6. Excitable Cells: Neural Communication Flashcards
(300 cards)
1
Q
Define the membrane potential.
A
The electrical potential difference that exists between the inside of a cell and its surrounfdings.
2
Q
How can membrane potentials be studied?
A
Using glass microelectrodes.
3
Q
Describe which cations and anions have higher concentration intracellularly and extracellularly.
A
Higher intracellularly:
- Potassium
- Phosphate
- Protein
- Magnesium
- Hydrogen
Higher extracellularly:
- Sodium
- Chloride (NOTE: Varies a lot intracellularly)
- Calcium
- Hydrogencarbonate
4
Q
What are the two types of gradient involved in membrane potentials?
A
- Chemical gradients
- Electrical gradients
5
Q
What is the equilibrium potential?
A
For each ion, it is the potential at which the chemical gradient balances the electrical gradient so that there is no movement of ions through the respective channel.
6
Q
What is another name for the equilibrium potential?
A
The reversal potential.
7
Q
In a neurone, what is the equilibrium potential for potassium, sodium and chloride ions?
A
- Potassium = -90mV
- Sodium = +58mV
- Chloride = -60mV
8
Q
What is the symbol for the membrane potential?
A
Em
9
Q
What is the symbol for the equilibrium potential of potassium and sodium?
A
- Potassium = EK
- Sodium = ENa
10
Q
Describe briefly how the membrane potential relates to the equilibrium potential of the different ions.
A
- The membrane potential is a balance between the different equilibrium potentials
- The membrane potential will be closer to the equilibrium potential of the ions with higher permeability
11
Q
Describe why the resting membrane potential is closer to the potassium equilibrium potential than the sodium equilibrium potential.
A
The permeability of the membrane to potassium is much higher than to sodium.
12
Q
What happens to the membrane potential when the permeability to a given ion is increased?
A
The membrane potential tends towards the equilibrium potential of that ion.
13
Q
What equation can be used to calculate the membrane potential at any time?
A
Constant Field Equation (a.k.a. Goldman, Hodgkins & Katz equation)
NOTE: This is not core material.
14
Q
State the Constant Field Equation (GHK equation).
A
15
Q
What equation can be used to calculate the equilibrium potential for an ion?
A
Nernst equation
16
Q
State the Nernst equation.
A
17
Q
In the Nernst equation, when are the concentrations taken?
A
At equilibrium
18
Q
When drawing models of cell membranes, what is it important to remember about the ions?
A
The ions of each side should usually ensure osmotic equilibrium.
19
Q
What is the effect of increasing extracellular potassium concentration (K+o) on the membrane potential? Describe an experiment that showed this.
A
- As the extracellular potassium concentration increases, the membrane potential increases
- In Hodgkin and Horowicz’s 1959 experiment, this is a linear increase (according to the Nerst equation), but below 10mM potassium concentration, the line slopes off to the right -> This is due to some slight permeability of the membrane to sodium ions
20
Q
State how the Nernst equation simplifies at RTP.
A
21
Q
How can the effect of changing the extracellular concentration of an ion on the membrane potential be predicted?
A
By looking at the Constant Field equation.
22
Q
What is the Gibbs-Donnan effect?
A
The movement olf charged particles near a membrane due to charged particles that are impermeable to the membrane, but can still exert an electric force.
23
Q
Describe the double-Donnan distribution of cell membranes at rest.
A
- Intracellularly, there are negatively-charged proteins.
- Extracellularly, there is an excess of sodium ions.
- Both of these contribute to electric forces, but also ensure an osmotic equilibrium since the total number of ions intra and extracellularly should be roughly equal.
https://derangedphysiology.com/main/cicm-primary-exam/required-reading/cellular-physiology/Chapter%20121/gibbs-donnan-effect
24
Q
Describe the pump-leak model of ion homeostasis.
A
- An Na+/K+-ATPase is used to pump 3 sodium ions out of the cell for every 2 potassium ions being pumped in
- This counteracts the leakage of sodium and potassium through leak channels and help to maintain constant concentrations at rest
25
In terms of homeostasis, what is the importance of the Na+/K+-ATPase moving sodium out of the cell?
Maintaining the extracellular sodium concentration is important because it helps to maintain osmotic equilibrium.
26
Describe how the action of the Na+/K+-ATPase is adapted to different conditions.
The graph for the rate of sodium efflux against the intracellular sodiu concentration is a sigmoidal curve, showing how the pump is smart and responds to the concentration of sodium.
27
What word describes the Na+/K+-ATPase?
Electrogenic
28
What is Ouabain and what is its effect on resting membrane potential?
Ouabain is an inhibitor of Na+/K+-ATPases, so it causes slow depolarisation of the membrane.
29
How does the Na+/K+-ATPase respond to an injection of sodium into a cell?
* The injection causes hyperpolarisation (since ENa is reduced)
* The Na+/K+-ATPase increases this hyperpolarisation by increasing its action so that the membrane potential is hyperpolarisaed further
* If ouabain is injected prior to the sodium, the downwards dip is much smaller, showing that the Na+/K+-ATPase increases this hyperpolarisation
30
What is the effect of these on cell membrane potential:
* Increase in extracellular potassium
* Increase in intracellular sodium
* Increase in extracellular potassium -\> More positive Em
* Increase in intracellular sodium -\> More negative Em
(In some ways this is not intuitive, but think about it in terms of the equilibrium potential of each ion changing according to the Nernst equation)
31
Describe how the resting membrane potential arises in cell membranes.
* The outwards potassium current (through potassium leak channels) combines with the outwards current from Na+/K+-ATPase to balance the inward sodium leak currents.
* The Na+/K+-ATPase works by moving three sodium ions out of the cell for every two potassium ions moved into the cell (using energy from the hydrolysis of ATP), so it is electrogenic, but it also helps maintain the sodium and potassium gradients that are being run down by action potentials.
* There may also be some contribution from inwards chloride fluxes (contributing to the potassium and ATPase), but this is a small effect.
32
What is the best way to visualise the effect of changing intracellular or extracellular ion concentrations?
I think of it this way:
* If the concentration on one side is increased, then there will be more flow to the other side, causing the potential to change in the way that is intuitive from there
33
State Ohm's Law in terms of conductances.
I = Vg
(where g = conductance)
This is because g = 1/R.
34
Describe how the size of the current of a single ion across a membrane can be determined.
For potassium (for example):
IK = (Em - EK) x gk
35
Draw an electrical analogue of a cell membrane.
36
State the conductance equation.
| (Note: Not core material)
37
State the equation for the charge stored on a membrane and how this is used.
Q = VC
Where:
* Capacitance of biological membranes = 1μ/cm
This means that the charge stored on a membrane is very very small per unit surface area.
38
Draw the shape of an action potential.
39
What peak does an action potential typically reach?
30-40mV
40
What is a typical resting membrane potential?
About -70mV
41
What is a typical membrane potential during the hyperpolarisation of an action potential?
-90mV
42
What is a typical threshold for an action potential?
-55mV
43
Describe the events underlying an action potential.
* Rapid voltage-dependent activation of voltage-gated Na+-channels
* Influx of sodium ions leads to membrane depolarisation
* Rapid depolarisation due to positive feedback opening more voltage-gated Na+-channels
* Inactivation of voltage-gated Na+-channels
* Depolarisation also triggered voltage-gated K+-channels (delayed rectifier), which open more slowly
* Efflux of potassium leads to membrane repolarisation
* Hyperpolarisation occurs due to the delayed closing of potassium channels
* Repolarisation causes the sodium channels to go from inactivated to closed state
44
Draw the conduction curves for sodium and potassium during an action potential.
These occur due to the opening of voltage-gated sodium and potassium channels.
45
What type of potassium channels are involved in an activation potential?
Delayed rectifier
46
State some evidence for the ionic mechanism of action potentials.
* Sodium-free solution -\> No AP possible (graded reduction in sodium produces reduction in overshoot)
* Radiotracers -\> Show entry of sodium and potassium
* Voltage-clamp and patch-clamp techniques
* Genes and proteins identified for sodium and potassium channels (+ X-ray crystallography shows structure and function)
47
What is the differences between a voltage-clamp and patch-clamp?
A voltage-clamp is concerned with an entire membrane, while a patch-clamp aims to study a single channel.
48
Describe how a voltage-clamp can be used to study ionic currents in an action potential.
* The membrane potential is measured
* A required membrane voltage is input
* The two are compared and current is injected proportional to the difference, so that the membrane is maintained at a given potential
* The current injected is equal and opposite to the currents through channels in the membrane
* The currents through a single type of channel can be studied by using channel blockers of the other channels
49
What are the blockers of voltage-gated sodium and potassium channels?
* Tetradotoxin (TTX) -\> Sodium channel blocker
* Tetraethylammonium (TEA) -\> Potassium channel blocker
50
Draw the voltage-current graphs for sodium and potassium channels (involved in an action potential).
The x-intercept of sodium shows the reversing potential.
51
Why is an action potential all or nothing?
The initial depolarisation must open enough voltage-gated sodium channels to trigger the rapid positive feedback explosion that leads to full depolarisation.
52
Where is positive and negative feedback seen in an action potential?
* Positive feedback -\> Rapid opening of sodium channels
* Negative feedback -\> The opening of potassium channels leading to the repolarisation that causes them to close
53
Describe why the conductance of the membrane to sodium decreases after some time in the action potential.
* The voltage-gated sodium channels inactivate after some time
* This means that they do not allow sodium to pass through them
* The channels can only recover from inactivation when the membrane repolarises
54
What are the three states of voltage-gated sodium channels in an action potential?
Open, closed, inactivated
55
When can recovery of sodium channels from inactivation occur?
When:
* The membrane repolarises (AND)
* Sufficient time has passed
56
What are the two phases of the refractory period?
* Absolute refractory period
* Relative refractory period
57
What is the absolute refractory period and what causes it?
The first part of the refractory period when not enough sodium channels have recovered from inactivation to allow for another action potential to be triggered. No AP is possible at all.
58
What is the relative refractory period and what causes it?
The second part of the refractory period where sufficient sodium channels have recovered for an action potential to be triggered, but the threshold for the stimulus is higher than usual.
59
Describe the patch-clamp technique used in action potential experiments.
* Glass pipette is pressed against membrane around a single channel and suction is applied
* Electrically tight seal is formed
* When pulled away, the patch of membrane along with the channel is excised and held in the pipette tip
60
Describe a patch-clamp experiment to show the activity of sodium and potassium channels in an action potential.
When the membrane is suddenly depolarised:
* Sodium channels exhibit a single, short inwards current (before becoming inactivated), which is almost immediate
* Potassium channels open randomly and stay open, so the outwards current starts at any point and is prolonged until repolarisation occurs
61
Describe the features of a voltage-gated sodium channel.
Transmembrane protein with 4 transmembrane domains:
* Pore through centre with a narrow region that acts as a selectivity filter
* Charged, moving, voltage-sensing region
* Activation gate coupled to the voltage-sensing region with inactivation particle
62
Remember to read up on potassium channel structure.
Do it.
63
Draw a graph to demonstrate the absolute and relative refractory period on an action potential.
64
Explain this graph.
* Point A is the normal resting potenetial where there is no net flow of current.
* The arrows towards A show how the membrane will equilibrium to that potential below the threshold.
* Point B is the threshold, above which the membrane potential does not return to A, but instead there is a rapid net inward current that depolarises the membrane.
65
Draw a diagram of a neuron.
66
Describe how an action potential is propagated in an unmyelinated axon.
* Sodium influx in an active patch of membrane generates a strong local depolarisation
* Current flows from the active region along the axon, depolarising adjacent regions of the axon
* This depolarisation leads to the triggering of an AP in these parts of the membrane, which regenerates the strength of the signal
67
In an unmyelinated axon, why is there a need for the constant regeneration of the action potential?
There is some outward leak from the axon, so the depolarisation is weakened and must be constantly regenerated by the opening of sodium channels (i.e. triggering new APs).
68
Describe the local currents that exist during impulse propagation in an unmyelinated axon.
The outwards leak currents complete local currents that include flow in both forward (orthodromic) and backward (antidromic) directions.
69
What is the clinical relevance of local currents in action potential propagation?
Local currents lead to changes in surface potential, which can be detected by ECGs. Extracellular electrodes can also be used in experiments to monitor the propagation of signals.
70
What is some experimental evidence for the local currents that exist during action potential propagation?
* A nerve axon can be fixed with recording electrodes at different points along it
* A block is placed on the axon between the first two recording electrodes
* The size of the depolarisation is massively reduced between the first two recording electrodes, and degrades more at the subsequent ones
71
What is the name for the flow of charge along an axon without propagation?
Electronic conduction
72
What is the name for the way in which an axon can be modelled electrically?
Cable theory
73
Describe cable theory of axons, with the help of a diagram.
The membrane can be split into a series of sections, each with:
* Rm = Membrane resistance
* Rl = Longitudinal resistance -\> Due to resistance from the cytosol
* Cm = Membrane capacitance
74
What are the two main passive properties of a membrane that affect conduction velocity?
* Time constant (τ)
* Length constant (λ) - The distance a potential can travel along a neurone before it degrades to 37% of the original amplitude
75
For the length constant, state:
* Its symbol
* The definition
* What it is mathematically equal to
* λ
* The distance a potential can travel along a neurone before it degrades to 37% of the original amplitude.
* λ = √(Rm/Rl)
76
For the time constant, state:
* Its symbol
* The definition
* What it is mathematically equal to
* τ
* A measure of how quickly the membrane potential is able to rise or fall (i.e. the time for the membrane potential to fall to 37% of its original)
* τ = Rm x Cm
77
How can the length and time constant be represented on a graph?
* The graphs are voltage-distance and voltage-time respectively.
* Both show an exponential decrease, with the constant being the distance or time for the voltage to fall to 37%.
78
Name the main factors that affect conduction velocity in nerves.
* Myelination
* Axon diameter
* Temperature
* Other factors (e.g. age)
79
Describe the structure of a myelinated axon and how conduction along it works.
* The membrane of a Schwann cell wraps several times around an axon, creating a myelin sheath
* The gaps between the Schwann cells are nodes of Ranvier, in which voltage-gated channels are concentrated
* Between nodes of Ranvier, the conduction is purely electrotonic
* The nodes act as regenerating stations, so that the signal is reinforced
80
Describe how myelination affects conduction velocity.
* Myelination increases conduction velocity
* This is because it increases the resistance of the membrane (Rm), so that the length constant is increased -\> λ = √(Rm/Rl)
* Increasing the length constant increases the conduction velocity because more distance areas ahead of the impulse can be depolarised to threshold
* Although the Cm is decreased, the time constant is unaffected because the increase in Rm is roughly proportional to the decrease in Cm -\> Otherwise, a decrease in time constant causes an increase in conduction velocity
81
Describe how axon diameter affects conduction velocity.
* The larger the axon diameter, the faster the conduction
* This is because the longitudinal resistance (Rl) is decreased, so that the length constant is increased -\> λ = √(Rm/Rl)
* Increasing the length constant increases the conduction velocity because more distance areas ahead of the impulse can be depolarised to threshold
82
Derive the equation showing the effect of axon diameter on the length constant.
When x is the diameter of the axon:
* The longitudinal resistance is proportional to the area, so: Rl ∝ 1/x2
* The membrane resistance is proportional to the circumference, so: Rm ∝ 1/x
* These can be inserted into λ = √(Rm/Rl), which can be manipulated to give: λ ∝ √x
83
Myelination or axon diameter - Which has a greater effect on conduction velocity in axons and why?
Myelination has a larger effect because it is roughly directly proportional to the conduction velocity, whereas the square root of the axon diameter is roughly directly proportional to the conduction velocity.
84
Describe how the time constant affects the conduction velocity in an axon.
* The smaller the time constant, the faster the conduction velocity
* This is because depolarisation of the membrane ahead of the active patch can occur more rapidly, so the signal is transmitted more quickly
85
State some experimental evidence for the structure of myelinated axons.
Fluorescently-labelled antibodies for sodium channels and potassium channels can be prepared and then observed under a microscope, showing the location of the channels.
86
Describe the importance of having densely-packed channels in the nodes of Ranvier.
The large number of channels increases the magnitude of the inwards sodium current, thus making the local depolarisation more effective.
87
What is the name for the process of conduction in myelinated axons?
Saltatory conduction
88
Describe the effect of temperature on conduction velocity in axons.
* Positive (often linear) correlation observed between temperature and conduction speed.
* Because in cold conditions, there is a slowed opening of voltage-gated channels, so that the action potential is more slowly propagated.
* This is of particular importance in the population of elderly people and those with impaired blood circulation, who may struggle with responding to a cold external environment.
89
Give some examples of other factors that affect conduction velocity in axons.
* BMI
* Age
* Height
* Sex
However, these are only weakly supported by experimental evidence. There are often confounding factors -\> e.g. Sex is often correlated with height, so that sex differences might not be explained by the sex.
90
Draw a graph of conduction velocity against axon diameter for myelinated and unmyelinated axons.
91
Are myelinated neurons always faster than unmyelinated neurons?
No, below about 0.8 micrometer axon diameter, unmyelinated axons are faster (see graph).
92
Remember to clarify evidence for saltatory conduction.
Do it.
93
What are the different nerve fibre types in mammals? What are their diameters, conduction velocities and functions?
From fastest to slowest conduction velocity:
* A
* α -\> 12-20μm, 70-120m/s -\> Proprioception, Somatic motor
* β -\> 5-12μm, 30-70m/s -\> Touch, Pressure
* γ -\> 3-6μm, 15-30m/s -\> Motor to muscle spindles
* δ -\> 2-5μm, 12-30m/s -\> Pain, Cold, Touch
* B -\> Less than 3μm, 3-15m/s -\> Preganglionic autonomic
* C (unmyelinated)
* Dorsal root -\> 0.4-1.2μm, 0.5-2m/s -\> Pain, Temperature, Mechanoreceptor
* Sympathetic -\> 0.3-1.3μm, 0.7-2.3m/s -\> Postganglionic sympathetic
94
For Aα fibres, what is the diameter, conduction velocity and function?
Aα:
* 12-20μm
* 70-120m/s
* Proprioception (position and movement of the body), Somatic motor
95
For Aβ fibres, what is the diameter, conduction velocity and function?
Aβ:
* 5-12μm
* 30-70m/s
* Touch, Pressure
96
For Aγ fibres, what is the diameter, conduction velocity and function?
Aγ:
* 3-6μm
* 15-30m/s
* Motor to muscle spindles (force generation)
97
For Aδ fibres, what is the diameter, conduction velocity and function?
Aδ:
* 2-5μm
* 12-30m/s
* Pain, Cold, Touch
98
For B fibres, what is the diameter, conduction velocity and function?
B:
* Less than 3μm
* 3-15m/s
* Preganglionic autonomic
99
For C sympathetic fibres, what is the diameter, conduction velocity and function?
C sympathetic:
* 0.3-1.3μm
* 0.7-2.3m/s
* Postganglionic sympathetic
100
For C dorsal root fibres, what is the diameter, conduction velocity and function?
C dorsal root:
* 0.4-1.2μm
* 0.5-2m/s
* Pain, Temperature, Mechanoreceptor
101
What is a compound action potential (CAP) and where does it occur?
* A compound action potential is the series of action potentials in the different nerve fibres (A-alpha, A-beta, etc.) that are caused by a single stimulation of the nerve
* Due to the different conduction velocities, the CAP arrives as a series of peaks
* You need to appreciate that they ocur in peripheral nerves.
102
What is the effect of TTX on the action potential?
* TTX is a sodium-channel blocker
* So the initial depolarisation phase is inhibited and action potentials cannot fire
TTX is found in pufferfish and poisoning is extremely dangerous.
103
What is the effect of TEA on the action potential?
* TEA is a potassium-channel blocker
* So although the initial depolarisation is the same, repolarisation takes much longer (it is only due to sodium channel inactivation) and there is no hyperpolarisation overshoot
* The resting membrane potential is not affected because the potassium channels involved in that are different
The prolonged action potential can, for example, cause release of excess neurotransmitter into a synapse, so it can be used to to reverse the effects of non-competitive blockers, such as curare.
104
What are some diseases related to the myelination of neurons?
* Guillaine-Barré syndrome -\> Autoimmune condition characterised by a fast destruction of neurones (axonal type) or their Schwann cells (demyelinating type) in the peripheral nervous system. Often underlied by infection. The demyelinating variant (the most common type) has early symptoms such as tingling and weakness of the limbs, which can develop into severe mobility problems, pain, difficulty breathing and paralysis. Rarely, the syndrome may result in death. These symptoms are caused by improper signal conduction down the neurones, since the signal cannot be contained within the partially unmyelinated neurone and its strength dissipates rapidly between nodes of Ranvier.
* Multiple Sclerosis -\> Demyelinating disease in which the CNS myelin sheaths are degraded.
* Diptheria (in about 10% of cases)
105
Can a nerve innervate multiple muscle fibres?
Yes, but each muscle fibre can be only be innervated by one nerve.
106
What is an endplate?
The location of the muscle fibre where each of the axon expansions terminate (i.e. the location of the NMJ).
107
What is a synapse?
A specialised structure at which information transfer takes place without physical interactions between the two participating cells.
108
Describe how a synpase works.
The electrical signal (action potential) is converted to a chemical signal (neurotransmitter) that diffuses between the presynaptic and postsynaptic cells.
109
Describe an experiment that proved the chemical process of synaptic transmission.
* Otto Loewi placed two beating frog hearts, each in its own perfusion chamber – one preparation had the vagus nerve intact, while the other was denervated
* Next, he stimulated the vagus nerve supplying the first heart, causing it to beat more slowly
* When Loewi applied the perfusate of the first heart to the second heart, it too slowed down, as if its vagus nerve had been stimulated as well
* He named the inhibitory factor ‘vagusstoff’, which is known today as acetylcholine
110
Draw a diagram of a NMJ.
111
What is the neurotransmitter used at NMJ?
Acetylcholine (ACh)
112
Describe the structure of acetylcholine.
It is an ester of acetic acid and choline.
113
Describe the process of storage of acetylcholine at the NMJ.
* ACh is stored in vesicles in the presynaptic neuron
* Proton pump (V-ATPase) is used to move hydrogen ions in the vesicle, acidifying it
* The proton gradient created by this is exploited by antiporter called the vesicular acetylcholine transporter (VAChT), which moves ACh in using the outwards H+ gradient
114
Name the proteins involved in the packing of ACh in vesicles.
* Proton pump (V-ATPase) -\> Pumps H+ into vesicles
* Vesicular acetylcholine transporter (VAChT) -\> Uses the H+ gradient to move ACh into cells
115
Describe the process of release of ACh at the NMJ.
* Action potential causes depolarisation of presynaptic membrane
* This triggers the opening of voltage-gated calcium channels and causes an influx of calcium into the presynaptic neuron
* The Ca2+ ions bind to calcium-sensitive proteins on the vesicular surface, namely **synaptotagmin**.
* This leads to docking of the calcium to the membrane, by the joining of SNARE proteins.
* v-SNARE proteins on the vesicular surface attach to t-SNARE proteins on the presynaptic membrane, and a tight complex (a SNARE-pin) is formed. This is seen when **synaptobrevin** on a vesicle interacts with **syntaxin** and **SNAP-25** on the presynaptic membrane surface.
* The complex enables calcium-dependent fusion of the vesicle with the membrane and exocytosis of the ACh can occur.
It is worth noting that the “kiss and run” mechanism of ACh release occurs occasionally, where the vesicle only partially fuses with the membrane before sealing shut again, but this is rare and does not allow for complete release of ACh into the synaptic cleft.
116
Describe the proteins involved in the fusion of a vesicle with the presynaptic membrane at the NMJ.
* Synaptotagmin -\> Vesicular protein that senses cytoplasmic Ca2+
SNARE proteins:
* v-SNARE -\> Synaptobrevin -\> Vesicular protein that binds to t-SNARE proteins
* t-SNARE -\> SNAP-25 and syntaxin -\> Protein on presynaptic membrane that binds to v-SNARE proteins
117
How can you remember the different v-SNARE and t-SNARE proteins?
* _v_-SNARE proteins are on the **_V_**esicles
* _t_-SNARE proteins are the **_T_**arget
* Synaptobre**_v_**in is a _v_-SNARE protein
* Syntaxin and SNAP-25 are therefore the t-SNARES
118
What drugs can influence the storage of ACh?
* Proton pump inhibitors
* VAChT inhibitors (e.g. vesamicol)
119
What drugs can influence the release of ACh at the NMJ?
* Botulinum toxin -\> Proteolytic so it can cleave the 3 main SNARE proteins at different sites
* Verapamil or Magnesium -\> Block calcium channels (so release is indirectly prevented)
* Vesamicol -\> Inhibition of VAChT so that vesicles are not filled
* Black widow spider venom -\> Massive release and depletion of vesicles
120
What are some clinical uses of botulinum toxin?
* Blepharospasm (uncontrolled contraction of eyelid)
* Salivary drooling
* Axillary hyperhidrosis
* Achalasia (oesophageal spasm)
* Cosmetic reasons
121
How is a steep concentration gradient maintained across the NMJ?
* Quantal secretion of ACh
* Acetylcholinesterase is found in pits on the post-synaptic membrane, while the acetylcholine receptors are on the projections.
122
What receptors are present on the postsynaptic membrane of the NMJ?
Nicotinic AChR (nAChR)
123
Describe the diffusion and binding of ACh at the NMJ.
* Diffusion occurs due to Fick's Law (J = D x A x ΔC/Δx)
* ACh binds to nAChR, a non-selective cation pore is formed, allowing Na+ to diffuse into the postsynaptic membrane and K+ to diffuse into it, depolarising the membrane
* This depolarisation of the membrane is called the end-plate potential (EPP)
* An EPP can trigger an action potential in the muscle due to the depolarisation of the membrane, which voltage-gated sodium and potassium channels in the membrane are sensitive to
124
What is the potential at the postsynaptic membrane of the NMJ called?
End-plate potential
125
Is the end-plate potential propagated?
No, but it is usually strong enough to trigger an action potential, which is propagated and causes muscle contraction
126
What are miniature end-plate potentials (mEPPs)?
* Small depolarisation of the postsynaptic membrane which do not trigger an AP
* They are caused by the random exocytosis of a single vesicle containing ACh
127
Describe the breakdown of ACh at the NMJ.
* Acetylcholine in the synaptic cleft is broken down by acetylcholinesterase (AChE), which occurs after it dissociates from nicotinic receptors -\> This action is so fast that about half of the ACh is broken down before it even reaches the receptors
* Products of ACh hydrolysis are acetate and choline
128
Where can AChE be found at the NMJ?
* In the pits of the postsynaptic membrane
* It is secreted by the muscle and is attached to the basal lamina by collagen connections
129
What type of reaction is the breakdown of ACh?
Hydrolysis
130
What are the products of the breakdown of ACh?
Acetate and choline
131
What is the difference between acetate and acetic acid?
Acetate is the conjugate base of acetic acid (i.e. it doesn't have the H).
132
What are some chemicals that can influence the breakdown of AChE?
* Novichok
* Neostigmine (used to treat myasthenia gravis)
133
Describe the mecahnism and effects of Novichok.
* Inhibits AChE action, which causes ACh build up in the synaptic cleft
* So ACh permanently occupies the nicotinic receptors
* The result of this is that the pores formed by the nicotinic receptors remain open, allowing free flow of sodium and potassium ions, which gradually depolarises the membrane above its threshold. After a brief period of involuntary muscle contraction (due to the rapid firing of action potentials), the voltage-gated sodium channels in the muscle are permanently inactivated and are forced into a permanent absolute refractory period, so that further action potentials in the muscle cannot be triggered. This is because the channels cannot exit their inactivated state until the membrane returns to its resting potential.
* There are many antidotes to Novichok, such as pralidoxin (which activates acetylcholinesterase), but without intervention death occurs due to cardiac arrest or suffocation within minutes.
134
What type of drug is neostigmine?
It is a anti-cholinesterase.
135
What is myasthenia gravis and how can it be treated?
* Autoimmune neuro-muscular condition caused by auto-antibodies against nAChR at the NMJ
* Antibodies act in three ways:
* Steric block of ACh
* nAChR internalisation and degradation
* Encourage immune system attack on muscle cell membrane
* Symptoms: Tiredness, muscle weakness, eyelids drooping, double vision
* Treatments: NEOSTIGMINE (anti-cholinesterase), Surgical removal of thymus, Removal of antibodies from blood
136
Describe how neostigmine works.
* Binds to the active site of AChE
* So more ACh are able to reach the small number of nAChR
* This helps to trigger an action potential
137
Describe the process of recyling and synthesis of ACh.
* Choline is taken back into the presynaptic neurone via choline transporters, which is a sodium-dependent process
* Choline may also be obtained from the diet in the form of phosphotidylcholine (a phospholipid)
* Acetate is not recycled
* Instead glycolysis converts glucose to pyruvate (in the presynaptic neuron), which is then converted to acetyl-CoA (in mitochondria)
* Choline acetyl transferase (CAT) is the enzyme that transfers an acetyl group from acetyl-CoA to choline, forming ACh
138
What drugs can be used to target the synthesis of ACh?
The CAT enzyme is a drug target that is currently being studied, because stimulation of its activity (or prevention of its inhibition) would lead to an increase in ACh production, allowing for an increased release of ACh from vesicles after an action potential. This offers the potential for alternative treatments for myasthenia gravis and Alzheimer’s disease, amongst others.
139
Describe an experiment to show that the EPP is variable in size and is not propagated.
* Muscle fibre in a bath, with microelectrodes inserted at 1mm intervals along the fibre
* Stimulate using motor axon
* Record the shape of the end-plate potential at each microelectrode
140
Describe an experiment to show that ACh release is calcium-dependent.
Flash photolysis in the Calyx of Held (a particularly large synapse in the mammalian CNS):
* Fill presynaptic terminal with caged calcium (calcium ion trapped within a 'cage')
* Flash light to release Ca2+ and use intensity to vary [Ca2+]
* Record the excitatory postsynaptic current
141
Describe an experiment to show that the release of ACh is quantal.
* Record a series of EPPs
* Plot a bar chart of the frequency of each EPP amplitude value
* The peaks exist at 0.4mV intervals, showing that the release is quantal
142
Describe an experiment to show the action of nAChRs.
Patch-clamping can be used to show the currents through the non-selective pore that forms in the nAChR.
143
Describe the structure of an nAChR.
* 4 transmembrane regions
* M2 region is pore-forming
* Two alpha units when ACh binds -\> 2 ACh molecules bind to each receptor
144
Name all of the experiments involved in the NMJ.
* Otto Lowi's frog hearts
* EPP microelectrodes
* Flash photolysis of caged Ca2+
* Patch clamping
* Plotting EPP sizes
145
Name the main mechanisms of neuromuscular blocking drugs and give examples of each.
* Competitive non-depolarising -\> Tubocurarine, Vecuronium
* Depolarising -\> Suxomethonium
* Vesicular releasem-\> Botulinum toxin
146
Name the non-depolarising blockers at the NMJ and explain how they work. Also state the main clinical uses.
Examples: Tubocurarine, Vecuronium
* Antagonists that compete with ACh for AChR binding sites
* Therefore do not depolarise membrane but prevent transmission
Clinical uses: Muscle relaxation in anesthesia
147
Name the depolarising blockers at the NMJ and explain how they work. Also state the main clinical uses.
Examples: Suxamethonium -\> Structure is like 2 acetylcholine molecules joined by a methyl group
* Are similar to ACh so they can bind to nAChR and cause depolarisation of the postsynaptic membrane, just like ACh does
* However, they are much more resistant to breakdown by AChE, so they can keep binding to the AChR and causing depolarisation
* Phase 1 is characterised by muscle twitches, while phase 2 involves paralysis due to the sodium channels in the postsynaptic membrane being inactivated (they are unable to return to normal due to the membrane potential never returning to rest)
Clinical use: Anesthesia
148
Describe how each type of neuromuscular block can be reversed.
* Non-depolarising block -\> Reversed by anticholinesterases, such as NEOSTIGMINE, because they can increase the concentration of ACh so that is out-competes the blocker
* Depolarising block -\> Harder to reverse
149
Describe the structure of local anaesthetics.
Homologues of cocaine with:
* Hydrophobic group
* Ester or amide linkage
* Ionisable group (usually an amine)
150
Name some widely-used local anaesthetics. Give their potency and duration.
* Procaine -\> Potency: 1, Duration: Short
* Lidocaine -\> Potency: 4, Duration: Medium
* Tetracaine -\> Potency: 16, Duration: Long
* Bupivacaine -\> Potency: 16, Duration: Long
151
Describe the mechanism of action of local anaesthetics.
* Local anaesthetics block voltage-gated sodium channels
* So action potentials cannot be initiated
* So sensory and motor information cannot be conveyed
152
Why is the effect of lidocaine on sodium-channels use dependent?
* The degree of blocking increases when some action potentials are sent through the nerves
* This is because the lidocaine is more effective at binding to the sodium channels when they are open
(Check this)
153
Which nerve fibre types are involved in the sensing of noxious stimuli and how is this converted into an action potential?
* Aẟ and C fibres sense different modalities of noxious stimulus (e.g. pain, heat, etc.)
* This is converted into electrical activity by transient receptor potential channels (TRP channels) and purinergic channels
* This is amplified by sodium channels to elicit an action potential
154
Describe the effect of neuron diameter on sensitivity to local anaesthetic.
* Narrower neurons are more sensitive to block
* So myelinated Aẟ and unmyelinated C fibres carrying pain information are the most sensitive
155
Describe the equilibrium that exists within local anaesthetics and what the importance of this is.
BH+ -\> B + H+
* Only the B can diffuse through the lipid bilayer to get into the cytoplasm
* This is required for the LA to work
* Therefore the effectiveness of the LA is dependent on the pKa and pH
156
What are some factors that influence local anaesthetic activity and what effect do they have?
* pKa -\> The pKa is the pH at which the number of ionised and unionised fractions of the drug is in equilibrium. The lower the pKa, the more the unionised fraction is present for any given pH and hence the faster the onset of action.
* pH -\> The lower the pH, the lesser the potency because of the equilibrium (BH+ -\> B + H+) so there is less of the unionised form to cross the membrane and block sodium channels
* Lipid solubility -\> The more lipid soluble the LA, the higher the potency, the faster the onset of action and the longer the duration, since more of the drug can cross the lipid bilayer and form a depot of it in the cytoplasm
* Intermediate chain -\> The longer the intermediate chain, the more potent the LA
* Protein-binding -\> The higher the degree of protein-binding, the longer the duration of action
157
Describe some experimental evidence for the model of local anaesthetic action.
* QX-314 and QX-222 are permanently charged LAs
* They only work when induced straight into the cytoplasm, not outside the neuron
* This shows that the active form of LAs (the charged form) can only enter and bind on the cytoplasmic side of the sodium channels
* They also bind cumulatively with each depolarising pulse and then don't unbind at rest
158
What are the two main pathways for local anaesthetics to access sodium channels?
* Hydrophilic pathway
* Hydrophobic pathway
159
Draw and explain the hydrophilic pathway of local anaesthetics accessing sodium channels.
160
Draw and explain the hydrophobic pathway of local anaesthetics accessing sodium channels.
161
Describe the structure of a voltage-gated sodium channel.
[EXTRA]
* Made up of 3 subunits: α, β1 and β2.
* The alpha subunit consists of 4 transmembrane domains, each of which is made up of 6 membrane-crossing segments that are alpha helices
* The S5 and S6 segments from all 4 domains make up the pore of the sodium channel
* The S4 segments contain a sequence of positively charged residues that sense the voltage across the membrane and move in response to a change in membrane potential
* The inactivation gate is the loop between the III and IV domains
162
Do local anaesthetics bind permenently or reversibly?
Reversibly
163
Is the action of local anaesthetics confined to blocking sodium channels in nerves?
* No, they can also block other channels, such as nicotinic acetylcholine receptors, ryanodine receptors and K+ channels.
* No, they can also block channels in the heart.
164
Give another clinical use of local anaesthetics.
They can be used as anti-arrhythmic drugs (e.g. lidocaine).
165
Name a more potent sodium channel blocker that local anaesthetics and describe its action.
* TTX
* It is a more specific and potent blocker than LAs.
* It can bind sodium channels extracellularly.
166
Describe the differences between the metabolism of different local anaesthetics.
* Esters (e.g. procaine, tetracaine) -\> Metabolised in the blood by plasma cholinesterases or liver esterases. These have a short half-life.
* Amides (e.g. lidocaine, bupivacaine) -\> Amides are widely distributed via the circulation and are metabolised exclusively by enzymes in the liver. Half-life of these compounds is therefore longer.
167
What are some of the factors affecting local anaesthetic absorption?
* Dosage
* Injection site
* Vasoconstricting agents
* Drug-tissue binding
168
What is the effect of vasocontricting agents on local anaesthetic action?
* There is loss of LA from the site of local application into the circulation.
* Vasoconstricting agents can cause the local anaesthetic effect to be prolonged and circulating anaesthetic levels to be reduced.
169
Describe some of the toxic effects of local anaesthetics.
If the LA gets into the circulation:
* CNS toxicity (initial light-headedness, but may lead to convulsions and eventually respiratory depression and coma).
* Cardiovascular effects (myocardial depression with reduced heart rate and stroke volume, vasodilatation, reduced blood pressure).
170
What are some of the methods of administrating anaesthetics?
* Surface
* Direct infiltration into tissues
* Injection close to nerve trunk
* Regional anaesthesia
* Spinal anaesthesia
* Epidural anaesthesia
171
Draw a diagram of how regional anaesthesia may be administrated and state what this is called.
Bier's block
172
What is a more specific method of anaesthesia and how may it be achieved?
* Nociceptor-specific anaesthesia may sometimes be preferrable
* It could be achieved by targetting TRVP1 channels on small unmyelinated C-fibres that selectively express this channel
173
Describe the sequence of local anaesthetic blockade.
* Pain first
* Then general sensory
* Motor last
174
In spinal and epidural anaesthesia, where is the injection site? Draw a diagram of this.
* Spinal -\> In the L2-L5 region, Lumbar CSF which bathes the cauda equina
* Epidural -\> In the lumbar epidural space (but can also be in the thoracic and sacral areas)
175
In spinal anaesthesia, what tissues are transversed by the needle?
* Skin
* Fat
* Interspinous ligament
* Ligamentum flavum
* Epidural space
* Dura
176
Describe the patient positioning for spinal anaesthesia injection.
The patient should be sitting down, curled up so that the area between the vertebrae is opened up on the posterior side.
177
How is spinal anaesthesia used to reach different dermatomes?
Gravity is used to reach different dermatomes. The curvature of the spine is useful for this.
178
What are the uses of spinal and epidural anaesthesia?
* Spinal -\> Pain relief and muscle paralysis during surgery
* Epidural -\> Analgesia in labour, Pain relief and muscle paralysis during surgery
179
Briefly summarise the differences between spinal and epidural anaesthesia.
180
What type of molecule is acetylcholine and how can you remember this?
It is an ester. You can remember this because it is broken down by acetylcholinESTERase.
181
Where in the body may acetylcholine be found?
* All autonomic ganglia
* All NMJs
* At many autonomically innervated organs
* At many sites in the CNS
182
Describe the divisions of the nervous system.
Note: The enteric nervous system may also be listed alongside the sympathetic and parasympathetic divisions of the autonomic nervous system.
183
Can the CNS function on its own?
No, it requires the sympathetic and parasympathetic systems to provide a link between the CNS and peripheral organs.
184
Describe the structure of the autonomic efferent pathway.
There are two neurons arranged in series. These are the pre-ganglionic and post-ganglionic and these are joined at an autonomic ganglion.
185
Where is the autonomic nervous system is acetylcholine released?
* By all pre-ganglionic and post-ganglionic parasympathetic neurons
* By all pre-ganglionic sympathetic neurons (and in the post-ganglionic too with sweat glands ONLY)
186
Draw a diagram illustrating the different sites of ACh release in the peripheral nervous system.
187
Describe the difference between the two types of ACh receptor.
* Nicotinic -\> Ligand-gated ion channel permeable to monovalent and divalent cations, but not anions
* Muscarinic -\> G-protein coupled receptors
188
Are acetylcholine receptors only found on the postsynaptic membrane?
No, there can also be autoreceptors on the presynaptic neuron, which serve in negative feedback in signal transduction.
189
Describe the different locations of acetylcholine receptors in the body, acounting for nAChR and mAChR.
* **CNS** -\> **nAChR** and **mAChR**
* **Autonomic** -\> **nAChR** and **mAChR** -\> After all pre-ganglionic and post-ganglionic parasympathetic neurons, and after all pre-ganglionic sympathetic neurons (and post-ganglionic too in sweat glands ONLY)
* **Somatic (NMJ)** -\> **nAChR**
Note that mAChRs are not always quoted as being in autonomic ganglia because transmission is mostly due to nAChRs (see flashcard about this).
190
Where does cholinergic transmission stimulate mAChRs and mAChRs exactly?
mAChRs:
* Autonomic ganglia
* Organs innervated by parasympathetic nervous system
* CNS
nAChRs:
* Autonomic ganglia (+ adrenal medulla)
* NMJ
* CNS
191
What is the main neurotransmitter released at organs innervated by the sympathetic nervous system and what is the main exception to this?
Norepinephrine (a.k.a noradrenaline), except at sweat glands, where ACh is released instead.
The way to remember this is Akerman going "**N**AAAAAW" to symbolise being sympathetic.
192
What acetylcholine receptors are found at autonomic ganglia?
Off Wikipedia, so double-check this:
* ACh is always used as the transmitter within the autonomic ganglion. Nicotinic receptors on the postganglionic neuron are responsible for the initial fast depolarization of that neuron. As a consequence of this, nicotinic receptors are often cited as the receptor on the postganglionic neurons at the ganglion.
* However, the subsequent hyperpolarization and slow depolarization that represent the recovery of the postganglionic neuron from stimulation are actually mediated by muscarinic receptors, types M2 and M1 respectively.
193
Describe the structure of mAChRs.
Single polypeptide containing 7 transmembrane regions that are alpha helices.
194
Are mAChRs ever found presynaptically? What is the purpose of this?
In the CNS, they are found both presynaptically and postsynaptically. The ones of the presynaptic membrane are used to modulate the release of ACh.
195
Describe the different types of muscarinic receptors and their functions.
* M1 -\> Excitatory -\> **CNS** responses such as memory, arousal, attention and anaesthesia + **exocrine** glands
* M2 -\> Inhibitory -\> Lowers **heart** rate
* M3 -\> Excitatory -\> **Smooth muscle**
* M4 -\> Inhibitory -\> **CNS**
* M5 -\> Excitatory -\> **CNS**
196
Which mAChRs are excitatory and what type of GPCR are they? How do these work?
* M1, M3 and M5
* They couple to Gq proteins
* Which activate the inositol phosphate pathway
197
Which mAChRs are inhibitory and what type of GPCR are they? How do these work?
* M2 and M4
* They couple to Gi/Go proteins
* Which inhibit adenylate cyclase and so reduce intracellular cAMP
198
Describe the different types of nictonic ACh receptors and their locations and functions.
* N1 (or Nm) -\> At NMJ
* N2 (or Nn) -\> Autonomic ganglia, CNS and adrenal medulla
199
Compare the structures of N1 and N2 nAChRs.
* N1 (at NMJ) -\> 2 alpha, 1 beta, 1 gamma and 1 delta subunit -\> Each subunit has 4 transmembrane regions
* N2 (neuronal and ganglionic) -\> Only made of 2 subtypes: alpha and beta
200
Describe the agonists and antagonists of nAChRs and mAChRs.
nAChRs:
* Agonists -\> ACh and nicotine
* Antagonists -\> Tubocurarine
mAChRs:
* Agonists -\> ACh and muscarine
* Antagonists -\> Atropine
201
What is the name for the depolarisation of the postsynaptic cell due to opening of channels in AChRs?
* At the NMJ -\> EPP (End-plate potential)
* In nerves -\> EPSP (Excitatory postsynaptic potential)
202
Compare the onset and duration of nAChR and mAChR action.
* nAChR -\> Rapid and short-lasting
* mAChR -\> Slow onset and long-lasting
203
What is the name for the change in membrane potential of the postsynaptic cell due to the action of mAChRs?
* ESPS (Excitatory postsynaptic potential) -\> When the membrane is slowly depolarised
* IPSP (Inhibitory postsynaptic potential) -\> When the membrane is slowly hyperpolarised
204
Summarise simply everything about nAChRs and mAChRs.
* The nervous system is split into the CNS and PNS
* The CNS uses both nAChR and mAChR which bind ACh, but it also makes use of many other neurotransmitters -\> The excitatory receptors are N2 and M1 and M5, while the inhibitory ones are M4.
* The PNS is split into the somatic and autonomic nervous system
* The somatic nervous system only features one efferent neuron from the CNS to the effector (i.e. no ganglia), so the only neurotransmitter released is at the NMJ where ACh binds to nAChR
* The autonomic nervous system is split into the sympathetic and parasympathetic (and also the enteric, but this is sort of like the parasympathetic)
* The autonomic nervous system features two efferent neurons from the CNS to the effector (joined by an autonomic ganglion)
* At the ganglion, there are nAChRs (N2) which are predominantly used to allow transmission here, although there are also mAChR which play a role in repolarisation (check this)
* The sympathetic autonomic nervous system uses mostly norepinephrine (noradrenaline) at the effector organ (this is adrenergic transmission), and the only major exception to this is sweat glands, at which mAChR receptors are used (M2)
* The autonomic nervous system uses mAChRs at the effector organs, with M2 receptors being inhibitory for the heart muscle, while M1 receptors are excitatory for exocrine glands and M3 are excitatory for smooth muscle
* mAChRs that are excitatory are Gq-coupled, which work by activating the inositol phosphate pathway
* mAChRs that are inhibitory are Gi/Go-coupled, which work by inhibiting adenylate cyclase and so reduce intracellular cAMP
205
What are the physiological effects of ACh at low and high concentrations? Describe an experiment to show this.
206
What is Endothelial Derived Relaxing Factor (EDRF)?
Nitric oxide (NO) - it causes smooth muscle relaxation.
207
Describe the importance of snake toxins in physiology and medicine.
Bind very tightly and selectively to nAChRs at the NMJ, so:
* Useful for identification and purification of nAChR protein
* Useful for studies of NMJ transmission and ion channel function
* Potential treatments for MS, infections caused by herpes viruses, and retroviruses
208
What are the different types of drugs that affect cholinergic transmission?
* Muscarinic agonists/antagonists
* Ganglion-stimulated drugs
* Ganglion-inhibiting drugs
* Neuromuscular-blocking drugs
* Acetylcholinesterases and other drugs that enhance cholinergic transmission
209
What is the effect of muscarinic agonists and what is another name for these drugs?
* They closely mimic the effects of parasympathetic nerve stimulation (decreased heart rate, increased smooth muscle contraction, increased exocrine secretions)
* They may be called **parasympathomimetics** because of this
210
Give some clinical uses of parasympathomimetics (muscarinic agonists).
* Glaucoma -\> Pilocrapine constricts the pupil and improves drainage, reducing the pressure that is characteristic of glaucoma
* Suppression of atrial tachycardia (rarely)
211
What is another name for muscarinic antagonists?
Antimuscarinic drugs
212
What are some effects of muscarinic antagonists?
* Inhbition of exocrine secretions
* Tachycardia (very fast heart rate)
* Pupillary dilation
* Relaxation of smooth muscle
213
Give some clinical uses of antimuscarinic drugs.
* Cardiovascular -\> Treatment of bradycardia (slow heart rate)
* Ophtalmic -\> Pupil dilation
* Respiratory -\> Asthma (due to smooth muscle relaxation)
* Gastrointestinal -\> Decrease secretions
* Smooth muscle -\> Urinary incontinence
214
What types of drug are ganglion stimulants?
nAChR agonists
215
Do all nAChR agonists act on both neuronal and NMJ nAChRs?
No, because there are different types of nAChR at each (N1 and N2). For example:
* Nicotine -\> Autonomic ganglia + CNS
* Lobeline -\> Autonomic ganglia
* Suxamethonium -\> NMJ
216
What is the only depolarising blocking drug currently in use?
Suxamethonium
217
What is the location and duration of action of suxamethonium?
* Only lasts a few minutes (before it is metabolised by plasma esterase)
* Acts at NMJ because it is not broken down by AChE at the NMJ
218
What are the non-depolarising blockers of nAChR at the NMJ and at ganglia?
* NMJ -\> Tubocurarine + Vecuronium
* Ganglia -\> Mecamylamine
219
Both depolarising and non-depolarising blockers may be used in surgery. Compare how easily each is reversed.
* Non-depolarising blockers (e.g. tubocurarine and vecuronium) are easily reversed by neostigmine (AChE inhibitor) since they are competitive blockers and the increase in ACh can reduce their effect
* Depolarising blockers (e.g. suxamethonium) are not easily reversed
220
What is the most common cholinesterase inhibitor?
Neostigmine
221
What is an important condition for when cholinesterase inhibitors will work?
They only work when there is pre-existing ACh release.
222
Give some examples of clinical uses of anticholinesterases.
* Skeletal muscle -\> Reversing NMJ block + Treatment of myaesthenia gravis
* CNS -\> Treatment of Alzheimer's disease (questionable benefits)
* Eye -\> Treatment of glaucoma
223
Give a short summary of the drugs influencing cholinergic transmission.
224
Give some experimental evidence to demonstrate that the nAChRs in ganglia are different from those at the NMJ.
* Hexamethonium is a non-depolarising blocker that acts at **autonomic ganglia**
* Decamethonium is a non-depolarising blocker that acts at **NMJ**
Their structures are similar.
225
Draw a nice diagram showing the organisation of the different divisions of the nervous system.
Note that the sensory division is not divided into sympathetic or parasympathetic -\> Check
226
Draw a summary of the efferent pathways of the different nervous sytem divisions.
227
Compare briefly the somatic and autonomic nervous system transmission methods.
228
Aside from cholinergic and catecholaminergic transmission, what are some other types of signalling? Define each and give an example of each.
* Purinergic -\> Extracellular signalling mediated by purine nucleotides and nucleosides (e.g. adenosine and ATP)
* Gaseous -\> Signalling mediated by gases (e.g. nitric oxide)
* Neuropeptide -\> Signalling mediated by small protein-like molecules (peptides) used by neurons to communicate with each other (e.g. Neuropeptide Y and vasoactive intestinal peptide)
229
What is the concept of co-transmission?
* The control of a single target cell by two or more substances released from one neuron in response to the same neuronal event, does occur in experimental situations.
* It has not been shown to occur in the normal operation of an animal, but the likelihood that it does is great.
230
Remember to reivse muscle structure in OB deck 6.
Do it.
231
What intracellular change is responsible for muscle contraction?
Increase of cytosolic Ca2+.
232
Draw an EM image of a sarcomere, showing the different bands and lines.
233
Draw a diagram to show how sarcomeres change in appearance upon contraction.
234
What is excitation-contraction coupling?
The physiological process of converting an electrical stimulus to a mechanical response. It is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction.
235
Describe excitation-contraction coupling in skeletal muscle. [IMPORTANT]
* An action potential causes release of ACh at an NMJ
* This ACh binds to AChR on the muscle fibre, leading to depolarisation of the sarcolemma, which travels along the membrane and into the T-tubule
* Each T-tubule is flanked by 2 cisternae of the sarcoplasmic reticulum (called a triad)
* Sarcolemma depolarisation causes opening of L-type Ca2+ channel (LTCC) in the sarcolemma
* Each LTCC is mechanically coupled to a ryanodine receptor (RyR) on the sarcoplasmic reticulum
* Calcium exits via the RyR into the cytosol
* Some of the Ca2+ entering via the LTCC can also activate the RyR to open (calcium induced calcium release), but this pathway is not essential in skeletal muscle
* Calcium activates troponin C and triggers contraction
236
Label this. What muscle type is it?
It is a triad in skeletal muscle.
237
What proteins are involved in release of calcium in skeletal muscle once depolarisation reaches T-tubules?
On the T-tubule:
* L-type Ca2+ channel (LTCC) -\> a.k.a. DHP receptor
On the sarcoplasmic reticulum:
* Ryanodine receptor (RyR) -\> a.k.a. Ca2+ release channel
238
What is the form of coupling in skeletal muscle between the L-type calcium channel and ryanodine receptor?
* Mechanical coupling
There is also calcium-induced calcium release -\> But this is not required for contraction like in cardiac muscle.
239
Describe the arrangement of the contractile filaments in skeletal muscle at rest.
* There are thin actin filaments that have tropomyosin wrapped around it in a helical fashion
* This tropomyosin blocks the heads of thick myosin filaments from binding to the actin (at myosin binding sites)
* There is a troponin complex that regulates the position of troponin
240
What are the three components of the troponin complex?
* TnT -\> Binds to **_T_**ropomyosin
* TnI -\> **_I_**nhibits the myosin binding site
* TnC -\> Binds to **_C_**alcium
241
Describe how an increase in cytosolic calcium leads to contraction of skeletal muscle.
Calcium binds to troponin C, which causes tropomyosin to undergo a conformational change so that troponin I no longer blocks the myosin binding site and cross-bridge cycling can occur.
242
Draw the process of cross-bridge cycling in skeletal muscle.
243
Describe the different points at which ATP is required during cross-bridge cycling.
* Binding of ATP to myosin head causes release of the head from the binding site
* ATP hydrolysis causes the head to return to its relaxed state (but the free phosphate remains bound)
* Once a new cross-bridge is formed, release of the phosphate causes the power stroke to occur
244
What change must occur in order for skeletal muscle contraction to end?
The cytosolic calcium levels must drop.
245
Describe how skeletal muscle contraction is terminated and what proteins are involved.
Most important:
* Ca2+-ATPase on sarcoplasmic reticulum -\> SERCA pump
Less important:
* Ca2+pump on plasma membrane -\> PMCA
* Na+/Ca2+-exchanger on plasma membrane -\> NCX
These lower the levels of cytosolic calcium by pumping it out of the cell or into the SR.
246
What does SERCA stand for and why is the most important mechanism for terminating contraction of skeletal muscle?
* Sarco-endoplasmic reticulum calcium ATPase
* It is the most important mechanism because it is on the SR and intracellular calcium must be conserved
247
What are the 3 ways in which the tension produced by skeletal muscle can be regulated?
* Selective recruitment of a greater or smaller number of motor units
* Changes in the frequency of stimulation
* Changes in the starting length of the relaxed muscle
248
Draw how motor unit recruitment can be used to vary the tension produced by skeletal muscle.
The more motor units are recruited, the greater the tension in the muscle will be.
249
What is the name for increasing the number of motor units recruited in a skeletal muscle in order to produce a greater tension?
Spatial summation
250
Describe how modulation of stimulation frequency can be used to vary the tension in skeletal muscle.
Since muscle contraction lasts longer than an action potential, multiple action potentials in sequence can cause the tension to be increased.
251
What is the name for increasing the frequency of stimulation of skeletal muscle in order to produce a greater tension?
Frequency summation (or temporal stimulation)
252
What is tetany and what are the two types?
* It is the prolonged, constant contraction of skeletal muscle resulting from a very frequency of action potentials, so that the muscle contractions fuse
* The two types are fused tetanus and unfused tetanus
253
Describe how modulation of muscle length can be used to vary tension.
In general, the more you stretch a muscle, the less force it can exert when contracted.
254
What are the two types of muscle contraction?
* Isometric
* Length is constant
* But tension is generated
* Isotonic
* Length changed
* Tension is constant
255
Draw the relationship between the length of muscle and the tension that can be generated. [EXTRA?]
256
What parts of cardiac muscle allow it to act as a electrical and mechanical syncytium?
* Gap junctions -\> Electrical coupling
* Desmosomes -\> Mechanical coupling
These are two components of intercalated discs along with adherens junctions.
257
Summarise briefly the idea of the Frank-Starling mechanism.
The greater the filling of a cardiac chamber, the greater individual myocardial fibres are stretched, and hence the greater the subsequent force of contraction .
258
Draw the shape of an SAN action potential and explain the currents that are part of it. [IMPORTANT]
* Phase 0: Depolarisation due to inward calcium current
* Phase 3: Repolarisation dependent on outward potassium current
* Phase 4: Pacemaker potential due to funny current and also changes in potassium and calcium currents
259
What is the symbol for the funny current?
If
260
What channel carries the funny current?
HCN -\> Hyperpolarisation-activated cyclic nucleotide channel
261
How does the funny current work?
It is an inward current that slowly depolarises SAN cells in response to hyperpolarisation.
262
What are the different currents that contribute to the pacemaker current?
* Inward Ca2+ current
* De-activation of outward K+ current
* Funny current (Inward cation current)
* NCX current
* Background inward Na+ leak
263
Draw the shape of a ventricular action potential and explain the currents that are part of it. [IMPORTANT]
* Phase 0: Fast upstroke. Due to inward calcium and sodium currents.
* Phase 1: Rapid repolarization. Almost total inactivation of sodium and calcium currents.
* Phase 2: Prolonged plateau. Continued entry of Ca2+ or Na+ ions through their major channels and via NCX1 exchanger.
* Phase 3: Repolarization due to outward potassium currents.
* Phase 4: Electrical diastolic phase
264
Show the different refractory periods in the ventricular action potential.
265
Compare the action potential in sub-epicardial cells vs sub-endocardial cells of the ventricles.
The action potential is longer in sub-endocardial myocytes.
266
What are some differences between the SAN action potential and ventricular action potential?
* Ventricular action potential features a steeper upstroke -\> For speed of transmission
* Ventricular action potential features a prolonged plateau -\> Enables plentiful entry of calcium into the cell
267
Describe the process of excitation-contraction coupling in cardiac muscle.
It is essentially the same as in skeletal muscle, except the LTCC and RyR are not mechanically coupled.
268
What makes excitation contraction coupling different in cardiac muscle compared to skeletal muscle?
Calcium-induced calcium release is required, since the L-type calcium channel and ryanodine receptors are not mechanically coupled.
269
What is different about the T-tubules in cardiac muscle compared to skeletal muscle?
The T-tubules only have one sarcoplasmic reticulum cisternae next to them, rather than two.
270
Explain the concept of CICR.
Calcium induced calcium release:
* When depolarisation travels into a T-tubule, it causes L-type calcium channels in the membrane to open
* In cardiac muscle, the LTCC are not coupled to ryanodine receptors (RyR)
* Instead, the calcium that enters the cytosol via the LTCC causes release of calcium from the SR via RyR
* Therefore, calcium flowing in from the outside is NECESSARY for contraction to occur, unlike in skeletal muscle
271
Describe the feedback regulation of calcium levels in the cytosol of cardiac myocytes.
* Increased cytosolic Ca2+ inhibits LTCC so that there is less influx of calcium
* It also increases NCX activity
272
What are the 2 main ways in which cardiac output can be modulated?
* Inotropy -\> Change in force of contraction
* Chronotropy -\> Change in frequency of contraction
273
What are the main ways in which inotropy and chronotropy can be regulated?
Inotropy:
1. Starling's Law -\> Change in cell length
2. Cardiac nerves and circulating hormones
Chronotropy:
* Nerves and hormones
274
Draw a graph to show Frank-Starling and the mechanism that explains it.
It happens in two ways:
* Stretch leads to changes in double actin overlap
* Stretch leads to changes in troponin complex affinity for calcium
275
Draw a graph of tension in heart muscle against intracellular calcium.
276
Describe which parts of the heart can be regulated in terms of inotropy and chronotropy. Which type of innervation enables this?
* Inotropy
* Ventricles
* Sympathetic innervation only
* Chronotropy
* SAN/AVN
* Sympathetic and parasympathetic innervation
277
What is phospholamban and what does it do?
Protein that inhibits the SERCA pump.
278
Draw the mechanism for the sympathetic regulation of inotropy.
This occurs in ventricular myocytes.
279
Draw the mechanism for the sympathetic regulation of chronotropy.
This occurs at SAN and AVN cells.
280
Draw the mechanism for the parasympathetic regulation of chronotropy.
* It is the opposite process to symnpathetic regulation, since it involves inhibition of cAMP synthesis rather than driving of cAMP synthesis.
* ALSO, the βγ subunit of the G-protein activates K+-channels, leading to hyperpolarisation of the cell
281
Describe the effects of cardiac glycosides. Give an example. [IMPORTANT]
Digoxin:
* Inhibits sodium-potassium ATPase pump in cardiac muscle cells to alter their function.
* So intracellular sodium concentration therefore increases.
* Raised intracellular sodium levels inhibit the function the NCX (sodium-calcium exchanger), which is responsible for pumping calcium ions out of the cell and sodium ions in.
* Thus, calcium ions are also not extruded and will begin to build up inside the cell as well -\> Leads to increased calcium uptake into the sarcoplasmic reticulum (SR) via the SERCA transporter.
* Raised calcium stores in the SR allow for greater calcium release on stimulation, so inotropy is increased.
* The refractory period of the AV node is increased, so cardiac glycosides also function to decrease heart rate.
* For example, the ingestion of digoxin leads to increased cardiac output and decreased heart rate without significant changes in blood pressure; this quality allows it to be widely used medicinally in the treatment of cardiac arrhythmias.
282
Is calcium required for smooth muscle contraction?
Yes, just like with all types of muscle.
283
What are the different things that can regulate smooth muscle contraction?
* Myogenic activity
* Sympathetic / Parasympathetic innervation
* Hormones and local compounds
284
Describe how innervation, myogenic activity and hormones can evoke an increase in calcium in smooth muscle cells.
Myogenic activity:
* Voltage-gated Ca2+ channels (VGCCs) can open upon membrane depolarisation
* This increases intracellular calcium
Sympathetic innervation and hormones:
* Bind to ligand gated Ca2+ channels
OR
* Bind to Gq-coupled GPCRs
* This leads to an increase in IP3 (second messenger)
* IP3 binds to IP3 receptors on the sarcoplasmic reticulum
* This leads to calcium release
NOTE: Don't worry too much about parasympathetic innervation for now.
285
What types of calcium channels are found on smooth muscle cells?
L-type calcium channels
286
How does increased cytosolic calcium lead to contraction of smooth muscle?
Relies on a change in myosin structure:
* Calcium binds to calmodulin (CaM) to give a Ca2+-CaM complex
* This in turn activates myosin light chain kinase (MLCK) to phosphorylate myosin on serine 19 on the light (regulatory) chain.
* This exposes the site that binds to actin, preparing the myosin for cross-bridge formation.
* The Ca2+-CaM complex also stimulates ATPase, which is necessary for the cross-bridge cycle.
287
What are two important enzymes involved in control of contraction of smooth muscle? How do they do this?
* Myosin light chain kinase (MLCK) -\> Stimulates contraction
* Myosin light chain phosphatase (MLCPh) -\> Inhibits contraction
MLCK and MLCPh work by phosphorylating and dephosphorylating myosin. When it is phosphorylated, the site that binds to actin is exposed, so cross-bridge cycling is enabled.
288
What does myosin light chain kinase do and what things can regulate its activity? [IMPORTANT]
* Drives smooth muscle contraction by phosphorylating myosin so that it cross-bridge cycling can occur
* Activated by: Calcium (in Ca2+-calmodulin complex)
* Inhibited by: PKA and PKG
289
What does myosin light chain phosphatase do and what things can regulate its activity? [IMPORTANT]
* Terminates smooth muscle contracting by dephosphorylating myosin so that the sites that bind to actin are not exposed and therefore cross-bridge cycling cannot occur
* Inhibited by: PKC
290
What are the effects of PKA, PKC and PKG on smooth muscle contraction?
* PKA and PKG inhibit MLCK -\> Therefore lead to relaxation
* PKC inhibits MLCPh -\> Therefore lead to contraction
291
How can agonists (e.g. hormones) cause sensitisation of smooth muscle to calcium?
* Bind to GPCR that activates an enzyme called Rho-kinase
* Rho kinase inhibits MLCPh (which usually drives relaxation of smooth muscle)
* Therefore, the phosphorylation of myosin is prolonged and therefore the muscle remains more contracted
292
How does cAMP affect smooth muscle and why? [IMPORTANT]
* Inhibits contraction
* By inhibiting the actions of MLCK
293
How does cGMP affect smooth muscle and why? [IMPORTANT]
* Inhibits contraction
* By stimulating the actions of MLCPh
294
What is meant by L-type calcium channels?
Long-lasting -\> They stay open for as long as the membrane is depolarised.
295
Describe how the endothelium of arteries allows for vascular smooth muscle relaxation. [IMPORTANT]
296
Draw a summary of the different things that can cause smooth muscle contraction or relaxation.
297
Draw how sympathetic innervation can affect smooth muscle contraction.
This shows that the receptors that are present on the muscle determine what response the muscle has to sympathetic stimulation.
298
How does viagra work?
* It is a phosphodiesterase 5 inhibitor, which means it blocks the breakdown of cGMP to GMP
* Therefore, the cGMP can continue to cause smooth muscle relaxation
299
What two things can lead to smooth muscle relaxation?
* Drop in intracellular calcium
* MLCPh activity
300
What are the 3 classes of calcium-blocking drugs that act on smooth muscle? [EXTRA]
