What does Resting Membrane Potential mean in terms of charge?
Inside is negative relative to outside
What does excitable mean in terms of cells?
- Ability to be activated/generate action potentials
- Repolarisation (return to resting state)
Give examples of Excitable cells
- Neurones (nerves) - Many types & functions - Fast
- Muscle - Skeletal, Cardiac, Smooth - Slower
- "Others" (membrane potential oscillations)
- Fibroblasts, macrophages, megakaryocytes
- Pancreatic islet ß-cells
What are the cellular potential changes between Action Potentials & Subthreshold responses?
- Action Potential - Active, regenerative process
- Subthreshold responses - Passive, electrotonic process
What is the importance of passive processes?
- Integration of neuronal inputs depends on passive membrane properties:
- Temporal & Spatial Summation - Processes by which synaptic potentials generated in different regions of the neurone combine to generate an A.P
- Passive processes also determine conduction velocity
What do passive properties determine?
What is the lipid bilayer and what does it act as?
- Is an insulator
- Acts as a capacitor
How fast the capacitor charges depends on what? How do these affect the charge time?
Current & Capacitance.
- Smaller current leads to slower charging
- Larger capacitance leads to slower charging
What is time constant T and what is it's formula?
T is a measure of how fast voltage changes.
T = RC
What happens to T when R (resistance) is increased?
What happens when capacitance is increased?
Increasing resistance slows charge.
Increasing capacitance slows charge.
What does current leak through? What is leak carried mainly by?
Leak carried mainly by K+ ions
What is the equation for exponential rise of voltage with time?
Vt = Vinf(1-e(-t/T))
What is the equation for exponential fall of voltage with time?
Vt = V0e(-t/T)
Why are neurones NOT like Paramecium?
- Have more complex geometry:
- Not spherical
- Have long, thin extensions (axon, dendrites)
- Use 'cable theory' to understand passive processes in neurones
Describe Cable Theory in cables and in neurones.
- Core conductor - Copper, low resistance
- Insulation material - High resistance
- Insulation thick - Low capacitance
- Core conductor - Saline, higher resistance
- Insulation leaky membrane - Lower resistance
- Insulation thinner - Higher capacitance
In neurones, how does current spread and how does it fall off when injected?
- Current spreads longitudinally in both directions
- Falls off as a function of distance from injection site, exponentially
What does Fall Off of current depend on?
Ratio of membrane resistance (rm) and cytoplasmic resistance (ri).
Describe how ratio of rm and ri affects current.
- rm / ri high → Current will spread further
- rm / ri low → Current will leak out sooner
What is Length constant (λ) and what is it's formula?
Length constant is distance over which potential falls to approx. 37% of max value.
λ = (rm/ri)½
With Length Constant, what is the fall of potential with distance caused by?
Current leaking out (transversely) through the membrane
Describe the fall of voltage with distance.
Voltage falls exponentially with distance.
A constant proportion of the available current leaks out for each given length of the axon.
What size of diameter of axons has longer length constants?
Larger diameter axons have longer length constants than narrower axons.
Do myelinated or unmyelinated axons have longer length constants?
What is the typical value range of length constants?
0.1 to 1.0mm
What is the importance of length constant for conduction?
The larger the value of lenght constant, the further the actual distance depolarised above threshold by local circuit currents & the quicker the action potential propagates.
What does propagation speed depend on?
Time taken for depolarisation to spread along axon depends both on resistance & capacitance.
What does resistance affect?
What does capacitance affect?
What does larger capacitance mean?
That current must flow for a longer time to produce a given depolarisation (because the time taken to charge it is greater).
Describe effect of decreasing capacitance (C) by myelination.
- Myelination is wrapping of glial cell membranes around an axon
- Functionally this corresponds to an increase in thickness of axonal membrane
- Since C decreases as thickness of insulator increases, myelination decreases C
- For a given current, smaller C charges in shorter time
- Therefore, conduction velocity is increased
How does myelination allow saltatory conduction?
Nodes of Ranvier:
- Contain high density of sodium channels - Generate an intense depolarising inward sodium current in response to the passive spread of depolarisatiom from the axon upstream.
- Regularly distributed nodes thus boost the amplitude of A.P periodically, preventing it from dying out.
- A.P 'jumps' from node to node.