Skeletal Muscle Flashcards
Describe how the sliding filament model corresponds to the isometric muscle length and force curve
See pg. 237
To measure the force generated during an isometric twitch, a muscle if fixed in position such that force can be measured without the muscle shortening (isometric). The muscle is stimulated electrically with a single AP, and the force (or tension) generated by the muscle is measured as a function of time.
There is a latent period between the stimulation and contraction = delay between AP and the activation of the crossbridge cycle.
Tension builds during the contraction phase of the twitch and tension declines during the relaxation phase (contraction generated by a single AP is called a twitch). Furthermore, the contraction and relaxation periods closely parallel the levels of Ca2+ in the sarcoplasm that surrounds the sarcomere (Ca2+ present in contraction & absent in relaxation).
Total tension = passive tension (initial, prior to contraction) + active tension. When the muscle is at the optimum length the number of active crossbridges between thick & thin filaments is the greatest = max active tension. Muscle length greater or less than the optimum = less crossbridges = less force.
Organize these terms from largest unit to smallest unit: sarcomere, myofibril, muscle fascicles, muscle, muscle fiber, myofilaments
muscle, muscle fascicles, muscle fiber = myofiber, myofibril, myofilaments, sarcomere
Thick filaments
Myosin
Thin filaments
Actin
Z line
marks the boundary between two sarcomeres
I band
contains only the actin thin filaments, which extend from the Z line toward the center of the sarcomere
A band
Myosin thick filaments are found in the dark A band
H zone
there is no overlap between actin and myosin
M line
is at the center of the sarcomere and is the site at which the thick filaments are linked with each other
Do the fill-in-the-blank sarcomere sheet
-
Sarcomere is the
contractile unit of muscle from one z line to the next z line
Sliding filament theory: In the fully contracted muscle, the ____disappears, ____ becomes very narrow, the length of ____ is constant, & the ____ move closer together. As the sarcomere shortens, so does the ____?
H zone disappears
I band becomes very narrow
A band is constant
Z lines move closer together
As the sarcomere shortens, so does the muscle
The sliding filament theory states that the ____ and ____ filaments slide past each other during muscle contraction.
thick
thin
Myosin generates a pulling force on actin towards the?
M line or the middle
Describe the chemical and mechanical steps in the crossbridge cycle
- An AP travels along a motoneuron & initiates an AP in the muscle cell.
- The nerve endings secrete Ach which acts on the sarcolemma to open Ach-gated ion channels.
- Opening of these channels permits Na+ to flow into the muscle, thus depolarizing the muscle & initiating a second AP which propagates along the muscle fiber membrane.
- The muscle AP propagates down the T-tubule (transverse tubule) membranes into the interior of the muscle fiber to the triad junction, where it causes release of Ca2+ from the sarcoplasmic reticulum.
- The increased concentration of Ca2+ in the sarcoplasm causes the actin and myosin filaments to interact with each other, resulting in a sliding motion that shortens the length of the sarcomere.
- The Ca2+ are then ACTIVELY pumped back into the sarcoplasmic reticulum by the Ca-ATPase ion pump located in the sarcoplasmic reticulum membrane, thus reducing the concentration of free Ca2+ in the sarcoplasm, which allows the muscle fiber to relax. The rate at which Ca2+ is sequestered by the sarcoplasmic reticulum of skeletal muscle during a twitch is directly related to the rate of relaxation.
- Lengthening of the muscle is achieved by contraction of an antagonistic muscle, e.g. contraction of the triceps lengthens the biceps (& vice versa).
@ rest (no Ca2+), the ______ active sites on thin filaments cannot attach to the thick filament myosin heads.
Troponin C
When Ca2+ released from the SR enters the cell it binds to _______ & causes _______ to shift, exposing the troponin sites on actin so it can attach to the myosin heads.
Binds to Troponin C
Tropomyosin to shift
Describe the steps of the crossbridge cycle
1) ATP binds to myosin, causing the myosin head to detach from actin.
2) Myosin ATPase cleaves ATP. Hydrolysis of ATP causes cross bridge, still separated from actin, to change conformation to a “cocked” orientation.
3) A “cross-bridge” forms as the myosin head binds to actin.
4) The myosin head then releases the inorganic phosphate (P), which causes a conformational change in crossbridge to tilted position, constituting the power stroke that drags the actin filament towards the M-line.
5) ADP is then released in the final step of the cycle.
Describe why muscles are contracted in rigor mortis
No ATP around, but Ca2+ is present so it binds with Troponin C and causes tropomyosin to shift, allowing for crossbridging to occur between myosin and actin proteins = they are stuck in the contracted state.
Total tension =
Total tension = passive tension + active tension
Discuss how the A band is _____ & the I band ____ in a muscle contraction from the perspective of microscopy
A band constant; I band narrows
Both the A (anisotropic) and I (isotropic) bands are visible in polarized light and the I band disappears (A band stays constant) as the muscle contracts.
Satellite cells are thought to contribute to muscle fiber growth and repair by
Directly fusing with the myofiber (muscle fiber).
Surprisingly under conditions of injury or increasing work demands satellite cells fuse to the adjacent myofiber, contributing their cytoplasmic volume and nucleus to the enlarging myofiber. The nucleus contains genes that encode sarcomeric proteins (e.g., actin, myosin, tropomyosin) that get turned on after cellular fusion.
The pharmaceutical industry has some interest in myostatin as a drug target for muscle wasting diseases like muscular dystrophy because
Low myostatin levels promote satellite activation and fusion with the muscle fiber.
Myostatin is a circulating growth factor that restrains muscle growth. Patients with no myostatin have exaggerated muscle growth and strength. The pharmaceutical industry believes that inhibition of myostatin should increase muscle strength, and that this will occur by promoting satellite cell activation and fusion with the muscle fiber.
Describe skeletal muscle nuclei structure
multinucleated
