Lecture 20

Question 1

What does ion selectivity depend on?

Answer 1

• ion selectivity depends on diameter and shape of the ion channel and amino acids that line it

Question 2

What is the selectivity filter?

Answer 2

• narrowest part of the channel: only allows ions of appropriate size and charge to pass
• ions shed water molecules in order to pass through

Question 3

are ion channels continuously open

Answer 3

• no they are not continuously open

Question 4

what is the difference between ion channels and transporter

Answer 4

• 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

Question 5

Resting membrane potential

Answer 5

• 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

Question 6

Name 3 gated ion channels

Answer 6

1. voltage gated: probability of being open is controlled my membrane potential
2. ligand gated: opening is controlled by the binding of a ligand
3. mechanically gated: opening is controlled by mechanical force

Question 7

mechanically-gated ion channels

Answer 7

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

Question 8

Neurons

Answer 8

• 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

Question 9

action potential

Answer 9

• 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!

Question 10

Na+ channels

Answer 10

• 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

Question 11

Role of voltage-gated K+ channels

Answer 11

• 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

Question 12

Action potential propagation

Answer 12

• when a patch of the plasma membrane becomes depolarized, the self amplifying depolarization spreads outwards

Question 13

voltage-gated Ca2+ channels

Answer 13

• 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

Question 14

Describe the nerve terminal

Answer 14

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

Question 15

Patch-clamp recording

Answer 15

• 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

Question 16

Neurotransmitters (process)

Answer 16

• 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

Question 17

transmitter-gated ion channel

Answer 17

• a subclass of ligand gated ion channels that convert the chemical signal of a neurotransmitter back into an electrical one

Question 18

neurotransmitter can _____ or ______ a postsynaptic cell

Answer 18

excite or inhibit

Question 19

Excitatory neurotransmitters

Answer 19

like acetylcholine and glutamate are ligand gated cation channels

Question 20

Inhibitory neurotransmitters

Answer 20

like GABA and glycine are ligand gated Cl- channels

Question 21

psychoactive drugs

Answer 21

• 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

Question 22

cocaine

Answer 22

• 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

Question 23

synaptic plasticity

Answer 23

ability of a synapse to adjust its strength depending on how heavily it has been used in the past

Question 24

synaptic plasticity is triggered by

Answer 24

• 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

Question 25

optogenetics

Answer 25

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