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MSK exam 3 > muscle excitation and contraction > Flashcards

muscle excitation and contraction Flashcards

1
Q

Define a motor unit

A

One motor neuron and all the muscle fibers it innervates

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2
Q

Where are the cell bodies of efferent neurons?

A

The CNS - ventral horn of spinal cord (except cranial nerves)

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3
Q

Where do axons typically synapse on a skeletal muscle fiber?

A

At a single point called the neuromuscular junction or end plate, midway along the length of a skeletal muscle fiber

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4
Q

How are neuromuscular junctions similar to synapse?

A
  1. Both consist of two exciteable cells seperated by a narrow cleft which prevents transfer of electrical activity
  2. Axon terminals of both contain neurotransmitters released by Ca2+ induced exocytosis of storage vesicles in response to an AP
  3. Binding of N.T. with receptor opens channels in the membrane permitting ionic movements that change the membrane potential (graded potential in both cases)
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5
Q

What are the differences between neuromuscular junctions and synapses?

A
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6
Q

What are the 5 parts of the NMJ?

A
  1. Active Zones
  2. Postjunctional Folds
  3. Synaptic Cleft
  4. Nicotinic Acetylcholine Receptors
  5. Acetylcholinesterase
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7
Q

Describe the Active zones of the NMJ

A
  • Area for fusion of synaptic vesicles & release of ACh
  • Clustering of synaptic vesicles
  • Above secondary postsynaptic clefts between adjacent postjunctional folds
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8
Q

Describe the postjunctional folds of the NMJ

A
  • Increase surface area of muscle plasma membrane
  • Invaginations on postsynaptic membrane opposite nerve terminal
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9
Q

Describe the synaptic cleft. Characterize the speed of impulse transmission.

A

•~ 50 nm, time delay in impulse transmission with ACh diffusion

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10
Q

Describe Nicotinic Acetylcholine Receptors

A

• High density expression at crests of postjunctional folds

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11
Q

Describe Acetylcholinesterase (AChE):

A

• High concentration

associated with synaptic

basal lamina

(basement membrane)

• Terminates synaptic

transmission after AP

• Hydrolyzes ACh →

choline & acetate
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12
Q

Where are neurotransmitter vesicles produced? How are they transported?

A
  • Motor neuron cell bodies in spinal cord produce vesicles for NT
  • Fast axonal transport translocates vesicles to nerve terminal(microtubule-mediated process)Vesicles for ACh (& other non-peptide NTs) travel empty down axon
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13
Q

What does Choline acetyltransferase do?

A

•Synthesizes ACh from choline + acetyl coenzyme A

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14
Q

What is the purpose of the ACh-H+ exchanger?

How does it function?

A
  • ACh uptake by synaptic vesicle
  • Driven by vesicular proton electrochemical gradient

(ACh influx coupled with H+ efflux; due to positive voltage & low pH inside)

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15
Q

What are the 4 synaptic vesicle membrane proteins?

A
  1. Synaptobrevin
  2. Synaptotagmin
  3. Syntaxin
  4. SNAP-25
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16
Q

What is the role of synaptobrevin?

A
  • Essential for transmitter release
  • Forms complex with SNAP-25 & syntaxin (presynaptic membrane proteins; t-SNAREs)
  • Helps drive vesicle fusion
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17
Q

What is the role of synaptotagmin

A
  • Ca2+ receptor of synaptic vesicle
  • Detects rise in [Ca2+]i and triggers

exocytosis of docked vesicles

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18
Q
  1. & 4. Syntaxin & SNAP-25 (t-SNARES), what do they do? How does synaptobrevin interact with them?
A
  • Presynaptic membrane of nerve terminal; key role in fusion process
  • Synaptobrevin coils around free ends of syntaxin/SNAP-25, bringing the vesicle closer to the presynaptic membrane
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19
Q

What happens when synaptotagmin binds Ca2+?

A

Triggers vesicle fusion and exocytosis of N.T.’s

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20
Q

What are the neurotoxins that block the fusion of synaptic vesicles, and what are their targets?

A

Tetanus and botulinum toxins B,D,F,G - Synaptobrevin

Botulinum Toxins A and E - cleave SNAP-25

Botulinum toxin C1 - Cleaves Syntaxin

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21
Q

describe the Acetylcholine Receptor

A

Ionotropic, Nicotinic AChR Channel:

Nonselective cation channel on muscle endplate, permeable to cations (Na+, K+, Ca2+( minimal) ). Not permeable to anions

 Function to raise Vm above     threshold
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22
Q

Opening of AChR channel at the muscle end plate leads to… (regarding Na+ and K+)

A
  • Na+ and K+ become equally permeable
  • Result: increase the normally low (resting) permeability of Na+ relative to K+ →
  • Vm shifts to a value between EK (−80 mV) and ENa (+50 mV)
  • End-plate Potential = graded potential of end plate, small depolarization
23
Q

What is an end-plate potential?

A
  • a type of Graded Potential
  • Excitatory Postsynaptic Potential (EPSP)
    - Produced by transient opening of AChR 
- Increased Na+ conductance   hypopolarizes Vm of end-plate
24
Q

Can inhibition of skeletal muscle be accomplished at the NMJ?

A

No, only in the CNS through IPSP at dendrites and cell body of motor neuron

25
Q

Describe steps 1-5 shown below

A
26
Q

Describe steps 6-9 below

A
27
Q

Describe a sarcomere

A

Sarcomere = Z line to Z line

  1. A band:

Myosin (thick) filaments;

Partial overlap with actin (thin) filaments

  1. H zone:

Middle of A band;

Part of myosin where actin does not overlap

  1. M line:

Extends vertically down center of A band

  1. I band:

Part of actin not overlapping myosin; no project into A band

  1. Z line:

Thin filament attachment

28
Q

During contraction of a sarcomere which bands contract and which are unchanged in length?

A

A bands remain the same, I bands shorten

29
Q

On myosin heavy chains, what are the two important binding sites, and what are they for?

A

Heads of heavy chains (S1 fragments) each have 2 binding sites:

  1. Actin binding site (for cross-bridge formation)
  2. Myosin ATPase site (for binding and hydrolyzing ATP)
30
Q

Actin is bound by what two regulatory proteins?

A
  1. Tropomyosin - interacts with 7 actin monomers
  2. Troponin - interacts with 1 tropomyosin molecule and actin
31
Q

What is the one important binding site on actin?

A
  1. Myosin Binding Site

• Blocked by Tropomyosin at rest

32
Q

When Ca2+ combines with troponin, what happens?

A

tropomyosin slips away from its blocking position between actin and myosin

33
Q

What is Titin (connectin) involved in?

A

•Appears to be involved in the elastic behavior of muscle by maintaining the resting length of muscle during relaxation (passive stiffness)

34
Q

Describe excitation-contraction coupling

A

Action potential of sarcolemma (excitation) —> Increased [Ca2+]i allowing actin & myosin binding (coupling) ——> Power stroke

35
Q

How do action potentials propagate from the sarcolemma to the interior of muscle fibers?

A

Transverse (T) tubules

36
Q

Where are T tubules found?

A

Surrounding myofibrils at the junction of A and I bands

37
Q

Where is calcium stored?

A

• Sarcoplasmic reticulum (specialized endoplasmic reticulum) storage organelle for intracellular Ca2+

38
Q

Describe the Triad. What is it’s role? What are the two important channels associated with it?

A

•T-tubule membrane & 2 associated cisternae (specialized regions of the sarcoplasmic reticulum)
•Crucial role in linking excitation to contraction
–Propagation of AP into T tubules depolarizes triad
–Results in Ca2+ release from lateral sacs of the sarcoplasmic reticulum
–Two important channels

  1. Dihydropyridine receptor
  2. Ryanodine receptor
39
Q

Describe the L-type Ca2+ Channel: Dihydropyridine (DHP) receptor.

A
  • Voltage-gated channels
  • Role: Voltage sensor• Associated with T-tubule membrane• Tetrads: cluster in groups of 4
40
Q

What do conformation changes in 4 DHP channels lead to?

A

•Conformational changes in 4 L-type

Ca2+ channels → induces a

conformational change in 4 subunits of

the Ca2+-release channel
41
Q

Describe the Ca2+- Release Channel: Ryanodine (RyR) receptor

A
  • Role: Releases stored Ca2+ from the SR
  • Associated with the SR membrane

• Cluster at the portion of the SR membrane opposite the T tubule

42
Q

Summarize the events at the triad Steps 1-3 only.

A

(1) Depolarization of voltage-sensor L-type Ca2+ channel (Dihydropyridine) on the T-tubule membrane
(2) Mechanical activation of Ca2+-release channel (Ryanodine) in the SR
(3) Ca2+ stored in the SR rapidly leaves through the Ca2+-release channel

43
Q

How does an increase in intracellular calcium trigger contraction?

A

It removes the inhibition of cross-bridge cycling

44
Q

Describe the power stroke

A
  1. Calcium causes troponin to remove tropomyosin from actin.
  2. Myosin binds to actin
  3. at rest myosin heads are cocked
  4. Interaction between myosin and actin “pulls trigger” using stored potential energy
  5. Myosin pulls the actin toward the center of the sarcomere
  6. Sarcomere shortens
45
Q

What happens when new ATP binds to myosin head?

A

Actin detaches from myosin, and the cross bridges return to original position to repeat power stroke cycle

46
Q

Describe the steps depicted below.

A
47
Q

myocyte relaxation is an active process that requires ATP. How is calcium removed from the intracellular space?

A

Calcium pumps and calcium binding proteins

48
Q

What are the two calcium removal pump systems involved in relaxation?

A
  1. Na+- Ca2+ Exchanger and Ca2+ Pump
  2. Sarcoplasmic and Endoplasmic Reticulum Ca2+-ATPase (SERCA)— type Ca2+ pump
49
Q

What is the most important mechanism for returning [Ca2+]i to resting levels in skeletal muscle?

A

Sarcoplasmic and Endoplasmic Reticulum Ca2+-ATPase (SERCA)— type Ca2+ pump

•Ca2+ re-uptake into the SR

50
Q

What impact does high [Ca2+]i have on SERCA activity? What role do calcium binding proteins have related to this?

A
  1. Inhibits activity of SERCA
  2. Ca2+-binding proteins in the SR lumen can delay inhibition of Ca2+ pump activity
51
Q

What are the two major Calcium binding proteins?

A

Calsequestrin and calreticulin

52
Q

Describe calsequestrin

A

•Major Ca2+-binding protein in skeletal muscle
–Localized in SR at triad junction
–Forms complex with Ca2+-release channel (RYR)
–Facilitates muscle relaxation by buffering Ca2+ AND unbinds Ca2+ near Ca2+-release channel

53
Q

Desribe calreticulin

A

Ca2+-binding protein in smooth muscle

54
Q

Describe the 7 major steps from the action potential at the NMJ to relaxation.

A

MSK exam 3 (12 decks)

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