What produces the striated appearance of skeletal muscle?
The organization of the sarcomeres.
What makes up a sarcomere?
One Z line to the next is one sarcomere
What is contained within the I band?
Only actin thin filaments, which extend from the line toward the center of the sarcomere.
Where are myosin thick filaments located?
The A band (darker)
In the H zone, is there overlap between actin and myosin?
What is the M line in skeletal muscle?
The M line is the center of the sarcomere and is the site at which the thick filaments are linked with each other.
Skeletal muscle is composed of __1__ that are in turn comprised of __2__ muscle fibers. These fibers are composed of smaller __3__, which contain sarcomeres, the site at which __4__ and __5__ filaments produce contraction.
1. Fascicles 2. Multinucleated 3. Myofibrils 4/5. Actin, myosin
How do the Z lines move during skeletal muscle contraction?
They move closer together.
How does the length of the A band change during skeletal muscle contraction?
It stays constant.
How does the length of the I band change during skeletal muscle contraction?
It gets shorter.
How do thick and thin filaments move during skeletal muscle contraction?
They slide past each other.
Muscle myosins are organized into what?
Each myosin head contains what important features?
1. An ATPase catalytic site 2. Actin-binding site
What structure is responsible for phosphorylating the regulatory light chain of myosin (RLC) in striated muscle?
Ca-dependent MLCK (myosin light chain kinase)
What is the first step in creating a muscle contraction?
A motor AP travels along a motor neuron to the motor end plate at the neuromuscular junction.
The nerve endings at the neuromuscular junction secrete what substance, upon being excited?
What effect does acetylcholine have that the neuromuscular junction?
It opens acetylcholine-gated ion channels in the sarcolemma. These open channels permit sodium to flow into the cell, thus depolarizing membrane potential, and initiating an AP which is propagated along the length of the muscle fiber membrane.
How does a muscle AP propagate?
It travels along a t-tubule (transverse tubule) into the interior of the muscle fibers to the triad junction.
When a muscle AP reaches the triad junction, release of what occurs?
Release of Ca from the sarcoplasmic reticulum.
How are calcium ions pumped back into the sarcoplasmic reticulum after a muscle AP?
Ca-ATPase ion pump located in the SR membrane.
What happens when Ca is removed from the sarcolemma?
The muscle fiber can relax.
Lengthening of one particular muscle is achieved by contraction of what?
An antagonistic muscle, for example, contraction of the triceps in order to lengthen the biceps (pairs).
Each myofibril is surrounded by what?
2 cisternae + 1 t-tuble = ?
The sarcoplasmic reticulum has ____ which abut t-tubules
What is the function of tropomyosin?
Tropomyosin blocks myosin from binding to the thin filament.
What does the C in troponin C mean?
It means the subunit that calcium binds to
Troponin is a complex of what 3 subunits?
1. Troponin C binds calcium 2. Troponin I has a strong affinity for actin and inhibits actomyosin ATPase 3. Troponin T binds tropomyosin
The troponin complex binds ____ to the thin filament?
What happens when troponin C binds calcium?
Tropomyosin moves and exposes the active sites on actin, permitting the interaction between the myosin head and actin.
Tropomyosin in resting muscle serves what function?
To inhibit the force-generating interaction between actin and myosin.
Name the five main steps in the crossbridge cycle, starting with ATP binding to myosin.
1. ATP binds to myosin causing tilted cross bridge to separate from actin. 2. Myosin ATPase cleaves ATP but hydrolysis products ADP and phosphate remain bound to myosin. 2b. Cleavage of ATP creates the cross bridge (still separated from actin), to change to cocked conformation. 3. A cross bridge forms as the myosin head binds to actin 4. The myosin head then releases the phosphate which causes a conformational change in the cross bridge to a tilted formation (power stroke dragging the actin filament towards the M line). 5. ADP is released
What percentage of energy of ATP is transformed into mechanical work?
Contractions which occur at constant muscle length are known as:
The muscle contraction generated by a single AP is called:
Extraocular muscles have what kind of fibers that confer the ability to move quickly?
Fast twitch fibers. They show very rapid isometric twitches.
What are some structural differences between fast twitch and slow twitch muscles?
1. Speed of SR elevation and clearance of calcium 2. Type of myosin heavy chain involved
Various myosin heavy chain genes encode proteins that differ how?
In the rates of ATP hydrolysis, which thereby influence the speed of the cross bridge cycle.
What kind of muscle has the most mitochondria?
Type I (slow twitch) Red, fatigue-resistant, low glycogen
What kind of muscle has the most glycogen?
Type IIb, fast twitch Fatiguable, white, glycolytic metabolism (NOT oxidative)
Red color in muscle means high what?
In an isometric experiment, the muscle notable, does NOT do what?
Shorten during a contraction
In an isometric experiment, explain the two curves being drawn.
Difference between upper and lower curves is the active tension generated my muscle contraction.
Active tension by sending impulses through it (Total)
Passive tension by stretching the muscle (Passive)
What is the relationship between tension and thick/thin filament overlap?
Linear. As tension increases, thick and think filament overlap also increases (until the sarcomere is stretched too far, like diminishing returns, then tension goes down)
When can muscle tension decrease?
2. If the muscle is shortened below its optimal length for active tension generation. A plateau is reached when the thin filaments reach the H-zone. Tension decreases as the thin filaments cross the M-line and then interfere with formation of cross bridges on the other side of the sarcomere. The tension decreases to zero when the thick filaments hit the Z line.
1. If the sarcomere is stretched beyond its limit
When muscle is at optimal length, the number of active cross bridges is ???
The greatest. When the muscle is stretched beyond this length the number of activce cross bridges decreases.
Beyond the ideal muscle stretch length, why does the number of active cross-bridges decrease?
The overlap in actin and myosin fibers has decreased.
When the muscle becomes shorter than its optimal length, what happens to the thin filaments?
They overlap with each other instead of with the thick filaments, interfereing with each others' movement. This results in a show decrease in tension.
What is the normal range of muscle length?
100-115% its resting length, which corresponds tot he maximum overlap between thick and thin filaments.
Why are satellite cells called as such?
Because they lie at the periphery of mature muscle fibers within the basal lamina.
When are satellite cells mobilized to proliferate, differentiate and fuse into multinucleat myofibers?
Following muscle injuries, or in response to increasing work demands.
What is the effect of losing myostatin in a muscle?
Muscle hypertrophy and altered (increased) satellite cell activation
Emerging studies suggest that exercise and growth hormone has what effect on myostatin?
They decrease myostatin levels.
What protein is being studied with respect to its potential in treating muscular dystrophy?
Myostatin (MYO-029 study)