Lecture 3.1: Neuron at Rest Flashcards
Membranes prevent the diffusion of charged molecules:
Membranes create a ______
Composed of ______
Prevent movement of ____, allow diffusion of ______, general anesthetics
Membranes create a barrier, delimit the cell
Composed of glycerophospholipids
Prevent movement of ions, allow the diffusion of amphiphillic molecules, general asthetics
What structure allows the movement of ions across the plasma membrane of a cell?
Transmembrane proteins such as ion channels, transporters or gap junctions allow movement of ions
Upon stimulus, ion channels can switch from a ____ state to an ____ state
Stimuli can include ______ or ______
This moves ions closer to their ______
Upon stimulus, ion channels can switch from a closed conformation to an open state
Stimuli can include changes in voltage of the presence of a ligand
This moves ions closer to their electrochemical gradient
Transporter examples include ______ and ______ (both of which use ATP to move ions against their gradient)
_______ traverse the extracellular space, connecting the intracellular space of two adjacent cells, allowing small molecules to pass through
Transporter examples include pumps and exchangers; they use ATP in order to move ions against their gradient
Gap junctions traverse the extracellular space, connecting the intracellular space of two adjacent cells… allowing small molecules to pass through
The two electrical components of a biological cell are ________ and ________
Explain their electrical equivalents
Lipid bilayer is like a capacitor (holds/stores charge)
Ion Channels are like resistors (allow things to go through them but slowly)
What is the equivalent circuit of a biological membrane?
A simple/classic RC circuit
with a resistor (ion channels)
and a capacitor (lipid bilayer)
What are the units for current?
Current is an Amphere (A) which is C/sec
or coloumbs per second
Explain resistance in terms of pipes
What is conductance?
Resistance: narrower channels or pipes present GREATER resistance to flow
Conductance is the inverse of resistance (conductance is higher in wide channels)
Explain capicitance
Capacitance is analogous to membrane surface area
Voltage changes slowly across a high capacitance membrane (water fills a large glass slowly)
Voltage changes rapidly across a low capacitance membrane
Voltage changes slowly across a _______ capacitance membrane and rapidly across a _____ membrane
Voltage changes slowly across a high capacitance membrane and rapidly across a low-capacitance membrane
Explain “Resting Potential” of a neuron
- Definition
- failure to keep it causes what?
Resting Potential:
- “DEFAULT” electrical potential on the neuron in the absence of any inputs
- Failure to keep the RP disrupts neuronal function
Explain Subthreshold Potentials
Subthreshold potentials or “graded potentials” are electrical inputs that do not cause an action potential (AP) to fire
Other examples of these are EPSPs and IPSPs
The Resting Potential of neurons is a result of…
Resting potential in neurons is a result of uneven distribution of charged molecules across the membrane
The mechanism used by the neuron to maintain its Resting Potential determine that neuron’s _____
Explain
The mechanism used by a neuron to maintain the RP (during rest or when receiving inputs that deviate the membrane potential from the RP) detremine the neuron’s excitability
- Neurons that deviate easily from the RP are more excitable, they can reach threshold for an AP quickly
- Neurons that deviate briefly and rarely from RP are less excitable
- Two neurons with the same RP are not necessarily similar in their excitability, they can have different electrochemical mechanisms
What is the range for the RP of neurons?
The resting potential (RP) of neurons is between
-50 mV to -70 mV
What is the value of the Nernst Potential for the following ions:
Na
K
Ca
Ena = +60 mV
Ek = -90 mV
Eca = +120 mV
What is the concentration of the following ions inside and outside of the cell:
Na
K
Ca
Three ions move across the membrane at rest
Which three?
At steady state, the chemical and electrical forces are _________
Three ions move across the membrane at rest (K+, Na+, and Cl-)
At steady state, the chemical and electrical forces are equal and opposite to so that no net K movement occurs
The Nernst equation calculates what?
What does Nernst potential indicate?
The Nernst equation calculates the electrical potential of a given ion
It has electrical and chemical contributions
For example, the nernst potential for K is -92 mV
At -92 mV ions are at a steady state, no net flux occurs
What is the Nernst potential for K?
What happens when the system gets perturbed?
(What happens when the potential is more positive vs more negative?
Normally, K inside the cell is a lot higher (155 mM)
Nernst potential for K+ is -92 mV
Thus, at a more positive value than that, K+ will LEAVE the cell to make the inside more negative and shift back to equilibrium
At values more negative than that, K+ will enter the cell in order to bring in more positive charges and bring the potential back up to nernst
If [K}outside increases
- what happens to the chemical gradient?
- What happens to the electrical gradient?
If [K] outside the cell increases (aka in hyperkelemia)
- the chemical gradient will be reduced
- The electrical gradient will increase
and then K+ ions will enter the cell until a new resting potential is achieved
Remember, this causes issues with firing APs
The RP (resting potential) depends on the distribution of all permeable ions: (___, ____, and ____)
Eplain what the GHK equation calculates and accounts for
The RP depends on the distribution of all permeable ions: Na, K, Cl
The GHK equation is a more accurate way than Nernst to calculate/predict cell behavior in regards to membrane potential
Unlike nernst, GHK accounts for ion permeability
What is the GHK equation?
What is the permeability of the following ions: Na, Cl, and K
GHK equation is shown in the picture below
Pk = 1.00 (60-70% at rest)
Pcl = 0.45 (or 25-35% at rest)
Pna= 0.004 or (3-4% of the current at rest)
Explain the following Cl ion transporters:
NKCC
KCC
NKCC is a Na/K/Cl transporter that moves Cl ions INTO the cell
KCC is a K/Cl carrier that moves Cl ions out of the cell
The RP is maintained by moving only _____ of the free ions in cells
A gradient must be maintained to keep the cell functioning normally, thus Na+ and K+ ions are ___________
Explain the Na/K ATPase
The RP is maintained by moving only 1/1000th of the free ions in the cells
A gradient must be maintained to keep the cell functioning normally, thus Na+ and K+ ions are “pumped” against their gradient, using energy, to keep those ion concentration differences inside and out of the cell
The Na/K ATPase hydrolizes ATP and pumps 3 Na out and 2 K+ in, thus causing hyperpolarization inside the cell
What happens when you inhibit the sodium potassium ATPase?
Give some examples of substances that block this pump
The Na/K ATPase works in order to re-establish the gradient of Na high outside and K high inside (it pumps 3 Na out and 2 K+ inside, hyperpolarizing the inside of the cell/making the inside negative)
If you inhibit this pump, the membrane will depolarize
Examples of substances that do this are digitalis, ouabain, and digoxin. This causes paralysis, and can be used to treat irregular heartbeats
Immature neurons lack what kind of Cl ion transporter?
What does that mean if you use GABA on immature neurons?
Immature neurons LACK a KCC
GABA is usually inhibitory neuron (usually moves Cl inside, hyperpolarizes the cell and makes it less likely to fire an AP)
BUT in immature neurons, GABA is excitatory (Cl will exit the cell)
Synaptic Inputs:
Currents that cause depolarization are called ____
Currents that hyperpolarize are ______
Synaptic Inputs:
Currents that cause depolarization are called inward (example, inward Na or Ca influx)
Currents that cause hyperpolarization are outward (example, outward K or inwards Cl)
What do nonspecific cation channels (like ionotropic receptors) do?
What is the Erev for ionotropic receptors?
Non-specific cation chanels like ionotropic receptors move Na in and K out (with a net inward, depolarizing current)
Erev (the reversal potential) for these = 0
What is input resistance?
What are synaptic inputs?
Input resistance is the TOTAL resistance across a membrane, defined by the ion channels present in a cell. A high input resistance means those channels are closed.
Synaptic inputs are any signal that the neurons receive. Neurons constantly receive these synaptic inputs.
These inputs will cause either opening or closing of ion channels. This can result in :
- Generation of synaptic currents
- Alteration of input resistance
A synaptic input affect’s a neuron’s excitability through changes in _____ and _____
Synaptic inputs are governed by Ohm’s Law which is _____
A synaptic input affects a neuron’s excitability through changes in input resistance and membrane potential
Synaptic inputs are governed by Ohm’s Law
V = I * R
(I is the capacitance, and R is the total resistance… aka all the ion channels)
A synaptic input will cause a _____ current
That current will elicit a change in memrbane voltage that must follow Ohm’s Law and will therefore be proportional to the synaptic current multiplied by the imput resistance:
- The synaptic current is more effective at changing the membrane potential if _____
A synaptic input will cause a synaptic current
Due to Ohm’s Law (V= IR):
The synaptic current is more effective at changing the membrane potential if the Rinput is high (AKA channels are closed)
The synaptic current is less effective at changing membrane potential if Rinput is low (channels are open)
Explain how synaptic inputs can be excitatory vs inhibitory
Synaptic inputs can be:
- Excitatory: (make neuron more likely to fire AP) usually cation influx and depolarization…. and in this cause the depolarizing synaptic currents dominate over the negative effect of Rinput
To explain this: closure of K channels at rest results in cell depolarization, the input resistance increases and outwards K stops, causing net accumulation of positive charges in side
- Inhibitory: they make the neuron less likely to fire an AP… outward K current or inward Cl current will decrease Rinput therefore the synaptic current will have less impact… a hyperpolarizing current will also bring the resting potential to more negative values
Excitatory Ionotropic Receptors:
What is the excitatory NT?
What are the receptors?
Excitatory Ionotropic Receptors:
Ligand gated GLUTAMATE receptors:
AMPA and NMDA…. both cause an EPSP
Permeable to cations
Summation of EPSP can cause firing of an AP
AMPA receptor:
- permeable to Na and K, reversal potential is O mv
- Na and K move with their gradients, cause a quick depolarization and fast return to resting potential
Inhibitory Ionotropic Receptors:
What are the two inhibitory NTs
What kind of ion are the receptors permeable to?
What do they cause?
INHIBITORY NT is GABA or Glycine
The receptors are ligand-gated GABA or glycine receptors
They are permeable to anions
Cause an IPSP, which reduces the probability of firing an AP
GABAa receptor: permeable to Cl, reversal potential is -70mV
Anions are unevenly distributed inside and outside the neuron:
Some molecules essential for life are negatively charged, they are abundant in the cell and do not exist outside:
1.
2.
3.
Some molecules essential for life are negatively charged, they are abundant in the cell and do not exist outside:
- ATP and ADP
- Nucleic Acids: DNA (nucleus). mRNA and tRNA (cytosol) usually bound to Mg
- Amino Acids: glutamic acid and aspartic acid
