Lab 2 Flashcards
Change in membrane potential during action potential propagation
Simulus gated sodium channels open
Sodium floods into neuron with the opening of more sodium channels
Voltage gate sodium channels close
Voltage gated potassium channels open
Returns to resting membrane prtential and both channels are now closed
Define conductance as it applies to ions moving through a cell membrane
Conductance is the inverse of electrical resistance - if the conductance of the membrane is low, then the resistance to movement of that ion across the membrane is high
Depends on permeability (ion channels) and equilibrium potential (driving force)
Changes in the membrane potential in a local area of a neuron’s membrane will result from …
changes in membrane permeability
Intracellular recordings
Recordings taken from inside of an axon or neuron
measurement of the electrical charge in the internal environment relative to the electrical charge in the external environment outside the membrane
Units that are normally used for membrane potential
mV
Why does an action potential peak at approximately +40mV?
Because it is going towards sodium conductance
Extracellular recordings
Recordings taken from electrodes placed at 2 different points on the outside of an axon or neuron
Records the electrical activity in the external environment of a group of axons such as a whole nerve
How do the properties of the individual axons within a nerve vary?
Myelination and diameter
Whole nerve potential
An action potential representative of the combined electrical activity of ALL the different axons within a nerve. All individual axons within a nerve summate electrically to produce a WNP
The summed electrical activity from all icons within the nerve
What feature in some axons might increase the membrane resistance (rm) and therefore aid in the spread of the electrical signal along the cable?
Myelin
Amplitude of WNP is
Phase 1 - upswing
2 phases of a WNP
Upswing and downswing
Whole nerve action potential is …
biphasic
Why is WNP biphasic ?
When you stimulate a whole nerve APs propagate at different speeds along different axons. Some axons are slow and some axons are fast. This means the WNP travels down the nerve as a wave, not a single point, with the APs from the fast neurons at the front and the APs from the slow neurons at the back.The biphasic trace occurs because of the way we record the wave of APs across two electrodes.
What is happening within each axon within the nerve during the whole nerve potential?
• If threshold is reached within a neuron, an action potential will propagate along its axon. • Not all neurons have the same threshold.
Latency
Time delay between the stimulus and the response
Are all the axons doing the same thing at the same time during a WNP?
No - APs propagate at different speeds along an axon according to axon diameter (rin) and the extent of axon myelination (rm)
Threshold response
The point at which you first see a response after seeing no response below that threshold on the wave form
Maximal response
The point at which increasing the pulse height no longer causes an increase in the amplitude of the response
What behaviour did the rat sciatic nerve exhibit in response to increasing stimulation intensity? Was it all-or-nothing? If not, why not?
Increasing stimulation intensity caused more axons within the sciatic nerve to be activated.
• This increased the amplitude of the response.
• In a whole nerve the all-or-nothing response does NOT occur because all axons are not activated at the same time. The all-or-nothing response (action potential) occurs only within individual axons.
Subthreshold stimulus - Whole nerve vs single axon
Whole nerve = no response
Single axon = sub threshold response (local potential)
Threshold - Whole nerve vs single axon
Whole nerve = First signs of WNP
Single axon = Action potential (all or nothing)
Increasing towards maximum response - Whole nerve vs single axon
Whole nerve = WNP increases as the stimulus increases
Single axon = action potential (all or nothing)
At maximum response - Whole nerve vs single axon
Whole nerve = WNP is at maximum once all axons within the nerve are activated
Single axon = action potential (all or nothing)
Above maximum response - Whole nerve vs single axon
Whole nerve and single axon - same as at maximum response…
Whole nerve = WNP is at maximum once all axons within the nerve are activated
Single axon = action potential (all or nothing)
What happened to the second whole nerve/compound action potential ?
The second whole nerve potential progressively decreased in amplitude because the second stimulus occurred during the absolute refractory period in some axons (but not all axons)
The second whole nerve potential eventually disappeared because absolute refractory period was occurring in all axons
What physical property of the axons within the whole nerve contributes most to determine the threshold stimulation required?
Internal resistance (diameter) - axons that are able to get to threshold with less ion flux i.e. smaller diameter means that they are more likely to fire action potentials in the same amount of time
Wole nerve general trends with stimulation and single axon general trend with stimulation
Whole nerve shows graded response
Single axon nothing until it reaches threshold then all or nothing response
Which property of voltage gated sodium channels determines the absolute refractory period?
Inactivation gate – stops channel reactivating too soon. ‘Ball and chain’
Velocity =
distance/time
mm/ms is the same as
m/s
Is the conduction velocity you measured in you rat sciatic nerve the value you might expect in a living rat?
No - different environment, temperature, dying nerves at it is out of the body, handling, preparation
Human value of conduction velocity in sciatic nerve approximately
40-100 m/s
How does myelination improve conduction velocity?
Increases electrical insulation which increases membrane resistance by reducing current leakage
What else, in vertebrate axons, helps conduct electrical signals along the full length of the axon of a large motor neuron?
• Nodes of Ranvier: Gaps in the myelin sheath where regeneration
of AP occurs to improve the
propagation along an axon
Large axon diameter
Nodes of Ranvier
Gaps in the myelin sheath where regeneration
of AP occurs to improve the
propagation along an axon.
Lignocaine
A local anaesthetic that blocks voltage gated sodium channels
Lignocaine and the electrodes
Action potential affected at the second electrode because the sodium channels are affected before this electrode therefore the first potential remained relatively the same and the second potential that occurs at the second electrode slowly decreases in amplitude
Lignocaine and what does it influence first?
More active neons are affected first and this is why dentist inject and scratch gums to make pain receptors active therefore are affected by ligonocaine
Lignocaine binds to voltage-gated Na+ channels and prevents them from opening. What properties of lignocaine will be important for its local anaesthetic properties compared with, for example, the deadly toxin tetrodotoxin (TTX)?
Lignocaine: Low affinity block - reversible
TTX: High affinity block – not reversible
Using the knowledge that you have gained from today’s lab and your lectures, why do you think shellfish poisoning can cause paralysis, and do you think a cure for severe poisoning of this type is possible?
Saxitoxins
• Red tides, reddish dinoflagellates
• Ingested by shellfish then eaten by people
• Causes irreversible poisoning – paralytic shellfish poisoning
• Death due to respiratory failure - blocks the franc nerves which innervates the diaphragm so within minutes that patient can sop breathing
Treatment
• Induced coma/life support
• Wait for insertion of new Na+ channels into cell
membrane
Irreversible blocking of voltage gated sodium ion channels so no more action potential can be propagated
WNP is also called a
compound action potential
Action potentials propagate more slowly along
small diameter axons compared to larger diameter axons
The property of voltage gate sodium channels that determines the duration of the absolute refractory period is the
speed of opening of the inactivation gate
Signal filters can be applied to
isolate one signal of interest when the recorded signal contains multiple overlaid frequencies
WNP - extracellular or intracellular recording?
Extracellular recording
WHat does a WNP represent?
collective response of axons within a nerve
WNP varies on both
myelination and diameter of the axons in the nerve
Electrodes measure
electrical potentials at a set location
