Buffl

histo-musclular tisue

ds
von dawn S.

(just read)

MECHANISM OF CONTRACTION

− Filaments do not change their length.

− Results as the overlapping thin and thick filaments of each

sarcomere slide past one another

− Induced when an action potential arrives at a synapse, the

neuromuscular junction (NMJ), and is transmitted along the T

tubules to the sarcoplasmic reticulum to trigger Ca2+ release.

1. A nerve impulse triggers release of ACh from the synaptic knob

into the synaptic cleft. ACh binds to Ach receptors in the motor

end plate of the neuromuscular junction, initiating a muscle

impulse in the sarcolemma of the muscle fiber.

2. As the muscle impulse spreads quickly from the sarcolemma

along T tubules, calcium ions are released from terminal

cisternae into the sarcoplasm.

3. Calcium ions bind to troponin. Troponin changes shape,

moving tropomyosin on the actin to expose active sites on

actin molecules of thin filaments. Myosin heads of thick

filaments attach to exposed active sites to form

crossbridges.

4.Myosin heads pivot, moving thin filaments toward the

sarcomere center. ATP binds myosin heads and is broken

down into ADP and P. Myosin heads detach from thin

filaments and return to their prepivot position. The

repeating cycle of attach- pivot-detach-return slides thick

and thin filaments past one another. The sarcomere

shortens and the muscle contracts. The cycle continues as

long as calcium ions remain bound to troponin to keep

active sites exposed.

5. When the impulse stops, calcium ions are actively

transported into the sarcoplasmic reticulum, tropomyosin

re-covers active sites, and filaments passively slide back to

their relaxed state


(check later if i can get something from this)

INNERVATION

− Myelinated motor nerves branch out within the perimysium connective tissue - gives rise to several unmyelinated terminal twigs that pass-through endomysium and form synapses with individual muscle fibers. Schwann cells enclose the small axon branches and cover their points of contact with the muscle cells; the external lamina of the Schwann cell fuses with that of the sarcolemma

− Motor end plate (MEP), or NMJ- a dilated termination of each axonal branch

− Within the axon terminal are mitochondria and numerous

synaptic vesicles containing acetylcholine.

− Between the axon and the muscle is a space, the synaptic cleft.

− Adjacent to the synaptic cleft, the sarcolemma is thrown into numerous deep junctional folds, to provide greater

postsynaptic surface area and more transmembrane

acetylcholine receptors.

− Nerve action potential reaches the MEP – acetylcholine is

liberated from the axon terminal - diffuses across the cleft - binds to its receptors in the folded sarcolemma.

− Acetylcholine receptor contains a nonselective cation channel opens upon neurotransmitter binding – influx of Na+ - depolarizing the sarcolemma - producing the muscle action potential.

− Acetylcholine quickly dissociates from its receptors, and all free neurotransmitter is removed from the synaptic cleft by the extracellular enzyme acetylcholinesterase, preventing prolonged contact of the transmitter with its receptors.

− Motor unit -single axon and all the muscle fibers in contact with its branches

− “All or nothing” contraction

− To vary the force of contraction, the fibers within a muscle fascicle do not all contract at the same time.

− An axon from a single motor neuron can form MEPs with one or many muscle fibers.

− The lesser the number of fibers innervated by one axon the greater the precision of its control and movement

SKELETAL MUSCLE FIBER TYPES

A. ________

• Slow oxidative

• Adapted for slow contractions over long periods without

fatigue, having many mitochondria, many surrounding

capillaries, and much myoglobin, all features that make

fresh tissue rich in these fibers dark or red in color.


B. ________

• Fast, Oxidative-Glycolytic Fibers

• Have physiological and histological features intermediate

between those of the other two types.


C. ________

• Fast, Glycolytic Fibers

• Specialized for rapid, short-term contraction, having few

mitochondria or capillaries and depending largely on

anaerobic metabolism of glucose derived from the stored

glycogen, features that make such fibers appear white.

Rapid contractions lead to rapid fatigue as lactic acid

produced by glycolysis accumulates.

SKELETAL MUSCLE FIBER TYPES

A. Type I

• Slow oxidative

• Adapted for slow contractions over long periods without

fatigue, having many mitochondria, many surrounding

capillaries, and much myoglobin, all features that make

fresh tissue rich in these fibers dark or red in color.


B. Type IIa

• Fast, Oxidative-Glycolytic Fibers

• Have physiological and histological features intermediate

between those of the other two types.


C. Type IIb

• Fast, Glycolytic Fibers

• Specialized for rapid, short-term contraction, having few

mitochondria or capillaries and depending largely on

anaerobic metabolism of glucose derived from the stored

glycogen, features that make such fibers appear white.

Rapid contractions lead to rapid fatigue as lactic acid

produced by glycolysis accumulates.

Author

dawn S.

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