Action Potential Flashcards
Action Potential
An action potential is a rapid sequence of changes in the voltage across a membrane. The membrane voltage, or potential, is determined at any time by the relative ratio of ions, extracellular to intracellular, and the permeability of each ion
Excitable Membrane
membrane capable of generating, maintaining, and conducting action potentials
Resting Membrane Potential
normal difference in charge across neuronal membrane
theoretically -70 Vm, but varies. Sits here due to electrical potential of K+ and Na+, and their relative permeabilities
Ion
atom/molecule with net electrical charge
Cation
Positive Ion
Anion
Negative Ion
Movement of Ions
Influenced by diffusion down the concentration gradient and electrical force.
ion channel
protein embedded in a membrane with 4-6 parts that create a polar pore that allow ions to pass across the neuronal membrane down the concentration gradient
ion selectivity
Ions can only pass through certain channels based on diameter of pore and polarity of internal R groups
ion pump
Use ATP to pump ions against concentration gradient
ion gating
Opening and closing of channels is determined by local environment in the cell. Eg local electrical charge
diffusion
Passive movement of a molecule from somewhere of high concentration to low concentration
concentration gradient
the gradual change in the concentration of solutes in a solution as a function of distance through a solution
established via ion pumps
electrical current
I measured in Amps
Movement of electrical charge
electrical potential
force exerted on a charged particle; reflecting difference in charge between anode and cathode
voltage
V measured in volts
electrical potential
electrical conductance
g measured in siemens(S)
relative ability of a charge to migrate from one area to another
electrical resistance
R measured in Ohms
relative inability of a charge to migrate
inverse of conductance (1/g)
Ohm’s law
I = gV
Current = conductance x electrical potential
Both g and v ≠ 0 for a current to be present
membrane potential
voltage across neuronal membrane at any point in time
microelectrode
glass rod with microneedle filled with conductive salt that is inserted into neurons to measure their potential difference
equilibrium potential
Eion
the electrical potential that balances an ionic concentration gradient so that the net movement of ions across the membrane = 0
ionic driving force
Where and how the ions are moving
(Vm-Eion)
Nernst Equation
takes into account temperature, charges of ion, ratio of concentration - check equation
Sodium Potassium Pump
Uses ATP to move 3 Na+ ions outside of the cell and 2 K+ ions into the cell
Consumes about 70% of all ATP in the brain
Calcium Pump
Pushes Ca2+ outside of cells to maintain a normal level via ATP
Goldman Equation
Accounts for relative permeability of ions, and does not account for leakage
See reference and practice
blood-brain barrier
blood vessels regulate movement and concentration of ions
rising phase
Na+ channels open so Sodium ions rush in as gNa increases. This leads mV to depolarise until approximately 40 mV
overshoot
when mV is > 0
falling phase
Rapid repolarization.
Na+ channels close and K+ channels open leading to potassium efflux
undershoot
Vm returns to resting potential
hyperpolarization
Hyperpolarization is when the membrane potential becomes more negative at a particular spot on the neuron’s membrane
depolarization
depolarization is when the membrane potential becomes less negative
threshold
Miniumum mV for the channels of a certain proton to open
absolute refractory period
the interval of time during which a second action potential cannot be initiated, no matter how large a stimulus is repeatedly applied
relative refractory period
The relative refractory period is the interval of time during which a second action potential can be initiated, but initiation will require a greater stimulus than before.
optogenics
Mutating brain cells so that ion channels become sensitive to light, allowing scientists to trigger action potentials with light
Channel Rhodospin 2
ChR2
Mutated channels used to trigger light activation
voltage clamp
membrane potential can be set a specific voltage to see if membranes will open
voltage gated sodium channel
A sodium channel that cannot open without a voltage threshold being reached. Consist of 4 polypeptides, each made of 6 alpha helices. 4 helix senses voltage and twists to open the channel. After a time, a globular protein will block the channel so that the membrane potential can return to normal. Activate during the rising phase.
patch clamp
Pipette used to isolate ion channel so that charge flow can be monitored and measured for one distinct channel
channelopathy
diseases that develop because of defects in ion channels caused by either genetic or acquired factors
tetrodotoxin
TTX
A toxin found in pufferfish that works by blocking Na+ channels.
voltage gated potassium channels
Works like a voltages gated sodium channel.
Opens during falling phase
Saltatory conduction
Some axons are wrapped in myelin sheaths to aid in the propogation of the action potential. Impulses only exist in the nodes between sheaths. This prevents the charge from diffusing, and speeds up the message
spike-initation zone
potential
seperation of electrical charge from across a membrane
net differences in electrical charge in relation to membrane
measured right next to membrane
capacitance
electrical charge gathers around membrane due to polarity on either side so in a way membrane holds a charge. This property is capacitance
important ions
Na+, K+, Ca2+, Cl-
Eion and Temperature
Proportional
Eion and charge
Inversely Proportional
Distribution of Ions
K+ inside
Na+ and Ca2+ outside
Nernst at Body Temp
Only need respective concentrations
Relative permeability of Na and K
Pk is 40x PNa
Shakers
Fruit flys that were not put to sleep by ether vapours and instead shook. This was found to be due to a mutation in a specific type of potassium channel. This gene was then mapped and used to find the code for all other potassium channels, which helped understanding of selective permeability.
Potassium Channels
A transmembrane polypeptide made of 4 subunits, whose R groups interact to create a pore loop that only allows K+ ions to pass through
Scorpions
scorption toxin works by blocking potassium channels and binding to a site, this allowed the stretch of genes that allowed potassium specificity to be deciphered = 3D structure of potassium channel
Mutations in Potassium Channel
Weaver mice - move weirdly and die early
epilepsy
Lethal injections
administer sedative then overload body with K+ to induce cardiac arrest.
Firing frequency
reflects magnitude of depolarizing current
Saxitoxin
Toxin that works similarly to TTX. Found in forms of shellfish
Batrachotoxin
Toxin that causes Na+ channels to open too long and early voltage wise
A-Alpha Fibre
Myelinated, biggest and quickest fibre
Eg: Somatic Motor
A-beta Fibre
Myelinated, big and quick fibre
Eg: Touch
A-gamma Fibre
Myelianted, medium, and quickish fibre
eg: motor to intrafusal fibres of muscle spindles
A-delta Fibre
Myelinated, medium, and quickish fibre
eg: pain and cold
B Fibre
Myelinated, smallish, average speed fibre
eg: preganglionic autonomic
C Fibre
Unmyelinated, small, slow fibre
eg: Pain, and Oflaction
length constant
lambda in neuro
the rate of exponential decay of membrane voltage as a function of distance from the location where current is injected
review equation
time constant
the product of the membrane resistance (rm) and membrane capacitance (cm)
What increases the speed of an action potential?
Decreased Capacitance
Increased Axonal Diameter
Increased Myelination
Increased Internodal Distance
Nodes of Ranvier
pecialized regions in the axonal membrane that are not insulated by myelin. Where Na+ channels are centralised and where AP propagate
Multiple Sclerosis
Common symptoms include tiredness, vision problems and problems with walking or balance.
Caused by issues with axonal integrity and myelination damage (CNS), often caused by the immune system
Guillian Barre Syndrome
Nerve damage beginning in hands and feet that will spread if untreated.
Caused by issues with axonal integrity and myelination damage( PNS).
Motor Neuron Disease
Neurodegeneration of motor neurons
Vincristine Neuropathy
Drug causes nerve damage
Local Anaesthetic
Most common one is lidocaine, that works by upsetting AP by messing with Na+ channels
