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Principles > Action Potentials > Flashcards

Action Potentials Flashcards

1
Q

What is depolarization?

A

Depolarization is when the membrane potential becomes less negative

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2
Q

What is hyperpolarization?

A

Hyperpolarization is when the membrane potential becomes more negative

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3
Q

What is the driving force of Na ions?

A

Net movement of Na ions occurs into the cells.

Its driving force is -140mV. Equilibrium potential of Na ions is + 60 mV.

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4
Q

What is the driving force of K ions?

A

Net movement of K ions occurs outside the cells. It’s driving force is +20 mV. Equilibrium potential of K ions is -100mV.

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5
Q

What is the membrane potential for most neurons?

A

-80mV

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6
Q

Effect of opening Na and K channels?

A

Na channels open first, leading to depolarization (+60mV) followed by hyperpolariation (-100mV) to allow movement of K ions out of the cell

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7
Q

Action potentials propagate with increasing magnitude and decreasing velocity?

A

False, action potentials propagate at constant magnitude and velocity.

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8
Q

Which channel opens more rapidly?

A

Na channels open rapidly, K channels open after a short delay.

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9
Q

Mechanism of action of Na channels

A

Positive feedback. Opening of a few channels causes further opening of channels; self-reinforcing and causing increased depolarisation.

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10
Q

Mechanism of K channels

A

Activation of K channels is self limiting. Outward movement of K ions causes repolarization which turns off stimulus for opening. Negative feedback.

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11
Q

What is the refractory period of Na channels?

A

After voltage activation, Na+ channels enter an open state but quickly enter a non-conducting inactivated state during depolarization. Repolarization is required for the channel to enter the closed state in readiness for opening and generation of action potential

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12
Q

When can a second action potential be elicited?

A

When a stronger than normal stimulus is applied during the relative refractory period. However, no response however strong can elicit a second action potential during the absolute refractory period.

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13
Q

Local current speed depends on?

A

The longer the length constant (λ) the greater the local current spread. The distance over which current spreads depends upon membrane resistance (rm) and the axial resistance of the axoplasm (ri) – increasing the ratio rm/ri increases λ

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14
Q

Strategies to increase current spread

A 
Decrease ri (possible by increasing axon diameter)
Increase rm (possible by adding an insulating material – myelin – provided by Schwann  cells in the PNS and oligodendrocytes in the CNS)
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15
Q

What causes the undershoot in action potential?

A

Undershoot is due to briefly delayed closure of voltage-activated K+ channels

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16
Q

Overview of Neurochemical transmission in ANS

A

Uptake of precursor
Synthesis of transmitter (T), or intermediate
Storage of transmitter (T), or intermediate
Depolarization by action potential
Ca2+ entry via voltage-activated Ca2+ channels
Ca2+- induced release of transmitter (exocytosis)
Receptor activation
Enzyme-mediated inactivation of transmitter (cholinergic), or Reuptake of transmitter (adrenergic)

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17
Q

What are the receptors for Acetylcholine?

A

Nicotinic Ligand Gated Ion Channels

Muscarinic G Protein Coupled Receptors

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18
Q

What happens to Acetylcholine (ACh) after activation of receptor?

A

Degradation of Acetylcholine (ACh) to Choline and Acetate by Acetylcholinesterase (AChE); terminates transmission

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19
Q

What is the structure of the Nicotinic Acetylcholine Receptor?

A

It consists of five glycoprotein subunits that form a central, cation conducting channel

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20
Q

Drugs that block the autonomic ganglia, once used to control hypertension, are ‘blunt tools’, no longer of significant clinical use. True or False?

A

True

21
Q

What molecule is shown to exhibit open channel block?

A

Hexamethonium exhibits open channel block, a form of non-competitive antagonism for all ganglionic transmission.

22
Q

Muscarinic GPCR at parasympathetic neuroeffector junction

A

M1 - Gq - Stimulation of Phospholipase C - Increased acid secretion
M2 - Gi - Inhibition of Adenylyl cyclase, opening of K+ channel - Decrease heart rate
M3 - Gq - Stimulation of Phospholipase C - Increased oral secretion, bronchoconstriction

23
Q

Both sympathetic and parasympathetic preganglionic neurons are cholinergic

A

True, they release acetylcholine (Ach) at the synapse in the ganglion. In the parasympathetic system, postganglionic neurons are also cholinergic. However in the sympathetic system, postganglionic are not all the same.

24
Q

In sympathetic postganglionic neurons, what happens to the noradrenaline neurotransmitter after exocytosis?

A

The noradrenaline is taken up by transporters (U1 and U2) and metabolised by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)

25
Q

Adrenoceptor GPCR subtypes at Sympathetic neuroeffector junctions?

A

Beta 1, beta 2 - Gs - Stimulation of Adenylyl Cyclase - Increase heart rate and force, bronchial and vascular smooth muscle relaxation
Alpha 1 - Gq - Stimulation of phospholipase C - Contraction of vascular smooth muscles
Alpha 2 - Gi - Inhibition of adenylyl cyclase - Inhibition of NA release

26
Q

Difference between post ganglionic parasympathetic and sympathetic neurones?

A

Post ganglionic parasympathetic nerve terminal consists of muscarinic receptors whereas sympathetic post ganglionic nerve terminals consists of Alpha and Beta receptors.

27
Q

What role do autoreceptors play in the negative feedback loop in the neurones?

A

A presynaptic neuron releases a neurotransmitter across a synaptic cleft to be detected by the receptors on a postsynaptic neuron. Autoreceptors on the presynaptic neuron will also detect this neurotransmitter and often function to control internal cell processes, typically inhibiting further release or synthesis of the neurotransmitter. Thus, release of neurotransmitter is regulated by negative feedback.

28
Q

Action of Cocaine in the autonomic nervous system?

A

Cocaine blocks U1 transporters, increasing the concentration of noradrenaline in the post sympathetic synaptic cleft causing increased adrenoceptor simulation. Peripheral actions cause vasoconstriction and (Alpha 1 stimulation) and cardiac arrhythmia (Beta 1 stimulation)

29
Q

Action of Amphetamine in the autonomic nervous system?

A

Is a substrate for U1 and enters the noradrenergic terminal where it inhibits MAO, enters the synaptic vesicle and displaces noradrenaline into the cytoplasm. Noradrenaline exits the terminal on U1 ‘running backwards’ and accumulates in the synaptic cleft causing increased adrenoceptor stimulation
Peripheral actions cause vasoconstriction and (Alpha 1 stimulation) and cardiac arrhythmia (Beta 1 stimulation)

30
Q

Action of Prazosin in the autonomic nervous system?

A

Selective, competitive, antagonist of alpha1. Does not block alpha2, beta1, or beta2. Vasodilator used as an anti-hypertensive agent

31
Q

Action of Atenolol in the autonomic nervous system?

A

Selective, competitive, antagonist of beta1. Does not block beta2, alpha1, or alpha2. Used as an anti-anginal and anti-hypertensive agent

32
Q

Action of Atropine in the autonomic nervous system?

A

Competitive antagonist of muscarinic ACh receptors, does not block nicotinic ACh receptors. Blocks all muscarinic ACh receptors with equal affinity (1, 2, 3) – exerts widespread effects by blockade of the parasympathetic division of the ANS
Used to reverse bradycardia following MI and in anticholinesterase poisioning

33
Q

What division of the ANS has thoracolumbar outflow from spinal cord?

A

Sympathetic division

34
Q

What division of the ANS has craniosacral outflow from spinal cord?

A

Parasympathetic division

35
Q

Primary neurotransmitters in postganglionic neurons of the parasympathetic division?

A

Acetylcholine (ACh); neurons are cholinergic

36
Q

Primary neurotransmitters in postganglionic neurons of the sympathetic division?

A

Adrenaline and noradrenaline; neurons are adrenergic

37
Q

Where do sympathetic preganglionic synapse with postganglionic neurons?

A

Paravertebral ganglia or prevertebral ganglia; both are close to spinal cord and hence have shorter axons

38
Q

One preganglionic neuron can innervate a single postganglionic neuron?

A

False, sympathetic preganglionic fibres branch extensively to synapse with many postganglionic neurones located in one, or several, pre- or para-vertebral ganglia.

39
Q

Parasympathetic preganglia have long axons

A

True, parasympathetic preganglionic neurones synapse with postganglionic neurones in terminal ganglia that are distant to the CNS and often located in the walls of the target organ. Their axons (fibres) are thus long.

40
Q

What are motor B fibres?

A

Typically, preganglionic fibres, both sympathetic and parasympathetic are myelinated and are termed motor B-fibres. They appear white and constitute the white matter of the spinal cord. By contrast, postganglionic fibres are largely unmyelinated and appear grey and are termed motor C-fibres.

41
Q

What is the difference in innervation of the adrenal gland in the sympathetic outflow?

A

The innervation of the adrenal gland (Chromaffin cells) is preganglionic and the transmitter is Acetylcholine (ACh) via the Splanchnic nerve Ex

42
Q

What is the level of spinal nerves in the sympathetic division?

A

T1 to L2/L3

43
Q

Example of postganglionic sympathetic fibres that use Acetylcholine as a neurotransmitter.

A

Postganglionic fibres (sudomotor neurones) innervating the thermoregulatory (eccrine) sweat glands, and a few blood vessels are cholinergic. However, the postganglionic fibres innervating the stress (apocrine) sweat glands are adrenergic and activate adrenoceptors

44
Q

What nerves are involved in the Parasympathetic division?

A

Cranial nerves -

Oculomotor (3), Facial (7), Glossopharyngeal (9), Vagus (10) and Spinal nerves S2 - S4

45
Q

What other neurotransmitters are used at post ganglionic junctions in the sympathetic division?

A

Adenosine Triphosphate (ATP) and Neuropeptide Y (NPY), rapid and slow response respectively

46
Q

What other neurotransmitters are used at post ganglionic junctions in the parasympathetic division?

A

Nitric Oxide (NO) and Vasoactive Intestinal Peptide (VIP), intermediate and slow response respectively

47
Q

What are nicotinic Acetylcholine receptors?

A

Nicotinic Acetlycholine receptors are ligand gated ion channels that mediate a fast synpatic transmission

48
Q

What are muscarinic Acetylcholine receptors?

A

Muscarinic Acetylcholine receptors are G-protein coupled receptors (GPCR) that mediate slow response

49
Q

What is the rank order of potency of agonists in adrenoceptors?

A

α-adrenoceptor:
noradrenaline > adrenaline > isoprenaline
β-adrenoceptor:
isoprenaline > adrenaline > noradrenalin

Principles (14 decks)

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