Download Skeletal muscle fiber (image)

The Muscle System An overview
Objectives
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Contrast the structural and functional characteristics of skeletal, smooth and cardiac muscle tissue
•
Describe the structure of a skeletal muscle and a skeletal muscle fiber, and identify their parts on a
diagram
•
Describe the structure and function of a neuromuscular junction
•
Describe the physiology of contraction, including the roles of cellular respiration, ATP and creatine
phosphate
Objectives continued
•
Contrast the all-or-none response of muscle fibers with the graded contractions of whole muscles
•
Describe the major disorders of the muscle system
•
Define the key terms
Key Terms
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Antagonist – a muscle whose contraction opposes an action of another
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Insertion – the end of a muscle that is attached to a movable body part
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Motor unit – a motor neuron and the muscle fibers that it controls
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Muscle fiber – a single muscle cell
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Myofibril – one of many contractile fibers with in a muscle cell
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Origin – the end of a muscle that is attached to an immovable body part
Key Terms continued

Prime mover – the muscle in a muscle group whose contraction is primarily responsible for movement

Sarcomere – the smallest contractile unit of a myofibril
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Synergist – a muscle that helps a prime mover to produce a movement

Tetanic contractions – a sustained muscle contraction
Structure of Skeletal Muscle (image)
Structure of Skeletal Muscles
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Skeletal muscle is the primary organ of the muscular system
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Skeletal muscles are held together by connective tissue
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Each muscle contains many fibers

Cells that are arranged in small groups
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Each cell is wrapped in connective tissue (ct)
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Each small group of fibers are also wrapped in ct
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Each muscle is covered by a dense layer of ct called deep fascia
Muscle Fiber image
Structure of Skeletal Muscles
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Connective tissue extends to the end of the muscle where it either forms a tendon or aponeurose

Tendon – cord like structure which combines with periosteum (outer covering of the bone) to attach
muscle securely.

Aponeurose – wide sheet like structure which attaches the muscle to a bone or other ct.
Skeletal Muscle Fibers

Skeletal Muscles

Many nuclei

Long , thin, cylinder, rounded ends
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Extends full length of muscle
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Cell membrane called sarcolemma
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Cytoplasm called sarcoplasm

Contains myofibrils
▪
Extend length of cell
Skeletal Muscle Fibers
▪
Contractile elements
▪ Called sarcomere
▪
Two basic types of protein filaments
▪ Actin – thin; Myosin – thick
▪ Create striated appearance of striations in skeletal muscles
Skeletal muscle fiber (image)
Skeletal Muscle Fibers “Fun Fact”
The number of skeletal muscle fibers cannot be increased after birth. However, heavy exercise, such as weight
training, increase the number of myofibrils, which increases the diameter and strength of the muscle
fibers and of the whole muscle itself.
Anabolic steroids, substances similar to the hormone testosterone, have been used by some athletes to
promote muscle development and strength. However, physicians have warned that such use can produce
a number of harmful side effects including damage to kidneys, increased risk of heart disease and liver
cancer, and increased irritability. Other side effects include decreased testosterone and sperm production
in males, and increased facial hair and deepening of the voice in females.
Neuromuscular Interactions
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Contraction of muscles is the result of communication between a motor neuron and a muscle fiber

A motor neuron is attached to each muscle fiber, and the fiber contracts only when stimulated by the
motor neuron.
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The motor neuron originates in either the brain or the spinal cord
Motor Units
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Consist of a motor (action-causing) neuron and connecting muscle fibers

One muscle fiber will only connect to one neuron but one neuron may connect to many different muscle
fibers (3-2,000)

Whenever a motor neuron is activated, it stimulates contraction of all the muscle fibers that it controls.
Motor Unit (image)
Neuromuscular Junction
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The connection btwn the terminal end of the axon and the sarcolemma is the neuromuscular junction
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Axon tips fit into depressions in the sarcolemma – synaptic clefts
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Neurotransmitter used is acetylcholine
Neuromuscular junction (image)
Physiology of Muscle Contractions
Complex process using many structures and chemical changes.
Mechanisms of Contraction

Sliding Filament Mechanism

A signal is sent to a motor neuron to release acetylcholine into the synaptic cleft, this kicks off a
long chain of events
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The acetylcholine triggers the cell to release the stores of Ca ++ it has in side of it
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The calcium triggers the active sites on the actin filaments to be exposed
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The exposure of these sites enables the cross-bridges of myosin to attach there
Mechanisms of Contraction
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The cross-bridge bends to pull the actin filaments toward the center of the A band
•
The cross-bridges separate from the first site on the actin filaments, bond with the next site
and produce another power stroke.
•
Repeat until the sacromeres contract the entire muscle fiber.
Mechanisms of Contractions (image)
Mechanisms of Contractions
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Once the contraction is initiated the sarcolemma releases cholinesterase to break down the acetylcholine
in the synaptic cleft.

Preps muscle for next contraction
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Prevents continued stimulation
Energy for Contractions

Comes from ATP molecules

Remember this is a product from cellular respiration!

Andesine triphosphate