lecture 6 action potential Flashcards
what does oscilliscope do
records membrane potential as a function of time
what causes rapid changes in membrane potential during action potential
ions flowing in and out of neuronal cell
why do ions flow during action potential
differences between membrane and equilibrium potential
at rest in neurons, what is the order of membrane potential in cations
pK>pCl>pNa (membrane potential near Ek)
what does nerst equation do
tells us the voltage that will balance the unequal concentration ratio across the membrane for an ion
what is ohms law
v=ir (voltage= current times resistance)
what is r=i/g
where g is conductance, how much an ion will flow
more conductance…. (what’s the inverse)
less resistance
what is driving force
(V) the difference between the membrane potential and the ion’s equilibrium potential
how do you calculate current with driving force
ionic current = ion conductance * driving force
Gk- absolute value of membrane potential - equilibrium potential
what two things affect current
driving force and conductance
what is the driving force of Cl at -65mv membrane potential
0 because absolute value of (-65- (-65) is 0
how do you calculate driving force
absolute value of membrane potential - equilibrium potential
what ions are responsible for the shape of the action potential
na+ and k+
what is the refractory period
small time period between action potentials where another action potential cannot be elicited
what is rising phase of action potential
rapid
depolarizing stage of the
action potential when the
there is an increase in Na+
current and membrane
potential quickly rises from
around -65 mV (resting
potential) to almost +60 mV
what is falling phase of action potential
phase of
action potential after the
peak (Vmax) when the Na+
current decreases and the K+
current increases causing the
membrane potential to
hyperpolarize and fall to or
below the resting potential
(Vrest
what are the parts of an action potential
threshold, rising phase, overshoot, falling phase, undershoot
what are the three kinds of membrane potential
reception potentials, action potentials, synaptic potentials
differences between receptor, synaptic, and action potentials
-action potentials all or none, other two graded
-action potentials propagate with same amplitude and dont decrement with distance, other two do
-local potentials can give rise to action potentials (in same cell), and action potentials can give rise to local potential in different cell
if na channels are open where is na going
in (wants to make cell more positive toward its equilibrium potential of +60)
if k channels are open where is na going
out (wants to make cell more negative toward its equilibrium potential of -80)
if ca channels are open where is na going
in (wants to make cell more positive toward its equilibrium potential of +134)
if cl channels are open where is na going
in or out (depends on if membrane potential is hyperpolarized, equal to, or depolarized relative to Ecl)
at peak of action potential, cl is flowing…
in
at peak of undershoot, cl is flowing
out
how is na positive feedback
if na channels open because of depolarization from local potential, na flows down concentration into neuron, causes more depolarization, more na channels open and brings Vm closer to Ena
(na channels opening encourages other na channels to open)
how is k negative feedback
depolarization with delay opens sluggish k (delayed rectifier) channels
during overshoot driving force of k is huge so extra open k channels cause k to leave positively charged interior of neuron
loss of k causes inside of neuron to become more negative
hyperpolarization closes delayed rectifier channels
(k channels opening discourages other k channels opening)
difference between bipolar, unipolar, multipolar neurons
amount of processes extending in opposite direction from soma
what kind of neuron are motor neurons
multipolar
what kind of neuron are sensory neurons
unipolar
is the 1a axon neuron efferent or afferent
afferent- it’s psuedo unipolar like other sensory neurons
what do synapses cause
release of neurotransmitters and opening of chemical gated channels, causing hyperpolarization or depolarization
what does conductance facilitate
flow of current (how many ion channels are open)
what does driving force facilitate
the propensity of the ion to move across the membrane
Overshoot
part of the
action potential when the
membrane potential is
greater than 0 mV
Threshold or Threshold Voltage
minimal membrane potential that a neuron must be
depolarized to open voltage gated channels and generate an action potential. Typically
around -45 mV. Since action potentials are an “all or none” event, if the threshold voltage is
reached, an action potential will fire. A subthreshold depolarization is a depolarization that
does not reach the threshold voltage, and thus does not trigger an action potential. A local
depolarization that is suprathreshold or depolarizes the neuron above the threshold voltage
to trigger an action potential is often due to synaptic potentials or a sensory stimulation
undershoot
part of the action potential when the membrane potential is below (more
hyper polarized than) the resting potential due to the increased K+ current compared to rest.
The membrane potential returns to the resting potential as the ionic conductances for all of
the ions return to the resting conductance (or resting permeabilities). During the undershoot
the membrane potential (Vm) will approach EK. The undershoot is also known as the afterhyper polarization (AHP)
Refractory Period
the period of time after an action potential when it is more difficult (i.e.
needs more depolarization) to reach threshold. This period is divided into the absolute and
relative segments
absolute refractory period
when it is impossible to generate
another action potential.
relative refractory period
when more current, is required to
generate another action potential.
Conductance
elative ability of charge to move or pass through a material.
Conductance is the inverse of resistance (R = 1/g) and can be thought as analogous to
permeability in the GHK equation
Define driving force, conductance and current and know how they are related by Ohm’s
law
Diagram an action potential as a function of voltage over time and label the following
stages:
i. resting potential
ii. rising phase
iii. falling phase
iv. overshoot
v. undershoot
Which part of most neurons occupies the most volume?
The axon
Local potentials (like receptor potentials) vary proportionally to the _______ of the stimulus.
size and duration
In the mayotatic stretch reflex, what is the approximate time latency between hitting a tendon and getting a muscle contraction ?
20 milliseconds
The ability or ease of an ion to flow is the _____
conductance
Of the following options, when is the driving force on K+ the highest (given typical neuronal concentrations)?
At the equilibrium potential for Na+
Sensory motor reflex
arc
1) Stretch (spindle)
2) Receptor Potential (1a Axon sensory spiral ending)
3) Action potential (starts at 1a axon trigger zone then
propagates)
4) Synaptic Potential (motor neuron dendrite)
5) Action potential (starts at motor neuron axon initial
segment then propagates)
6) Synaptic potential (extrafusal muscle fiber
neuromuscular junction)
7) Action potential (initiated at neuromuscular junction
then propagates along muscle fiber) which
contracts the muscle relieving the stretch
when Na channels are open sodium will flow
in
When K channels are open, K current will flow
out
Calcium channels open Ca++ will flow
in
Feedback: positive
• If some Na channels open because of slight depolarization (from a local potential) then Na flows down its
concentration and the electrical gradient INTO the neuron.
• The extra positive charge further depolarizes the cell’s membrane potential causing more Na channels to open
• This causes more depolarization and more channels to open and brings the cell membrane potential close to
Ena
• This is POSITIVE feedback because the channel opening encourages more channels to open
Feedback: negative
Depolarization with a delay opens the more sluggish K (delayed rectifier) channels
• During the overshoot the driving force on K is huge and so the extra open K channels (in addition to the leak
channels) cause K to leave the now positively charged interior of the neuron.
• The loss of K makes the inside of the neuron progressively more negative (and the Na channels are no longer
open)
• This hyperpolarization closes the delayed rectifier channels
• This is NEGATIVE feedback because the K channel opening discourages more K channels to open
During the rising phase of an action potential, is the magnitude of IK (potassium current) or INa
(sodium current) larger
Sodium current is larger. During the rising phase, the sodium conductance increases and the
driving force (especially at the beginning of the rising phase) on sodium is strong which causes
a large sodium current (need to mention both for full credit). Additionally, the neuron keeps
depolarizing – more sodium flowing in than potassium flowing out
Which ion is primarily responsible for the rising phase of the action potential? Which direction
(into or out of the neuron) does that ion flow
Sodium flowing into the cell is primarily responsible for the rising phase.
During the undershoot of the action potential, what is the membrane potential “under” (why is it
called the undershoot)? Which ion’s equilibrium potential does the membrane potential
approach during the undershoot?
During the undershoot the membrane potential is under (more negative) than the resting potential. The
membrane potential approaches potassium’s equilibrium potential (EK)
