L19 - nerve cells & excitability: action potential Flashcards
how do nerve cells communicate?
via dendrites, axons and terminals
what does it mean by neurones care polarised?
resting membrane potential
what is RMP?
electrical charge across plasma membrane where the inside is more negative compared to the outside. -70mV
what are action potentials?
change in voltage across membrane
describe depolarisation of membrane
- excitatory signals at dendrites cause ligand gated sodium channels to open
- causing Na+ ions to diffuse into the cell
- this decreases the negative charge inside the cell and makes the membrane less negative
- so cell becomes less polarised
why is axon hillock known as trigger zone?
- influx of Na+ diffuses inside neurone and produces current that travels towards axon hillock
- a.p usually starts at the axon hillock
- as a.ps are produced by voltage gated ion channels which are most concentrated at axon hillock
all or none law
- if stimulus is strong enough A.P occurs
- frequency of A.P rather than strength
- all A.P same size
- no weak or strong A.P
hypOpolarisation
initial increase of membrane potential to the threshold potential (-55mV)
depolarisation
membrane potential goes from resting membrane potential (-70mV) to LESS negative values
overshoot (peak)
peak of A.P at +40mV
repolarisation
membran potential going back to more negative value (resting memb potential)
hyperpolarisation/ undershoot
potential becomes even more negative than RMP
voltage gated ion channels
- closed at RMP
- open when memb is depolarised (more positive)
- change in voltage triggers opening
- close when rmp is restored
- responsible for initiaiton of Na+ and termination of K+ of action potentials
for A.Ps to be generated what must there be?
a signal strong enough to bring membrane voltage to threshold (-55mV), as this is the minimum needed to open voltage gated ion channels
describe how sodium and potassium channels open at threshold
- Na+ channels open quickly
- K+ channels slowly
- so initial effect is due to sodium influx
- causes cell to become even more positive, leading to further depol
describe in depth how A.P is triggered, talk about the channels and how they relate to the graph
- sodium channels open causing influx of Na+ ions to enter cell
- memb even more depol
- A.P triggered once men is positive enough
- by the time sodium channels start to close, K channels are fully opened
- so K+ ions rush out of cell
- rmp restored
- potassium gates slow to close so there is a undershoot called hyperpolarisarion (more neg)
absolute refractory period
A.P can’t be evoked as voltage gated sodium channels are inactivated and cannot be activated again until memb is depolarised and resting state returns (this is from depol to rmp)
relative refractory period
memb potential hyperpolarised by potassium ions, AP an be generated only if stimulus is strong enough to overcome the hyperpol to reach the threshold (during hyperpol)
action potential propagation
- sodium influx spreads along axon
- sodium currents diffuse in both directions
- but refractory properties of ion channels make sure a.p is propagated in ONE DIRECTION
- as the part that just fired is unresponsive
what does the speed of propagation depend on?
- depends on axon diameter (size)
- and myelination
speed of propagation for a large axon
- less resistance to current
- larger diameter = faster propagation
myelin sheath
- electrically insulated axon so currents can jump from node to node
- as voltage gated sodium and potassium channels are only at nodes of ranvier
pain is one of the…
slowest sensations bodies can send as they are sent from smaller fibres without myelin
faster signals are sent by?
larger, myelinated neurones
Guillain-barre syndrome
destruction of schwann cells in PNS
ms
loss of oligodendrocytes in brain and spinal column
what do ms and Guillain barre syndrome cause?
- muscle weakness
- numbness
- tingling
when myelin coating of nerves degenerates and the nerves are afferent (sensory)
- signals are diminished
- or destroyed completely
- causes numbness & tingling
- as sensations are not travelling the way they should
when myelin coating of nerves degenerates and the nerves are efferent (motor)
leads to weakness as brain gives out lots of energy but still can’t actually move the affected limbs. limbs are especially affected as they have longest nerves, longer nerve = more myelin to be destroyed
graded potentials
-changes in membrane potential in small region of membrsne
- size of potential changes with the strength of stimulus
- if the threshold for sodium channels is reached A.P is generated
- 2 types: excitatory & inhibitory
excitatory graded potentials
increases likelihood of A.P by depolarising the membrane
inhibitory graded potentials
decrease likelihood of A.P via hyperpol
no summation
2 stimuli at separate times
temporal summation
2 excitatory stimuli occur close in time
spatial summation
2 stimuli occur at sae time at different locations
spatial summation of EPSPs and IPSPs
changes in membrane potential can cancel each other out
