Membrane potentials and action potentials

Question 1

What is a flux?

Answer 1

The number of molecules that cross a unit area per unit of time

Question 2

what is voltage and current?

Answer 2

Voltage is a potential difference that provides energy to supply movement of ions from a high voltage to a low voltage. Current is the movement of ions due to that potential difference

Question 3

What is the zero volt level?

Answer 3

the potential difference of a cell measured by placing an electrode outside the cell

Question 4

Describe generation of membrane potential based on K+?

Answer 4

membrane permeable to K+. K+ moves from compartment 2 to 1 (where there is no K+) down concentration gradient taking its positive charge with it. You reach a point where the concentration gradient is balanced by the electrochemical gradient. Positive charge further accumulates in compartment 1 preventing further influx of K+(as like charges repel eachother). This is the state of ELECTROCHEMICAL EQUILIBRIUM where the electrical forces balance the diffusion forces. Allowing for a stable transmembrane potential to be achieved The same can happen with Na+ ions just in opposite direction e.g Na+moves from compartment 1 to 2 instead.

Question 5

What is the equilibrium potential?

Answer 5

The potential at which electrochemical equilibrium has been reached. It is potential that prevents diffusion of the ion down its concentration gradient

Question 6

How can you calculate the equilibrium potential?

Answer 6

using the Nernst equation

Question 7

How can you simplify the Nernst equation?

Answer 7

Assume T=37 degrees=310 kelvin convert natural log to common log State E in mV make compartment 2 the inside of the cell and compartment 1 the outside Use typical concentrations of K+: 150mM inside and 5mM outside

Question 8

What are the individual equilibrium potential fro K+ and Na+?

Answer 8

Ek = -90mV ENa= +72mV

Question 9

Why do membrane potentials not rest at Ek or ENa?

Answer 9

Because membranes have mixed K+ and Na+ permeability

Question 10

What is the Goldman Hodgekin Katz (GHK) equation?

Answer 10

Describes the resting membrane potential taking into account the relative permeability of each of the ions at one time. K+,Na+ and Cl- concentrations all contribute to the real membrane potential. The size of each ions contribution is proportional to how permeable the membrane is to the ion. P in the equation stands for permeability or channel open probability If P is: 0=100% clossed 1=100% open 0.5=open 50% of the time The subscript besides P indicates the ion and the subscript next to the conc of each ion indicates the con inside or outside the cell

Question 11

What do the following terms mean : Depolarisation Repolarisation Overshoot Hyperpolarisation ?

Answer 11

Depolarisation-Membrane potential increases from negative towards 0 Repolarisation - Membrane potential decreases towards resting potential Overshoot- when the membrane potential increases form 0 to become more positive Hyperpolarisation- When the membrane potential decreases below resting potential

Question 12

What changes membrane potential?

Answer 12

Stimuli e.g. a large stimulus can cause a lot of depolarisation whereas a small one would cause a small amount of depolarisation Also different types of stimulus can result in depolarisation or hyperpolarisation

Question 13

What are graded potentials?

Answer 13

temporary charges in the membrane. The size of the graded potential and duration is dependent on the size of the stimulus and the nature of the stimulus

Question 14

what is meant by decremental spread of graded potentials

Answer 14

Graded potential travel to neighbouring membrane regions. As the impulse propagates( travels further from initial site) the amplitude diminishes and charge leaks from the axon

Question 15

What is the membrane potential dependent on?

Answer 15

Not just solely due to the sodium potassium pump is also due to movement of potassium through channels

Question 16

What are the 5 stages of an action potential?

Answer 16

Resting membrane - set up by K+ flowing out of cell taking positive charge with it and leaving behind negative charge and there’s also a small amount of Na+ flux into the cell Depolarising stimulus- Stimulus cause response of receptor producing a graded potential, when this potential reaches a threshold value the action potential occurs Upstroke- Threshold value reached. This is the voltage that the Na+ channels respond to and open QUICKLY= increased Na+ permeability = Na+ enters cell down electrochemical gradient causing the membrane potential to move towards the Na+ equilibrium potential. Voltage gated K+ channel also open but SLOWLY, this causes K+ to leave the cell down the electrochemical gradient . Repolarisation- Na+ channels close = Na+ entry into the cell stops. The previous depolarisation cause K+ channels to open, these remain open causing K+ to leave cell causing the membrane potential to ove towards the K+ electrochemical potential as (decrease) Hyperpolarisation- K+ continues to leave and the K+ channels dont close fast enough causing mebrane potential to decrease below resting membrane potential.

Question 17

What happens during repolarisation?

Answer 17

Na+ channel does 2 things when it is effected by voltage: - It opens up to allow Na+ into the cell (depolarisation/upstroke) - It has a part of the protein that goes in and plugs the membrane at one end of the channel preventing sodium moving through the channel(Repolarisation). This evokes a phase called the REFRACTORY PERIOD in which you cannot restimulate the nerve to cause an action potential as the Na+ channel is temporarily inactivated due to being blocked by that protein.

Question 18

Why do graded potentials decrement down the axon?

Answer 18

Graded potentials decay down the axon as they don’t reach threshold level. If you have a large diameter neurone this happens more slowly due to them having a lower resistance.

Question 19

Why is the action potential conveyed down the axon?

Answer 19

The adjacent area to the peak of the action potential becomes depolarised because some Na+ flows sideways into this neighbouring area. If this area reaches threshold you’ll get opening of Na+ channels here and therefore Na+ influx here. Some will move down the axon further. If this adjacent area gets more depolarised and reaches threshold then Na+ channels open here also and this continues causing the action potential to move down the axon.

Question 20

What is saltatory conduction?

Answer 20

Nodes of Ranvier are the parts of the axons which are not covered in mylin sheath and they are concentrated with Na+ channels. The action potential travels down axon by jumping to the nodes of ranvier = quicker conduction

Question 21

What 2 things effect conduction down axon?

Answer 21

Diameter- increased diameter = increased speed of conduction due to less resistance Myelination- increased myelination = increased speed of conduction due to acting as an electrical insulator and allowing the formation of the nodes of Ranvier

Question 22

What diseases reduce myelination?

Answer 22

MS and diptheria

Question 23

What are the 3 main factors that influence the movement of ions across the membrane?

Answer 23

Concentration of ions on either side of the membrane Charge of the ion Voltage across the membrane

Question 24

Why is the K+ equilibrium potential negative and the Na+ equilibrium potential positive when both are positive ions?

Answer 24

More K+ inside the cell than outside so tend to flow out of the cell while more Na+ outside the celll than inside so tends to flow into the cell. A potential of -70mV is needed to attract K+ and stop the net outward flow, while a positive charge of 40mV is needed to repel Na+ from entering the cell.

Question 25

What is the relative refractive period?

Answer 25

occurs after hyperpolarisation Inactivation gate is open but stronger than normal stimulus needed to stimulate an action potential

Question 26

Describe neuromuscular junction?

Answer 26

same as normal neurotransmission across synapse but: -ACh is always the neurotransmitter release -the post synaptic membrane is always SKELETAL MUSCLE -Receptors that bind to ACh on post membrane=NICOTINIC ACh receptors

Question 27

What is excitation contraction coupling?

Answer 27

an action potential causes muscle contraction 1-ACh binds to Nicotinic ACh receptor 2-depolarisation in sarcolemma(muscle fibre cytoplasm) 3-wave of depolarisaton continues down t-tubules 4-cause sarcoplasmic reticulum to release Ca2+ 5-calcium binds to actin =trophomyosin removes =myosin head of myosin filament can bind to actin here and move the filament =contraction

Question 28

What are some disorders of the neuromuscular junction?

Answer 28

Botulism-botulinum toxin irreversibly disrupts stimulation induced ACh release from presynaptic terminal Myasthenia Gravis-autoimmune disorder-antibodies directed against ACh receptor. causes weakness Lambert-Eaton myastenic syndrome(LEMS) - autoimmune disorder antibodies directed again VGCC

Question 29

What are the 3 synaptic organisation of the post synaptic neurons that allows communication between nerves?

Answer 29

Axodendritic synapse: connection between presynaptic terminal → neuronal dendrite Axosomatic synapse: connection between presynaptic terminal → neuronal soma Axoaxonic synapse: connection between presynaptic terminal → neuronal axon