biophysics

Question 1

ohm’s law

Answer 1

current, voltage, conductance and resistance
current = conductance x voltage
conductance = 1 / resistance
current = voltage / resistance

Question 2

equilibrium

Answer 2

state of system where no further changes are possible

Question 3

Nernst equation at 37 degrees Celsius for monovalent cations

Answer 3

Ex = (RT/zF)log([X]o/[X]i)

simplified - Ex = 61.5log([X]o/[X]i)

the inside and outside ion cons are switched for anions (invert the ratio)

Question 4

ion equilibrium potentials (skeletal muscles)

Answer 4

potassium - -98mV, sodium - 67mV, chloride - -90mV, calcium - 110mV

Question 5

Goldman-hodgkin-katz equation

Answer 5

resting membrane (Vm) is calculated using the Nernst equation where the difference is, is that all the concentrations of ions are added together where outside are added on top of the ration (opposite for anions) and the inside concentrations are added on the bottom of the ration ( opposite for anions), P is 1 combined

Question 6

ion channels

Answer 6

conduct small ionic current along electrochemical gradient, selective permeability, open/close in milliseconds underlying rapid membrane potential changes, when closed generates resistance, when open generates conductance

Question 7

transporters/exchangers

Answer 7

transport or exchange along electrochemical gradient, bind/ release ions, don’t normally conduct ionic current

Question 8

ion pumps

Answer 8

transport ions agains electrochemical gradient, requires ATP (sodium potassium ATPase, plasma membrane calcium ATPase

Question 9

voltage-gated ion channels

Answer 9

ion conducting pore and gates controlling the pore, pore has selectivity filter region allows permeable ion to move through easier than others, gate coupled to voltage sensor - moveable portion of protein molecule sensitive to to voltage changes across membrane movement of voltage sensor controls ion channel gate, important in action potential generation, each channel contains 4 domains/subunits each made of 6 transmembrane helices

Question 10

potassium channel selectivity filter

Answer 10

potassium channel pore mimics hydration of potassium ion but sodium is too small so oxygens will be too far away from normal hydration shell of sodium, energetically unfavourable for it to lose water and gain nothing in potassium channel pore the T V G Y G residues that are conserved and create the selectivity filter in the pore have oxygens that are at the same distance are the water surrounding potassium so these are easy to swap out

Question 11

sodium channel gate

Answer 11

activation gate (m gate) opens rapidly on depolarisation allowing sodium to flow, inactivation gate (h gate) responds to depolarisation by plugging channel pore after brief delay (5ms), h gate remains in place for short time - refractory period, further sodium influx not possible

Question 12

membrane potentials and ions roles

Answer 12

potassium brings the cell membrane towards hyperpolarisation whereas sodium and calcium bring it towards depolarisation

Question 13

neuronal ion channel activity

Answer 13

potassium (-90mV) - hyperpolarises membrane and increases voltage needed to reach action potential firing threshold, sodium (+65mV) - depolarises membrane, calcium (+110) - depolarises membrane, cations (~0mV) - non selective cation channels (don’t discriminate between small cations) depolarises membrane, chloride (varies ~-90mV) - inhibitory

Question 14

driving force of ion

Answer 14

Vm-Eion

+ve = outward driving force
-ve = inward driving force

Question 15

reversal potential

Answer 15

membrane potential at which direction of current flow reverses

Question 16

voltage clamp

Answer 16

feedback system, allows voltage control (voltage clamp) across plasma membrane, essential for analysis of channels who’s activation is voltage dependant, voltage controlled and current across membrane recorded - directly tests properties of active ion channels

Question 17

current clamp

Answer 17

feedback system, allows current control (current clamp) across plasma membrane, essential for recording membrane potentials, controls current flowing through cell and voltage responses recorded - mimic action potentials and synaptic input

Question 18

role of sodium channels in action potential

Answer 18

voltage gated, open on membrane depolarisation, influx of sodium into cell, influx causes more depolarisation - more sodium channels open, sodium influx drives membrane too and past 0 mV towards Esodium, explosive sodium influx only occurs if threshold reached (~55-50 mV), channels inactivate rapidly - no new action potentials possible - absolute refactor period

Question 19

role of potassium channels in action potential

Answer 19

voltage gated, open in response to depolarisation at more positive potential then sodium channels (delayed), small fraction of specific voltage gated and non gated potassium channels open at rest (leak channels) - maintain rising membrane potential near potassium equilibrium, delayed opening main factor responsible for depolarisation phase, potassium ions flow out cell (+ flow out) cell becomes more negative, repolarisation creates a temporary more negative membrane potential than rest (afterhyperpolarisation) - voltage gated potassium channels deactivate and resting membrane potential restored

Question 20

calcium activated potassium channels

Answer 20

some neurones, internal calcium trigger specific family of potassium channels, contribute to action potential repolarisation, respond to activation of voltage gated calcium channels which contribute to depolarisation similar to sodium channels, calcium-gated potassium channels often open longer than voltage gated potassium channels and can prolong the afterhyperpolarisation, create a ‘shoulder’ on action potential readings

Question 21

action potential propagation

Answer 21

action potential originates at axon initial segment, in myelinated axons action potential jumps from one node of ranvier tot he next

Question 22

channel recordings

Answer 22

single channel recordings - current through single ion channel, scrutinise io channel property
whole cell recording - gating properties of ion channels can be revealed

Question 23

M current (excitatory effect on cell by acetylcholine binding to M1)

Answer 23

depolarisation activated noninactivating potassium channel, M channel activity defines firing patterns, muscarinic acetylcholine receptor excites sympathetic neurones by inhibiting M channels, acetylcholine binds to M - hetrotrimeric G protein activates enzyme PLC beta - PIP2 broken down into DAG which activates PKC and Ins(1,4,5)P2 which activates calcium release both pathways inhibit Kv7/ M channel, PIP2 all inhibits it

Question 24

GIRK channels (inhibitory effect on cell by acetylcholine binding to M2)

Answer 24

muscarinic acetylcholine receptors inhibit cardiac muscle cells by activating GIRK channels, acetylcholine binds to M2 - beta gamma unit of G protein binds to GIRK channel opening it and allowing potassium to enter