Neural Signalling - Ions, APs and the Synapse

Question 1

In Muscle Cells, what Voltage means contraction and what means relaxation? (2)

Answer 1

1. Contraction = +20mV

2. Relaxation = -90mV

Question 2

What is Voltage? (2)

Answer 2

The difference between electric potential energy per unit charge between two points.

Question 3

What is meant by Equilibrium Potential? (4)

Answer 3

1. Equilibrium Potential = Favourite Voltage for an ion.
2. When the ion channel for that ion opens, the membrane potential moves towards that ion’s favourite voltage.
3. This is driven by the conductance (permeability) of the ion channel for the ion.
4. If the ion’s channel conductance is > than other ion channels put together, then membrane potential will become nearly Equilibrium potential for that ion.

Question 4

What are the Equilibrium potentials for:

Ca2+
Na+
Cl-
K+ (4)

Answer 4

Mneumonic from highest to lowest - CaNCK

1. Ca2+ = +123mV
2. Na+ = +60mV
3. Cl- = -40mV
4. K+ = -90mV

Question 5

What occurs is a cell membrane is permeable to exactly two ions at once? Give an example (2)

Answer 5

1. The membrane potential would be average between their equilibrium potentials.
2. For e.g for Na+ and K+ = -90 + 60 = -30mV/2 = -15mV

Question 6

What is the Membrane when at rest compared to when Action potential occurs? (2)

Answer 6

1. At rest membrane potential is -70 mV

2. During AP = +40 mV (ranging from +20mV to +60 mV). Cel normally becomes permeable to Na+ and/or Ca2+.

Question 7

Draw a graph and describe the events of a neuronal action potential. (5)

Answer 7

1. The resting mV of the cell is -70mV.
2. When a stimulus is received, it must reach threshold of -55mV or it does not shoot (all or nothing).
3. Depolarisation = When it reaches threshold, Na+ ion channel opens and Na+ ions flow through, reaching AP at +40 mV.
4. Repolarisation: Once AP is reached, Na+ channels start closing and K+ channels open, this Repolarises the cell.
5. Hyperpolarisation: The K+ channels are slow to close so overshoot below resting mV, Na+ channels still working help the mV go back up to Resting mV. This is Refractory period and no new APs can be fired during that time.

Question 8

What is a neurotransmitter? (3)

Answer 8

1. It is an endogenous chemical;
2. which is transmitted extracellularly by a neuron;
3. across a synpase to signal another cell (under physiological conditions).

Question 9

What are some examples of small neurotransmitters and large neurotransmitters?

Answer 9

Small neurotransmitters: Acetylcholine, Amino acids, Catecholamines

Large Neurotransmitters: Neuropeptides (substance P in pain).

Question 10

What are 3 examples of Catecholamines? (3)

Answer 10

1. Dopamine
2. Noradrenaline
3. Adrenaline

Question 11

Describe the events of Neurotransmitter transport across a synapse (5)

Answer 11

1. Neurotransmitter is synthesised in cell body.
2. Neurotransmitter packaged into vesicles and transported to membrane.
3. Action Potential is triggered, opening Ca2+ channel which allows Vesicle to fuse to membrane and release neurotransmitters.
4. Neurotransmitter reaches next cell, binds to and activates/inhibits post-synaptic receptors.
5. The rest of the neurotransmitters are transported/metabolised back into initial axon terminal.

Question 12

How can drugs disrupt the neurotransmitter activity across the synapse? (2)

Answer 12

1. They can inhibit enzymes that breakdown the neurotransmitters e.g Acetylcholine esterase inhibitor.
2. They can inhibit protein channels which transport neurotransmitters back into cell. e.g Fluoxetine : Selective serotonin reuptake inhibitors (SSRI).

Question 13

What neurotransmitter triggers action potential in muscle cell (myocyte)?

What type of receptor does it bind to on myocyte membrane?

What molecule eliminates the neurotransmitter? (3)

Answer 13

1. Acetylcholine triggers action potential.
2. Nicotinic acetylcholine receptor. A non-selective cation channel as lets Na+ go inwards and K+ out.
3. It is eliminated by Acetylcholineesterase (AChE)

Question 14

Draw and label a sacomere. (3)

Answer 14

Allocate:

1. Thin (actin) and thick (myosin) filaments
2. Bands - H (just myosin) , I (just actin) and A band (Overlap)
3. Lines - M and Z lines

Question 15

Describe the changes in different parts of a sarcomere upon contraction. (3)

Answer 15

1. Myosin pulls actin, so Z lines pull towards M line.
2. Upon contraction, H and I bands decrease in width.
3. A band width never changes.

Question 16

What polymers make up actin?

What are the components that make up the regulatory complex on actin (3)

Answer 16

1. G actin makes double helical strand; F actin makes regulatory complex with groove on each side.
2. Tropomyosin occupies groove - Blocks Actin, Myosin bind site
3. Troponin protein complex attaches to tropomysosin and actin - controls tropomyosin

Question 17

What is the second messenger once Action Potential is triggered in myocyte? How does it work. (3)

Answer 17

1. Calcium is second messenger.
2. There is a direct physical connection between calcium channels in membrane and calcium channel in Sarcoplasmic reticulum.
3. Once AP triggered, membrane is depolarised, channels undergo conformational change, Ca2+ released into cytosol.

Question 18

What is twitch and tetany? (4)

Answer 18

1. Twitch is the period of stimulation to contraction and then complete relaxation (as Ca2+ pumped back into SR) in a myocyte. If frequent, series of twitch can fuse together by SUMMATION. This is slower than AP.
2. Tetany is continuous maximum muscle stimulation. This occurs as relaxation too short for Ca2+ to be pumped back into SR.

Question 19

What are the 3 subunits that make up troponin? (3)

Answer 19

1. Troponin T (tropomyosin-binding)
2. Troponin C (Calcium binding)
3. Troponin I (inihibitory/actin binding part)

Question 20

Describe the Actin-Myosin Cross Bridge cycle. (4)

Answer 20

1. Mysoin releases Actin, ATP binds to myosin.
2. Myosin head cleaves ATP (phosphorylated) activating myosin.
3. Myosin binds to actin (once Ca2+ binds to troponin to uncover binding site)
4. Power Stroke (swivels from 90 degrees to 45 degrees).

Question 21

How do each of these drugs work in relation to Acetylcholine:

Carbochol (for treating glaucoma)

Rocuronium (trachael intubation)

Curare (poison) (paralysis of diaphragm)

Donepezil (Alzheimers treatment). (4)

Answer 21

Carbochol : it is an agonist for AcH Nicotinic Receptor (mimics acetylcholine).

Rocuronium and Curare are both antagonists to AcH Nicotinic receptor.

Donepezil is an inhibitor to acetylcholinesterase, preventing the breakdown of Acetylcholine.

Question 22

Why does Rigor Mortis occur after death? (3)

Answer 22

1. ATP depletes after death.
2. Myocyte initially contracts due to Ca2+ not being taken up back into SR.
3. No ATP to uncouple Myosin from Actin. Rigor mortis only ends 3 days after when tissue degrades.