2 ways to change the potential inside cells
1. change in membrane permeability 2. ion concentrations change
potential inside cell becomes more negative. i.e potential moves closer to EK+ and further from ENa+ (e.g. -70 to -75)
potential inside cell becomes less negative i.e. potential moves closer to ENa+ and further from EK+ (e.g. -70 to -60)
action potential =
an AP is a brief fluctuation in membrane potential, caused by a transient opening of voltage-gated ion channels, which spreads like a wave along an axon
when will an action potential occur?
when the membrane potential reaches the threshold = -55mV
how is information carried by an AP?
information is carried in the frequency of the AP, NOT size of AP
APs are the key element in the process of signal transmission along axons true/false
Outline steps of AP
- slow depolarisation caused by stimulus
- once MP reaches threshold of -55mV (from -65mV), fast depolarisation to +30mV
- AHP (after-hyperpolarisation)
absolute refractory period =
period of time coinciding with the Na+ channel activation and inactivation. A second action potential cannot be initiated during this time, even with a strong stimulus.
relative refractory period =
period of time when voltage-gated K+ channels are still open after inactivated Na+ channels have returned to resting state. AP can only be generated with a larger than normal stimulus.
what is overshoot?
reversal of polarisation, when MP goes from -ve to +ve
2 types of stimulus
1. chemical (synaptic excitation) 2. physical (electric current, light, stretch)
what happens once threshold is reached
sudden activation (opening) of voltage-gated Na+ channels (PNa+ increases). Rapid influx of Na+.
how does the permeability of Na change in comparison to K, once threshold is reached?
goes from resting of PK+/PNa+ = 40/1 to 1/20. this causes MP to shift more towards Na+. overshoots to +30mV.
opening of voltage-gated Na+ channels short lived because... (two causes)
1. inside potential becomes more positive (so electric gradient is decreasing) 2. Na+ channels inactivate (inactivation gate plugs channel)
what follows inactivation of voltage-gated Na+ channels?
TRANSIENT opening of voltage-gated K+ channels, leading to repolarisation and after-hyperpolarisation (AHP)
how does the permeability of K change in comparison to Na, in AHP?
PK+/PNa+ becomes 100:1
why is the flow of Na+ into the cell, via voltage-gated channels so rapid?
Na+ is moving down both its electrical and its chemical gradient.
does the AP amplitude stay constant or change depending on stimulus intensity
AP amplitude stays constant at ~100mV. the intensity of the stimulus doesn't matter, as long as it is SUPRATHRESHOLD
what happens if the stimulus is subthreshold?
nothing. These stimuli are graded. Each AP is an "all or nothing" event
when current flows into the cell, there is a _____ ________. When current flows out of the cell, there is a _____ ________.
when current flows into the cell, there is a _local_ _hyperpolarisation_. When current flows out of the cell, there is a _local_ _depolarisation_.
action potentials are first generated in the axon _____ _______ (also called axon _______) which has the ______ threshold, and thus serves as the trigger zone for action potentials
action potentials are first generated in the axon _initial_ _segment_ (also called axon _hillock_) which has the _lowest_ threshold, and thus serves as the trigger zone for action potentials
what are excitatory postsynaptic potentials (EPSPs)?
depolarising currents which spread PASSIVELY from the dendtrites to evoke the depolarisation to threshold (-55mV)
how are APs transmitted along the axon?
ACTIVELY, away from cell body/soma
in which region would voltage-gated K+ channels open and then close?
Voltage-gated K+ channels open at stage 2 (repolarisation) and close at the end of stage 3 (AHP).
which region of the curve shows where PK is less than PNa ?
produced by opening of channels in the postsynaptic membrane that are permeable to several cations (Na+, K+ and often Ca2+).
produced by opening of channels in the postsynaptic membrane that are permeable to K+ and Cl-