Lecture 10: electrical properties of neuron cell membrane Flashcards
Current (I)
amount of charge moving past a point per sec
Voltage (V)
Difference of electrical potential energy available to move charges from one point to another
V=RI
Resistance (R)
“friction” opposing the flow of electrical current
Capacitance (C)
a capacitor stores charges, it is made of two conducting surfaces placed near one another and separated by an insulator
-when charged, one surface has -ve charges and the other +ves, they are attracted to each other by kep seperate by the insulator
Q=CV
Q=the amount of charge on each surface of capacitor, V=diff. of electrical potential energy across the surfaces; C=capacitance: amount of charges that can be stored by 1 volt
Membrane Capacitor is made of…
Two conducting materials: Intracellular fluid (ICF) and extracellular fluid (ECF), and insulating layer of phospholipids
Membrane Canacitance (Cm)
depends on 1. membrane surface area (A), increasing area=increasing capacitance
and 2. insulating layer of thickness (d), where increasing thickness decreases capacitance:
Cm=e*A/d
Membrane potential change:
V=Vmax*e^(-x/lambda)
Length constant lambda:
-distance over which change in Vm (GP) dies down
lambda=squr(Rm/(Ri+Re)); Re is usually low and constant
=squr(Rm/Ri)
-lambda is largest when Rm is high and Ri is low
when neuron is exposed to a stim, the potential does not change instantaneously, why is the change of Vm delayed?
- membranes acts as capacitors
- capacitance delays the change in Em following stimulation
- greater the capacitance, the more charges the membrane can separate and store for a given potential difference and therefore the more time it takes for ions to be redistributed in response to stim.
time constantr “r”:
- time it takes for change in Vm (GP) to reach it’s max value
- aka time taken for change in MP to reach 63% of its max value
- time needed to charge membrane capacitor is described by the time constant=RmCm
Importance of length and time constants
- lambda: is distance over which change in Vm (GP) will decrease by 37% of its original value
- “r”: time taken for change in Vm (GP) to reach 63% of its max value
- lenght and time constant are important for both integration of graded potentials and conduction of AP
to compare two length or time constants…
look at the x-axis value at 50% decreases and compare the distance of the length or time constants
A neuron has a greater length constant than another because it has
a larger membrane resistance, allowing for greater integration of GP and thus a stronger AP
greater the length or time constant,
the higher the AP
a neuron has a greater time constant than another because it has
a larger membrane resistance, allowing for greater integration of GP and stronger AP
There is great variation in signal conduction speed of APs between:
species AND neuron types
2 important componenents of AP conduction to conduction velocity:
- Regeneration of APs (opening of VGates and Na+ diffusion) is the slow component of conduction
- Electrotonic current flow is the FAST component of conduction, but elecrtontonic current flow decays and only travels short distances: APs need to be reinitiated to keep current going over distance
the higher the length constant…
the further the electrotonic current by AP can be propagated, the faster the conduction of APs along the axon (look at threshold potentials post-depolarization to compare length constants)
Length constant is greater with…
high membrane resistance and low cytoplasmic resistance, and lower membrane permeability to K+
the lower the time constant:
- the faster the capacitor becomes charged
- the faster depolarization of membrane
- faster the electrotonic current will start spreading
- from all the above, the faster the conduction velocity
- lower time constant=lower capacitance
Overall increasing axon diameter…
increases conduction velocity
increasing axon diamter in relation to Rm
- increases surface area
- increases the number of leak channels
- decreases Rm
- *length constant still increases when Rm decreases due to axon diameter because Ri decreases MORE
Increasing axon diameter in relation to Ri
Increases volume, decreasing Ri
Increasing axon diameter in relation to capacitance
increases surface area, increasing capacitance
**time constant changes very little because changes in R, and Cm cancel each other out
Rm is inversely proportional to surface area; increasing axon diameter,
-increases surface area
-increases number of leak channels
-decreases Rm (Rm=1(2pirh)
decreases length constant
Ri in inversely proportional to volume; increasing axon diameter…
increases volume
-decreases R (Ri=1/(pir^2h)
increases length constant
Overall, increasing axon diameter INCREASES length constant
lambda= squr(r/2)
-net effect of increasing axon diameter is to increase speed of conduction
Neuron design objectives:
- to maximize computing power, need high density and therefore neurons diameters must be small
- minimizing response time to changes requires fast conduction and therefore large diameter since it increases speed of conduction
disadvantages of giant axons:
takes up lots of space, limiting number of neurons in the nervous system
-have large volumes of cytoplasm, making them expensive to produce and maintain
Effect of myelin on velocity:
precense of myellin increases conduction velocity
Myelination…
- found in many vertebrate neurons
- increases conduction speed without greatly increasing space required, therefore better than giant neurons
- it is an insulating layer of lipid-rich Schwann cells wrapped around the axon
- Increases Rm by a factor of 5000
- decreases Cm by factor of 50
- this increases length constant and decreases capacitance, reducing the time constant
- overall increasing axon diameter and increases conduction velocity
Myelin acts as insulation, increasing membrane resistance Rm
- decreases current loss through leak channels
- increases Rm, length constant, and conduction velocity
effect of myelin on length constant;
-spreads electrotonic current, increasing distance from point of repolarization to return to threshold potential
Nodes of Ranvier
-areas of exposed axonal membrane between schwann cells
internodes
myelinated regions
-current spreads electrotonically through internodes while new APs occur at nodes of ranvier
Saltatory conduction
“leaping” of AP from node to node, due to the channels being there and the current spreading through internodes of sheath to the next channel
Demyelination
-patients with multiple sclerosis lose myelin sheath, and current leaks and Em changes due to AP decay occur faster
