The Electrical Signals of Excitable Cells Flashcards
Types of bodily signals
Electrical signals
* Graded potential and action potential
Chemical signals
* Neurotransmitters (e.g. acetylcholine) and hormones (e.g. adrenaline)
Excitable cells
- Contain excitable cell membranes, which are capable of producing action potential
- Include all neurones and muscle cells (skeletal, smooth and cardiac)
Neurone
- Specialized to initiate, integrate and conduct electrical signals
General structures of neurone
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
Membrane potential (MP)
- 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 )
Resting membrane potential (RMP)
RMP of a neurone is the weighted average of each ion’s equilibrium potential
Neurone: -70mV
Skeletal and cardiac muscle: -90mV
Contributors of RMP
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
Equilibrium potentials
- 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)
Extracellular fluid (ECF)
Contains plasma in blood vessels and tissue (interstitial) fluid that nourishes tissue cells
Intracellular fluid (ICF)
Fluid contained within a cell
Chemical force
Movement of ions down its concentration gradient
Electrical force
Attraction or repulsion of charges
Potassium equilibrium potential (-90 mV)
- K+ diffuses out of the cell, chemical driving force: out of the cell
- Inside of cell becomes more negative, electrical driving force acts to pull K+ back into the cell (pulling force)
- Cell eventually reaches an equilibrium when the chemical and electrical driving forces become opposite in direction but equal in magnitude
Sodium equilibrium potential (+60 mV)
- Na+ diffuses into of the cell, chemical driving force: into the cell
- Inside of cell becomes less negative, electrical driving force acts to push Na+ out of the cell: resistance
- Cell eventually reaches an equilibrium when the chemical and electrical driving forces become opposite in direction but equal in magnitude
Nernst equation (to determine the equilibrium potential of a single ion)
E ion = 61/z log ( [ion]out / [ion]in)
Modification of Nernst equation
The greater the membrane permeability to an ion species, the greater the contribution that ion species will make to the membrane potential
The Goldman-Hodgkin Katz (GHK) equation
V m = 61 log (relative membrane permeability x [ion] out + … (cation out, anion in) / relative membrane permeability x [ion] in + … (cation in, anion out))
Repolarization
Potential moving back to the RMP
Depolarization
Potential moving from RMP to less negative values
Hyperpolarization
Potential moving away from the RMP in a more negative direction
* Occurs because channels do not close in time
A stimulus is required
- 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
Graded potential (GP)
- 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
Action potential (AP)
- 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
Purpose of graded potential
- 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)
Threshold potential
- Minimal level of depolarization necessary for excitable cell to fire AP
- A sub-threshold stimulus will not generate an action potential
Describe the formation of AP
- Steady RMP is near eqm potential of K, MP of K > MP of Na due to K+ leak channels (non-gated channel)
- Local membrane is brought to threshold voltage by a depolarizing stimulus
- Opening of voltage-gated Na+ channels causes Na+ influx by facilitated diffusion and rapidly depolarizes the membrane, causing more Na+ channels to open
- Na+ channels are inactivated and delayed opening of voltage-gated K+ channels halts membrane depolarization (blocked by inactive protein)
- K+ efflux through open voltage-gated K+ channels via facilitated diffusion repolarizes the membrane back to a negative potential
- 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
- Closure of voltage-gated K+ channels returns the MP to its resting value to undergo another round of excitation
Driving force of ion = Vm - Ex
-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+
Why the resting membrane potential is much closer to E K+ ?
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
Major factors determining the RMP
- Activity of Na+/K+ - ATPase
- Ionic concentration of ions and electrical differences
- Permeability of membrane
- Charge of ions
- Temperature
