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).
