Skeletal Muscle Flashcards
Give the anatomical arrangement of skeletal muscle
Muscle fibers - myofibrils - myofilaments (contractile element)
___ lines delineate the sarcomere
Z
__ bands represent the thin filaments that are anchored to Z lines and extend in each direction.
I
Do I bands overlap with thick filaments?
no
The ___ band includes the ___ zone, which is only myosin thick filaments with no overlapping thin filaments and some degree of overlap of thin and thick and thin filaments
A; H
Thin filaments are polymers composed of two strands of F-actin wrapped around each other in a helical arrangement with ___ and ____
troponin; tropomyosin
In regards to thin filaments, ____ is arranged in parallel with the actin strands
tropomyosin
For each of the following Troponin subunits, give its function:
Troponin T
Troponin I
Troponin C
Troponin T: binds troponin to tropomyosin
Troponin I: inhibits the interaction between myosin and actin
Troponin C: binding site for Ca2+
Thick filaments are composed of several hundred ___ molecules. It is made up of __ heavy chains and ___ light chain head units. The head units have ___ activity.
myosin; 2; 2; ATPase
This protein connects the Z line to the M line of the sarcomere. It acts as a molecular spring and contributes to the passive elasticity of muscle, and contributes to force transmission at the Z line and resting tension at the I band
Titin
This protein is a large molecule that forms a rod connecting the actin filaments to the transmembrane protein B-dystroglycan in the sarcolemma by smaller proteins in the cytoplasm known as syntrophins (forms scaffold that connects myofibrils to external environment)
Dystrophin
Disruption in the structure of Dystrophin can lead to specific muscular pathologies such as ___ ____
muscular dystrophy
T-tubules come in close contact with the SR in skeletal muscle, forming ____
triads
The SR contains the Ca2+ binding protein, ____, which facilitates the storage of large quantities of Ca2+
calsequestrin
Skeletal muscle fibers are innervated by ___ motor neurons. These are large, myelinated neurons with rapid conduction velocity.
alpha
The resting membrane potential in skeletal muscle is approx ___ mV
-90
The AP duration in skeletal muscle is only __-__ ms, while the duration of muscle contraction is about ___-___ ms
3-5; 20-100
Voltage-gated Ca2+ channels called _____ receptors are present in the t tubules.
dihydropyridine (DHPR)
Activation of the DHPR physically opens the ____ receptor in the terminal cisternae of the SR, allowing Ca2+ to leave
ryanodine
The Ca2+ released from the SR into the cytosol is pumped back into the SR by what?
SERCA (ATPase)
SERCA has a greater affinity for Ca2+ than troponin C and starts pumping Ca2+ back into the SR immediately after it is released into the cytosol. True or false?
true - very transient event
The increase in cytosolic Ca2+ results in Ca2+ binding to troponin C which weakens the interaction between troponin ___ and ____.
I; actin
note: this causes conformational shift of both the troponin and tropomyosin that uncovers the actin binding site for myosin and leads to the formation of anti-myosin crossbridges
The binding of actin and myosin results in the release of ___ and ___
ADP; Pi
After ADP and Pi have been released, there is a conformational change in the myosin head unit that moves the ___ filament relative to the ___ filament and constitutes the “power stroke”
thin; thick
The power stroke shortens the sarcomere by about ___ nm
10
After the power stroke, ATP binds to the open site on myosin. What is the result?
detachment of the myosin head from the thin filament
After the myosin head detaches from the thin filament, the ATP is hydrolyzed and Pi is released, causing “___” of the myosin head unit
cocking
If there is no ATP available, the actin remains bound to the myosin leading to a state of “___” If the transport of Ca2+ back into the SR is inhibited, the muscle will remain in a state of “____”
rigor; contracture
Ca2+ is pumped back into the SR by SERCA, and as cytosolic levels fall, Ca2+ detaches from troponin C leading to what?
cessation of the actin-myosin interaction
The coordinated shortening of many sarcomeres results in force generation and movement. Muscle has elastic components that must be overcome to accomplish muscle shortening. These are in ___ and in ___ with the contractile elements
parallel; series
Parallel elastic elements include?
sarcolemma, intracellular architectural components, extracellular CT
Series elastic elements represent the ___ and regions within the ___
tendons; myosin
____ contractions represent a single contractile event resulting in a brief (20-100 ms) contraction.
twitch
In regards to twitch contractions, the velocity of shortening is inversely related to the initial load on the muscle. If the load exceeds the force generated during the contraction, what is the result?
no shortening, and the velocity will be zero (isometric contraction).
In regards to twitch contractions, as the load ___, the velocity of shortening increases to a theoretical Vmax at zero initial load.
decreases
____ contractions are characterized by contractions that result in sarcomere shortening (internal shortening), but do not result in external shortening (same length). In such cases, the load exceeds the ability to overcome the elastic elements and there is no external work
isometric
_____ contractions are characterized by contractions that result in sarcomere shortening and external shortening. In such cases, the load is less than the tension that can be generated by the contractile elements resulting in external work.
Isotonic
_____ contractions occur when the muscle contractile elements are contracting, but the muscle is being externally lengthened forcefully. These type of contractions usually result in muscle damage.
eccentric
Contractile elements do not display ____ so as soon as another action potential fires, another contraction can be initiated. If the cytosolic Ca2+ is still elevated the resulting contractile tension will be greater because there is a greater intracellular Ca2+ level
refractoriness
Describe incomplete tetany
if the stimulus frequency is increased, the developed tension will increase. As the stimulation frequency increases, the phasic nature of the twitch contraction begins to disappear and tension does not return to baseline
Describe tetanic contraction
if the stimulation frequency increases sufficiently there will be less time for relaxation between stimulations and muscle force will be sustained, resulting in a complete or fused tetany.
At compltete tetany, the muscle tension will be about ___ times greater than a single twitch contraction
4
When muscle is stretched from an unstressed length, the resting or ____ tension increases as a function muscle length.
passive
note: this is due to the progressive stretch on the elastic elements within the muscle
If the muscle is contracted at each of the same muscle lengths, from the unstressed length, the ___ tension development will increase to some maximum
active
The sarcomere length at which active tension is greatest represents the “optimal length” and is typically ___-___ um
2.0-2.2
Define motor unit
single alpha motor neuron and all the muscle fibers it innervates
The exception of a single motor neuron leading to an AP (“all or nothing”) will cause every muscle fiber in that motor unit to contract simultaneously. True or false?
true
Motor neurons also influence muscle fibers in a trophic manner. What does this mean?
the properties of all muscle fibers in the motor unit are similar
The following describes what type of muscle fiber: diameter: small myosin ATPase: slow rate of Ca2+ sequestration: moderate mitochondria: abundant oxidative enzymes: high glycolytic enzymes: moderate myoglobin: abundant capillary density: high tension/motor unit: low control: fine
Type I
The following describes what type of muscle fiber: diameter: moderate myosin ATPase: fast rate of Ca2+ sequestration: high mitochondria: abundant oxidative enzymes: moderate glycolytic enzymes: high myoglobin: moderate capillary density: high tension/motor unit: moderate control: moderate
Type IIA
The following describes what type of muscle fiber: diameter: large myosin ATPase: fast rate of Ca2+ sequestration: high mitochondria: sparse oxidative enzymes: low glycolytic enzymes: high myoglobin: none capillary density: low tension/motor unit: high control: coarse
Type IIB
Motor units are recruited in an orderly manner, from smallest to largest, as the need for tension generation increases. This is known as the ___ _____ and permits smooth, graded development of muscle tension
size principle
Maximum tension development can be attained when all of the largest motor units are recruited. However, while these large motor units generate the greatest tension, they are not well-suited for what?
prolonged activity (fatigue very quickly)
