block 2 Flashcards
why is the lipid bilayer considered a good resistor?
because it doesn’t allow ions to pass through it without a conductor
-if the ion channels are open = higher resistance
what is the movement of ions influenced by?
-concentration gradient
-electrical gradient
describe a equilibruim potential of -70mv
-there are more positive k+ ions inside the cell than outside therefore postive ions move out of the cell down their concentration gradient = so the cell is more negative inside than out
equilibrium potentials are calculated using the Nernst equation
- see lecture
electrical current
-the movement of electrical charges (ions)
ohns law
V=I x R
v= voltage
R=resistance
I=electrical current
resistance
-the measure of friction a component presents to the flow of current
-the more ion channels open= the more resistant
-conuctance increases and so its easier for ions to move inside the cell
conductance
-how easy it is for current to flow through a conductor
-opppsire of resitance
G= I/R
g=conductance
capacitance
-the store of charge that builds up on a membrane
-the charge required to change membrane potential by a given value= to imitate a action potential
-membrane capacitance act slow to change membrane protentional
e.g. the lipid bilayer because it stores charge
RC circuit
capacitor= 2 conductors separated by a thin insulator
-the current will either charge the capacitor
-or flow through the conductor and out of the cell
actions of the RC circuit
- initally all the charge will change the capacitor
- as charge starts to flow in the capacitor its voltage starts to change
-as the capacitor charge the rate of voltage change drops and more and more current is diverted through the resistor
-when the capacitor is fully charged the current goes through the resistor
what is capacitance influenced by
-cell size and insulator thickness e.g. myelination
c= surface area/insulator thickness
gilal cells
-wrap around many axons at once.
-Larger axons are surrounded by several glial cells instead of just one.
-These glial cells help guide the electrical signals along the axons, acting like a pathway or channel that helps the signal move smoothly.
However, this process is different from myelination
oligodendrocyte
cells that cause myelination in the CNS
schwarnn cells
cells that cause myelination in the PNS
how does myelination occur?
Oligodendrocytes/ schwarnn cells wrap around the axon multiple times, forming multiple layers of their cell membrane around it. These layers stack on top of each other to create the myelin sheath, which is the thick, insulating layer that helps speed up the nerve signals traveling along the axon
Oligodendrogenesis
Neural Progenitor Cells → turn into → Oligodendrocyte Precursor Cells (OPCs) → develop into → Oligodendrocytes.
how do we know which axons are going to be myelinated?
-small axons arent going to be myelinated between 0.1 micromemtres to 0.4 micrometres
-the larger they get the more curvature occurs = increased diameter= increased myelination
Wnt / β – catenin intracellular regulation of the myelination pathway in the CNS
-can inhibt and promote myelination
PI3K / AKT / mTOR intacellular pathway regulation of the myelinated pathway of the CNS
-Inhibition of mTOR results in the impaired
initiation of myelination and hypomyelination
Knock-out models show reduced
myelin thickness due to
oligodendrocytes being unable to
produce sufficient myelin proteins
ERK / MAPK
ERK / MAPK intracellular pathway of myelinated pathway of CXNS
Knock-out models show reduced
myelin thickness due to
oligodendrocytes being unable to
produce sufficient myelin proteins
activity-dependent myelination
-the more active and engaged your brain is through different kinds of experiences, the better it can develop these important myelin layers. This helps the brain adapt and improve its functioning
- info can be used as therapeutic target
the role of myelinated sheath
Insulation of the axon
* Increases membrane resistance
* Reduces capacitance
* Increases speed of passive conduction
Not continuous
role of caspr cells
separate important parts of the nerve, like sodium and potassium channels, so that electrical signals can jump quickly along the nerve at the nodes of ranvier .
Without Caspr, these structures can become disorganized, leading to problems with how nerve signals travel.
Saltatory Conduction in myelinated neurons
siginals jump from node to node = faster conduction
how does myelination affect action potentials?
Myelination: the effect on action potential conduction velocity
Nodes of Ranvier
* Short time constants
Membrane resistance and capacitance
* Cm and Rm low
* high density of open channels
The Internodes
* Long length constants
Membrane resistance
* Rm: Ra very high
* Low levels of ion channels
demyelination
pre-existing myelin sheaths are damaged and subsequently lost
multiple sclerosis
demyelination in MS causes nerve signal breakdown, leading to the physical and cognitive symptoms seen in the disease. such as muscle weakness, trouble with coordination, and vision problems, because the brain can’t communicate properly with the rest of the body
passive conduction
the passive flow of current along a neuron in the absence of action potentials. no energy is required
what part of the neuron does action potentials occur?
- the axon
whats the importance of passive signalling?
its needed for:
-temporal and spacial summation
-motor skills
-sensory integration
-cognitive function
what determines how siginals are passesd along a neuron?
-membrane capacitance
-membrane resiatance
-intracellular axial resitance
length constant
-a measure of how far a siginal can passively travel along neuronal process
- signal diminishes as you go so it bnascially measure how far the current spreads before leaking out
3 components of resistance the length constant is dependent on?
- membrane resistance = Rm
-cytoplasm/Axial resistance =Ra
-resistancce of extracellular fluid =R ext
cable theory = neuronal processes as leaky cables
-as current flows along cytoplasm some of it leaks across the membrane.
-resistance in series is additive= resistance increases as we go along the axon
-signals can either pass through the axon or tgrav el across the plasma memb rane and leak out if ion channels are open
-look at length constant equatgion= don’t need to memorise but needed
when is the length constant greatest
- Rm is high due to maybe low channel density or high insulation
-Ra is low due to diameter of membrane process is high
how can we measure length constants?
-inject current
-place a number of electrodes in neuron knowing the precise distance they are from stimulating electrode
-record the signal to get numbers(voltage)
-plot this to get an exponential curve
-length constant= distance from current injection point (d) when voltage signal has decreased to 37%
-from graph find the maximum value of voltage at zero
-find 37% of that value
-find point on graph that correlates to the distnace in which you reach 37% of the graph
time constant in neuronal funcation
important i
-temporal summation
-why doesnt current flowing across a membrane immedieately change the membrane potential
-capacitors introduce a time delay to changes in voltage (membrane potential)
-vm rises slowest in larger cells and cells with high membrane resistances
time constant
-a measure of how rapidly the membrane potential changes with time
-ie. how long it takes for thee voltage to decay to 37% of maximum
what does an increase in conductance for an ion channel represent
a increase in the number of open channels for that ion
summarization of action potential
stimulus causes sodium (Na⁺) channels in the neuron’s membrane to open, letting Na⁺ ions rush in and depolarize the cell (make it more positive). As the charge inside reaches a threshold, more Na⁺ channels open, causing a rapid rise in voltage.
Next, potassium (K⁺) channels open, allowing K⁺ ions to exit the cell, repolarizing (making it more negative again) and eventually restoring the original charge balance
absolute refractory period
-impossible to trigger an AP. nearly all t6he Na+ channels are inactivated
relative refractory period
-hard but not impossible
-excitability returns to normal as the number of Na+ channels In the inactivated state decreases and the volatge-gated k+ channels close.
what is the voltage clamp technique
- a way of controlling membrane potentional to measure the movement of chartered ions across the membrane as electric current
-can be used to measure very rapid current events
describe the voltage clamp technique
- the voltage command electrode= where you control the voltage you want to measure at
-the membrane potential
-membrane potential (Vm) electrode= measures the membrane potential of that specific axon
-feedback amplifier=constantly monitors the actual membrane voltage and compares it to the desired voltage that the experimenter wants to maintain.
If there’s a difference between the actual voltage and the target voltage, the feedback amplifier quickly adjusts the current to bring the membrane voltage back to the set point.
current output=hen the cell’s membrane voltage starts to drift from the set level, the current output supplies just the right amount of current to counteract this and bring the voltage back to the target value.
Measuring Ion Flow: The amount of current outputted is directly related to how much ion flow (like sodium or potassium ions) is happening across the membrane. By measuring this current, researchers can understand the behavior of specific ion channels
how do we remove na+ urrent so your only measuring k+ current in the voltage clamp technique
- add tetrodoxtoxin which blocks sodium current
how can we look at just the sodium current?
- subtract the potassium current from the original current
conduction velocity
the speed at which an action potential travels along a neuron
-governed by passive electrical properties of the membrane
conduction velocity=
distanced traveled/time taken
-distance travelled= distance between 2c points where action potentials are being measured
-time=time taken to propagate from 1 action potential to the next
