NROB60 Exam With Complete
Solution
Action Potential Answer Dramatic redistribution of electrical charge across membrane.
Rapid reversal of charge across membrane such that the interior becomes relatively
positive (depolarization caused by influx of sodium)
How to determine MP Answer Microelectrode in cell, voltmetre used to measure
electrical potential between tip of intracellular and extracellular microelectrode
Membrane potential at rest Answer ~-65mV
Key parts of Action Potential Answer Rising phase, overshoot, falling phase,
undershoot/after-hyperpolarization, restoration of resting potential
Length of Action Potential Answer ~2msec
Rising Phase Answer Rapid depolarization of membrane until ~40mV; Na+ rush into cell
through open Na channels
Overshoot Answer Section of AP where inside positively charged wrt outside; MP goes
to value close to ENa+ (greater than 0) because relative permeability of membrane
greatly favours sodium
Falling Phase Answer rapid repolarization (efflux of potassium) until membrane more
negative than resting potential (undershoot); result of Na+ channel inactivation, K+
channels opening, K+efflux.
Perception of Sharp Pain (thumbtack) Answer 1. Thumbtack enters skin
2. Membrane of nerve fibres in skin stretched
3. Na+ permeable channels open, Na+ ions enter nerve fibre down [gradient]
4.entry of Na+ depolarizes membrane; depolarization reaches threshold potential;
ACTION POTENTIAL!
"all or none" Answer Increasing depolarization has no effect until crossing threshold
Multiple Action Potentials: Rate and Frequency Answer Rate depends on magnitude of
continuous depolarization
Frequency reflects magnitude of depolarization current (max: 1000Hz)
Absolute Refractory Period Answer ~1msec; impossible to initiate another AP; Na
,Channels must be de-inactivated by sufficiently negative MP
Relative Refractory Period Answer several msec; relatively difficult- amount of current
required for depolarization to threshold elevated (result of
hyperpolarization/undershoot)
Intracellular Recording Answer impale neuron w microelectrode; challenging (SIZE);
goal is to measure potential difference between intracellular electrode and grounded
electrode; I. Electrode filled w concentrated KCl; potential difference between electrode
and ground displayed w oscilloscope
Extracellular Recording Answer Detecting sequence of ionic movements across
neuronal membrane by placing electrode near membrane; when AP arrives at recording
position, +ve charges flow away from recording electrode into neuron; as AP passes by,
+ve charges flow out across membrane towards recording electrode; EC AP
characterized by brief, alternating voltage difference between recording electrode and
ground. Voltage changes seen w oscilloscope, heard by connecting output to
loudspeaker (Popping sound)
Concentration Gradients Answer Na-K pumps continuous work to establish and
maintain concentration gradients; K+ efflux until inside -ve and Vm=Ek. Net movement of
K+ is electrical current (Ik); # K+ channels open proportional to electrical conductance
(gk)
Ik=gk(Vm-Ek)
Ideal Neuron Answer Only permeable to K+; Vm=Ek=-80mV
Action Potential in Reality Answer When membrane depolarized to threshold; transient
increase in gNA-> entry of Na+, depolarizes neuron. Return to RP by transient gK
increase during falling phase.
Voltage Clamp Answer Kenneth C. Cole; Hodgkin and Huxley could Clamp at any chosen
MP Value
Showed rising phase of AP caused by transient increase in gNa, influx of Na+; falling
phase associated w increase in gK, efflux in K+
Voltage Gated Sodium Channel Answer Protein forms pore in membrane that is highly
selective to Na+ ions; pore is opened and closed by changes in Electrical potential of
membrane
Sodium Channel Structure Answer Single long polypeptide; 4 distinct domains (I-IV);
Each domain made of 6 transmembrane alpha helices (S1-S6); domains clump together
to form pore between them; pore loops assembled into selectivity filter (12x more
permeable to Na+ than K+); Voltage sensor in S4- +ve aa residues regularly spaced
along helix coils, depolarization twists S4 causing gate to open
Patch Clamp Answer Method of studying ionic currents passed through individual ion
, channels; entails sealing tip of electrode to very small patch of neuronal membrane.
Patch can then be torn away from neuron, ionic currents across it measured. W luck,
patch has single channel.
Voltage Gated Na+ Channel Characteristics Answer 1.) Open w little delay
2.)stay open for ~ 1msec then inactivate
3.) cannot be opened again by depolarization until MP returns to native value near
threshold
Why threshold? Answer single channels do not open till critical value of depolarization
Why is rising phase so rapid? Answer Rapid opening of channels in response to
depolarization
Why is AP so brief? Answer Short time channels stay open before inactivating
Generalized Epilepsy w Febrile Seizures Answer Cause: single aa mutations in EC
regions of one Na channel (AP prolonged). Epileptic seizures result from explosive,
highly synchronous brain electrical activity, occur in response to fever, confined to
early childhood (3mos-5years)
Channelopathy Answer human genetic disease caused by alterations in structure and
function of ion channels
Tetrodoxin Answer clogs Na+ permeable pore by binding tightly to specific site on
outside of channel; blocks al Na dependent AP, therefore usually fatal upon injestion
Saxitoxin Answer channel blocking toxin, concentrated in clams, mussels, and other
shellfish that feed on certain kind of protozoa
Batrachotoxin (columbian frog); Vetradine (lilies); Aconitine (buttercups) Answer
causes channels to open at more negative potentials and stay open much longer than
usual, scrambling AP info
Patch Clamp Method Answer 1. gently lower tip of glass recording electrode, 1-5nm in
diameter, onto neuron membrane
2. apply suction through electrode tip forming tight (gigaohm) seal between walls of
electrode and underlying membrane patch, leaving ions only one path to take
3.if electrode then withdrawn, membrane patch can be torn away, ionic currents can be
measured as steady voltages applied across membrane. Amplitude reflects channel
conductance, duration of current reflects time channel is open
Voltage Gated K+ Channel Characteristics Answer don't open immediately upon
depolarization, take ~1msec (delayed rectifier)
function to diminish further depolarization
Solution
Action Potential Answer Dramatic redistribution of electrical charge across membrane.
Rapid reversal of charge across membrane such that the interior becomes relatively
positive (depolarization caused by influx of sodium)
How to determine MP Answer Microelectrode in cell, voltmetre used to measure
electrical potential between tip of intracellular and extracellular microelectrode
Membrane potential at rest Answer ~-65mV
Key parts of Action Potential Answer Rising phase, overshoot, falling phase,
undershoot/after-hyperpolarization, restoration of resting potential
Length of Action Potential Answer ~2msec
Rising Phase Answer Rapid depolarization of membrane until ~40mV; Na+ rush into cell
through open Na channels
Overshoot Answer Section of AP where inside positively charged wrt outside; MP goes
to value close to ENa+ (greater than 0) because relative permeability of membrane
greatly favours sodium
Falling Phase Answer rapid repolarization (efflux of potassium) until membrane more
negative than resting potential (undershoot); result of Na+ channel inactivation, K+
channels opening, K+efflux.
Perception of Sharp Pain (thumbtack) Answer 1. Thumbtack enters skin
2. Membrane of nerve fibres in skin stretched
3. Na+ permeable channels open, Na+ ions enter nerve fibre down [gradient]
4.entry of Na+ depolarizes membrane; depolarization reaches threshold potential;
ACTION POTENTIAL!
"all or none" Answer Increasing depolarization has no effect until crossing threshold
Multiple Action Potentials: Rate and Frequency Answer Rate depends on magnitude of
continuous depolarization
Frequency reflects magnitude of depolarization current (max: 1000Hz)
Absolute Refractory Period Answer ~1msec; impossible to initiate another AP; Na
,Channels must be de-inactivated by sufficiently negative MP
Relative Refractory Period Answer several msec; relatively difficult- amount of current
required for depolarization to threshold elevated (result of
hyperpolarization/undershoot)
Intracellular Recording Answer impale neuron w microelectrode; challenging (SIZE);
goal is to measure potential difference between intracellular electrode and grounded
electrode; I. Electrode filled w concentrated KCl; potential difference between electrode
and ground displayed w oscilloscope
Extracellular Recording Answer Detecting sequence of ionic movements across
neuronal membrane by placing electrode near membrane; when AP arrives at recording
position, +ve charges flow away from recording electrode into neuron; as AP passes by,
+ve charges flow out across membrane towards recording electrode; EC AP
characterized by brief, alternating voltage difference between recording electrode and
ground. Voltage changes seen w oscilloscope, heard by connecting output to
loudspeaker (Popping sound)
Concentration Gradients Answer Na-K pumps continuous work to establish and
maintain concentration gradients; K+ efflux until inside -ve and Vm=Ek. Net movement of
K+ is electrical current (Ik); # K+ channels open proportional to electrical conductance
(gk)
Ik=gk(Vm-Ek)
Ideal Neuron Answer Only permeable to K+; Vm=Ek=-80mV
Action Potential in Reality Answer When membrane depolarized to threshold; transient
increase in gNA-> entry of Na+, depolarizes neuron. Return to RP by transient gK
increase during falling phase.
Voltage Clamp Answer Kenneth C. Cole; Hodgkin and Huxley could Clamp at any chosen
MP Value
Showed rising phase of AP caused by transient increase in gNa, influx of Na+; falling
phase associated w increase in gK, efflux in K+
Voltage Gated Sodium Channel Answer Protein forms pore in membrane that is highly
selective to Na+ ions; pore is opened and closed by changes in Electrical potential of
membrane
Sodium Channel Structure Answer Single long polypeptide; 4 distinct domains (I-IV);
Each domain made of 6 transmembrane alpha helices (S1-S6); domains clump together
to form pore between them; pore loops assembled into selectivity filter (12x more
permeable to Na+ than K+); Voltage sensor in S4- +ve aa residues regularly spaced
along helix coils, depolarization twists S4 causing gate to open
Patch Clamp Answer Method of studying ionic currents passed through individual ion
, channels; entails sealing tip of electrode to very small patch of neuronal membrane.
Patch can then be torn away from neuron, ionic currents across it measured. W luck,
patch has single channel.
Voltage Gated Na+ Channel Characteristics Answer 1.) Open w little delay
2.)stay open for ~ 1msec then inactivate
3.) cannot be opened again by depolarization until MP returns to native value near
threshold
Why threshold? Answer single channels do not open till critical value of depolarization
Why is rising phase so rapid? Answer Rapid opening of channels in response to
depolarization
Why is AP so brief? Answer Short time channels stay open before inactivating
Generalized Epilepsy w Febrile Seizures Answer Cause: single aa mutations in EC
regions of one Na channel (AP prolonged). Epileptic seizures result from explosive,
highly synchronous brain electrical activity, occur in response to fever, confined to
early childhood (3mos-5years)
Channelopathy Answer human genetic disease caused by alterations in structure and
function of ion channels
Tetrodoxin Answer clogs Na+ permeable pore by binding tightly to specific site on
outside of channel; blocks al Na dependent AP, therefore usually fatal upon injestion
Saxitoxin Answer channel blocking toxin, concentrated in clams, mussels, and other
shellfish that feed on certain kind of protozoa
Batrachotoxin (columbian frog); Vetradine (lilies); Aconitine (buttercups) Answer
causes channels to open at more negative potentials and stay open much longer than
usual, scrambling AP info
Patch Clamp Method Answer 1. gently lower tip of glass recording electrode, 1-5nm in
diameter, onto neuron membrane
2. apply suction through electrode tip forming tight (gigaohm) seal between walls of
electrode and underlying membrane patch, leaving ions only one path to take
3.if electrode then withdrawn, membrane patch can be torn away, ionic currents can be
measured as steady voltages applied across membrane. Amplitude reflects channel
conductance, duration of current reflects time channel is open
Voltage Gated K+ Channel Characteristics Answer don't open immediately upon
depolarization, take ~1msec (delayed rectifier)
function to diminish further depolarization
