The Action potential - L18 Flashcards
What allows us to do everything? How is info transferred?
Through APs
What is inside and outside the cell
Salty H2O
Conc. of Na and Cl outside the cell
High
Two forces acting on Na+ and K+
Chemical and electrical gradients –> electrochem gradients
conc. of K+
Lots of potassium inside the cell, less outside
RMP
-70mV inside the cell, attracts +ve cell resulting in a attraction of cations
How do ions pass through the membrane?
Through ion channels made of proteins
When the cell is at rest, the ion channels are:
Closed
When can ions be opened or closed?
Selectively, unless they are signalled to
What is a chemically gated ion channel?
Channel that is only activated by the presence of a specific chemical
Chemical binds to the extracellular binding site.
How do ions pass down the electrochem gradient?
pore opens in the protein which is fluid filled, allowing specific ions to flow through.
How is membrane permeability reduced?
The chemical on a chem gates ion channel unbinds and the protein changes shape and closes, preventing flow of the specific ions
What are voltage gated ion channels?
Influenced by change in charge of the membrane
Where are vgic common?
areas of excitable membrane
Why are vgic important?
Generation and conduction of APs and synaptic transmittion
Structure of VGIC at rest
the activation gate on the outside of the cell is closed
Structure of VGIC in the refractory period i.e at +30mV
The inactivation gate on the inside is closed until the membrane is repolarised
The VGIC has what that allows it to change shape when it detects a change in charge
Charged resides on the protein
What is mechanically gated ion channels?
Physical change in the membrane causing the opening and closing of protein channels
What do dendrites do?
Increase the SA of the cell
Another name for axon
Nerve fibre
Channels found at nerve endings
Ca2+ VGIC
Type of ion channels found most abundant on the dendrites and cell body of neurons
Chem gated ion channels
What is a local potential
Localised change in voltage across the membrane after interaction with neurotrans with receptor of another neuron
What type of channels are on the axons?
VGIC
What determines the magnitude of the voltage change?
The strength of the stimulus
What is a graded AP
A small or big change in voltage
*Depends on how much neurotrans was released
*How many channels opened because of neurotrans
What is depolarisation?
When positive i.e Na+ moves into cell, the RMP becomes less negative due to opening of ion channels
hat is a local potential?
Change in voltage only occurs in one area and doesn’t spread
What causes chem gated ion channels to open
A chem stimulus i.e through neurotrans like ACh
What happens when CGIC open?
Cell membrane depolarises locally
After threshold, what happens
Cell gets repolarised and stimulus is removed through the Na+ K+ pump
What is hyperpolarisation?
When positive charge leaves the cell or negative charge enters the cell, causing the cell to become inhibitory
At +30mV, the cell becomes
Excitatory
What channels are involved in local potentials
CGICs
If there are two nerve terminals that are close to one another…
Their signals add together to create a larger signal but if they are not actively progagated, the charge dissipates and nothing happens
What is another word for the AP of depolarisation?
Excitatory postsnaptic potential
What is another word for the AP of Hyperpolarisation?
Inhibitory postsnaptic potential (IPSP)
What is summation?
When 2 or more nerve terminals excrete excitatory neurotrans, it results in local potential whose effects add up
Effect of local potentials is summed over…
Time and space
Spatial summation
If nerve terminals are close to one another
Temporal summation
Whether they arrive in time close to one another
What is threshold
+10mV change from -70mV to -60mV
What determines if info is passed as an AP
The balance between excitation and inhibition
What makes up the axon hillock?
Initial segment, segment 2 and 3
What is the initial segment
The point where the cell body and axon join
What is unique about the initial segment
Very dense in VG Na+ C
*Where AP occurs
Sensitive to change in membrane potential
How is AP generated
Balance of excitation and inhibition at every moment in time depolarises the mem —> causes VGIC to open
hen does VG Na+ C open at the initial segment
When the cell mem potential reaches -60mV
What happens as VG Na+ C opens?
Positive charge brought into the cell and voltage across mem becomes more positive at that point
What causes a bigger voltage change?
As more voltage gated Na+ channels open
How long does the reversal of the transmem voltage take?
2-3 ms
What kind of potential brings the cell to threshold?
A graded potential
What happens to the shape of the VG Na+ C as the cell reaches +30mV?
Polarity of voltage across the protein changes causing protein to change shape and the inactivation gate to close (Sodium channel inactivation)
What happens at +30mV
VG Potassium channels open,
electrical gradient is outwards and takes K+ out while Na+ stops coming in
Causes repolarisation of the cell (cell mem becomes more negative)
What happens at -40mV
The VG Na+ C has their activation gates close inactivation gates open i.e back to resting position
*Potassium channels begin to close
What happens at -60mV
*Potassium channels begin to close, Na+ channels begin reactivating and cell brought back into RMP
What happens during the absolute refractory period?
Another AP cannot be generated no matter how large the stimulus is
What happens during the relative refractory period?
AP can be generated in response to a v large stimulus only as enough of the Na+ channels that are in resting can be reactivated (not all are in resting)
Why does a refractory period occur?
Prevents AP from travelling backwards
When can another AP be conducted?
At -40mV in response to a v big stimulus as Na+ channels will be in their resting state
When is the refractory period and for how long
From the -60mV to -40mv
Lasts 1ms
What happens as the MP at the initial segment depols to +30mV
There is flow of charge in the forward, diagonal, backwards and downwards directions, allowing those parts of the axon to come to threshold
Why isn’t the AP conducted backwards even though there is a backflow of charge
The MP is in its refractory period hence it is not affected
Time taken for APs to be conducted along unmyelinated axons
1.5m/s
How is conducted v increased?
Using myelin
What is under the myelin
A low density of VGICs
Why does myelin help to transmit the AP faster?
As the charge does not dissipate
What happens at the node of Ranvier?
The local current from the initial segment causes a graded depol due to the high density of VGICs at the node of Ranvier, which brings MP to threshold
How fast does AP travel in an myelinated axon
20-100 m/s
