lecture 18 Flashcards
what are chemically gated ion channels
Chemically gated ion channels or ligand gated ion channels, open when they bind to specific chemicals. The receptors for ACh at the neuromuscular junction are chemically gated ion channels.
describe voltage-gated sodium channels in the axon
these have two gates. One that opens on stimulation, activation gate, and one that closes to stop ion entry, inactivation gate. For these to open another stimuli must cause the resting membrane potential to change to -60mV, at which point the activation gate opens.
what are mechanically gated ion channels
these open in response to mechanical stimuli that physically distort the neuron membrane surface.
what is a local or graded potential
this is a neuron and neurotransmitter interaction, the specific part of the neuron is the dendrites. This interaction results in a localised change of membrane potential. The magnitude of these is regulated by the strength of the stimulus.
what causes a local potential
due to the result of Neurotransmitters opening Na+ chemical gated channels and causing depolarisation. However the signals can also open K+ channels to lower the depolarisation, perhaps even hyperpolarise.
where are chemically gated ion channels common
in the dendrites of nerve cells
where is the action potential initally propogated
the initial segment of the axon hillock
what needs to happen to cause an action potential
local potentials must summate well enough to cause depolarisation to -60mV, ie threshold must be met in the cell, in the initial segment of the axon hillock
action potential can be briefly defined as
a short increase in permeability to Na+, once threshold is reached, followed by termination of said permeability and then an increase in K+ permeability, which results in a quick hyperpolarisation.
local potentials can be what or what
inhibitory or excitiory
whats special about the inital segment
it has a high density of sodium gated ion channels, it is the site where an AP is propogated
step one of propagation of an AP in a neuron
The stimulus that initiates an AP is a graded depolarisation that is large enough to open voltage gated sodium channels. The opening of these gated channels occurs at a membrane potential called the threshold.
step 2 of propagation of an AP in a given area of an axon
Activation of sodium ion channels and rapid depolarisation
When a Na+ ion activation gate opens, the plasma membrane becomes more permeable to Na+. Sodium ions then rush into the cytosol, rapidly depolarising the cell.
step 3 of propagation of an action potential in a given area of axon
Inactivation of sodium ion channels and activation of potassium channels:
As the membrane potential reached 30mV, the inactivation gates of the voltage-gated sodium channels close. This is called sodium channel inactivation, this occurs as the voltage-gated K+ channels open. Positively charged K+ moves out of cytosol, lowering the membrane potential, thus repolarisation begins.
step 4 of propagation of an AP in a given axon segment
The voltage gated sodium channels remain inactivated until near threshold voltage is reached. At this time the voltage gated sodium channels move back to normal status of closed but ready to open. The voltage gated K+ channels begin closing as the cell membrane potential repolarises to near -70mV. Until all potassium channels close, the K+ keeps leaving so some small hyper polarisation momentarily occurs.