Describe currents:
Current is the rate of flow of charge Q.
It has units Amperes [A]:
1 Ampere = 1 Coulomb charge/ second
1 A = 1 C/s
Charge can be electrons (e.g. in wire) or ions (e.g. in neurons)
Define voltage:
A potential difference, or voltage, of 1 Volt means that 1 Joule of energy has been expended in moving 1 Coloumb of charge.
Voltage is measured in Volts[V].
In living systems, voltages are on the order of (tens of) millivolts [mV].
What is resistance?
Materials which allow easy movement of charge are called good conductors, while materials which do not allow easy flow of charge are insulators.
Electrical resistance is measured in…
Ohms [Ξ©].
Ohms Law:
voltage = current x resistance
or
V = IR
When a voltage is applied across a resistor…
A current flows which is proportional to the voltage
When electrical charges accumulate on one side of an insulator…
a voltage builds up across the insulator.
capacitance = charge / voltage
C = Q/V
Capacitance depends on the permitivity of the insulator and the distance between the conductor plates.
How does the lipid bilayer act as an insulator?
Insulates inside from outside. Ions cannot pass.
If ions accumulate on one side (surplus of charge Q), a voltage builds up: π=πβπΆ.
This is called the membrane potential VM .
What is the Membrane potential VM?
The membrane potential is the voltage difference between the inside and the outside of the neuron measured at a certain location.
In voltage-gated channels, these transitions are favoured/disfavoured by the…
membrane potential.
The state of ion channels determines the…
permeability of the cell membrane for different ion types.
What drives ions across the membrane?
Drift is a movement driven by difference in electric potential (=voltage)
Caused by Coulomb forces
Affects equally charged ions in the same way
What is difussion?
Diffusion is a movement driven by difference in concentration, from high to low
Specific for ion type (Na+, K+, Ca2+, Cl-)
Independent of charge
How will ions move between two compartments due to both drift and diffusion?
This is described by the Nernst equation.
The Nernst equation is…
ππππ [ππ]=25/π§
ππ([πππ]_ππ’π‘/[πππ]_ππ )
Modified Ohmβs law incl Nernst potential:
πΌπππ=((ππβ ππππ)/π πππ )
Ions will flow through a resistance Rion according to the…
difference of membrane potential VM (Coulomb force) and their Nernst potential Vion (Diffusion force).
The net current of inflowing Na+ ions and outflowing K+ ions charges the membrane capacitance and thus determines how the membrane voltage changes over time.
Describe this in terms of net inflow and outflow.
A net inflow (more Na+ entering than K+ leaving) makes the membrane potential more positive.
A net outflow (more K+ leaving than Na+ entering) makes the membrane potential more negative.
When a neuron is resting on the outside of the membrane there is:
- a thin layer of positive charges
- a thin layer of negative charges inside.
The AP is a…
change of membrane potential VM over time with a very particular shape (not a specific voltage!)
An AP can be βtriggeredβ by…
the arrival of a signal that raises the membrane potential above a characteristic threshold.
The AP has three phases:
depolarization, repolarization, and hyperpolarization. The first two determine the duration of the AP and the latter the βrefractory periodβ.
During resting potential, the Vm is at a negative potential, meaning…
- few K+ channels are open, most Na+ channels are closed
- Na+ influx equals K+ efflux
- resting VM
During depolarization, the Vm has increased meaning…
- Na+ channels start opening quickly
- strong Na+ influx overcomes modest K+ efflux
- raises VM further, and then quickly further (positive feedback)
During repolarization, the Vm is very positive meaning…
- open Na+ channels inactivate; K+ channels open with a delay
- strong K+ efflux overcomes small Na+ influx
- VM starts dropping rapidly
