What is the role of the cell plasma membrane?
Acts as a barrier separating the contents of the cell from the outside
How are the distribution of ions maintained between the outside and the inside?
by ionic pumps and exchangers and so consumes energy.
What is the result of the ionic imbalance?
An electrical potential difference between the cytoplasm and the external medium.
How do the cells use the electrochemical gradients?
By allowing ion flow across the cell membrane under well-controlled conditions.
What do the ion channels mediate?
The ion transfer in the cell
Describe the process of diffusion?
Dissolved ions distribute evenly. Ions flow down the conc. gradient when the channels are permeable to this ion and there is a conc. gradient across the membrane.
Describe the process of the electrical movement of ions?
Electrical current influences the ion movement. Resistance=1/electrical conductance.
What is the distribution of the potassium ions across the membrane?
Potassium are more conc. inside. 5mM (outside):100mM (inside) Negative Eion
What is the distribution of the sodium ions across the membrane?
Sodium more conc. outside. 150: 15 (mM) Positive Eion
What is the distribution of calcium ions across the membrane?
Calcium is more conc. outside. 2: 0.0000002 (mM) Positive Eion
What is the distribution of the chloride ions across the membrane?
More conc. outside. 150: 13 (mM) Negative Eion
Name the two ways in which the electrochemical gradient is created?
1. Active transport (sodium pump) 2. Selective membrane permeability to certain ions and molecules.
What ions are movement during rest membrane potential?
Sodium will tend to leak in and potassium to leak out.
What ion is said to be permeable to the cell?
How is the resting membrane potential -65mV?
Due to the potassium being pumped out at rest
When is the potassium ions at equilibrium?
When the concentration and electrical gradients balance.
When is equilibrium achieved?
When the movement down conc. gradient equals movement down electrical gradient.
What is the importance of the nernst equation?
Gives the value of the membrane potential at equilibrium.
What is the Nernst equation?
Ek = -RT/zF . ln [K]i/ln[K]o
What receptor is used to distribute the potassium and sodium ions?
Why isn't the resting membrane potential equal to the resting potassium equilibrium (-80mV)?
As there is always some leakage of sodium at rest
How can some ions have a large conc. gradient but little influence in the overall potential?
As the membrane is less permeable to that ion.
What does the nernst equation display?
The theoretical balance between electrical and conc. gradients.
What does the goldman equation display?
The real world situation of finite and variable ionic permeabilities.
What is the goldman equation?
Vm= -60 log (Pk [K]i+Pna[Na]i + Pcl [Cl]o / Pk[K]o + Pna [Na]o+Pcl [Cl]i)
Why is the chlorine outside on the top line of the goldman equation instead on the bottom like the potassium and sodium?
Due to its ionic charge being negative.
Describe the characteristics of an action potential?
Rapidly propagated All-or-none Electrical message that passes along axons of the nervous system.
Describe the depolarisation phrase of the action potential?
Sodium influx. -70 -> 50mV
Describe the depolarisation phrase of the action potential?
Potassium efflux. 50 -> -70mV
Describe the afterhyperpolarisation phrase of the action potential?
The sodium channels are still inactivated state therefore the cell becomes more repolarisated (-70 -> -90mV)
Describe subthreshold potential?
A graded potential starts above threshold at its initiation point but decreases in strength as it travels through the cell body. At the trigger zone, it is below threshold and therefore does not initiate an AP.
Describe suprathreshold potential?
A stronger stimulus at the same point on the cell body creates a graded potential that is still above threshold by the time it reaches the trigger zone. Causes AP.
What happens when you increase the extracellular potassium?
Depolarisation of the membrane potential.
Name the two ways in which the external potassium concentration is regulated?
1. Blood-brain barrier. 2. Potassium spatial buffering
How do you record a single channel voltage?
Patch clamp technique
Describe the 4 different patch clamp configurations?
1. Cell-attached 2. Inside-out patch 3. Whole- cell recording 4. Outside-out patch
What is the normal resting potential?
What is the chemical and electrical gradient of potassium at -65mV?
Outward chemical gradient > inward electrical. Hence, potassium current is outwards.
What is the chemical and electrical gradient of potassium at -85mV?
Inward electrical gradient > outward chemical gradient Hence, potassium current is inwards.
Name the two ways in which the ion channels are controlled in the ion movement?
1. Gating 2. Inactivation
Describe channel gating?
Most channels spend the vast majority of time in this closed state. Requires energy to open it.
How is gating regulated?
By protein conformation Independent of permeability control
Name the two types of gating channel?
1. Voltage gated channels. 2. Ligand gated channels.
Describe channel modulation when looking at gating process?
It is a secondary factor that alters the gating process. The ion channel only conducts certain ions but some will enter the pore but are not permeable and thus block the channel.
What is needed to change the gate from a closed (resting) state to the open (conducting) state?
A change in the membrane potential.
What are the two possible routes for an open state channel to return to the non-conducting state?
1. Deactivation 2. Inactivation
Describe the deactivation state?
Channel return to the closed state. Driven by repolarisation.
Describe the inactivation state?
Maintained depolarisation drives the gate into a different non-conducting state.
Describe the structure of the voltage-gated ion channel?
4 subunit. 6 transmembrane domain in each subunit. 4th transmembrane domain is positively charged.
How is the voltage-activated channels opened?
Cause of a depolarisation current. If there is a negative charge in the inside the S4 domain will be more attracted to the inside of the cell and thus opening the receptor.
How does the voltage-activated channels go into the inactivated state?
Hydrophobic intracellular 'gate' between transmembrane 3 and 4 domain that binds to the intracellular mouth of the pore and inactivates it.
This is the triad of amino acids that form the inactivation gate.