Action Potentials 1 Flashcards
What changes potential in a neruon
Membrane permeability or ion concentration changing
What happens when potential becomes more negative
Hyperpolarisation - potential inside moves closer to Ek
What happens when potential becomes less negative
Depolarisation - potential inside cell moves away toward Ena
What’s a action potential
Fluctuation caused by short opening of vgic that spreads along a axon.
Occur after membrane potential reaches certain voltage called threshold 55mV
Significance of action potentials
Frequency encodes information (language which neurons communicate)
Stimulus examples
Physical - electric current or mechanical stretch
Chemical - drug or synaptic excitation
3 stages of AP
- Membrane potential reaches threshold followed by fast depolarisation
- Repolarisation
- After hyperpolarisation
What happens when membrane potential MP reaches threshold
Sudden activation of voltage gated Na+ channels (Pna increases) MP shifts toward Ena
What happens after channels inactivate quickly
A transient opening of voltage gated K+ channels leading to repolarisation
What happens after hyperpolarisation
Membrane potential shifts toward Ek+
What happens when voltage threshold is reached
Sodium channels open and Na+ ions move into cell along both concentration and electrical gradients
Influx of Na+ slows and stops when:
Inside potential becomes positive (moves toward Ena) and attracts Na+ less
Na+ channels inactivate
Is amplitude affected by stimulus
No. Each AP is all or nothing and usually constant amplitude of 100mV
How can action potentials be evoked externally
Under experimental situations. You can provide electrical stimulation via a battery
How can action potentials be evoked internally
Under physiological situations, post synaptic potentials build up
What happens to adjacent voltage gated ion channels when a action potential is generated
Open and allows flow of ions. Further opening adjacent VGIC leading to transmission
What path changed RMP of AP
Across membrane inside axon.
When current flows
OUT to IN = hyperpolarisation (mp becomes more negative)
IN to OUT = depolarisation (mp becomes less negative)
How are AP generated internally in CNS neurons
AP First made in axon initial segment as it has lowest threshold.
Depolarisation to threshold evoked by excitatory postsynaptic potentials spreads passively from dendrites.
Then AP go along axon away from cell body.
2 types of axons
Unmyelinated - small (1nm), transmits AP slow, continuous
Myelinated - larger (5-10nm) transmits AP fast and in large steps
2 stages of AP transmission
Passive spread
Generation of AP
What’s depolarisation
Positive charge inside cell
Passive spread of current
Passive current flow ?????
Speed in unmyelinated axons
Passive current flow is fast but AP must be regenerated at every point on membrane so conduction velocity is slow as it takes time
Why do myelinated axons conduct AP much faster
Due to saltatory conduction. Jumps from nodes of Ranvier (space in myelin sheath)
2 types of glial cells
Oligodendrocytes and Schwann cells
How does myelination increase passive spread of current
Due to insulating properties of myelin there’s less dissipation as it flows along the axon
Where’s are APs generated in myelinated axons
Only at nodes of Ranvier.
Why does myelination increase action potential conduction velocity
As APs don’t need to be regenerated at every part of the cell membrane
How are APs generated in sensory neurons
Stimulus acts
Evokes a graded depolarisation (receptor potential) which spreads to trigger zone where APs are generated.
APs then spread along axon towards CNS
