NROB60 Exam Questions With 100%
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Action Potential - answerDramatic 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 - ...
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
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