Module 6: Neurons Flashcards
What are some general facts about neurons?
- They are the primary method of communication between the CNS and PNS.
- There are about 100 billion neurons in the brain alone
- Connections are determined by genes and other cues
What do dendrites do?
Receive chemical signals at their synapses
What do axons do?
Send electrical signals (APs)
What part of a neuron (besides dendrites) can receive electrical stimuli?
The cell body
What is membrane potential and RMP?
Membrane potential is the voltage across the cell membrane. RMP (Resting Membrane Potential) is the membrane potential when the cell is at rest.
Where is the normal RMP for a neuron?
About -70mV
What are the three factors which influence MP?
- Difference in concentration gradients across the membrane for Na+ and K+
- Difference in permeability of Na+ and K+
- Electrogenic action of Na+/K+ pump
How does the difference in concentration gradients of Na+ and K+ determine MP?
Determines the electrochemical gradient and the extracellular fluid, and therefore the (degree of) ion movement
How does the difference in Na+ and K+ permeability affect MP?
Affects how much each ion will move in either direction
How does the electrogenic action of the Na+/K+ pump affect the MP?
It sets up the electrochemical gradients
What is the difference in permeability of Na+ and K+, and why does this difference exist?
There are many more non-gated ion channels for K+ than for Na+. Therefore, the resting permeability of K+:Na+ is 40:1
What does the Na+/K+ pump do?
It actively accumulates K+ in the cell, and forces Na+ out of the cell. This forms a high concentration of K+ inside the cell, and a high concentration of Na+ outside the cell.
Does RMP depend more on the Na+/K+ pump, or the non-gated ion channels?
Non-gated ion channels.
Why is the RMP so negative?
K+ is in a greater concentration and has greater permeability inside the cell. Therefore a lot of K+ leaks out, and each ion which leaves makes the cell interior one unit more negative. As K+ leaves on a large scale, and is not replaced by the introduction of more Na+, the overall effect is to make the RMP negative.
How do negatively charged proteins affect RMP?
They don’t as the cell is impermeable to them
How does Cl- affect the RMP?
It doesn’t, as Cl- doesn’t undergo active transport and so tends to sit at equilibrium.
What allows a membrane potential to be upheld?
The cell membrane’s ability to separate charge.
What is the patch clamp technique?
A method of measuring membrane potential.
It involves applying a pipette and small amount of suction to a cell membrane so that no ions can flow between the two. Therefore, when a single ion channels open, all ions must be released into the pipette. The current it generates can be measured with an amp connected to the pipette. This allows it to be known when the channel is open or closed, and the difference between the inside and outside of the cell.
What is a second method of measuring membrane potential?
Using a micro-electrode inserted into the cell, and recording cell voltage relative to a reference electrode.
What are the two equations which can be used to calculate MP?
The Nernst Equation
The Goldman Equation
What does the Nernst Equation measure?
It calculates the MP of cells where the membrane is only permeable to a single ion, so the RMP also depends on that single ion.
What is the Nernst equation’s formula?
E(ion) = 61.5mV x log([ion]o / [ion]i)
Where:
E(ion) = the equilibrium potential of the ion
[ion]o = the concentration of the ion outside the cell, and
[ion]i = the concentration of the ion inside the cell.
(These two can also be used as a ratio).
What is the assumption associated with the nernst equation?
Equilibrium is at body temperature
What are the limitations of the Nernst equation?
This does not account for permeability or ion conductance. It cannot accurately calculate the MP for a membrane permeable to more than 1 ion.
What are the positives of the Nernst equation?
No foreknowledge of permeability is required
It gives us the equilibrium voltage of a single ion (so what the voltage of a cell would be if it were only permeable to a single ion)
For what type of cells can the Nernst equation be used to calculate RMP?
Glia cells, as they rely only on [K+]
What does the goldman equation measure?
It measures the permeability and concentrations of multiple ions, allowing a representative RMP to be calculated.
What is the formula of the goldman equation?
Vm = 6.15mV x log ( (Pk[K+]o + Pna[Na+]o) / (Pk[K+]i + Pna[Na+]i) )
Where: P= permeability to
Vm = RMP
What is the effect of the cell membrane being more permeable to a certain ion?
RMP is shifted toward the more permeable ion’s own equilibrium potential.
What does membrane permeability mean for a comparison between neuron and glia cells’ RMPs?
As glia cells are only permeable to K+, their RMP is closer to K+’s eq. potential of -80mV than neurons are, as neurons also take into account Na+’s +60mV eq. potential.
What is an AP?
A brief fluctuation in membrane potential, caused by the transient opening of voltage-gated ion channels, spreading from there along the neuron.
What is the first step in the generation of an AP?
A stimulus causes the membrane to become more and more positive by changing the cell’s permeability to ions (especially Na+).
What are the different forms of stimulus?
Physical (light, current, stretch)
Chemical- drugs, synaptic excitation
What is the step in the generation of an AP involving a threshold?
Once the membrane has been depolarized to a point called threshold (approx. -55mV), a full on action potential is generated.
What happens after threshold is reached?
Voltage-gated Na+ channels suddenly open all at once, causing a great influx of Na+ and depolarizing the cell. This increases past 0mV, and the membrane potential becomes positive (called the overshoot)