Regents Biology

36.2 – The Muscular System

Prepared by Kim Foglia. Adapted and Modified by Nhan Pham.

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Muscles

  The function of the muscular system is movement.   More than 40% of the mass of the average human

body is muscle.

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Muscle

voluntary

involuntary, contract on their own

involuntary evolved first

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Skeletal  usually attached to bones   responsible for voluntary movements  have many nuclei (multi-nucleated)   sometimes called striated muscles

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Smooth  are usually not under voluntary control –

can work without nervous stimulation   spindle-shaped  have one nucleus, not striated   found in many internal organs and blood

vessels

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Cardiac  only found in the heart   striated  may have one or two nuclei   connected to each other by gap

junctions

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Muscle Contraction  The fibers in skeletal

muscles are composed of smaller structures called myofibrils.

 Each myofibril has smaller structures called filaments.

 The thick filaments contain a protein called myosin.

 The thin filaments contain a protein called actin.

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  Filaments are arranged along the muscle fiber in units called sarcomeres.

  When a muscle is relaxed, there are no thin filaments in the center of a sarcomere.

  During muscle contraction, the actin filaments slide over the myosin filaments, decreasing the distance between the Z lines.

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Muscle Contraction

Binding sites Myosin

Cross-bridge

Actin

Movement of Actin Filament

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Muscle Contraction   When many myosin cross-bridges change shape in a

fraction of a second, the muscle fiber shortens with considerable force.

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Control of Muscle Contraction  A neuromuscular junction – point of contact between

a motor neuron and a skeletal muscle cell   Vesicles in the axon terminals of the motor neuron

release a neurotransmitter called acetylcholine.   Acetylcholine produce an impulse in the cell

membrane of the muscle fiber.   The impulse causes the release of calcium ions within

the fiber.   The calcium ions affect regulatory proteins that allow

actin and myosin filaments to interact.

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  A muscle cell remains contracted until the release of acetylcholine stops and an enzyme produced at the axon terminal destroys any remaining acetylcholine.

  Then, the cell pumps calcium ions back into storage, the cross-bridges stop forming, and contraction ends.

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Muscles Movement   Muscles do work by contracting

-  skeletal muscles come in antagonistic pairs (flexor vs. extensor)

-  contraction moves skeletal parts

-  tendons connect bone to muscle

-  ligaments connect bone to bone

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Fast twitch & slow twitch muscles   Slow-twitch muscle fibers

-  contract slowly, but keep going for a long time

-  more mitochondria for aerobic respiration

-  long distance runner

-  “dark” meat = more blood vessels

  Fast-twitch muscle fibers

-  contract quickly, but get tired rapidly

-  sprinter

-  “white” meat

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Muscle limits   Muscle fatigue

-  lack of sugar, ATP, O2

-  lactic acid lowers pH which interferes with protein function

-  nerve fatigue (due to loss of chemical transmitters)

  Muscle cramps

-  build up of lactic acid

-  lack of energy (ATP)

-  ion imbalance

-  massage or stretching increases circulation

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Diseases of Muscle tissue   ALS

-  amyotrophic lateral sclerosis -  Lou Gehrig’s disease -  motor neurons degenerate

  Myasthenia gravis -  auto-immune -  antibodies to

nerve transmitter chemical

Stephen Hawking

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Botox   Bacteria Clostridium botulinum toxin

-  blocks release of nerve transmitter -  botulism can be fatal!

muscle