Lecture 6: Action Potentials Flashcards
What does the term “potential” refer to?
the separation of electrical charge across the membrane
What is resting potential?
the membrane potential when a neuron is at rest
What is graded potential based on?
the stimulus received by the neuron
What may an action potential lead to?
synaptic activity on the next neuron
What is diffusion?
net movement of ions from regions of high concentration to regions of low concentration
What are two important factors which determine how much current will flow?
electrical potential (V) and electrical conductance (g) I = g x V
What does the movement of any ion through a protein channel depend on?
the concentration gradient and the difference in electrical potential across the membrane
What is equilibrium in a selectively permeable membrane?
when there is no net movement of
ions across the membrane, leaving a charge difference between the two sides
What are the opposing forces which keep K+ at equilibrium in the cell?
concentration gradient drives K+ out of the cell, while negatively charged proteins pull K+ back inside the cell
Why is energy required to keep a cell at resting membrane potential?
sodium-potassium pumps require energy to push ions across the membrane against their concentration gradients
What is the state of voltage gated Na+ channels at -65mV?
they are closed
What is the state of voltage gated Na+ channels at -55mV and what happens?
they are open
if there are enough open channels there will be a significant influx of Na+ into the cell
if threshold is met then the membrane will become depolarised and the voltage will rise (rising phase and depolarisation)
How long are Na+ channels open for and what happens when they close? When will the channel become active again?
open for around 1ms
this will stop the influx of Na+ into the cell and the peak of the action potential is reached
the channel will not become active again until the cell has reached back to -65mV
How is the cell repolarised following an action potential?
at 30mV K+ channels are open allowing for an efflux of K+ out of the cell
this will repolarise the neuron and bring it back down to RMP
How is hyperpolarisation handled by the cell?
equilibrated by the Na+ and K+ leak channels
What is the absolute refractory period?
the period during which the Na+ channels are open and the point at which they are closed and cannot be opened again
another action potential cannot be fired
What is the relative refractory period?
the period during which the cell is hyperpolarised and a sufficiently excitatory response is required for another action potential to occur
What is saltatory conduction?
when action potentials skip from one node to the next as they travel down a myelinated axon
Where are action potentials generated?
the axon initial segment (AIS) is the initiation zone
Which two proteins are expressed near the axon initial segment and what is their role?
β4-spectrin and ankyrin-g are proteins that are essential to the organisation of the axonal cytoskeleton
β4-spectrin links ankyrin-g which clusters voltage gated sodium channels and potassium channels to the AIS and the nodes of Ranvier
What happens when there are mutations in the genes encoding ankyrin?
severe neurodevelopmental disorders including congenital hypotonia, severe intellectual disability, and motor axonal and auditory neuropathy
What is the role of Nav1.9?
amplification of subthreshold stimuli, ultraslow kinetics
What is the role of Nav1.1, 1.2, 1.3?
contributes to amplification of subthreshold stimuli, low activation threshold, fast kinetics
What is the role of Nav1.7?
contributes to rising phase and amplifies subthreshold stimuli, low activation threshold, fast kinetics
What is the role of Nav1.6?
contributes to rising phase, rapid activation and moderate activation threshold
What is the role of Nav1.8?
main contributor to rising phase, high activation threshold, slow kinetics
When does an action potential occur?
if the combination of graded potentials exceeds a threshold
What is the role of dendrites and the soma membrane?
dendrites integrate information
the soma membrane generates graded potentials
What must happen in order for an action potential to be generated at the axon initial segment?
the summation of graded potentials must exceed the neuronal threshold
Where are voltage gated channels highly expressed?
within the AIS and the nodes of Ranvier with high levels of Nav within the nodal and Kv within the paranondal and juxtaparanodal aspects of the nodes of Ranvier
