Flashcards in Skeletal muscle organization and mechanics Deck (60):
The alpha motorneuron acts as the final common pathway for motor unit activation because?
It integrates excitatory and inhibitory inputs
Where are cell bodies for motor neurons?
What is included in a "motor unit"?
motor neuron plus all the muscle fibers it innervates
What is the difference between slow twitch motor units and fast twitch motor units?
slow twitch: small motor neuron bodies with innervate relatively few muscle fibers which have smaller cross sectional area.
Fast twitch: large motor neuron body that innervates a large number of fibers with larger cross sectional area
What causes release of Ach from presynaptic nerve?
Action potential reaches the end of the nerve fiber, opens voltage-gated calcium channels, which leads to influx of calcium into neuron and fusion of vesicles with the presynaptic membrane --> release of ACh
What kind of receptors are on muscle fibers? autonomic ganglion?
Describe nicotinic Ach receptors
ligand gates Na channels that bind 2 Ach
Binding of 2 Ach molecules to nicotinic Ach receptors cause _____.
influx of Na into the muscle fiber and a graded depolarization
Describe how action potential is generated in the muscle fiber after Ach has bound receptors and opened Na channels?
1. A sufficient number of receptors must be activated in order to trigger an action potential in the muscle fiber (individual graded depolarizations from NM channels must be summed and reach threshold)
2. The action potential propagates along the muscle fiber membrane like it would in an unmyelinated nerve axon
3. Because the muscle fiber is really long and really thin, the action potential moves almost completely parallel to the long axis of the muscle fiber
How is Ach degraded? What happens to the products?
1. Acetylcholinesterase in the synaptic cleft degrades acetylcholine into acetate and choline
2. Choline is the transported back into the nerve terminal and combined with acetyl-CoA to generate new acetylcholine
What are T-tubules
Invaginations of the muscle fiber membrane which pass through the cell and bring the membrane close to the SR.
What is the sarcoplasmic reticulum?
Specialized endoplasmic reticulum that stores a butt load of calcium which would be toxic to the cell if allowed to remain in cytoplasm for very long
What steps couple excitation to contraction?
Depolarization of T-tubule--> conformation changes in voltage sensitive dihydropyridine receptor--> activates ryanodine receptor--> allows efflux of Ca
What happens to Ca and associated receptors after action potential?
The dihydropyridine receptor resumes its role of inhibiting the ryanodine receptor. Calcium is pumped back into SR into active transport.
subunit of muscle fiber contractile machinery spanning from one Z line to the next Z line
What is the Z line?
In middle of I band, where thin filaments are anchored
What does cross section through I band show?
ordered hexagonal array of thin filaments anchored to Z line
What does cross section through A band show?
ordered hexagonal arrangement of thick filaments
What does cross section through section where A and I band overlap show?
Interdigitation of thick and thin filaments
T or F. The muscle fiber can expand/contract withouth the A band changing length at all.
Components of thick filament
1.Myosin heavy chain: large, with filamentous tail, globular head. a) tend to form dimers b) Dimers polymerize to form thick filaments
2. Myosin light chains: 2 per head a) Role appears to be permissive and modulatory for ATPase activity of heavy chain globular head
What are the 3 parts of thin filament?
Actin, tropomyosin, troponin
_____ are fibrous proteins that extend along length of thin filaments, covering up the mysoin binding sites on actin moleucles.
What are the components of troponin ternary complex?
Troponin C (Ca binding), T (tropomyosin binding), I (keeps troponin complex in position covering the myosin binding sites when calcium is not present in high concentration.)
When does myosin bind actin?
When myosin has ADP bound to its head, it changes conformation into a high-affinity binding state for binding actin
Power stroke occurs when there is a conformational change of what part of myosin?
What happens to Z lines in power stroke?
Z lines are pulled closer to each other
what causes release of myosin from actin?
Once power stroke has occurred (bending of myosin at hinge region), myosin has a higher affinity for binding ATP than ADP--> ATP displaces ADP--> Causes myosin to bind with lower affinity to actin --> release
Active force is maximum at a specfic _______.
optimal muscle fiber length
What happens if muscle fiber is too short?
there is too much baseline overlap between thick and thin filaments: only a small amount of force can be generated from that position (steric hindrance of too many proteins in a tight space)
What happens if the muscle fiber is too long?
there may be zero overlap between thick and thin filaments and therefore myosin cannot interact with actin and zero force can be generated
At optimum length, what is the passive force?
A fibers response to a single action potential is _____
If you start in optimal position and apply one action potential, how does the muscle fiber contract?
Time to peak tension
how long it takes from action potential until peak tension is generated in the muscle fiber
Half relaxation time
length of time it takes to relax from peak tension to 1/2 peak tension
What is isometric twitch when there is weight attached to the muscle?
If the weight is so heavy that there is no contraction. There is also an initial period of isometric contraction until the weight starts to move
Once the weight is being lifted, the tension in the muscle fiber remains _____.
If you reduce weight on a muscle, what is the energy being used for?
less into development of tension, more into moving weight
What can we obtain from a force velocity graph?
We can see the inverse relationship between initial velocity and force (weight). X intercept is the peak isometric tiwtch tension
How to time to peak tensions and half-relaxation time compare between fast and slow fibers.
They are faster in fast fibers. This sounded like a better question in my head...
Isometric twitch tension is greater for fast or slow units?
T or F. For a given force, fast motor units will develop a greater velocity of shortening than slow units.
Do smaller neurons require fewer or more excitatory symapses to acheive threshold at the axon hillock.
If similar numbers of excitatory and inhibitory synapses are happening on a large motor neuron and a small one, the ____ one is more likely to actually fire an action potential
_____ are recruited frist
small neurons/slow twitch units
At lower Hz, which slow or fast motor units is more likely to have summation?
Slow. the fast will contract and relax before the next AP.
What is the tetanic tension?
summations of contractions of units that reach a max
Which is bigger, tetanic tension or twitch tension?
2 ways to achieve graded contraction
recruit more motor units, increase frequency of contraction for motor units already recruited
Describe recruitment/summation in order of force required?
1. Slow motor units are recruited first.
2. Slow motor units get summation
3. Fast units recruited
4. Summation of fast units
In myofibrils, ATP consumed by myosin is regenerated how?
donation of high-energy phosphate by from creatine phosphate (via action of creatine phosphate kinase, CPK) with creatine as the by product
What would suggest that too much creatine is being made?
elevated CPK or LDH and creatinine
Describe ATP metabolism in slow motor units.
They have more myoglobin and more mitochondria. They make ATP by oxidative phosphorylation. They consume ATP at a relatively slow rate.
Slow motor units are found in ____ muscles
postural (ones that need to contract for a long time)
Describe ATP metabolism in fast motor units
They have fewer mitochondria and fewer myoglobin. Almsot entirely based on glycogenesis. Consumes ATP rapidly and storage is limited--> muscle fatigue
What motor units tend to consume ATP more quickly than slow units but more slowly than fast units
What kind of adaptations do resistance tasks cause?
increases muscle fiber diameter, expression of fast myosin and amount of glycogenolytic enzymes
What kind of adaptations do endurance tasks cause?
that shift towards slow myosin expression and more oxidative metabolism (less glycogenolysis)