Signalling 2

Question 1

How does GPCR work?

Answer 1

Ligand binding to receptor –> conformational change in cytoplasmic domain allows G-protein to bind/ be activated by receptor.
Activated G protein activates intracellular enzymes.

Question 2

What type of G proteins do GPCR’s use?

Answer 2

GPCR’s use large, heterotrimeric G proteins, not small monomeric G proteins.

Question 3

What are the effects of GPCR activation?

Answer 3

There is lots of different targets of activated alpha and beta gamma subunits.
Targets will be activated or inhibited directly or indirectly.
It commonly causes the production of a 2nd messenger.

Question 4

How are G-proteins activated?

Answer 4

GTP is a similar molecule to ATP.
GTP bound G-proteins = active
GDP bound G-proteins = inactive
Activated G protein activates downstream effector proteins.

Question 5

What are the off switch molecules for G protein activation?

Answer 5

GAP/RGS proteins

Question 6

What is adrenaline, where is it produced and why?

Answer 6

-Adrenaline is a hormone produced by adrenal medulla after sympathetic NS stimulation in acute stress situations to increase heart rate etc.

Question 7

Using the example of adrenaline, explain GPCR activation?

Answer 7

• Adrenaline binds to GPCR, triggers activation of G-protein alpha subunit, this activates enzyme adrenyl cyclase to produce 2nd messenger cAMP.
• cAMP activates PKA which does 2 things:
  1. Phosphorylates and activates phosphorylase kinase. This phosphorylates and activates the enzyme glycogen phosphorylase, which catalyses breakdown of storage molecule glycogen, to produce glucose.
  2. PKA phosphorylates and inhibits enzyme glycogen synthase, therefore glycogen synthesis does not occur.

Question 8

Why is the adrenaline GPCR sequence like an on, off switch with no intermediate phase?

Answer 8

It wouldnt make sense as an intermediate phase would mean synthesis and degradation at the same time.

Question 9

How can the signal in the adrenaline GPCR sequence be switched off?

Answer 9

G-protein alpha subunit hydrolyses the GTP, restoring it to inactive GDP. It can no longer activate adenyl cyclase, so no new cAMP is produced.

Question 10

How are 2nd messengers broken down?

Answer 10

• Specific enzymes break down several 2nd messengers eg. phosphodiesterases remove cAMP
• Alternatively, Ca+ ions acting as 2nd messengers can be actively removed by using ion channel pumps, to pump them back into intracellular stores, or out the cell.

Question 11

How is specificity of cell signalling acheived?

Answer 11

Some genomes of cells in different locations for response however, there is diffrent expressions of GPCR’s, G proteins, effector molecules and 2nd messengers produced. There fore leading to different specific response in each tissue.
This means that the same hormone can have different effects in different tissues.

Question 12

How does electrical signalling work?

Answer 12

Electrical nerve impulses travel along the axon of a nerve cell.
Axons can be very long but nerve impulses travel very fast.

Question 13

What is a nerve impusle?

Answer 13

A wave of altered charge across the nerve cell membrane that sweeps along the axon.
aka. an action potential, depolarisation

Question 14

What is the ion conc. like in a resting cell?

Answer 14

At rest, there is a difference in ion conc. between inside and outside of cell. This is maintained by ion pumps in the plasma membrane.
Approx -70mV potential at rest, when the nerve cell is stimulated, action potential is generated.

Question 15

How is an action potential generated?

Answer 15

1.Some stimuli can directly stimulate a nerve cell eg.olfactory (small) neurons
2.A neurotransmitter from a nearby neuron can bind to a receptor on the nerve cell. In this case, receptor activation causes ion movement that triggers AP in 1st neuron.
Either way threshold of -50 to -55mV needed.

Question 16

What is the process of an action potential?

Answer 16

Na+ ion channels open; +charged Na+ ions flow into the cell and this increases membrane potential to around +40mV (depolarisation).
K+ ion channels open in response to depolarisation and Na+ channels close. K+ ions are pumped out (repolarisation).
K+ channels eventually close but usually overshoot (hyperpolarisation) at around -90mV. Slowly the Na/K pump restores the voltage to -70mV.
This AP moves along the axon.
When AP reaches end of neuron it must cross a synapse to set AP up in next neuron.

Question 17

What is a refractory period?

Answer 17

Time immmediately following AP when new AP cannot be initiated in same area of membrane.
Meaning AP’s have one way travel.
There is an absoloute (when Na+ channels open) and relative (after that) refractory periods both exist.

Question 18

What happens when there are multiple inputs to a nerve cell?

Answer 18

Some may be excitatory (ESP’s), some may be inhibitory (ISP’s).
They are added together at axon hillock by summation.

Question 19

What are the 2 types of summation and the difference?

Answer 19

• Spacial; summation of inputs from different areas, such as from dendrites.
• Temporal; where same input occurs multiple times within a short time period. When this occurs with ESP’s, the threshold value for an AP can be reached quicker.

Question 20

How is signal intensity communicated by an AP?

Answer 20

AP is always same magnitude, therefore info about signal intensity is transmitted not by magnitude but by frequency of AP generation.
More frequent AP’s result from a stronger signal and vice versa.

Question 21

What is the importance of chemical signalling in the NS?

Answer 21

It is vital as most neurons dont act alone, they act with other nerve cells and communicate via a synapse.
Neurons use biochemical signals to cross the synaptic cleft: neurotransmitters.

Question 22

What are synapses formed from?

Answer 22

From a presynaptic nerve cell axon terminus and a post synaptic nerve cell dendrite, with the synaptic cleft between.

Question 23

What do neurotransmitters do and when are they released?

Answer 23

• Transmit signals across synapse.

- Release when AP reaches pre-synaptic neuron termini.

Question 24

What is the action of neurotransmitters like?

Answer 24

Bind to receptors on dendrites of post-synaptic neuron.
They can be excitatory or inhibitory; will promote or inhibit formation of AP in receiving neuron.
Different neurotransmitters are associated with different nervous system functions and different locatiions in NS.
Many drugs act on neurotransmitter signalling.

Question 25

What are some examples of neurotransmitters?

Answer 25

1. Amino acid and derivatives: eg. glutamate - main excitatory NT in CNS
   eg. GABA - main inhibitory NT in CNS
2. Catecholamines: eg. dopamine, seratonin, derived from Tyr.
3. Acetylcholine: derived from choline at neuromuscular junction
4. Peptides: eg. substance P, endorphins

Question 26

Using GABA as an example, explain neurotransmitter action?

Answer 26

-Binding of ligand GABA to receptor causes conformational change that opens ligand gated ion channel.
- Cl- Ion moves through channel along conc. gradient
GABA released from presynaptic neuron into synapse.
GABA-A receptor in post synaptic neuron membrane.

Causes hyperpolarisation that makes it harder to reach AP threshold value for formation of AP in post synaptic cell.

Question 27

How are neurotransmitters removed?

Answer 27

By reuptake or enzymes that remove it.

Question 28

What are some molecules that potentiate GABA activation leading to more inhibition of AP’s –>sedative action?

Answer 28

Benzodiazepine, ethanol and some anesthetics.

Question 29

What is a neuromuscular junction?

Answer 29

A junction between a motor neuron and a muscle cell (similar to synapse between 2 neurons).

Question 30

Use the eg of acetylcholine to explain the neuromuscular junction?

Answer 30

It binds to nicotinic Ach receptor (nAChR) on the post synaptic muscle cell membrane.
The nAChR is a sodium ion channel receptor. Ach binding opens the channel and allows Na+ ion influx into muscle cell, causing depolarisation of muscle cell membrane.
This depolarisation initiates muscle contraction.

Question 31

How is acetylcholine removed after action in neuromuscular junction?

Answer 31

An enzyme called Acetylchesterase is present in cleft and breaks down the acetylcholine that has been released into the cleft, turning off signal transmission into muscle cell.

Question 32

What is a clinical example of neuromuscular junction use?

Answer 32

-Myasthenia Gravis (muscle weakness)
Autoimmune antibodies attack ACh receptors at neuromuscular junction.
Treatment- Acetylcholinesterase inhibitors are often used. These prevent breakdown of Ach in cleft o neuromuscular junction.
Increasing ammount of transmission at junction.

Question 33

Describe electrical signalling via gap junctions and give some examples?

Answer 33

• Connexin proteins form connection with channel
• Ions carry charge from one cell to other by diffusion.

eg. s-electrical synapses between neurons (most are chemical)
- cardiomyocytes