Electrical Properties of Neurons Flashcards
Charge (Q)
- The quantity of electricity
- Carried by ions
- Coulombs (C)
How much charge does a monovalent ion carry?
1.6 x 10 (^19)
Current (I)
The rate of flow of charge over time (amps)
Equation linking current (I) charge (Q) and time
Current = Charge / time
What does charge need to move?
- A mechanism to move (eg. ion channel)
- An ionic gradient or p.d.
Voltage (V), p.d.
- An unbalance in charge
- Arises due to difference in (+) and (-) ions across a membrane or boarder
- Provides ‘driving force’ for current
Resistance (R)
- The restriction of mvmt of charge
- Ohms
Conductance (G)
- Ease of mvmt of charged species
- Ohms (^1)
Ionic equilibrium
When ion mvmt due to charge and conc. gradients
Equilibrium potential (of a given ion)
The membrane voltage at which ionic equilibrium occurs
How can the membrane potential be changed v rapidly?
- Involves redistribution of ions:
- Need to alter membrane permeability (ie. alter membrane resistance or conductance through opening channels)
- Will allow ions to move across membrane
What information does the Nernst equation provide?
The factors affecting the equilibrium potential for a given ion.
At rest, what ion is the membrane typically permeable to?
K+
Difference btw Nernst eq. and Constant field eq.
The nernst equation only refers to the equilibrium potential of one ion whereas neural membranes are often permeable to more than one ion. GHK takes into account permeability of all the main permanent ions as membrane potential will reflect the equilibrium potentials of all the ions present.
What does the constant field equation calculate?
The membrane potential
What are the 3 properties of the neuronal cell membrane?
Acts as a variable resistor, a battery and capacitor.
Capacitance
The ability to store charge (Farads).
Why is the membrane like a battery?
Due to the potential difference btw intracellular and extracellular
How does the membrane act as a variable resistor?
Varies the resistance by opening and closing ion channels.
How does the membrane act as a capacitor?
Has the ability to store charge due to insulating / non-conducting phospholipid bilayer btw electrolytic conducting mediums each side with different voltages. (+) on one side is attracted to (-) on other side.
Relationship btw charge, capacitance and voltage.
Q = CV
Amount of charge is proportional to voltage difference across the capacitor.
How is a capacitor charged and discharged?
Applying a voltage across capacitor stores charge.
Reducing this p.d. causes charge to flow away.
Describe the capacitance of the plasma membrane.
Bilayer forms vv thin non-conducting layer btw extra and intra but has large surface area meaning membrane has a high capacitance.
What effect does a large capacitance have on neuronal signalling?
The larger the capacitance, the larger the amount of charge needs to move in order to change membrane potential and the slower the change is.
What is the difference btw the active and passive properties of axons?
Active conduction relies on voltage-gated ion channels
Internal resistance (Ri) vs membrane resistance (Rm)
Neurons have high internal resistance due to small amount of charged ions and large amount of non-conducting material. Membrane resistance is much lower due to the presence of permanently open ion channels ie. membrane ‘leakiness’.
What does the time constant measure?
The time it takes for the voltage to decay to 37% of its initial value.
What factor(s) effect the amount of capacitance on the phospholipid bilayer?
The thickness of the cell membrane - increased thickness leads to increased capacitance.
In passive conduction, what determines the magnitude of the voltage change that responds to an injection of current?
Ohms law - the size of I and the amount of membrane resistance at the point of injection.
Smaller current leads to smaller rise in voltage.
How quickly does the voltage change in response to injection of current?
Depends on the Rm and Cm (resistance and capacitance of membrane). Capacitance slows down the voltage change as any voltage change across the membrane must be accompanied by a corresponding change in charge.
What effect does capacitance have on the speed of neuronal signalling (1 point)?
Slows down signalling
What determines how far a voltage change will be propagated (passively) along an axon?
Depends on length constant - relationship btw the Ri and the Rm ie. ease of flow of charge through cytoplasm vs through membrane).
How does Rm influence the distance of passive conduction?
High Rm in relation to Ri - long distance
Low Rm in relation to Ri - short distance (as ionic current will leak out of membrane).
How does cable diameter influence internal resistance?
The larger the cable diameter, the lower the Ri.
How does insulation improve passive conductance?
It increases membrane resistance and decreases membrane capacitance.
