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NEURO lecture 2: action potentials and electrical signaling
Terms in this set (62)
can you explain the cellular basis of AP conduction?
can you explain ion flow and concentration gradients?
can you describe the basic properties of ion channels and transporter proteins?
please do tell
can you explain voltage-gated vs. ion-gated channels?
can you explain clinical links to FES, TENS, and electrodiagnostic testing?
what is current?
current is the amount of electrons transferred per unit time
it represents the flow of electrons through a conductive material
what is atomic number of an element?
number of protons
electron and proton charges
negative and positive, respectively
what does neutral charge mean?
number of electrons and protons are equal
what is an ion?
a molecule with unequal number of electrons and protons
negatively charged ion
positively charged ion
how do batteries work?
1) electrolyte 2) anode (-) 3) cathode (+)
collection of electrolyte (anion) becomes so great that it must flow down its concentration gradient towards the cathode via a pathway called a circuit ➔ this flow creates a current
what is threshold?
certain point the voltage much reach in order for AP to occur
what is AP threshold?
what is resting membrane potential?
electrical activity recorded in neuron at rest
-65mV (inside is negative relative to outside)
what is receptor potential?
electrical activity produced by neurons in response to stimuli
what is synaptic potential?
electrical activity transmitted from one neuron to another
membrane potential more negative (inside more negative than outside)
membrane potential more positive (inside more positive than outside)
neuron must depolarize to threshold in order for AP to occur
AP amplitude relationship with the current that caused it
it is independent of the magnitude of the stimulus/current that caused it
intensity of AP depends on?
frequency of AP, not the amplitude
electrical potential across cell membrane exists because?
1) active transporters maintain different concentrations of ions across the membrane AGAINST concentration gradient
2) selectively permeable ion channels allow ions to flow down (WITH) their concentration gradient
what does electrical gradient help?
keeps all ions from leaving the cell
electrical potential across a membrane at electrochemical equilibrium
describe K+ concentration at resting membrane potential
membrane more permeable to K+ efflux at rest because active transporters maintain higher concentration of K+ inside the cell than outside
what happens upon AP initiation re: Na+?
usually, active transporters keep Na+ mostly extracellular
when membrane potential reaches threshold, Na+ channels open and Na+ rushes down its concentration gradient into the cell, depolarizing the cell
equilibrium potential for K+
-58mV because the flow in and flow out of K+ (the chemical gradient) is balanced by the electrical gradient
is the equilibrium potential for Na+ be the same as K+?
idk look at the textbook
used to calculate equilibrium potential of a SINGLE ion
considers relative permeability of more than one ion (if more than one ion is present across a membrane)
which ion has the greatest concentration difference across the mammalian cell membranes?
why is Ca2+ concentration so tightly regulated?
because if too much of it goes intracellularly, it can kill the cell
period immediately following AP during which the neuron is not excitable
this happens for two reasons:
1) slow course to turn off voltage-dependent K+ conductance (it basically takes so long to open and close those K+ channels)
2) persisting Na+ conductance inactivation (the Na+ channels are stubborn and stay shut, so they can't open again for another depolarization)
how do APs differ from receptor and synaptic potentials?
1) all or nothing property
2) require reaching a threshold of -50mV
3) have a refractory period
membrane permeability is dependent upon?
Na+ and K+ channels are dependent upon voltage
Hodgkin and Huxley did a voltage-clamp technique to measure membrane permeability while controlling cell membrane potential
describe what "leaky axons" and length constant means
this means that injected currents dissipate within mm of injection, causing decreased voltage with increased distance
eventually, the voltage becomes too small to initiate an AP (the distance at which this occurs = length constant)
how can APs transmit long distances along axons?
their amplitude is constant, there is no decay
self-propagating/regenerating: when an AP occurs, Na+ rushes into the cell and some of that charge is passively shuttled to next part of the axon ➔ this causes a change in voltage that reaches threshold and causes a local depolarization that causes another AP
how is back-propagation of APs blocked?
how are APs propagated better in the body?
2) increasing diameter of axon
3) nodes of Ranvier
what does increasing the diameter of an axon do?
1) decreases resistance to current flow
2) increases length constant (current voltage does not dissipate as easily)
unmyelinated vs. myelinated conduction velocity
0.5 - 10 m/s unmyelinated
150 m/s myelinated
myelin-producing cells in CNS v. PNS
oligodendrocytes = CNS ➔ these will wrap many axons
Schwann cells = PNS ➔ these will wrap only one axon each
nodes of Ranvier
allows for saltatory conduction
voltage-gated ion channels are located here, so this is where the impulse is conducted
orthodromic neural signaling
in straight direction from dendrites ➔ cell body ➔ axon
antidromic neural signaling
in opposite direction from axon ➔ cell body ➔ dendrites
two types of gated channels
voltage-gated (responds to changes in membrane potential)
ligand-gated (responds to chemicals binding)
how does depolarization affect ion channels?
increases the probability of opening Na+ and K+ channels
also sequentially activates Na+ channels but not K+
(Na+ channel activation, Na+ inactivation K+ channel activation)
how does hyperpolarization affect ion channels?
closes Na+ and K+ channels
(*remember that Na+ channel inactivation and Na+ channel closure are different)
describe ligand-gated channels
respond to chemical signals
usually allow more than one ion to enter
some respond to signals inside the cell (ex: synaptic vesicles, ER)
convert chemical signal into electrical signal (ex: sensory signal transduction)
which type of channel is faster?
in addition to VGICs and LGICs there are what kind of receptors?
stretch and heat-activated receptors
contribute to pain and temperature, mediate inflammation, provide basis for stretch receptors and neuromuscular stretch reflexes
describe what active transporters are
create and maintain concentration gradients
slower than ion channels because they require ATP (energy) to transport ions AGAINST their concentration gradients
ex: ATPase pump, ion exchangers
describe how ATPase pump works
derive energy from hydrolysis of ATP in order to pump Na+ out and K+ in
describe how ion exchangers work
it uses the electrochemical gradient of one or more ions in order to power the exchange of other ions that need to go against their electrochemical gradient
antiporters = switch the ions
co-transporters = take the ions along for the ride
how important is the Na+/K+ ATPase pump and what does it do to cells?
CRITICAL in the CNS
20-40% of energy consumption goes to maintaining this pump!
it hyperpolarizes the cell because 3 Na+ go out for every 2 K+ that go in
what are some inhibitors of the Na+/K+ ATPase pump?
Ouabain (used by Somali tribesmen) is a poison that will block the pump but we also kinda make it endogenously??? LOL
Digoxin is used to treat the heart for Afib, Aflutter ➔ increases contractility of heart by blocking the pump
describe the Ca2+ pump
Ca2+ has the greatest concentration gradient across the membrane
requires ATP to maintain this great concentration gradient
found in sarcoplasmic reticulum in muscle cells and endoplasmic reticulum in neurons
how do ion transporters and channel proteins have complementary functions?
ion transporters maintain concentration gradients across the cell membrane and are "exploited" by ion channels allowing ions to move down their concentration gradients
ion channels (channel proteins) generate voltage-dependent conductances
when ion channels are affected by action potentials, they open or close in response to the change in membrane potential
what are a couple of clinical treatments that manipulate APs?
TENS = transcutaneous electrical nerve stimulation ➔ for pain modulation
FES = functional electrical stimulation ➔ used to facilitate movement where neuromuscular impairments are present
what can you test in nerves clinically?
neural conduction velocity
what are the three electrodiagnostic tests that can help identify radiculopathies, plexopathies, and polyneuropathies?
all of these are stimulating either sensor or motor neurons using an imposed current
M-wave = stimulating the efferent motor neuron itself coming from the ventral horn of the SC ➔ the biggest, fastest wave
H-wave = stimulating the afferent 1a sensory neuron entering the dorsal horn of the SC ➔ synapses with the motor neuron exiting the ventral horn of the SC (electrical equivalent of a tendon tap; happens later than M-wave because of the synapse)
F-wave = stimulating a motor neuron so hard that the current becomes antidromic and travels back to the cell body in the ventral horn of the SC, creating a new AP that goes back out ➔ happens even later than H-wave and is the smallest wave
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