muscle NMJ and choline

Question 1

distance and transmission time of NMJ

Answer 1

50nm gap between presynaptic terminal and post synaptic density
Delay of >500us between terminal release and muscle contraction

Question 2

how does curare affect the NMJ

Answer 2

curare acts as a competitive inhibitor for ACh at the post synaptic terminal

Question 3

how does alpha -bungarotoxin affect the NMJ

Answer 3

Alpha -bungarotoxin is known to bind competitively and in a relatively irreversible manner to the nicotinic acetylcholine receptor found at the neuromuscular junction, causing paralysis, respiratory failure, and death in the victim.

Question 4

structure and function of nAChR

Answer 4

Pentamers of two alpha subunits with a beta theta and delta subunit
Each has two Ach binding sights on interface of Alpha and other subunits
Opening of channels increases membrane permibility for Na+and K+

Question 5

where are nAChRs and voltage gated Na+ channels located on the post synaptic membrane?

Answer 5

nAChRs are localized in post synaptic density opposing presynaptic membrane
Voltage gated sodium channels are located in folds of the neuromuscular junction.

Question 6

comment on the end plate potential compared with an action potential in a myocyte

Answer 6

End plate potential smaller than action potential but fires in similar structure

Question 7

how can vesicles be imaged at the synapse

Answer 7

Because of active loading of synapes vesicles are verry acidic, and hence ph imaging can be used to monitor vesicles at a synapse

Question 8

describe life cycle of a synaptic vesicle

Answer 8

Adapter protein bind to clathrin causing blebbing of the membrane
Dynamin acts as molecular scissors separating vesicle from membrane
> bind with endosome
> active acid loading then loading with neurotransmitter
Synapsin holds full vesicles in non membranous fluid density region
Synaptotagmin Binds with snare proteins and holds vesicle by membrane
Ca2+ release triggers fusing with the membrane

Question 9

name and explain the function of the vesicular and membranous snare proteins

Answer 9

vesicular:
synaptobrevin- binds with SNAP-25 and syntaxin to hold the vesicle in place for release
synaptotagmin- is basis of Ca2+ sensing mechanism changes conformation to fuse with the membrane in presence of Ca2+
membranous:
SNAP-25 and syntaxin both function to hold the vesicles ready for docking

Question 10

state the location and function of acetyl choline esterase

Answer 10

The basal lamina is very rich in AChE acetylcholinesterase which breaks down ACh rapidly in the neuromuscular junction. Ach is broken down into choline and acetic acid

Question 11

how does the NMJ achieve high fidelity

Answer 11

Probability of any docked vesicle
being released <0.02
BUT
The entire frog NMJ has a total of 14,000–28,000 docked vesicles
350 vesicles released after every action potential
-> reliable suprathreshold response

Question 12

how does botulinum toxin work

Answer 12

it is up taken in vesicles then cleaves snare proteins in the presynaptic terminal preventing neurotransmitter release

Question 13

what is neuromyotonia

Answer 13

Autoimmune neuromyotonia is typically caused by antibodies that bind to potassium channels on the motor nerve resulting in continuous/hyper-excitability. Onset is typically seen between the ages of 15–60, with most experiencing symptoms before the age of 40.

Question 14

what is myasthenia gravis

Answer 14

Myasthenia gravis is an autoimmune disease associated with antibodies directed to the postsynaptic acetylcholine receptor. These antibodies reduce the number of receptors. Autoantibodies against AChR and other muscle antigens can be used for the diagnosis of myasthenia gravis and related disorders.

Question 15

what is lambert-eaton myathenic syndrome?

Answer 15

Lambert Eaton Myasthenic Syndrome (LEMS) is an autoimmune disorder characterised by proximal muscle weakness and autonomic symptoms. Approximately 60% of cases are paraneoplastic. Antibodies to presynaptic voltage gated calcium channels of the P/Q–type are detected in 92% of LEMS patients.

Question 16

define sarcolema

Answer 16

The sarcolemma is the plasma membrane of the muscle cell

Question 17

what is a sarcomere

Answer 17

the contractile unit of muscle cells

Question 18

what is the z line and what purpose does it serve

Answer 18

Main function to bind sarcomeres together

Protine dense region many other functions

Question 19

what is the M line

Answer 19

the m line binds heavy myosin chains together it also contains creatine kinase

Question 20

what is the A band

Answer 20

consists of the darker area of overlapped myosin and actin with the M line at the center

Question 21

what is the I band

Answer 21

the I band consists of the exposed actin centered around the very dark line of the Z band

Question 22

describe structure of a triad

Answer 22

two loops of the SR coupled by proteins spanning the 35nm gap to a central t-tubule

Question 23

describe how depolarization of the T-tubule causes Ca2+ release from the SR

Answer 23

1) Plasma depolarisation opens Ltype Ca2+ channels
2) Mechanical coupeling with the L- type calsium channel(LTCC) and Ryanodine receptor
3) Ca2+ exits via RyR activates tryponin C and triggers contraction
- Ca2+ entering via LTCC can also activate the RyR to open but this pathway (CICR) is not essential.

Question 24

what is a fascicle

Answer 24

a fasicle is a bundle of muscle fibers

Question 25

how does Ca2+ enable contraction

Answer 25

Ca2+ ions bind to tropnin Cor calmodulin in smooth muscle causing the troponin complex to shift this causes a confrontational change in the tropomyosin moving it deeper into the actin groove uncovering the myosin binding sites thus enabling myosin binding and contraction

Question 26

in what 3 ways is contraction terminated

Answer 26

Major method is H+/Ca2+ ATPase co transporter into sarcolema. 2 minor mechanisms, Ca pump PMCA or Na-Ca exchanger, both move calcium to extra cellular space.

Question 27

explain the cross bridge cycle

Answer 27

ATP binds to to the myosin head causing it to un-couple with the Actin chain The ATP is then hydrolyzed causing the myosin head to straighten such that its inline with a further point on the actin chain. here a cross-bridge then forms Release of the Phosphorous from the breakdown of ATP causes the 'power stroke' as the myosin head snaps back dragging the myosin along the actin chain here the ADP is then released and the cycle begins a new

Question 28

explain what the IR stands for and how it relates to muscle strength

Answer 28

innervation ratio is the number of muscle cells that a single motor neuron innovates. Generally increase in innervation is correlated to muscle strength.

Question 29

explain the function and location of gap junctions in myocardium tissue

Answer 29

Between myocytes at intercalated disks Allow ion movement between 6 connexin sub-units in conexome 2 connect form junction Ca2+ one of the substances that regulate the aperture size of gap junctions/ opening. it deceases aperture size

Question 30

what is sterling's law of the heart

Answer 30

As the more full the cardiac chambers the more stretch in the myocytes which confers greater contraction force of contraction due to the operating range of cardiac sarcomeres

Question 31

``` explain the ionic vents that create the following phases in cardiac ventricular muscle Phase 0: Fast upstroke. Phase 1: rapid repolarization. Phase 2: plateau. Phase 3: repolarization . Phase 4; electrical diastolic phase. ```

Answer 31

Phase 0: Fast upstroke. Due to both ICa and INa (blocked by TTX). Phase 1: rapid repolarization. Almost total inactivation of INa or ICa Phase 2: plateau. Is prominent in VM. Continued entry of Ca2+ or Na+ ions through their major channels and on a minor membrane current due to the Na-Ca exchanger NCX1. Phase 3: repolarization . It depends on IK Phase 4; electrical diastolic phase. Vm during phase 4 is termed the diastolic potential.

Question 32

how does TTX (tetrodotoxin) affect nerve cells

Answer 32

It inhibits the firing of action potentials in neurons by binding to the voltage-gated sodium channels in nerve cell membranes and blocking the passage of sodium ions

Question 33

explain the phases and ionic events in a sinoatrial node potential

Answer 33

Phase 0: Slow upstroke, due only to ICa Phase 3: repolarization . It depends on IK Phase 4; electrical diastolic phase. In SA and AV nodal cells, changes in IK , ICa, and I f produce pacemaker activity (pacemaker potential or pacemaker current) during phase 4.

Question 34

how does adrenaline act on the heart muscle

Answer 34

through Gs coupled receptors that lead to phosphorylation of the L-type Ca2+ channels incresing the plateau phase Phosphorylation of phospholamban (PLB), boosts Ca2+ re-uptake in SR

Question 35

how does adrenaline act on the san

Answer 35

* Phosphorylation of LTCC (ICa-L), accelerates pacemaker potential decay * Phosphorylation of delayed rectifier K+ channel (IKV), increase in the outward current IKv contributes to the shortening of the action potential as well as pacemaker potential decay

Question 36

compare the effects of acetylcholine on the heart muscle wit adrenaline

Answer 36

much the opposite slows HR contractile force ect as coupled to Gi proteins instead

Question 37

Common features of smooth muscle cells

Answer 37

Spindle shaped cells 4-8µm diameter, 80-200 µm length, much smaller than striated muscle • No striations: myofilaments not arranged as sarcomeres • No T-tubules • Sarcoplasmic reticulum present, but variable organization Innervated by autonomic nerves Organised into functional bundles

Question 38

Classification of smooth muscle:

Answer 38

Myogenic- contracts spontaneously e.g. the iris | Neurogenic- must be activated by nerves e.g. smooth muscles of arteries

Question 39

how is Ca2+ released into smooth muscle

Answer 39

ligand gated ion channels CaV channels Gq coupled receptors release out of SR

Question 40

How does calcium generate contraction in smooth muscle

Answer 40

Ca2+ binding protein, calmodulin (CaM) as Smooth muscle lacks troponin C Caldesmon and calponin both calmodulin binding thin-filament proteins, inhibits myosin ATP’ase. Inhibition removed by Ca2+-CaM Slow actin-myosin dissociation, ‘latch-state’ conserves energy Contraction also follows an increases in calcium sensitivity of contractile proteins

Question 41

3 Chemical classes of VGCC(voltage gated Ca2+ channels) Blocker

Answer 41

Dihydropyridines, e.g. nifedipine, amlodipine Phenylalkylamines e.g verapamil Benzothiazepines e.g. diltiazem All bind to the α1 subunit of L-type VGCCs, but at distinct sites Block calcium entry into smooth muscle cells

Question 42

how does doxazosin function

Answer 42

a1 adrenoceptor antagonists (doxazosin) blocks noradrenaline action from sympathetic nerves

Question 43

what effect does cGMP have on smooth muscle cells

Answer 43

decreases Ca2+ levels relaxing the cells

Question 44

ACh is an ester of what two chemicals

Answer 44

acetic acid and choline

Question 45

where is ACh found?

Answer 45

``` At al autonomic ganglions (n&m) Many autonomously innervated organs(n&m) At NMJ(n) ```

Question 46

describe the life cycle of acetylcholine

Answer 46

choline + Acetyle CoA(from mitchondria)---choline acetyltransferase----> ACh + CoA loaded into vesicles by H+ anti-porters and released into cleft where its broken down by AChE (acetylcholinesterase ) into Choline and acetate. choline is transported back into the cell and used to produce more ACh

Question 47

as nerves cannot produce enough choline where else is it acquired from?

Answer 47

Nerves cannot make enough choline so must be taken up from blood gained from diet and liver.

Question 48

mAChRs are found:

Answer 48

organs innervated by the parasympathetic division of the autonomic nervous system autonomic ganglia, and in the CNS In the cns Muscarinic receptors are located pre and post synaptically M1-5

Question 49

describe in detail nAChR receptor opening with reference to positive cooperativity

Answer 49

When inactive the 2nd membranous loop of each subunit has a kinked alpha helical structure that blocks pore conductance. ACh binding to the of the extracellular N terminus domains increases affinity of the second binding site for ACh this phenomenon is called positive cooperativity. Agonist binding causes a conformational alpha subunit twist that unkinks the alpha helical pore regions allowing ion conductance. The flow of Na+ and K+(equal permeability) down their electrochemical gradient causes an EPP (excitatory postsynaptic potential) in skeletal muscle and neurons.

Question 50

Name a mAChR antagonist

Answer 50

atropine

Question 51

name two toxins affecting ACh release

Answer 51

Alpha latrotoxin- massive release of vesicles causing depletion Botulinum toxin breaks down snare proteins no ACh release

Question 52

snake venom's can be useful in identifying receptors give an example

Answer 52

Snake venom useful in identifying receptor e.g. nAChR bound very tight by alpha cobratoxin

Question 53

Main effects of Muscarinic antagonists are:

Answer 53

* Inhibition of secretions * Tachycardia * Pupillary dilation * Relaxation of smooth muscle (Bladder, gut, etc.)

Question 54

why must competitive antagonists for ACh be verry high affinity or high concentrations to be effective

Answer 54

80% of ACh receptors must be blocked to prevent any stimulus passing threshold vecuronium and atracurium used clinically

Question 55

Acetylcholinesterases can also be used to decrese volontary movement name two such drugs

Answer 55

neostigmine and europhonium