3 - Ion Channels and Neuronal Signalling

Question 1

What does the ligand-activated ion channel do?

Answer 1

A ligand-activated ion channel will recognise its ligand, and then opens a gap (channel) in the plasma membrane through with ions can pass - these ions will then relay the signal

Question 2

What is involved in signal transduction on the ligand-activated ion channel?

Answer 2

• signal transduction on this type of cell surface receptor involves the recognition of the receptor of what we call a ligand
  -the ligand is a molecule that interacts with the receptor
• neurotransmitters can be called ligands, so can hormones

Question 3

What happens when the ion channel interacts with the ligand?

Answer 3

• the interaction opens up a gap in the cell membrane, and ions being very small can pass through this gap
• the transmission of ions through the gap in the cell membrane causes a change in charge on either side of the cell membrane, and this is detected by the cell and translated into a cell response

Question 4

Give an example of signal transduction of ion channels

Answer 4

An example of this mechanism is found in the receiving cell of a synapse

Question 5

What can cause an ion channel to open?

Answer 5

• an ion channel can also open when the receptor is activated by a change in cell potential, that is, the difference of the electrical charge on both sides of the membrane
• effectively, the opening and closing of ion channels underlies the movement of charge or an action potential along the axons of neurons

Question 6

What are ion channel coupled receptors?

Answer 6

Ion channel coupled receptors are ligand gated ion channels

Question 7

Where are ion channel coupled receptors found?

Answer 7

They are found on all cell membranes as the signal transduction they mediate is very strong

Question 8

What happens when the cell membrane potential changes (ion channel coupled receptors)?

Answer 8

• the cell membrane acts as an insulator and it doesn’t readily allow charged ions to pass through it
• the cell membrane can act to set up a charge difference or potential difference between the outside and inside of the cell

Question 9

What happens when the cell activity changes (ion channel coupled receptors)?

Answer 9

• when we have movement of charged ions across the cell membrane, a change in the cell membrane potential occurs, in response to that, there could be a change in the activity of the cell

Question 10

Give 4 examples of ion channel coupled receptors

Answer 10

1. nicotinic receptor for acetylcholine
2. GABA(A) receptor for GABA - a ligand gates Cl(-) channel
3. inotropic glutamate receptor - a cation channel
4. 5-HT(3) receptor - a ligand gated cation channel

Question 11

What is the basic structure of ion channel?

Answer 11

The structure of the ion channel is basically a cluster of protein segments that surround a channel that runs through the cell membrane

Question 12

How many conformations of the ion channel & what are they?

Answer 12

• the channel can be in two conformations - OPEN or CLOSED
• when it is open there is a channel in the centre of the receptor that allows ions to pass in and out
• when it is closed, the ions are prevented from passing across the cell

Question 13

What is acetylcholine?

Answer 13

• acetylcholine was the first neurotransmitter to be found and regulates muscle activity and the receptor itself was characterised initially pharmacologically for being able to interact with the chemical nicotine
• it was originally the NICOTINIC RECEPTOR although nicotine is not the natural ligand it the body - acetylcholine

Question 14

What is the GABA receptor?

Answer 14

• a ligand gated ion channel
• there are two types: GABA(A) and GABA(B)

Question 15

What is GABA(A)?

Answer 15

• a ligand gated ion channel
• the natural ligand for this receptor is gamma amino butyric acid, which is a small molecules neurotransmitter that interacts with the GABA receptor that opens the ion channel

Question 16

What does the cell membrane do to the charge?

Answer 16

• in the body, often the cell membrane causes a charge of potential difference on the outside and they inside of the cell leading to a negative charge
• so we an excess of electrons on the inside compared to the outside, and e can have a flow of ions which changes the potential

Question 17

How many binding sites are there on the nicotinic acetylcholine receptor?

Answer 17

There are 2 ligand binding sites for acetylcholine

Question 18

What happens when the nicotinic acetylcholine receptor is in the resting state?

Answer 18

The channel is interacting with the binding sites

Question 19

What happens when acetylcholine is released from a neurone?

Answer 19

• acetylcholine interacts with the two binding sites
• this causes a conformational change in the channel such that we now have a channel or space opening across the cell membrane in the centre of the protein cluster
• ions can now pass through the channel, into or outside the cell

Question 20

What is an ionotropic receptor?

Answer 20

A receptor that allows the low of ions across the cell (e.g., nicotinic acetylcholine receptor)

Question 21

What does an acetylcholine receptor antagonist do?

Answer 21

Bind to the binding site of the receptor, preventing acetylcholine from binding and keeping the ion channel in a closed conformation

Question 22

Function of the GABA(A) receptor?

Answer 22

A channel which allows the traffic of negatively charged chloride ions across the cell membrane

Question 23

List components of the neurone (6 features)

Answer 23

1. dendrites
2. cell body (perikaryon)
3. axon (myelinated or not)
4. nodes of ranvier
5. terminals
6. synapse

Question 24

Function of dendrites

Answer 24

• receive signals from other neurons
• when they receive a signal, it triggers an impulse of electrical charges that travel to the cell body

Question 25

Features of cell body

Answer 25

• contains nuclei
• contains ribosomes
• and contains all of the usual functional organelles

Question 26

Features of axon

Answer 26

• extends away from the cell body
• is often myelinated

Question 27

Features of nodes of Ranvier

Answer 27

• gaps in the myelin sheath
• allow the accelerated jumping or saltatory conductance of action potentials

Question 28

What happens when an impulse arrives at a neurone?

Answer 28

• the impulse arrived at the end of the nerve to the synapse where, at individual nerve endings, neurotransmitters are secretes
• these then cross the synapse to stimulate a receiving cell, which can be a receiving neurone or another cell type

Question 29

What is crucial to the mechanism of nerve conduction?

Answer 29

The sodium potassium pump

Question 30

What is the sodium potassium pump?

Answer 30

• a mechanism which continuously pumps Na+ out of the nerve cell, they causing a potential difference across the cell membrane
• there is some potassium entering/leaving the cell at the same time but the overall effect is the potential across the membrane of about -60mV

Question 31

What is an action potential?

Answer 31

An action potential is when a wave of depolarisation passes along the nerve axon at up to 100m/second

Question 32

What happens to the charge in the sodium potassium pump?

Answer 32

• a separation of charge - excess of sodium ions on the outside of the cell (along with some potassium ions) and on the inside a net negative charge
• because there are fewer sodium ions on the inside, the positive charge is balanced by chloride ions, found on the outside

Question 33

What happens when there is a generation of an action potential of the sodium potassium pump?

Answer 33

• the system collapses so that we have movement of ions inside the cell such that we alter the membrane potential so it rises to about -30 mV and then shoots up so there’s an inversion in charge
• there is then an undershoot where it returns to the resting potential of -60 mV
• called hyper polarisation

Question 34

Stages of nerve conduction (5 stages)

Answer 34

1. Resting state (membrane potential at resting potential - outside positive, inside negative)
2. depolarisation (sodium moves into the cell & the membrane potential reaches the threshold [-60 mV])
3. rising phase of the action potential (outside of cell now negative and inside positive - membrane potential peaks)
4. falling phase of the action potential (sodium can no longer enter the cell (inactivation loop) and potassium now passes through to the outside - outside positive, inside negative & membrane potential drops down to resting)
5. undershoot (membrane potential dips below resting) and then returns to normal

REPEAT

Question 35

What is the nerve trunk?

Answer 35

• axons surrounded by myelin are bundled together to form the nerve trunk
• basically a large insulated wire which is fed by blood vessels to provide the nutrients and their immune responses

Question 36

What are neurotransmitters?

Answer 36

• chemical messengers that traverse the synaptic gaps between neurones
• travel across the synapse and bind to receptor sites on the receiving neurone, thereby influencing whether it will generate a neural impulse

Question 37

Stages of synaptic signalling by neurotransmitters (6 stages)

Answer 37

1. Action potential arrives at axon terminal
2. voltage-gated calcium channels open and calcium ions enter the axon terminal
3. calcium entry causes neurotransmitter-containing synaptic vesicles to release their contents by exocytosis
4. neurotransmitter diffused across the synaptic cleft and binds to ligand-gated ion channels on the post synaptic membrane
5. binding of neurotransmitter opens ligand-gated ion channels, resulting in graded potentials
6. reuptake by the presynaptic neurone, enzymatic degradation and diffusion reuse neurotransmitter levels, terminating the signal

Question 38

Stages leading to the release of neurotransmitters (5 stages)

Answer 38

1. action potential arrives at the end of an axon
2. voltage gated calcium ion channels in the plasma membrane of the axon open
3. calcium ions diffuse into the neurone from the surrounding tissue fluid
   4, increase in calcium stimulates synaptic vesicles to fuse with the plasma membrane of the neurone
4. neurotransmitter molecules are released