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.
what is absolute refractory
At the +30mV point is absolute refractory as no more AP will be conducted as the Na+ ion channels aren’t available for use. As the inactivation gate is closed. This occurs till about -40mV,
what is relative refractory
-40mV, at this point some inactivation gates are open and some activation close, ie the voltage can can now allow Na+ through. This is Relative Refractory as it would take a large stimulated an AP but an AP can generate. The AP can generate as the activation gates can open to allow the movement of Na+ into the cell.
whats a local current
the movement of ions down the axon bringing membrane down the axon to threshold. moving down the axon as its down a concentration gradient.
step one of AP propogation
Propagation is progressive depolarisation. At this point, an AP has generated in the axon hillock so lots of Na+ is in the cell. the initial segment depolarises to 30mV
step two of AP propagation
The electrochemical gradient for Na+ is now down the axon within the cell. Thus Na+ will move down the axon in a graded potential. The movement makes further segments of the axon also reach threshold,
step three of AP propagation
as the second site is at threshold more Na+ enters the cell and then another local current moves down the axon. Making another region hit threshold and so on. and the initial segment begins refractory
step four of AP propagation
the graded potential in the second segment moves down to the 3rd segment causing threshold in that segment and the process repeats.
why is AP conduction in myelinated axons faster
between the myelin sheath is a node of Ranvier is like its own axon hillock. Myelin prevents the loss of Na+ ions through the membrane as travelling. The myelin means only small areas must be made to threshold not the whole axon. So its like the AP instantly “jumps” between nodes. Thus this process is fast at 20-100 meters per second.
what is repolarisation caused by
the exit of k+ from the inside of the cell membrane
Explain how the speed of conduction of the action potential can be enhanced by myelin
Myelin prevents short range local currents across the membrane, so current spreads further along the inside of
the axon to depolarize membrane to threshold at next Node of Ranvier. Increase CV up to 20-100m/sec. also prevents sodium ion leaks
