1a Membrane And Action Potenials Flashcards
What is a membrane potential?
The difference between the voltage inside and outside the neurone
What is the resting membrane potential?
-70mV
When does movement of ions occur?
When the concentration of ions is different across the membrane
What are ion channels?
Permeable pores in the membrane which open and close depending on the transmembrane voltage
Which are the four main ions involved in mediating membrane potentials?
Na+
Cl-
Ca2+
K+
What is equilibrium potential?
The potential at which electrochemical equilibrium has been reached
This prevents the diffusion of ions down the concentration gradient
What equation is used to calculate the equilibrium potential?
Nernst Equation
What is the Nernst equation?
X2 is intracellular ion concentration
X1 is extracellular ion concentration
z is charge of ion
Where is the concentration of sodium ions the highest?
Outside the neurone
Where is the concentration of potassium ions the highest?
Inside the neurone
Why do membrane potentials not rest at Na/K potentials?
Because the neurone has mixed permeability for both ions
What determines how much an ion contributes to membrane potential?
How permeable the membrane is to the ion
What does the P in the Goldman-Hodgkin-Katz equation represent?
The permeability/probability of the channel opening
0 is 100% closed and 1 is 100% open
How to increase the membrane permeability for a particular ion?
Open the ion hannels
What happens when Na+ ion channels are opened?
The membrane potential is shifted in the positive direction (becomes less negative)
What happens when K+ ion channels are opened?
Shifts the membrane in the negative direction
What is hyperpolarisation?
When the membrane potential becomes more negative than resting potential
What is the summation effect?
Excitatory + inhibitory impulses
Why do graded potentials decrease with time?
Potential decreases over the length of the axon as charge leaks from the axon
What are the 5 stages of an action potential?
- Resting membrane potential
- Depolarising stimulus
- Upstroke
- Repolarisation
- After hyperpolarisation
Changes in the membrane potential during an action potential are due to what?
Voltage gated ion channels
Why is the resting membrane potential for neurones closest to K+ eqm potential?
Because the permeability for potassium is greater than sodium
What causes depolarization?
The opening of voltage gated sodium ion channels resulting in an influx of Na+
What causes repolarisation?
More K+ ion channels opening and K+ flowing out the neurone, returning the potential to be more negative
Are the potassium ion channels open during depolarization?
Yes, however not many
What is the relative refractory period?
The period in which a stronger than normal stimulus is required for an action potential to be generated because most VGSCs are inactive
What is the absolute refractory period?
Period where the inactivation gate of VGSCs is closed thus a new action potential cannot be generated
What happens in stage 1 (resting membrane potential) of an action potential?
Permeability for K+ > Na+
Membrane potential is nearer the equilibrium for K+ (-90mV) than Na+ (+72mV)
What happens in stage 2 (depolarising stimulus) of an action potential?
Stimulus moves membrane potential in positive direction towards threshold
What happens in stage 3 (upstroke) of an action potential?
Increase in Na+ permeability as VGSCs open quickly
Increase in K+ permeability as VGKCs start to open slowly
Membrane potential moves towards Na+ eqm potential but doesn’t reach it as VGSCs also close quickly
What happens in stage 4 (repolarisation) of an action potential?
Decrease in Na+ permeability as VGSCs close
Increase in K+ permeability as VGKCs remain open
Membrane potential moves towards K+ eqm potential
What happens in stage 5 (after-hyperpolarisation) of an action potential?
VGKCs are still open
K+ continues to leave cell down gradient
Em moves closer to K+ eqm potential and VGKCs close
Em thus returns to resting potential
How is the membrane resting potential restored after hyperpolarisation?
K+ ions are pumped back into the neurone using an Na+K+ active pump which requires ATP
During the upstroke phase of an action potential what is the relative permeabilities of sodium and potassium ions?
Higher permeability for Na+, which results in depolarization
Why is the membrane more permeable to Na+ during depolarization?
The sodium ion channels open faster than the potassium ones
What are the nodes of ranvier?
Small gaps of myelin intermittently places along the axon which the action potential jumps between
What is the type of conduction where the action potential jumps between the nodes of ranvier?
Saltatory conduction
What does saltatory conduction do to the speed of the action potential conduction?
Increases it
What does the myelin sheath do to the action potential?
Prevents it from spreading as it has high resistance
What is known by the all of nothing mechanism of action potentials?
An action potential is only triggered if a certain threshold is reached
When does further depolarization stop?
Until the voltage gated sodium ion channels become inactive
What type of neurones will have a faster decay?
Small unmyelinated neurones as they have a larger resistance
What affect does myelin have on the rate of decay?
Slows the rate of decay and increases the distance required for decay along the neurone
How does the action potential spread down the neurone?
When sodium enters, it moves a small distance across the inside of the membrane and it’s positive charge depolarizers some more of the membrane, causing more Na+ voltage gated ion channels go opem
What is propagation along an unmyelinated neurone called?
Continuous conduction
Why is continuous conduction slower than saltatory?
Has to spread the whole length of the axon, no nodes of Ranvier
Where are most voltage gated channels located?
At the nodes of ranvier
What are the two factors which control the conduction velocity inside a neurone?
Myelination and axon diameter
What affect does a high axon diameter have on the conduction speed?
Increases
What else decreases the conduction velocity?
Cold, anoxia, compression and some drugs
What is an axondendritic synapse
Presynaptic terminal connected to a neuronal dendrite
What is an axosomatic synapse
Presynaptic terminal connected to a neuronal soma
What is an axoaxonic synapse?
Presynaptic terminal connected to a neuronal axon
What is paracrine signaling?
Cell produces a signal to cause changes in nearby cells
What is autocrine signaling?
Cell secretes an autocrine agent that binds to autocrine receptors on the same cell to trigger a response
What type of signaling is found in the neuromuscular junction?
Paracrine signaling
What neurotransmitter is used in the neuromuscular junction?
Acetylcholine
Which type of receptors are found on the sarcolemma?
Nicotinic acetylcholine receptors
What is botulism?
When botulin toxin causes the ACh to not be released from the vesicles in the pre-synaptic neurone resulting in muscle weakness
What is myasthenia gravis?
Antibodies directed against the nAChR on the sarcolemma, therefore ACh is not detected so less muscle movement
What is Lambert Eaton syndrome?
When there are antibodies directed against the voltage gated calcium channels, so no influx of Na+ therefore no action potential
What structure is involved in Ca2+ ion storage?
Sarcoplasmic reticulum
What happens once ACh binds to nACh receptors on the sarcolemma?
This causes an influx of sodium ions which result in an action potential being propagated down into the T tubules
What molecule breaks down ACh?
Acetylcholinesterase
How is Ca2+ released into the space surrounding the muscle myofibril?
The action potential is propagated down the T-tubules
The dihydropyridine receptor detects the change in membrane potential and changes the shape of the ryanodine receptor which allows Ca2+ influx
How is membrane potential measured?
A reference electrode is placed outside the cell, and another is placed inside the cell
The voltage inside and out are measured, and the difference between them is the resting membrane potential
What is the definition of flux?
The number of molecules that cross a unit area per unit of time
What three things might cause ion channels to open or close?
Transmembrane voltage, presence of activating ligands or mechanical forces
What is the Goldman-Hodgkin-Katz (GHK) equation used for?
To estimate membrane potentials when more than one ion is involved
What is the resting membrane potential almost entirely due to?
Almost entirely due to movement of K+ ions out of the cell
When does an action potential occur from a graded potential?
Action potentials (AP) occur when a graded potential reaches a threshold for the activation (opening) of many Na+ channels
What happens to the activation and inactivation gates during the process of repolarisation?
Start of repolarisation – activation gates open, inactivation gates closed
Later – both gates closed
What happens during the relative refractory period?
Some Na+ channels have recovered from inactivation – thus the gate is open
