L2b - Electrophysiology (2)

Question 1

Describe the path that action potentials take through the parasympathetic autonomic nervous system.

Answer 1

1) Central Nervous System (CNS)
2) Preganglionic Neurone
3) Synapse
4) Acetylcholine binds to Nicotinic Acetylcholine Receptor
5) Ganglionic Cell
6) Postganglionic Neurone
7) Acetylcholine binds to Muscarinic Acetylcholine Receptor
8) Target Organ

Question 2

Describe the 2 different paths that action potentials can take through the sympathetic autonomic nervous system.

Answer 2

1) Central Nervous System (CNS)
2) Preganglionic Neurone
3) Sympathetic Ganglion
4) Acetylcholine binds to Nicotinic Acetylcholine Receptor
5) Synapse
6) Postganglionic Neurone
7) Norepinephrine binds to Alpha Norepinephrine Receptor Subtype
8) Norepinephrine binds to Beta Norepinephrine Receptor Subtype
8) Target Organ

1) Central Nervous System (CNS)
2) Preganglionic Neurone
3) Adrenal Gland
4) Acetylcholine binds to Nicotinic Acetylcholine Receptor
5) Synapse
6) Postganglionic Neurone
7) Norepinephrine binds to Alpha Norepinephrine Receptor Subtype
8) Epinephrine binds to Beta Norepinephrine Receptor Subtype

Question 3

Describe epinephrine.

Answer 3

• Commonly known as adrenaline

- Secreted from the adrenal medulla

Question 4

Describe norepinephrine.

Answer 4

• Released into blood as a stress hormone / neurotransmitter

Question 5

Describe the pathway of action potentials through a neuromuscular junction.

Answer 5

1) Action potential arrives at axon terminal (depolarisation occurs as pre-synaptic membrane becomes more positive)
2) Voltage gated Ca2+ channels open
3) Ca2+ enters the cell
4) Ca2+ signals to vesicles
5) Vesicles move to the membrane
6) Docked vesicles release neurotransmitter by exocytosis
7) Neurotransmitter diffuses across the synaptic cleft and binds to nicotinic receptors

Question 6

Describe Vx Gas.

Answer 6

• Extremely toxic nerve agent
• Cholinesterase inhibitor (breaks down acetylcholine)
• ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase
• Postsynaptic membrane ion channel closed; ions cannot pass
• Glands and muscles are constantly being stimulated

Question 7

State the symptoms of a low/moderate dose of Vx Gas.

Answer 7

• Abnormally low / high blood pressure
• Blurred vision
• Chest tightness
• Confusion
• Cough
• Diarrhoea
• Drooling
• Drowsiness
• Excessive sweating
• Eye pain
• Headache
• Increased urination
• Nausea, vomiting and / or abdominal pain
• Rapid breathing
• Runny nose
• Slow / fast heart rate
• Small, pinpoint pupils

Question 8

State the symptoms of a large dose of Vx Gas.

Answer 8

• Convulsions
• Loss of consciousness
• Paralysis
• Respiratory failure (possibly leading to death)

Question 9

What are convulsions?

Answer 9

When body muscles contract and relax rapidly and repeatedly = uncontrolled actions

Question 10

What leads to summative contraction (tetanic contraction) in skeletal muscle?

Answer 10

Repeated, high frequency action potentials

Question 11

What does a repeated stimulus at the neuromuscular junction cause?

Answer 11

Greater number / frequency of skeletal muscle action potentials

Question 12

What happens when action potentials summate?

Answer 12

• Lead to a larger contraction
• Overexcitation of Nicotinic Receptors
• More action potentials in a short space of time causes larger contractions = tetanus

Question 13

What does overexcitation of Nicotinic Receptors cause?

Answer 13

• Sodium ion (Na+) build up
• Tetanus
• Involuntary contractions

Question 14

State the 4 different states in which tension developed in a muscle fibre.

Answer 14

1) Twitch
2) Wave summation
3) Unfused (incomplete) tetanus
4) Fused (complete) tetanus

Question 15

Sodium channels must first do what before another action potential is fired?

Answer 15

Must first recover from inactivation

Question 16

What leads to muscle contraction?

Answer 16

A single action potential leads to the release of calcium from the sarcoplasmic reticulum, which leads to contraction

Question 17

Describe the all or nothing principle.

Answer 17

• If stimulus is too small = few sodium channels will open = not enough to cause an influx of Na+ ions = NOTHING (no action potential sent)
• If stimulus is of sufficient size / threshold reached = Na+ ions influx = depolarisation of neurone = ALL (action potential sent)
• If stimulus is of large size / threshold surpassed = Na+ ions influx = depolarisation of neurone = ALL (action potential sent)

No difference caused by size of depolarisation

Question 18

Describe how voltage-gated ion channels inactivate.

Answer 18

• Ball and chain mechanism

- Protein at the end of n-terminus = ‘ball’

Question 19

Describe how the recovery from inactivation can be measured.

Answer 19

By applying another square pulse:

• A few milliseconds after the first one = little increase in current, few channels recovered
• A significant period after the first one = gradual increase in current, most channels recovered

Question 20

Describe the properties of the Absolute Refractory Period.

Answer 20

• Too few Na+ channels have recovered from inactivation

- Not possible to elicit an action potential (irrespective of how large stimulus is)

Question 21

Describe the properties of the Relative Refractory Period.

Answer 21

• A proportion of Na+ channels have recovered

- Possible to elicit an action potential with greater stimulus

Question 22

Define ‘Refractory Period’.

Answer 22

Time period during which another action potential cannot be elicited.

Question 23

Why can’t tetanic contraction occur in cardiac muscle?

Answer 23

Because sodium channels take longer to recover from inactivation (refractory period is longer)

Question 24

Describe cardiac action potentials.

Answer 24

• Cardiac action potentials do not elicit a summative contraction
• Each action potential leads to a single contraction
• They are no longer in duration
• Cannot cause tetanus because the refractory period is too long

(Time scale is a lot longer)

Question 25

Describe Myasthenia Gravis.

Answer 25

• Immune system mistakenly makes antibodies that attack the ACh receptor sites at the neuromuscular junction
• Weakness in muscles improves with rest, because ACh stores get built up again
• Edrophonium chloride (acetylcholinesterase inhibitor) improves condition because it prevents the breakdown of ACh, which allows for continued stimulation

Question 26

Describe Tetrodotoxin Poisoning.

Answer 26

• Powerful neurotoxin
• Blocks sodium conductance (Na+ channel blocker)
• Blocks neuronal transmission in skeletal muscle
• Inhibits firing of action potentials in neurones by binding to the voltage-gated sodium channels in the nerve cell membranes, and blocking the passage of Na+ into the neurone