S1B4 - Skeletal Muscle Flashcards
Decreased intracellular calcium concentrations leads to what process in skeletal muscle cells?
Skeletal muscle cell relaxation occurs with decreased intracellular calcium concentrations.
Steps leading to muscle cell relaxation:
- Decreased amount of acetylcholine bound to muscle cell membrane
- Muscle cell repolarizes
- Calcium release from sarcoplasmic reticulum decreases
- Calcium is transported back into sarcoplasmic reticulum by calcium ATPase
- Decreased calcium binding to troponin C
- Tropomyosin blocks actin-myosin binding sites
- Muscle relaxation occurs
Following muscle cell depolarization, what two receptors in skeletal muscle cells undergo sequential conformational changes, resulting in the release of calcium from the sarcoplasmic reticulum?
Once an end plate potential depolarizes the voltage-sensitive dihydropyridine receptor on the transverse tubule membrane, this induces a conformational change to the ryanodine receptor on the sarcoplasmic reticulum. As a result, calcium is released from the sarcoplasmic reticulum.
What type of receptor does acetylcholine bind to on the motor end plate?
Acetylcholine diffuses across synaptic cleft and binds to cholinergic receptors that form ligand-gated ion channels which allow the diffusion of sodium and potassium ions across the membrane. Acetylcholine binding results in an end plate potential. The end plate potential takes the muscle cell membrane to threshold and an action potential is propagated over the cell membrane into transverse tubules.
In skeletal muscle cross-bridge cycling, what occurs when ATP binds to the myosin head?
Steps in cross bridge cycling in skeletal muscle cells:
- ATP binds to the myosin head and myosin is released from actin
- ATP is hydrolyzed and the myosin head changes to a “cocked position”
- Myosin head binds to actin releasing ADP and phosphate
- Power stroke occurs
Upon binding of ATP to the myosin head, myosin is released from actin.
Which neurotransmitter is utilized in the excitation-contraction coupling process in skeletal muscle? What enzyme synthesizes the neurotransmitter in the presynaptic nerve?
Acetylcholine is utilized in the excitation-contraction coupling process in skeletal muscle. Acetylcholine is synthesized in the presynaptic nerve terminal by the enzyme choline acetyltransferase, stored in vesicles, and released upon appropriate stimulation.
The influx of what ion into the presynaptic membrane terminal leads to the release of acetylcholine into the neuromuscular junction?
Within the motor neuron axon terminal, acetylcholine is stored within vesicles. A wave of depolarization travels down the nerve and voltage sensitive calcium channels are activated. Calcium diffuses into the cell and causes the vesicles to fuse with the cell membrane releasing acetylcholine into the synaptic cleft.
Which regions of a sarcomere shorten or come closer together upon muscle contraction? Which band stays the same length?
Cross bridge cycling brings Z lines closer together and shortens the H zone and I band (“HI”), while the A band does not change length
Mnemonic:
HIZ shrinkage
A band is Always the same length.
In skeletal muscle cross-bridge cycling, what occurs when the ATP bound to the myosin head is hydrolyzed to ADP and phosphate?
When ATP is hydrolyzed, energy is released and the myosin head changes to a “cocked position”.
Which band does the Z line of a sacromere transect? What is the composition of this band?
The Z line transects the I band of two neighboring sarcomeres. The I band is composed of thin actin filaments.
In skeletal muscle cross-bridge cycling, what occurs when ADP and phosphate are released from the myosin head?
Once the myosin head binds to actin, ADP and phosphate are released and the power stroke occurs.
Why does rigor mortis occur?
After death, ATP is depleted. The muscles contract, but without fresh ATP, myosin remains bound to actin, a state called rigor mortis.
Which zone of the sarcomere is transected by the M line? What is the composition of this zone?
The M line transects the H zone. The H zone is within the center of the sarcomere and is composed of thick myosin filaments with no overlapping thin actin filaments.
Which ion plays a central role in initiation of skeletal muscle contraction? Where does this ion bind to initiate cross bridge formation in skeletal muscles cells?
Calcium plays a central role in the initiation of contraction. Calcium binds to troponin C which moves tropomyosin allowing the binding of actin and myosin to form cross bridges
What lines define the boundaries of a sarcomere? Which line defines the center of the sarcomere?
A sarcomere is defined as extending from Z line to Z line. The center of the sarcomere is defined by the M line.
What is the composition of the A band of a sacromere?
The A band is composed of the entire length of a single thick myosin filament. The amount of overlap with thin actin filaments depends on whether the muscle is contracting or not.
In skeletal muscle, what occurs when calcium binds to troponin C?
After calcium binds to troponin C, a conformational change in the protein complex occurs moving tropomyosin out of the way of the myosin-actin binding sites. As a result, actin and myosin are able to bind, force is generated, and the sarcomere is shortened.
Which of the following ions help cause a conformational change that moves tropomyosin out of the myosin-binding groove on actin filaments?
A) Na+
B) Cl-
C) Ca2+
D) PO4-
E) K+
Ca2+
Released Ca2+ binds to troponin C, causing a conformational change that moves tropomyosin out of the myosin-binding groove on actin filaments.
What do you need to know about parallel and series elastic elements in skeletal muscle?
Muscle has elastic components that must be overcome to accomplish muscle shortening. These are in parallel and in series with the contractile elements.
- Parallel elastic elements include the sarcolemma, intracellular architectural components, and the extracellular connective tissues.
- Series elastic elements represent the tendons and regions within the myosin, as well as the actin and myosin themselves.
