Contraction of Skeletal Muscle

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Chapter 6

myofibril

One of the longitudinal parallel, thread-like structure in striated muscle cells. It is made up of actin and myosin.Contractile elements surrounded by sarcoplasm.

actin

A globular protein with myosin binding sites; Tropomysin & Troponin are 2 proteins associated with the surface of this filament.

myosin

Consists of 2 twisted strands with globular cross-bridges projected outward along the strands.
Most abundant protein in muscle tissue and the main constituent of the thick filaments of muscle fibers

cross-bridges

globular heads that attach to myosin binding sites on the thin filaments during muscle activation; ATP binds here

cross-bridges ****

globular head of myosin molecule that projects from a myocin filament in muscle & in the sliding filament hypothesis of muscle contraction is held to attach temporarily to an adjunct actin filament & draw it into the A band of sarcomere between the myosin filaments

sarcomere ****

myosin cross-bridges now bind & pull on the actin filaments, causing these to shorten.

sarcomere

basic unit of contraction within a muscle fiber;small, repeating segments of myofibril between two adjacent z-lines

titin

"anchoring protein" that functions as a molecular ruler that specifoes the length of the thick (myosin) filament. It connects m line to z line, maintains the position of the myosin in the center of the sarcomere and prevents the sarcomere from being pulled apart (resting tension).

titin****

titin filamentous molecules keep myocin & actin filaments inplace. Titin is made of large number filamentous molecules of proteins.

sarcoplasmic reticulum

Membrane structure surrounding myofibrils;
contains calcium, which stimulates sarcomere contraction upon release

tropomysin

Larger of the two proteins that are part of actin filament.
protein in actin that blockades the myosin binding sites; in a relaxed muscle, it blocks it from being contracted all the time

tropinin

smaller of two proteins that are part of actin filament. Attached intermittently along side of tropomysin molecules are still other protein molecules called troponin.

Troponin I

binds actin

Tropinin T

binds tropomysin

Troponin C

binds calcium ions

Troponin Complex

believed to attach the tropmyosin to the actin. The strong affinity of troponin for calcium ions is believed to intiate the contraction process.

motor unit/motor end plate

A single motor neuron and the group of muscle fibers which it innervates. The size of these is an indication of how fine the precision of movement can be (the greater the number of fibers controlled by one neuron the less precision).

How many motor units take to control a muscle?

100 motor units

muscle contraction

A summary: The steps involved in contraction are as follows: Ach release from synaptic vesicles>binding of Ach to the motor end plate>generation of an electrical impulse in the sarcolemma>conduction of the impulse along T tubules>release of calcium ions by the SR>exposure of active sites on the thin filaments>cross-bridge information and contraction.

nebulin

Anchoring protein" functions as a molecular ruler that specifies the lenght of the thin (actin) filament.

power stroke

The step in the sliding filament theory during which myosin undergoes a conformational change to its low energy state, in the process dragging the thin filaments (and the attached Z lines) toward the center fo the sarcomere.

Three sources of energy by which ATP is restored....

1) phosphocreatine
2) glycolysis
3)oxidative metabolism

phosphocreatine

first source of energy used to reconstitute ATP;carries a high-energy phosphate bond similar to the bonds of ATP

glycolysis

second source of energy used to reconstitiute both ATP & phosphocreatine. Glycolosis of glycogen previously stored in muscle cells.

oxidative metabolism

third source of energy, it combines oxygen with the end products of glycolysis and with other cellular foodstuffs to liberate ATP.

oxidative metabolism

More than 95% of all energy used by muscles for sustained, long-term contraction is derived from this source.

Isometric contractions

Muscle develops tension but does not shorten during contraction

isotonic contractions

muscle length shortens and moves the load, the tension remains constant througout the contraction (concentric)

fast twitch muscles

white muscle/glycolytic; extensive SR for rapid release of calcium ions to intiate contraction; fewer mitochondria, fewer blood vessels, no myoglobin, fatique quickly, can't replenish ATP for prolonged contraction

slow twitch muscles

red muscle/slow oxidative
*type I fibers
*lots of mitochondria, myoglobin and capillaries (deep red color)
*adapted for aerobic respiration and fatigue resistance

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