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Two types of striated muscle
Skeletal and Cardiac
Def of muscle fiber
a single skeletal muscle cell.
Development of muscle fibers
Formed by the fusion of a number of undifferentiated mononucleated cells; called myoblasts into a cylindrical; multinucleated muscle fiber.
Def of myoblast
Undiiferentiated; mononucleated cells that come together during development to create multi-nucleated muscle fibers
If skeletal muscle is injured……
It cannot be replaced by the division of other skeletal muscle cells
Def of Satellite cells
Undifferentiated cells located adjacent to skeletal muscle fibers that can undergo differentiation much like myoblasts undergo during development. This will occur when/if skeletal muscle fibers are damaged. Cannot always restore a muscle to its original strength. Compensation for the loss of muscle fiber is achieved through the increase in size (hypertrophy) of the remaining muscle
Def of Muscle
A number of muscle fibers bound together by connective tissue
Def of myofibrils
Bundle of thick and thin contractile filaments in cytoplasm of striated muscle.

Exhibit a repeating sarcomere pattern.
Sarcomere
Repeating structural unit of myofibril, composed of thick and thin filaments.

Boundaries of a sarcomere are two adjacent Z lines
What are thick filaments composed of?
Myosin
What are thin filaments composed of?
  • Actin
  • Troponin
  • Tropomyosin
A Band
Wide dark band in the middle of the sarcomere.

This is where to find the thick filaments.
Z Line
Structure running down a sarcomere at each each end.


It anchors one end of the thin filaments; and titin (which anchors the thick filaments).


Adjacent Z lines are also the boundary for a sarcomere.
I Band
A transverse band of the sarcomere located between A Bands of adjacent sarcomeres and bisected by the Z Line.

This is where to find thin filaments and titin
H Zone
Narrow; light band in the center of the A Band. It cooresponds to the space between the opposing ends of the two sets of thin filaments in each sarcomere.
Each thick filament can affect how many thin filaments?
6
Each thin filament can be affected by how many thick filaments?
3
In the region of filament overlap, are there more thick or thin filaments?
There are twice as many thin as thick filaments.
Cross Bridges
Portions of myosin molecules that extend from the surface of the thick filaments towards the thin filaments.
Def of contraction
Does not necessarily mean "shortening." It refers to the activation of the force generation sites within muscle fibers - the cross bridges.
Description of Actin molecule
Globular protein composed of a simple polypeptide that polymerizes with other actins to form two intertwined helical chains. Each actin molecule contains a binding site for myosin.
Description of Myosin molecule
Composed of two large polypeptide heavy chains and four smaller light chains. These polypeptides combine to form a molecule that consists of two globular heads (containing heavy and light chains) and a long tail formed by two intertwined heavy chains.
Description of the globular head of myosin cross bridge
Each globular head contains two binding sites: one for actin; and another for ATP.

The ATP site also serves as an enzyme - an ATPase that hydrolyzes the bound ATP; harnessing its energy for contraction
Def of cross bridge cycle
The sequence of events that occurs between the time a cross bridge binds to a thin filament, moves, and then is set to repeat the process.
Four steps to the cross bridge cycle
  • Attachment of the cross bridge to the thin filament.
  • Movement of the cross bridge producing tension in the thin filament.
  • Detachment of the cross bridge from the thin filament.
  • Energizing the cross bridge so it can again attache to the thin filament.
Two distinct roles of ATP in the cross bridge cycle
  • The engergy released from ATP hydrolysis ultimately provides the energy for cross bridge movement.
  • ATP binding (not hydrolysis) to myosin breakes the link formed between actin and myosin; allowing the cycle to repeat.
What happens during Rigor Mortis?
The ATP concentration declines after death; and in the absence of ATP; the brekage of the link between actin and myosin does not occur. Thus, the thick and thin filaments remain bound together by immobilized cross bridges creating a rigid condition in which thick and thin filaments cannot move passed each other.
Tropomyosin
Covers the myosin binding site on each actin molecule; and thus prevents the cross-bridges from binding to actin.
Troponin
One molecule of troponin is bound to each molecule of tropomyosin and regulates access to the myocin binding sites on actin.

Troponin also has a binding site for calcium which, when bound, causes a conformational change and removes tropomyosin from the myosin binding site on actin, allowing for cross-bridge cycling.
What happens when calcium binds to troponin?
The binding of calcium to troponin produces a conformational change, which relaxes its troponin's grip and allows tropomyosin to move away from the myosin binding sites on actin molecule so cross-bridges can bind.
Sarcoplasmic Reticulum
A series of sleeve-like sacs surrouding each myofibril. At the end of each sleeve are lateral sacs. Calcium is stored and released from the lateral sacs
Transverse Tubule (T Tubule)
Lies directly between and is intimately associated with the lateral sacs. The lumen of the T tubule is continuous with the extracellular fluid surrounding the fiber; and can propogate action potentials. Action potentials travel into the interior of the fiber via the T tubules after being initiated at the plasma membrane.
Dihydrpyridine (DHP)
T Tubule protein that is a modified voltage -sensitive calcium channel. It is found at the junction of the T tubule and the Sarcoplasmic Reticulum
Ryanodine Receptor
Found on the surface of the sarcoplasmic reticulum and at the junctions of the SR with the T tubule; and forms a calcium channel
What occurs when an action potential reaches a foot process within a muscle fiber?
During a T tubule action potential, charged amino acid residues within the DHP receptor protein induce a conformational change which opens the ryanodine receptor channel. Calcium is thus released from the lateral sacs into the cystol activating cross bridge cycling.
Foot Process
Location at which the lateral sacs of the SR and the T tubule are connected
How is calcium removed from the cystol to relax the muscle fiber?
SR contains primary active transport proteins called Ca2+ ATPase pumps. These pumps pump calcium ions back into the SR from the cystol.
Three functions of ATP in skeletal muscle
  • Hydrolysis of ATP by myosin energizes the cross bridges.
  • Binding of ATP to myosin dissociates cross bridges bound to actin.
  • Hydrolysis of ATP by the Ca2++ ATPase pumps in the SR provides the energy for active transport of calcium ions back into the SR for muscle relaxation
How many neurons innervate one muscle fiber?
One
How many muscle fibers can a single neuron innervate?
Infinite
Def of Motor Unit
A motor neuron plus the muscle fibers it innervates.

Muscle fibers from a single motor unit are located in one muscle; however scattered throughout the muscle in no real organized fashion.
Motor End Plate
Region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon
Neuromuscular Junction
Junction of the axon terminal with the motor end plate
End Plate Potential (EEP)
Local depolarization of the motor end plate due to open sodium/potassium channels in reaction to the neurotransmitter Ach from a motor neuron.
How often does EEP (end plate potential) result in an action potential?
Always - every action potential in a motor neuron normally produces an action potential in each muscle fiber of its motor unit.
Neuromusclular junctions are of which type of junction?
Exitatory - all neuromuscular junctions are exitatory and none are inhibitory
Acetylcholinesterase
Breaks down Ach in the synaptic cleft. Choline is transported back to the motor neuron to be re-used.
Curare
Arrowhead poison. Binds to Ach receptors and does not open them,nor does acetylcholinesterase break it down. Thus, Ach cannot bind to its receptors on the motor end plate. Motor nerves still conduct normal action potentials and release Ach, there is no resulting EEP in the motor end plate and thus, no muscle contraction.
Organophosphates
Inhibits acetylcholinesterase. Ach is released normally via the motor neuron, however since it is not degraded by acetylcholinesterase, ion channels in the motor end plate remain open, causing continued muscle contraction. Eventually the Ach receptors on the motor end plate become insensitive to Ach and close, causing paralysis.
Botulinum Toxin
Enzyme that breaks down proteins of the SNARE complex; thus making it impossible for Ach vesicles to be released.
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