Neuromuscular Junction (7/14/15) Flashcards
What are the components of the Neuromuscular junction?
Myelin sheath Motor axon Muscle plasma membrane Acetylcholine vesicle Motor endplate (Muscle part of NMJ)
What is the sequence of events from and AP in a Motoneuron to an AP in the sacrolema of a muscle cell? (12 steps)
- AP travels along motoneuron
- AP invades motoneuron presynaptic terminal
- Ca++ influx in presynaptic terminal
- Vesicle fusion (exocytosis) with membrane of presynaptic terminal.
- Release of acetylcholine from fused vesicles
- Diffusion of ACh across neuron muscular cleft
- Binding of ACh to its receptor (AChR) in the posynaptic membrane (motor endplate).
- Opening of Na+ and K+ channels which are CHEMICALLY regulated at the motor endplate.
- Na+ influx and small K+ efflux across motor endplate.
- Generation of Endplate potential
- Opening of voltage-regulated Na+ channels in sarcolemma immediately surrounding the motor endplate.
- AP is initiated in Sarcolemma
Tell me about Endplate potentials (EPP’s)….
They occur following an AP in a motor neuron
- Not spontaneous
- They are graded, not all-or-none in amplitude.
- They do not propagate
- They can undergo summation, but usually lead to Ap in adjacent region of membrane.
- around 10mV (AP are around 130mV)
Tell me about Miniature Endplate potentials..
They occur at rest and spontaneously
- They are confined to endplate region
- They follow the release of the contents of one synaptic vesicle.
- Can undergo summation (Release of a few vesicles)
- Around 1-2mV
What is the Role of Acetylcholinesterase (AChE)? What would happen in its absence?
To Break down ACh. If it were not present we would see a continuous activation and contraction of the muscle cell.
Where is AChE found?
On the motor endplate and in the basal lamina.
What are the products of ACh breakdown and where do they go?
Acetic acid (acetate) = enters circulation Choline = taken up by the presynaptic terminal where t reacts with acetyl-CoA to from ACh again!
What does Curare do?
blocks ACh receptor (Nicotinic) so ACh cannot bind. ACh can still be released from presynaptic terminal, but Muscle AP will not occur.
What does botulism toxin do?
Blocks ACh release so skeletal muscle activation will not occur.
What do Organophosphates do?
Block action of AChE so ACh will not be hydrolyzed = cannot activate skeletal muscle in its presence because Na+ channels remain in a refractory state.
What is the hierarchy of organization of skeletal muscle?
Whole muscle -> Fascicle -> Muscle Fiber -> Myofibril -> Sarcomere -> Filament -> Protein
Describe the structure of thin muscle filaments…
Two intertwined helical chains of actin molecules.
Describe the Structure of Thick muscle filaments….
Myosin filament with myosin heads.
Myosin is a Hexomer:
- 2 Heavy chains = drives primary muscle action
- 4 light chains = fine tunes muscle action
*Myosin heads bind actin (thin filament and ATP)
What is Tropomyosin?
A thin strand that wraps around the actin (thin filament) that has cross bridge binding sites on it.
What is Troponin?
A molecule that has a Ca++ binding site that sits on top of the tropomyosin filament.
What is Troponin made of?
Troponin is comprised fo 3 subunits:
TnC = The calcium receptor/binding portion
TnL = Has inhibitory function (if you were to remove it, there would be sustained muscle contraction.)
TnT = Binds Troponin to Tropomyosin to from the Tropomyosin/Troponin complex.
What is an Isoform?
Same protein, but slightly different AA sequence yet similar function. Ex. Myosin Heavy chain Myosin Light Chain Actin TnC, TnL, TnT Tropomyosin
What is a sarcomere?
The functional unit of the contractile apparatus (muscle).
What is a Sarcomere composed of?
- Thick filaments
- Thin filaments
- Z-disks or Z-lines (One sarcomere is from Z-disk to Z-disk)
What do Sarcomeres do?
Shorten and generate force.
Draw a Sarcomere!
Okay done.
What is the I Band?
The area of thin filament that does not overlie the Thick filament (Just inside the z-disk)
What is the A band?
The area including all of the thick filaments
What is the H zone?
Middle area of sarcomere where thin filaments do not overlap thick filament at rest.
What is the Excitation-Contraction Coupling?
The mechanism by which AP in the sarcolemma initiates muscle contraction. *Ca++ play a critical role in the ACTIVATION of skeletal muscle. *The goal of the E-C coupling is to achieve a rapid, very large increase in the free Ca++ ion concentration inside the muscle cell.
______ binds Calcium in the lateral sacs.
Calsequestrin
Describe the Steps in the process of Excitation Coupling….
- Muscle AP propagated
- Ca++ released from lateral sacs
- Ca++ binding to troponin removes blocking action of Tropomyosin
- Crossbridge moves
- Ca++ is taken up
- Ca++ removal from troponin restores Tropomyosin blocking action.
What are the 2 Ca++ channels involved in the E-C coupling?
DHP receptor
Ryanodine Receptor
What is the Sarcoplasmic reticulum?
The Lateral sacs and Fenestrated collar.
- Ca++ ions are release from lateral sacs to the SR to Initiate contraction
- Ca++ ions are sequestered (taken up) by the fenestrated collar of the SR to cause relaxation
What happens at low cytosolic calcium levels?
Relaxed muscle = energized cross bridge (Myosin head) cannot bind to actin.
What happens at High cytosolic calcium levels?
Activated muscle = cross bridge (myosin head) binds to actin and generates force.
What is the Sliding Filament Theory?
Muscle shortens by a relative sliding of thick and thin filament. The Filaments DO NOT change length!
During the Sliding filament contraction, which bands get reduced?
The H zone and I band get reduced, the A band remains unchanged.
What is the Cross-bridge theory?
Filament sliding is due to repetitive crossbridge cycles:
One cycle = myosin head attaches to actin, followed by a conformational change in myosin and simultaneous sliding of thin filament followed by detachment. *Thick and thin filaments are not connected at rest!
What are the 4 stages of a cross bridge cycle?
- Cross bridge binds to actin
- Cross-bridge moves (This is the power stroke or working stroke of the cycle)
- ATP binds to myosin causing crossbridge to detach
- Hydrolysis of ATP energizes cross bridge for round 2.
What determines the amount of force generated during muscle contraction?
of Crossbridges attached
What is the source of energy for muscle contraction?
ATP (ATP —-Myosine—-> ADP + Phosphate)
The concentration of TP inside the muscle cells is buffered by ________.
Phosphcreatine (PCr)
Why is there no change in ATP levels during Muscle contraction?
Because ATP is so well buffered (by PCr) that it can keep conc. up (very important too).
What is the PCr buffering equation?
PCr + ADP —-Creatine Kinase—> ATP + Cr
*Creatine Kinase is the mist important protein at the M-line of sarcomere!
What are the 3 sources of ATP production?
- Creatine Phosphate
- Glycolysis
- Oxidative Phosphorylation
Myosin ATP-ase =
Contraction
Calcium ATP-ase =
Relaxation (Yes it takes ATP too)
What is the Length-tension relationship?
Basically that the amount of force a muscle can generate is dependent on its length…..so If a muscle is stretched or shortened from its 100% length it will generate less force from said position. *Stretching is worse. *Thin filament length affects length/tension relationship.
What is the Load-velocity relationship?
Muscles can shorten at higher velocity when moving a lighter load. *Interesting because it tells us something is affecting the rate of Cross-bridge cycling….
Note that the force sustained during a LENGTHENING contraction is _________.
greater than the maximum isometric tension = can cause muscle injury.
What are the different types of muscle contractions?
- Isometric = constant length
- Isotonic = constant load
What is a motor unit?
a single motoneuron and all the muscle fibers in innervates.
Most muscle fibers have _____ NMJ’s
one *and are innervated by single neuron.
Motoneurons can innervate _____ muscle fibers
More than one *100-2000
What are the two main types of muscle fibers?
Slow and fast
Slow, compared to fast:
- have smaller NMJ’s
- are smaller in diameter
- Contain differnet sarcomere protein isoforms
- contract more slowly
- are MORE fatigue resistant
How do slow and fast muscle fibers differ in there ATP production?
Fast = more glycolysis Slow = More Oxidative phosphorylation
Which type of muscle fiber is most efficient (among skeletal fibers)?
type I (slow)
What is the difference between type 2a and 2b fibers?
2a = tend to be smaller and depend more on oxidative metabolism and are less fatiguable, yet slower than 2b.
2b = faster, stronger, but less efficient and use glycolysis
Which type of muscle fibers fatigue quickest?
Fast-glycolitic
What causes Skeletal muscle cramps?
dehydration low sodium, magnesium, calcium, glucose or potassium Pregnancy Drugs Thyroid...
What is muscular dystrophy?
Deficiency or defect in dystrophin = links cytoskeleton proteins to membrane which causes membrane tears.
What is the main difference between a skeletal sarcomere and a cardiac sarcomere?
- The thin filaments are not all the same length = might be related to possible varied levels of contraction during filling vs pumping.
- 1 alpha heavy chain (Skeletal muscle is Beta?) *alpha is only found in masseter muscle.
- Nebulin is not present (probably due to difference in thin filament lengths.
What is nebulin?
A thin filament protein that is possibly used as a ruler to determine thin filament length.
Cardiac muscle cells are _____ than skeletal muscle cells.
Much smaller
What are attaches cardiac cells together (since there is no bone)?
Intercalated discs *Along the length of theses there are gap junctions that assist the heart in functioning as one unit, these ELECTRICAL synapsed do not involve chemical transmitters for AP transmission. *Mature skeletal tissue does not contain Gap junctions
Describe the differences in Time between Twitch (skeletal fiber) and Cardiac muscle?
- the events of the heart are much slower
- Entire Ap last 200mS in heart (3ms in skeletal)
- The AP in cardiac lasts until twitch tension is relaxed about 50% (Skeletal = AP is done before fibers begin to shorten.
- LONG REFRACTORY period in cardiac AP which prevents undergoing tetanic contraction which could be deadly.
Conductance change during Ventricular Ap: phase 0 =
Increase in conductance of Na+
Conductance change during Ventricular Ap: phase 1 =
Increase in conductance of K+
Decrease in conductance of Na+
Conductance change during Ventricular Ap: phase 2 =
Increase in conductance of Ca++
Decrease in conductance of K+
Conductance change during Ventricular Ap: phase 3 =
Increase in conductance of K+
Decrease in conductance of Ca++
Conductance change during Ventricular Ap: phase 4 =
Essentially no net flow!
What are the 2 sources of activating Ca++ ions in the heart?
- Ca++ ion enter cells from interstitial space by passing through channels in sarcolemma during plateau phase of AP.
- These Ca++ ions somehow trigger the release of many other Ca++ ions from the sarcoplasmic reticulum which bind to troponin C to cause activation of cardiac muscle. this process is called “calcium induced calcium release”.
After activation, how are Ca++ ions removed from the heart?
- Primary Mech. of removal of Ca++ from the sarcoplasm utilizes a Ca++-ATPase pump almost identical to the Ca-pump in skeletal muscle. (Up take by SR)
- Another Ca++-ATPase pump in the sarcolemma moves Ca++ ions out of the cell (across the sarcolemma)
- Another Mech, in the sarcolemma: Na+/Ca++ exchanger = this protein moves Ca++ out and Na+ in DOES NOT CONSUME ATP DIRECTLY! *Secondary active transport
_____ has little sarcoplasmic reticulum.
Smooth muscle
______ are homologous to the z lines in sarcomeres of striated muscle.
Dense bodies *Thin filament attach to dense bodies.
All innervation to smooth muscle is _______.
Autonomic nervous system (involuntary)
smooth muscle has ______ cell lengths over which force can be generated compared to striated muscle.
Greater
Rate of ATP splitting by myosin in smooth muscle is ______ than striated muscle.
10-100 times lower. Therefore there is virtually no fatigue!
What are the steps in Smooth muscle E-C coupling?
- Stimulus
- Increase in Ca++
- Ca + Calmodulin = Ca-calmodulin
- Ca-calmodulin + Myosin light chain kinase (MLCK)inactive ——> Ca-calmodulin-MLCK active
- Myosin-P = contraction
- Myosin + Pi = relaxed
What are the sources fro Ca++ activation in smooth muscle?
- Sarcoplasmic reticulum = Sparse
- Extracellular fluid = Ca++ ions enter the cell from he Extracellular space through the plasma membrane.
* AP’s in smooth muscle - inward flow of positive charge is carried by Ca++ ions during the rising potential (Instead of Na+)!
What are the 4 mechanisms of control of smooth muscle activation?
- Pacemaker potenial causes membrane potetial to reach threshold.
- Endings of motor neurons of autonomic nervous system release neurotransmitters in vincinty of smooth muscle…may be excitatory or inhibitory. *NO NMJ in smooth muscle.
- Hormones act like neurotransmitters
- Local factors - pH, oxygen level, nitric oxide, stretch
What are the 3 mech. of calcium removal in smooth muscle?
- Ca+ pump in sarcolemma
- Na/Ca exchanger
- Sarcoplasmic reticulum
* Same as striated muscle.
What are the 2 types of smooth muscle?
Single unit
- electrical synapses
- spontaneously active
- stretch activated
- EX. intestinal tract, uterus, small BV’s
Multi-unit
- each cell activated indenpendatly
- not spontaeously activated
- Gap junctions are rare
- Ex, large arteries and airways
