The Electrical Signals of Excitable Cells

Question 1

Types of bodily signals

Answer 1

Electrical signals
* Graded potential and action potential

Chemical signals
* Neurotransmitters (e.g. acetylcholine) and hormones (e.g. adrenaline)

Question 2

Excitable cells

Answer 2

• Contain excitable cell membranes, which are capable of producing action potential
• Include all neurones and muscle cells (skeletal, smooth and cardiac)

Question 3

Neurone

Answer 3

• Specialized to initiate, integrate and conduct electrical signals

Question 4

General structures of neurone

Answer 4

Cell body
* Contains nucleus

Dendrites
* Highly branched extensions of neurone cell body to receive information

Axon hillock
* Where an axon leaves the cell body, site of action potential origination

Axon
* Extension from neurone cell body, propagates action potential away from cell body

Axon terminal
* Forms chemical synapse, there are neurotransmitters waiting to be released

Question 5

Membrane potential (MP)

Answer 5

• An electrical potential difference exists across the membrane
• Measured in millivolts (mV) based on unpaired charges in the cytosol, which sets up some voltage (using microelectrode and voltmeter )

Question 6

Resting membrane potential (RMP)

Answer 6

RMP of a neurone is the weighted average of each ion’s equilibrium potential
Neurone: -70mV
Skeletal and cardiac muscle: -90mV

Question 7

Contributors of RMP

Answer 7

Plasma membrane Na+/K+ ATPase pumps
* Actively pumps 3 Na+ ions out and 2 K+ ions in (more negative, creates unequal ionic concentration)
* Maintains low intracellular Na+ concentration and high intracellular K+ concentration
* Ions move against concentration gradient

Permeability of cell membrane
* Cell membrane is much more permeable to K+ (due to more K+ non-gated ion channels present) than to Na+ ions
* Non-gated ion channel (leak channel) are always open and unregulated

Question 8

Equilibrium potentials

Answer 8

• MP that a single ion would produce if the cell membrane is permeable to that ion only
• Depends on ion concentration gradients (intra vs extra) and membrane permeability (K+ is the major ion contributing to RMP)

Question 9

Extracellular fluid (ECF)

Answer 9

Contains plasma in blood vessels and tissue (interstitial) fluid that nourishes tissue cells

Question 10

Intracellular fluid (ICF)

Answer 10

Fluid contained within a cell

Question 11

Chemical force

Answer 11

Movement of ions down its concentration gradient

Question 12

Electrical force

Answer 12

Attraction or repulsion of charges

Question 13

Potassium equilibrium potential (-90 mV)

Answer 13

1. K+ diffuses out of the cell, chemical driving force: out of the cell
2. Inside of cell becomes more negative, electrical driving force acts to pull K+ back into the cell (pulling force)
3. Cell eventually reaches an equilibrium when the chemical and electrical driving forces become opposite in direction but equal in magnitude

Question 14

Sodium equilibrium potential (+60 mV)

Answer 14

1. Na+ diffuses into of the cell, chemical driving force: into the cell
2. Inside of cell becomes less negative, electrical driving force acts to push Na+ out of the cell: resistance
3. Cell eventually reaches an equilibrium when the chemical and electrical driving forces become opposite in direction but equal in magnitude

Question 15

Nernst equation (to determine the equilibrium potential of a single ion)

Answer 15

E ion = 61/z log ( [ion]out / [ion]in)

Question 16

Modification of Nernst equation

Answer 16

The greater the membrane permeability to an ion species, the greater the contribution that ion species will make to the membrane potential

Question 17

The Goldman-Hodgkin Katz (GHK) equation

Answer 17

V m = 61 log (relative membrane permeability x [ion] out + … (cation out, anion in) / relative membrane permeability x [ion] in + … (cation in, anion out))

Question 18

Repolarization

Answer 18

Potential moving back to the RMP

Question 19

Depolarization

Answer 19

Potential moving from RMP to less negative values

Question 20

Hyperpolarization

Answer 20

Potential moving away from the RMP in a more negative direction
* Occurs because channels do not close in time

Question 21

A stimulus is required

Answer 21

• To alter the permeability of a section/area of the membrane to particular ion
• To cause the opening and closure of specific gated ion channels which leads to a change in MP

Question 22

Graded potential (GP)

Answer 22

• Smaller in magnitude
• Communicates over short distance as signals undergo decay (die out at short distance), hence is not good enough to trigger a successful action potential
• Occurs only at dendrites and cell bodies
• Does not require threshold potential for its generation

Question 23

Action potential (AP)

Answer 23

• Larger in magnitude, fixed and stereotypical size and shape
• Large and rapid alteration in the MP via facilitated diffusion (through voltage-gated ion channels) of Na+ and K+
• Communicate over long distances
• Occurs at axons after initiated at axon hillock
• Requires a threshold potential for its generation
• All-or-none response

Question 24

Purpose of graded potential

Answer 24

• determines whether an AP will occur at the axon hillock
• If the GP arrives at the axon hillock exceeds a depolarization value of -55mV (threshold potential), an AP will be inevitably and irreversibly fired (all-or-none principle: a successful AP is guanranteed)

Question 25

Threshold potential

Answer 25

• Minimal level of depolarization necessary for excitable cell to fire AP
• A sub-threshold stimulus will not generate an action potential

Question 26

Describe the formation of AP

Answer 26

1. Steady RMP is near eqm potential of K, MP of K > MP of Na due to K+ leak channels (non-gated channel)
2. Local membrane is brought to threshold voltage by a depolarizing stimulus
3. Opening of voltage-gated Na+ channels causes Na+ influx by facilitated diffusion and rapidly depolarizes the membrane, causing more Na+ channels to open
4. Na+ channels are inactivated and delayed opening of voltage-gated K+ channels halts membrane depolarization (blocked by inactive protein)
5. K+ efflux through open voltage-gated K+ channels via facilitated diffusion repolarizes the membrane back to a negative potential
6. Persistent K+ efflux through slowly closing voltage-gated K+ channels hyperpolarizes membrane towards eqm potential of K; Na+ channels return to its closed state without opening
7. Closure of voltage-gated K+ channels returns the MP to its resting value to undergo another round of excitation

Question 27

Driving force of ion = Vm - Ex

Answer 27

-ve sign just means inward movement of ion

As there is not much channel for Na+ to pass through, Na+ has a higher driving force than K+

Question 28

Why the resting membrane potential is much closer to E K+ ?

Answer 28

Ions with higher permeability will make greater contribution to RMP
Driving force for K+ is smaller as the membrane is largely permeable to K+
K+ permeability decreases —> RMP decreases
Na+ permeability increases —> RMP decreases

Question 29

Major factors determining the RMP

Answer 29

1. Activity of Na+/K+ - ATPase
2. Ionic concentration of ions and electrical differences
3. Permeability of membrane
4. Charge of ions
5. Temperature