Lecture 20 Flashcards
What does ion selectivity depend on?
- ion selectivity depends on diameter and shape of the ion channel and amino acids that line it
What is the selectivity filter?
- narrowest part of the channel: only allows ions of appropriate size and charge to pass
- ions shed water molecules in order to pass through
are ion channels continuously open
- no they are not continuously open
what is the difference between ion channels and transporter
- ion channels do not undergo conformational change each time a ion molecule passes through (transporters do)
- channels have a much faster rate of transport
- cannot facilitate active transport only passive
Resting membrane potential
- voltage difference across the plasma membrane when cell is not stimulated
- [K+] is high intracellularly and has the tendency to move out of the cell when K+ leak channels open
- K+ leak channels open randomly
- the K+ moves out of the cell until resting membrane potential is 0 because the [K+] gradient and voltage gradient are equal
Name 3 gated ion channels
- voltage gated: probability of being open is controlled my membrane potential
- ligand gated: opening is controlled by the binding of a ligand
- mechanically gated: opening is controlled by mechanical force
mechanically-gated ion channels
Example: auditory hair cells
- sound causes stereocilia to tilt which stretches a linking filaments, causing ion channels to open
- influx of positive ions stimulate the auditory nerve fibres and convey the auditory signal to the brain
Neurons
- receive, integrate, transmit signals from sensory organs to the central nervous system and then to target cells like muscle cells
- dendrites extend from the cell body and receive cell signals
- axon conducts electrical signals away from cell body towards nerve terminals
action potential
- a travelling wave of electrical excitation (used to carry messages by neurons)
- a stimulus causes the membrane potential to become less negative (depolarization), causing voltage-gated Na+ channels to open
- Na+ enters the cell, depolarizing the cell even more and causing more Na+ channels to open. Self-amplifying!
Na+ channels
- Na+ channels have an automatic inactivating mechanism, causing the channel to close
- it remains inactivated state until membrane potential has returned to its initial value
- even if another stimulus occurs, action potential cannot occur until Na+ channels returns to being closed
Role of voltage-gated K+ channels
- open in response to depolarization, but are delayed relative to the Na+ channels
- stay open as long as membrane is depolarized
- opening causes K+ ions to flow out of cell, helping to restore the membrane potential back to its resting state
Action potential propagation
- when a patch of the plasma membrane becomes depolarized, the self amplifying depolarization spreads outwards
voltage-gated Ca2+ channels
- when action potential reaches nerve terminal, the depolarization causes voltage gated Ca2+ to open
- this causes Ca2+ to enter, signalling the fusion of synaptic vesicles to the plasma membrane and release of the neurotransmitter
Describe the nerve terminal
presynaptic cell: cell transmitting message
postsynaptic cell: cell receiving message
synaptic cleft: space separating the two cells
- electrical signals cannot cross this cleft; instead, the electrical signal must be converted into chemical one which takes the form of neurotransmitter
- neurotransmitter are stored in synaptic vesicles
Patch-clamp recording
- allows the measurement of electric current in a living cell
- microelectrode is placed in contact with the plasma membrane
- the ion concentration can be adjusted on both sides and an electrode is placed on either side of membrane to measure current
- using a power source, a membrane potential can be set to study how ion channels are affected by membrane
- a single ion channel can be isolated in the patch being isolated
- ion channels are all or none
- even when environment is kept constant ion channels open and close randomly
- proportion of time that a channel is opened or close depends on cellular conditions
Neurotransmitters (process)
- neurotransmitters are stored in synaptic vesicles
- when action potential reaches nerve terminal, depolarization causes Cav channels to open
- the Ca2+ enters, signalling fusion of synaptic vesicles to the plasma membrane and release of neurotransmitter
- once neurotransmitter is released into the synaptic cleft it binds to onto the post-synaptic cell affecting the membrane potential
- neurotransmitters can be released, pumped back into the cell that released it, and taken up by neighbouring non-neuronal cells
transmitter-gated ion channel
- a subclass of ligand gated ion channels that convert the chemical signal of a neurotransmitter back into an electrical one
neurotransmitter can _____ or ______ a postsynaptic cell
excite or inhibit
Excitatory neurotransmitters
like acetylcholine and glutamate are ligand gated cation channels
Inhibitory neurotransmitters
like GABA and glycine are ligand gated Cl- channels
psychoactive drugs
- Sedatives like Valium and Restoril bind to GABA-gated Cl- channels making it easier for these channels to open
- The anti-depressents Prozac blocks the Na+ symport responsible for the reuptake of serotonin
cocaine
- dopamine is taken up by dopamine transporters which limit duration and spread of the signal
- cocaine blocks the uptake or removal of dopamine from the synaptic cleft
synaptic plasticity
ability of a synapse to adjust its strength depending on how heavily it has been used in the past
synaptic plasticity is triggered by
- the entry of Ca2+ in the post synaptic cell which leads to functional change in either side of the synapse:
- can adjust the amount of neurotransmitter released and can adjust how the postsynaptic cell responds
optogenetics
the channel protein channelrhodpsin allow flow of Na+ into cell in response to light
- ontogenetic: a technique that uses light to control activity of neutrons that have light-gated ion channels
