemical (concentration gradient) forces
Equilibrium is achieved when the tendency of an ion to move down its ___ gradient is exactly counter-balanced by an opposing ___ gradient, resulting in no net ion flux.
concentration
electrical
primary intracellular cation is ___
potassium (positive charge K+)
primary extracellular cation
sodium (Na+)
primary intracellular anion
proteins (negative charge)
primary extracellular anion
Cl-
how is resting membrane potential maintained
- Na+/K+ ATPase - Contributes only a small amount (~8-10mV) of the resting membrane potential
- K+ “leak” channels - Relatively selective permeability of the plasma membrane to K+ (via “leak” channels) contributes MOST of the resting membrane potential (K+ will leave the cell)
Eion
equlibrium(reversal) potential
equilibrium potential
•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 primarily by the intracellular and extracellular concentration of each ion.
- Unique for each ion
- Nernst Equation
Na/K pump
3 Na out
2 K in
(uses ATP)
helps maintain resting Vm at -70 → small amount 8-10 mV
How does Na/K pumps work to set up K leak channels
Na/K will pump 3 Na out, 2 K in (net loss of +1)
K channels will open- there is more K inside cell than out. K will move down concentration gradient and will move out of cell. inside of cell becomes more negative
most of resting Vm is from ___
the loss of K from the cell
(potassium leaves cell through channels due to concentration gradient set up by Na/K pumps)
what happens to K at -120 mV?
at -70?
at +70?
will flow into the cell
out of cell slowly (small current)
out of cell quickly (large current)
what does the chart tell you
what an ion will do at specific membrane potentials
wether it will go in or out of the cell and how fast it will leave or enter
Very ___ cell interior is needed to pull K+ into the cell against outward concentration gradient
negative
Very ___ cell interior is needed to push Na+ out of the cell against inward concentration gradient
positive
____ is determined by ion distribution (concentration differences) across the plasma membrane and the permeability of the membrane to each ion at a given time.
Actual membrane potential
•Direction of flow for a particular ion is determined by the relationship of its ___ to the actual membrane potential.
equilibrium potential
•Ion current (flow) amplitude is also determined by Vm; the electrical driving force (___).
Ohm’s law
____ is a very rapid change in membrane potential. It occurs when a “stimulation” of the nerve cell membrane depolarizes the membrane enough to allow voltage gated Na+ channels to open
An action potential
what is the threshold for action potential?
(threshold ~ -50mV).
cell must depolarize (become more + to -50) then it will launch the action potential down the axon
explain action potential graph
stimulus opens some Na channels
threshold at -50
voltage gated Na channels open (more + depolarization)
voltage gated K channels slowly open (more negative but not enough to cancel out rapid influx of Na- cell will continue to depolarize until peak at +40)
Na channels close (K channels still open cell becomes more negative and repolarization starts)
K channels close but slowly. Cell becomes too negative (hyperpolarization)
resting potential is fixed by Na/K pumps to get to -70 (refractory period)
Depolarization occurs when ligand-gated nonselective ___ channels at the post-synaptic membrane generate small excitatory depolarizing stimuli.
cation (+ go into the cell, make more +, depolarization)
how is threshold reached?
- ligand-gated nonselective cation channels at the post-synaptic membrane generate small excitatory depolarizing stimuli. (cell becomes more +)
- These signals allow Na+ to begin to pass into the cell, and the depolarization spreads to the axon hillock and initial segment where there are a large amount of voltage gated Na channels
where does action potential start?
axon hillock and initial segment
___ contains a high conc. of voltage-gated sodium channels.
initial segment
EPSP
EPSP - Excitatory Post-Synaptic Potentials
(positive charges into the cell); graded potential (depolarization)
little + into the cell that try to get to threshold to trigger action potential
IPSP
IPSP - Inhibitory Post-Synaptic Potentials
(negative charges into the cell); graded potential
small negative into the cell causing hyperpolarization (more negative) → further away from threshold
if IPSP + EPSP = 50 mV what will happen
summation
inhibitory(hyperpolarization- more negative) + excitatory(depolarization more positive)
action potential
temporal summation of PSP
multiple stimulus from same place add on top of each other
spacial summation
stimulus from different areas will add together
compare graded and action potentials
how do voltage gated sodium channels work?
open at threshold
inactivation gate closes after specific time
membrane returns to resting, gate closes and inactivation gate moves out of the way
ready for next threshold
absolute refractory period
Time immediately after action potential during which another AP cannot occur (due to Na+v channel inactivation and delay in K+v channel closing).
new action potential can not be triggered for specific time
relative refractory period
period of time during the refractory period that another action potential can be sent if the stimulus was very big
cell not back to normal but Na channels reset so technically they could go again if they had to
what can increase speed of action potential?
increase amount of voltage gated Na channels
increase diameter of the axon (decrease in resistance will increase current flow)
insulation (myelin)
___ is made by schwann cells and acts as insulation
myelin
how does myelin increase speed of action potential
gaps in myelin (nodes of ranvier) will make action potential jump from node to node (saltatory conduction)
___ are regularly spaced gaps in the myelin sheath
nodes of ranvier
jumping from node to node on an axon is called
saltatory conduction
•Saltatory conduction is metabolically efficient – ion movements limited to nodal regions (including Na+/K+ ATPase and K+ “leak channels”)
100x faster conduction velocities, necessary for rapid reflex
why does axon myelination do
decreases cation leakage (prevents + from leaving axon therefore keeping the strength of the AP all the way down the axon (more downstream current)
myelin will also spread out the + and - charge → lower capacitance this will mean more + available down stream to continue AP
how does myelin increase downstream current
prevent cation leakage
what happens to capacitance with myelinated axon?
decreased capacitance because + and - farther apart
lower capacitance= more downstream current
AP jumping from node to node
saltatory conductance
