Topic 2: Action potentials Flashcards
What is an action potential? (spike, nerve impulse, discharge)
Rapid reversal, for an instant, of the negatively interior of neuronal membrane. Inside of membrane becomes positively charged in relation to outside
What is the code used by neurons to transfer information from one location to another?
Frequency and pattern of action potentials
What is oscilloscope used for?
Study action potential, records voltage as it changes over time
What are the components of action potential shown on oscilloscope?
-Rising phase
-Overshoot
-Falling phase
-Undershoot (or after-hyperpolarization)
What is the rising phase characterised by?
Rapid depolarization of membrane –> change in membrane continues till Vm reaches peak of approx. 40mV.
-when inside of the membrane has a negative electrical potential there is a large driving force on Na+. Therefore, Na+ rushes into the cell through the open sodium channels, causing membrane to rapidly depolarise
What is the potential difference (voltage) of neuronal membrane at rest?
-65mV
What is the overshoot characterised by?
-The part of the AP where the inside of the neuron is positively charged with respect to the outside (i.e., the part above 0mV)
-because the relative permeability of the membrane greatly favours sodium, the membrane potential goes to a value close to ENa, which is greater than 0mV
What is the falling phase characterised by?
-The rapid repolarisation until the inside of the membrane is actually more negative than the resting potential
- The behaviour of 2 types of channels contributes to the falling phase, first the voltage-gated sodium channels inactivate. Second, the voltage-gated potassium channels finally open (triggered to do so 1 msec earlier by the depolarisation of the membrane).
-There is a great driving force on K+ when the membrane is strongly depolarised. Therefore, K+ rushes out of the cell through the open channels, causing the membrane potential to become negative again
What is the undershoot (or after-hyperpolarisation) characterised by?
-The part of AP where membrane potential dips lower (more negative) than the resting membrane potential
-The open voltage-gated potassium channels add to the resting potassium membrane permeability. The membrane potential goes toward Ek, causing a hyperpolarisation relative to the resting membrane potential until the voltage-gated channels close again
What happens after the undershoot phase?
Gradual restoration of resting potential, from beginning to end the action potential lasts about 2 msec
What is a generator potential?
A graded response to stimuli, or graded depolarisation, could produce AP if reaches threshold.
When is an action potential triggered?
Depolarisation of membrane beyond what is called threshold
What does the rate of action potential generation, or firing frequency depend on?
size of continuous depolarising current
What is the limit to the rate at which a neuron can generate AP?
about 1000Hz (1000 impulses per second)
Once AP is initiated how long is it impossible to initiate another AP? And what is this period called?
about 1 msec, absolute refractory period
What is the relative refractory period?
-Can be relatively difficult to initiate another AP after the end of absolute refractory period, the amount of current required to depolarise the neuron to AP threshold is elevated above normal
-The membrane potential stays hyperpolarised until the voltage-gated potassium channels close. Therefore, more depolarising current is required to bring the membrane potential to threshold
What is optogenetics?
introduces into neuron foreign genes that express membrane ion channels that open in response to light
What is channelrhodopsin-2 (ChR2)
-photopigment, introduced into mammalian cells, encodes a light-sensitive cation channel that is permeable to Na+ and Ca2+.
- channel opens rapidly in response to blue light –> in neurons the inward flow of cations is sufficient to produce depolarisation beyond threshold
Depolarisation of cell during the action potential is caused by what?
influx of sodium ions across the membrane
repolarisation of the cell during the action potential is caused by what?
Efflux of potassium ions across the membrane
What is the role of the pumps (such as sodium-potassium pumps)?
Work continuously to establish and maintain concentration gradients
What does intracellular recording require?
impaling neuron or axon with a microelectrode.
What does intracellular recording measure?
The potential difference between the tip of the intracellular electrode and another electrode placed in the solution bathing the neuron (electrically continuous with earth, and thus called ground)
What is the intracellular electrode filled with?
concentrated salt solution (often KCL) having high electrical conductivity
What is intracellular electrode connected to and what is the potential difference displayed on?
Connected to an amplifier that compare potential difference between electrode and ground. Potential difference can be displayed using oscilloscope
What is an oscilloscope?
sophisticated voltmeter that can record rapid changes in voltage (like AP)
What is extracellular recording?
placing electrode near membrane and can detect ionic movement across the membrane
What does the extracellular recording measure?
same as intracellular recording, measure the potential difference between the tip of the electrode and ground (solution around neuron)
What can be the electrode?
fine glass capillary filled with salt solution, but often simply thin insulated metal wire
How can we use sound to hear impulses?
Can be heard by connecting the output of amplifier to loudspeaker, each impulse makes a distinctive “pop” sound
what is the relationship between the ionic driving force, ionic conductance and the amount of ionic current that will flow?
I = g(Vm -E) (an expression of ohms law –> i=gV)
what happens if g = 0 or Vm-E (no difference in equilibrium potential and membrane potential)?
ionic current with be 0 for that certain ion according to the equation I = g(Vm -E)
What is the voltage clamp?
-Enables the “clamping” of the membrane potential of an axon at any value.
-Could then deduce the changes in membrane conductance that occur at different membrane potentials by measuring the currents that flowed across the membrane (Allows for control of transmembrane voltage and measuring of current through an ion channel after activation)
What is Hodgkin & Huxley’s Ionic theory of action potential?
-Rising phase of action potential is explained by an inward sodium current
-falling phase of action potential is explained by an outward potassium current
How did H&H use voltage clamp to identify ions channels in the squid axon membrane?
-Came up with the ionic theory of AP
-H&H proposed K+ channels possess voltage-sensitive “gate”,
-Na+ gates are “activated” (opened) by depolarization above threshold, and “inactivated” (closed and locked) when membrane acquires positive membrane potential. Gate “deactivated” (unlocked and enabled to be opened again) only after membrane potential return to a negative value
-found that squid axon action potential amplitude depends on extracellular Na+ concentration (less Na+ means less AP amplitude)
-Resting membrane potential does not depend on extracellular Na+ concentration
What is a voltage-gated sodium channel?
Protein forms a pore in the membrane that is highly selective to Na+, and pore is opened and closed by changes in membrane voltage
What is the voltage-gated sodium channel created from? And what is its structure?
-created from a single long polypeptide
-4 distinct domains, clump together to form pore, numbered I-IV –>each domain consists of 6 transmembrane alpha helices, numbered S1-S6
-pore closed at negative resting membrane potential.
-When membrane is depolarised to threshold the molecule twists into configuration that allows the passage of Na+ through the pore
What is the selectivity filter?
-Pore loops that assemble into selectivity filter
-Filter makes each channel specific to one type of ion
-Makes the sodium channel 12 times more permeable to Na+ than it is to K+
How does Na+ pass through the voltage-gated sodium channel?
-Retained water serves as sort of molecular chaperone for the ion, and is necessary for the ion to pass the selectivity filter
-the ion-water complex can then be used to select Na+ and exclude K+
Where does the voltage sensor reside in sodium channel and what does it contain?
Segment S4, in this segment positively charged amino acid residues are regularly spaced along the coils of the helix –> Depolarisation twists S4 and this conformational change in molecule causes the gate to open
What is the patch clamp?
-To study the ionic currents passing through individual ion channels
-Patch-clamp method entails sealing the tip (1-5 micrometres in diameter) of an electrode to a very small patch of neuronal membrane by suction (called gigaohm seal). This patch torn away from neuron. ionic currents across it can be measured as the membrane potential is “clamped” at any value the experimenter selects
What is the characteristic pattern of behaviour of voltage gated sodium channels?
-they open with little delay
-they stay open for about 1msec and then close (inactivate)
-they cannot be opened again by depolarization until membrane potential returns to a negative value near threshold
What is channelopathy?
A human genetic disease caused by alterations in the structure and function of ion channels
What is tetrodotoxin (TTX)?
-from puffer fish
-clogs the Na+ permeable pore by binding tightly to a specific site on the outside of channel –> TTX blocks all sodium-dependent action potentials and therefore usually fatal if ingested
What is batrachotoxin?
-interferes with NS function by causing the channels to open inappropriately
-causes channels to open at more negative potentials and to stay open much longer than usual –> thus scrambling the information encoded by AP
What can we learn from toxins?
-different toxins disrupt channel function by binding to different sites on protein. information about toxin binding and its consequences have helped researchers deduce the 3-d structure of the sodium channel
-toxins can be used as experimental tools to study consequences of blocking AP
When do potassium channels open? And why?
-about 1msec after depolarisation
-potassium conductance serves to rectify or reset the membrane potential –> this conductance called delayed rectifier
What is the structure of potassium channels?
-consist of 4 separate polypeptide subunits that come together to form a pore between them
- these proteins are sensitive to changes in electrical field across the membrane like sodium channels
-when membrane depolarised subunits twist into shape that allows K+ to pass through pore
What is the threshold?
Threshold is the membrane potential at which enough voltage-gated sodium channels open so that the relative ionic permeability of the membrane favours sodium over potassium
What is the absolute refractory period?
Sodium channels inactivate when the membrane becomes strongly depolarised. They cannot be activated again, and another AP cannot be generated, until the membrane potential becomes sufficiently negative to deinactivate the channels
Describe the propagation of the AP along the axon
-similar to propagation of flame along fuse.
-when patch of axonal membrane depolarised sufficiently to reach threshold, voltage-gated sodium channels pop open, the AP is initiated. The influx of positive charge spreads inside the axon to depolarise the adjacent segment of membrane, and when it reaches threshold, the sodium channels in the this patch of membrane also pop open
- in this way AP works its way down the axon until it reaches its way down the axon until it reaches the axon terminal
-initiated at one end of axon propagates in one direction and does not turn back –> because the membrane just behind is refractory, due to inactivation of sodium channels
What is orthodromic conduction?
AP from soma to axon terminal
What is antidromic conduction?
AP from axon terminal to soma
What does the speed of AP conduction depend on?
- AP conduction velocity increases with increasing axonal diameter
Do smaller axons require greater or less depolarisation to reach AP threshold and are they more or less sensitive to being blocked by local anesthetics?
require greater, and more sensitive
What are local anaesthetics?
Drugs that temporarily block action potentials in axons
What is lidocaine?
An anaesthetic can be dissolved into a jelly and smeared (called topical anaesthesia), can be injected directly into a tissue (infiltration anaesthesia) or a nerve (nerve block), can be infused into CSF bathing the spinal cord (spinal anaesthesia)
How does lidocaine (and other local anaesthesia) prevent AP?
By binding to sodium channels. Binding site for lidocaine has been identified as the S6 alpha helix of domain IV of the protein.
What is the good and bad thing about “fat” axons?
Good thing is they conduct AP faster, bad thing is that they take up a lot of space
What does myelin do? And what is it made up of in CNS and PNS?
-facilitates current flow down inside of axon, increasing AP conduction velocity
-Schwann cells in PNS, oligodendroglia in CNS
What are nodes of ranvier?
Breaks in the myelin sheath in which sodium channels are concentrated, usually 0.2 - 2mm long (fatter axons have larger internodal distances)
What is multiple sclerosis?
- a demyelinating disease
- MS attacks the myelin sheaths of bundles of axons in the brain, spinal cord, and optic nerves
What is saltatory conduction?
APs skip from node to node
Where does the axon originates from soma?
Axon hillock, is where AP begins –> often called spike-initiation zone
