Dr Mitcheson lecture 1 Flashcards
What types of electrical signalling occur within neurons?
Action Potentials - active process requiring energy to maintain electrical gradients. For example, maintenance of the sodium and potassium concentration gradients occur via the Na+/K+ ATPase.
Passive (electrotonic) conduction - No energy is required. For example, post synaptic potentials occur without the use of energy. Passive conduction is the underlying basis of neuronal signalling.
When does current flow?
When there is a potential difference (V) between the two points and if they are connected via a conductor.
What is current a flow of?
Charge (through ion movement)
What is the symbol and unit of current?
Symbol = I units = Ampere (A)
What is the symbol and unit of Charge?
Symbol = Q units = Coulomb (C)
What is Ohm’s Law?
The amount of current through a conductor for a given potential difference is proportional to the conductor’s conductance and inversely proportional to its resistance.
What equation is used to determine conductance?
G = 1/R
What equations are used to determine current?
I = G . V I = V / R
What is the basis of the electrical properties of biological membranes?
The mobility of ions within the intracellular and extracellular solutions. This mobility allows for charge to be carried across the membrane.
Which ions allow for charge to be carried across the membrane?
Na+, K+, Cl- and Ca2+.
Why is the lipid bilayer a bad conductor?
Ions are unable to pass through the lipid bilayer as the membrane is impermeable to them. To cross they must be transported via ion channels.
What determines membrane resistance?
The number of open ion channels
What is a capacitor?
2 conductors separated by a thin insulator.
Name the factors which affect capacitance.
Surface area - the bigger the cell the more charge can be stored across the membrane (proportional).
Insulator thickness - the thicker the insulator the less charge can be stored (inversely proportional).
What is capacitance?
The store of charge which builds up on the membrane for a given voltage. It may also be thought of as the quantity of charge required to change the membrane potential by a given value. The membrane capacitance aids in slowing the change of membrane potential.
Which equation, using capacitance, can be used to determine charge?
Q = C . V
Which equation, using capacitance, can be used to determine current?
I = C(dV/dt)
dt = rate of change of voltage with respect to time C = capacitance (farad) dV = Voltage change (Volts)
What factors affect the time course of changes to membrane potential?
Membrane capacitance and resistance
Define the time constant
The time taken for the membrane potential to change by 63% of the membrane potential max.
Give the equation for calculating the time constant.
t = Rm . Cm
What time course does changes in membrane potential follow?
Exponential
In which type of cell does membrane potential change more slowly?
Larger cells or cells with high membrane resistance.
What increases linearly with subthreshold stimuli?
Current
What is the length constant?
Distance from current injection point when voltage signal has decreased to 37%
How does the voltage potential decrease with distance from injection site?
Exponentially
Name the three components of resistance.
Rm, Ra and Rext
What does flow of current along a nerve process depend upon?
The leakiness of the membrane relative to the resistance of the cytoplasm
In which physiological conditions is the electrotonic conduction greatest?
High membrane resistance (low channel density or high insulation) as well as low axonial resistance (high membrane process diameter)
What does electrotonic conduction apply to?
1) The spread of passively propagating voltage signals such as post synaptic potentials (IPSP & EPSP) moving along the dendrites.
2) The conduction velocity of APs. The greater the electrotonic conduction the faster the APs will propagate since spread of depolarisation occurs via electrotonic conduction.
