Neuronal signalling

Question 1

How is voltage used for excitable cells

Answer 1

Voltage (across membrane) is used to signal changes
In muscle cells, +20 mV means the cell wants to contract (i.e. “on”), while -90 mV means the cell wants to relax (“off”) *E
Ion movements across membrane are what determine voltages and currents

Question 2

What is signalling

Answer 2

In excitable cells, ions are often used for “signalling” changes
“signalling” is when one part of the body tells another part of the body what to do
For example: when nerves tell muscles to contract

Question 3

Define voltage

Answer 3

Voltage (DV)= the difference in electric potential energy per unit charge between two points

In physiology, most voltages are across a membrane

Question 4

Define current

Answer 4

Current (I) = a flow of electric charge through a medium (through a surface, esp. X-section)

Question 5

Define resistance

Answer 5

Resistance (R) = the opposition to the passage of an electric current

Question 6

Define conductance

Answer 6

electrical conductance (g), measuring how easily electricity flows along a certain path

Question 7

Describe the selectively permeable membranes

Answer 7

The cell membrane is a “selectively permeable membrane”

Some ions can pass through, others cannot

Ionic permeabilities (and related conductances) depend on ion channels being open, so selectivity can vary over time

There are some man-made materials that also act as selectively permeable membranes

Also called a “semi-permeable membrane”

Question 8

Describe the Vm of selective membranes

Answer 8

Cell membranes have a transmembrane potential (Vm)

A measurement of the overall electrical potential energy across the membrane

Vm depends on the relative electrical currents and conductances of different ions

Fully permeable membranes will (generally) have a Vm of zero across them (e.g. death)

Question 9

Discuss Na+ and its effect on the membrane

Answer 9

When Na+ channels open, the membrane tends to become positive inside

[Na+] is higher outside the cell than inside

Membrane voltage is described in terms of what happens to the intracellular face of the membrane

When the inside of the membrane is positive with respect to the extracellular facing part of the membrane, we say, “The membrane potential is positive”

The extracellular space of all cells is electrically joined and thus has the same voltage everywhere

The extracellular fluid is considered the electrical ground

Question 10

Discuss K+ and its effect on the membrane

Answer 10

When K+ channels open  K+ flows from inside the membrane to the extracellular space  the membrane becomes negative inside

[K+] is higher inside than outside the cell

Question 11

Define gating

Answer 11

Gating = making channel conduct or not

Question 12

Discuss how channels are gated by voltage

Answer 12

Na+ channels open when inside of membrane becomes more positive (i.e. > -50 mV).

But then stop conducting automatically after a time delay

Na+ channels close when membrane potential is negative

There are many different types of K+ channels

The K+ channels for repolarisation open when the membrane becomes positive inside

But these channels are slow and open after a time delay

Question 13

Describe equilbrium potential controlling voltage

Answer 13

Each ion has an equilibrium potential (like a “favourite voltage”), and whenever ion channels for that ion open, the membrane potential moves toward that ion’s equilibrium potential.

The greater the permeability (or conductance) for the ion, the more the membrane potential is driven toward the equilibrium potential for that particular ion

If one type of ion channel is much more conductive than all the others put together, then the membrane potential will become (nearly) the equilibrium potential for that ion.

Question 14

State the equilibrium potentials of Na, K, Ca, Cl

Answer 14

ENa = +60  mV
EK  = -90  mV
ECa = +123 mV
ECl = -40  mV