09 Muscular System

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Skeletal muscle

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an organ of the muscular system, composed of skeletal muscle tissue, nervous tissue, blood, & connective tissues.

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Fascia

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is dense connective tissue that separates individual skeletal muscles

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A tendon

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is a cordlike structure that consists of dense connective, Tendons connect a muscle to a bone

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Aponeurosis

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is a sheetlike structure composed of dense connective tissue

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Epimysium

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is a layer of connective tissue that closely surrounds a skeletal muscle

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Perimysium

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is connective tissue that separates muscles into fascicles

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A fascicle

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is a section of a muscle

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Endomysium

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is connective tissue that surrounds individual muscle cells

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Subcutaneous fascia

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is fascia just beneath the skin

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Subserous fascia

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is connective tissue layer of the serous membranes covering organs in various body cavities & lining those cavities.

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A skeletal muscle fiber

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is a single muscle cell

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The sarcolemma

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is the cell membrane of a muscle cell

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The sarcoplasm

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is the cytoplasm of a muscle cell

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The sarcoplasm contains

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many small nuclei, mitochondria & myofibrils

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Myofibrils

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are threadlike structures & are located in the sarcoplasm, Myofibrils play a fundamental role in the muscle contraction mechanism

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Thick myofilaments are composed of

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myosin

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Thin myofilaments are composed of

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actin

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The organization of myofilaments produces

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the alternating light & dark striation characteristic of skeletal muscles

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A sarcomere

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is a repeating pattern of a myofibril

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Myofibrils

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may be thought of as sarcomeres joined end to end

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I bands are composed of

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thin actin filaments

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Z lines are structures

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that anchor I bands

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A bands are composed of

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thick myosin filaments overlapping thin actin filaments

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The H zone

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is a central region of an A band that only contains thick filaments

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The M line

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is a region of an A band which consists of proteins that help hold the thick filaments in place

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Titin connects

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proteins that connect myosin filaments to Z lines

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A sarcomere extends from

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one Z line to another Z line

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Each myosin molecule consists of

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two twisted protein strands with globular parts called cross bridges that project outward along their lengths

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Thin filaments consist of

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double strands of actin twisted into a helix

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Actin

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has a binding site to which the cross bridges of a myosin molecule can attach

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Troponin & tropomyosin associate with

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actin filaments

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Sarcoplasmic reticulum

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is endoplasmic reticulum of a muscle fiber

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Transverse tubules

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are membranous channels that extend into the sarcoplasm as invaginations continuous with the sarcolemma & contain extracellular fluid

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Cisternae

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are enlarged portions of sarcoplasmic reticulum

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A triad

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is formed by one transverse tubule & two cisternae

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Each skeletal muscle is functionally connected to

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an axon of a motor neuron

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A motor neuron passes outward from

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brain or spinal cord

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Normally, a skeletal muscle fiber contracts only

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upon stimulation by a motor neuron

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A neuromuscular junction is

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the site where an axon & muscle fiber meet

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A motor end plate

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is a specialized portion of the muscle cell membrane that is extensively folded

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A motor unit

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is a motor neuron & the muscle fibers it controls

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A synaptic cleft

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separates the membranes of the neuron & the membrane of the muscle fiber

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Synaptic vesicles

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store neurotransmitters

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Acetylcholine

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is the neurotrasnmitter that motor neurons use to control skeletal muscle

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ACh is synthesized and stored in

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the cytoplasm of the motor neuron & is stored in synaptic vesicles in axons

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When a nerve impulse reaches the end of an axon

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acetylcholine is released into the synaptic cleft

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ACh combines

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with ACh receptors on the motor end plate, & stimulates the muscle fiber

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A muscle impulse

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is an electrical signal that is like a nerve impulse

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A muscle impulse changes

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the muscle cell membrane in a way that transmits the impulse in all directions along & around the muscle cell

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Ultimately the muscle impulse reaches

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the sarcoplasmic reticulum & cisternae

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The sarcoplasmic reticulum has

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a high concentration of calcium

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In response to a muscle impulse

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the membranes become more permeable to calcium, & the calcium diffuses out of the cisternae into the cytosol of the muscle fiber

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When a muscle fiber is at rest

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the troponin tropomyosin complexes block the binding sites on the actin molecules

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Calcium ions

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bind to troponin, changing its shape & altering the position of the tropomyosin

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The movement of the tropomyosin molecule

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exposes the binding sites of the actin filaments, allowing linkages to form between myosin cross bridges & actin

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The functional unit of skeletal muscles is the

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sarcomere

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According to the sliding filament model, when sarcomeres shorten

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the thick & thin filaments slide past one another

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As contraction occurs

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the H zones & the I bands get narrower & the Z lines move closer together.

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The force that shortens the sarcomeres comes from

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cross bridges pulling on the actin filaments

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A myosin cross bridge attaches to

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actin in order to pull on the actin filament

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The cross bridge can

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release, straighten, & combine with another binding site further down the actin filament, & pull again

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Myosin cross bridges contain the enzyme

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ATPase

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ATPase catalyzes the breakdown of

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ATP to ADP

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The force for muscle contraction is provided by

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the breakdown of ATP into ADP

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Breaking down of ATP puts the myosin cross bridge in a

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“cocked” position

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When a muscle is stimulated to contract

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a cocked cross bridge attaches to actin & pulls the actin filament toward the center of the sarcomere, shortening the muscle

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When another ATP binds

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the cross bridge is released, & then breaks down the ATP to return to the cocked position

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The cross bridge cycle

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may repeat over & over as long as ATP is present & nerve impulses cause ACh release at the neuromuscular junction

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In order for a muscle fiber to relax

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acetylcholine must be decomposed by an enzyme called acetylcholinesterase.

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The action of acetycholinesterase

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prevents a single nerve impulse from continuously stimulating a muscle fiber.

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When acetylcholine breaks down

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the stimulus to the sarcolemma & the membranes within the muscle fiber ceases.

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The calcium pump moves

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calcium back into the sarcoplasmic reticulum

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When calcium is removed from the cytoplasm

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the cross bridge linkages break & tropomyosin rolls back into its groove, preventing any cross bridge attachment

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ATP is necessary for

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both muscle contraction & relaxation

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The trigger for contraction is

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the increase in cytosolic calcium in response to stimulation by ACh from a motor neuron

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Creatine phosphate

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is an energy source available to generate ATP from ADP

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Creatine phosphate includes

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a high energy phosphate bond

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As ATP is decomposed to ADP

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the energy from creatine phosphate is transferred back to ADP to produce ATP

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After creatine phosphate is used

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a muscle cell must depend on cellular respiration of glucose to synthesize ATP

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Typically a muscle stores glucose in the form of

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glycogen

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Glycolysis occurs

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in the cytoplasm & is anaerobic

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Glycolysis releases

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a few ATP molecules

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The complete break down of glucose occurs

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in mitochondria & requires oxygen

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The citric acid cycle & electron transport

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chain produce water, carbon dioxide & a large amount of ATP

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Oxygen is carried in the blood stream bound to

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hemoglobin

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Myoglobin is ____ in color

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red

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Myoglobin stores oxygen in

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muscle tissue

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Lactic acid threshold

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is the shift in cellular metabolism from breaking down pyruvic acid into carbon dioxide to pyruvic acid breakdown leading to lactic acid formation

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Under anaerobic conditions, glycolysis breaks down

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glucose into pyruvic acid & converts it to lactic acid

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Lactic acid is carried by

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the blood to the liver

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Liver cells can convert lactic acid to

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glucose

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Oxygen debt reflects the amount of

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oxygen liver cells require to use the accumulated lactic acid to produce glucose, plus the amount the muscle cells require to resynthesized ATP & creatine phosphate, & restore their original concentrations

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Fatigue

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is the condition in which a muscle fiber cannot contract

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Fatigue may result from

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decreased blood flow, ion imbalances across the sarcolemma, & the psychological loss of the desire to continue to exercise

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A cramp

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is a sustained, painful, involuntary muscle contraction

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The strenuous exercise of aerobic training

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stimulates new capillaries to grow within the muscles, supplying more oxygen & nutrients

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Heat

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is a by product of cellular respiration, Blood transports heat throughout the body, which helps to maintain body temperature.

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One way to observe muscle contraction is to

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remove a single muscle fiber from a skeletal muscle & connect it to a device that measures contraction

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An electrical impulse

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is usually used to produce muscle contraction.

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Threshold stimulus

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is the minimal stimulus needed to start a muscle contraction

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An impulse

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in a motor neuron normally releases enough ACh to bring the muscle fibers in its motor unit to threshold

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A twitch

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is the response of a single muscle fiber to the ACh released by a single action potential

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A myogram

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is a recording of the events of a muscle twitch

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Three periods of a muscle fiber contraction are

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latent, contraction, & relaxation

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During the period of contraction

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a muscle fiber is generating force or contracting

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The latent period

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is the period before contraction

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The period of relaxation

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is the period in which a muscle fiber is decreasing tension

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The refractory period

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is the brief moment when a muscle fiber remains unresponsive to stimulation

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An all or none response

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is one in which a muscle fiber contracts completely or not at all

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The length to which a muscle is stretched before stimulation affects

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the force it will develop when stimulated

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If a muscle is stretched well beyond its normal resting length

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the force will decrease

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At very short muscle lengths, the sarcomere becomes

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compressed & shortening is not possible

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In the whole muscle, the degree of tension reflects

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the frequency at which individual fibers are stimulated & how many fibers take part in the overall contraction of the muscle

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Twitches in a muscle can

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combine to become sustained

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Summation

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is the combination of the force of individual twitches

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Tetanic contractions

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are contractions that lack relaxation.

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The fewer muscle fibers

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in the motor units, the more precise the movements can be produced in a particular muscle

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All muscle fibers in a motor unit are

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stimulated at the same time

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Multiple motor unit summation is

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recruitment

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Recruitment

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is an increase in the number of activated motor units

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As the intensity of stimulation increases

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recruitment continues until all possible motor units are activated in a muscle

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During sustained contractions

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smaller motor units are recruited earlier

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The larger motor units respond

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later & more forcefully

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Muscle movements are smooth because

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the spinal cord stimulates contraction in different sets of motor units at different times

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Muscle tone

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is the amount of sustained contractions in a muscle

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Muscle tone is important for

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maintaining posture.

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An isotonic contraction

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is a type of contraction that produces movement of a body part

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A concentric contraction

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is an isotonic contraction in which shortening of the muscle occurs

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An eccentric contraction

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is an isotonic contraction in which lengthening of the muscle occurs

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An isometric contraction

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is a contraction in which muscle tension increases but no movements of body parts are produced

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An example of an isometric contraction is

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standing

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An example of an isotonic contraction is

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walking

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Type I fibers are

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slow twitch fibers

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Examples of type I fibers are

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those in the long muscles of the back

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Type I fibers are

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red in color because they contain a relatively large amount of myoglobin

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Type I fibers are

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resistant to fatigue

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Type IIa fibers are

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fast twitch glycolytic fibers, white in color because they contain relatively small amounts of myoglobin, they are fast twitch oxidative fibers

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All muscles include

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a combination of fiber types

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Compared to skeletal muscle fibers, smooth muscle fibers are

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shorter & they have single nuclei

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Two major types of smooth muscle are

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visceral & multiunit

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Multiunit smooth muscle is located

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in the irises & the walls of blood vessels

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Visceral smooth muscle is located

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in the walls of hollow organs except for the heart

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Fibers of visceral smooth muscle are connected by

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gap junctions

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Rhythmicity

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is a pattern of spontaneous repeated contraction

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Peristalsis

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is a wavelike motion produced by smooth muscle contraction

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Peristalsis helps

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force the contents of a tube along its length

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Compared to skeletal muscle fibers, smooth muscle fibers lack

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troponin & use calmodulin to bind calcium instead

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Two neurotransmitters that affect smooth muscle are

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acetylcholine & norepinephrine

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Hormones affect smooth muscle by

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stimulating or inhibiting contraction in some cases & lettering the degree of response to neurotransmitters in other cases

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Stretching of smooth muscle can trigger

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contractions

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Smooth muscle is slower to contract & relax than

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skeletal muscle

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Unlike skeletal muscle, smooth muscle fibers can change length without

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changing tautness

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Cardiac muscle appears only in

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the heart

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Cardiac muscle is composed of

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striated cells, forming fibers that are interconnected in branching, three dimensional networks

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Cardiac muscle fibers can contract longer than

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skeletal muscle fibers

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Intercalated discs are

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membrane junctions that join cardiac muscle fibers together

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Intercalated discs allow muscle impulses to travel

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rapidly from cell to cell.
F. A syncytium is a group of muscle fibers that contract as a unit

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Skeletal muscles generate

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a great variety of body movements

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The action of each muscle mostly depends upon

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the kind of joint it is associated with & the way the muscle is attached on either side of that joint

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Bones & muscles interact as

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levers

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The four basic components of a lever system are

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rigid bar, fulcrum, object that is moved against resistance, & a force

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In scissors, the handle & blades form

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a rigid bar

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The fulcrum of scissors is

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the screw

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The resistance of scissors is

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the material to be cut

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The force of scissors is supplied by

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the person on the handles

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In a first class lever system

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the parts are arranged resistance, fulcrum, force

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Besides scissors, other examples of first class lever systems are

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seesaws & hemostats

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In a second class lever system

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the parts are arranged fulcrum, resistance, force

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An example of a second class lever system is a

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wheelbarrow

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In a third class lever system, the parts are

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arranged resistance, force, & fulcrum

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An example of a third class lever system is

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a pair of tweezers

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In the action of bending the upper limb at the elbow

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the rigid bar is the forearm bones, the fulcrum is the elbow joint, the resistance is the hand, & the force is applied by muscles on the anterior side of the arm.

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Bending the arm at the elbow is an example of

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a third class lever system

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When the upper limb straightens at the elbow

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the rigid bar is forearm bones, the pivot is the elbow, the resistance is the hand, & the force is applied by the triceps brachii muscle located on the posterior surface of the arm

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Straightening the arm at the elbow is a first class lever system because

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the parts of the lever are arranged resistance, fulcrum, force

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An example of a second class lever system in the body is

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a movement produced at the temporomandibular joint (opening of the mouth)

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The origin of a muscle

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is the immovable end of the muscle

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The insertion of a muscle is

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the movable end of a muscle

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When a muscle contracts, its insertion is pulled toward

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its origin

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The head of a muscle is the part nearest

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its origin

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The origins of the biceps brachii are

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the attachment to the coracoid process of the scapula, & the attachment to a tubercle above the glenoid cavity of the scapula

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The insertion of the bicep brachii is

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the radial tuberosity of the radius

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When the biceps brachii contracts

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the elbow bends

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A prime mover

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is the muscle primarily responsible for producing an action

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A synergist

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is a muscle that assists the prime mover

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An antagonist

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is a muscle that resists the action of a prime mover

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The name of a muscle may reflect

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its size, shape, function, number of origins, attachment sites, or direction of its muscle fibers

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An example of a muscle named for it size is

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pectoralis major

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An example of a muscle named for its shape

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is deltoid

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An example of a muscle named for its function

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is extensor digitorum

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An example of a muscle named for its number of origins is

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biceps brachii

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An example of a muscle named for it attachments sites

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is sternocleidomastoid

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An example of a muscle named of the direction of its muscle fibers is

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external oblique

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As a group, muscles of facial expression connect

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the bones of the skull to connective tissue in region of the overlying skin

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Life Span Changes

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Signs of aging of the muscular system begin to appear one’s forties

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Life span changes At a microscopic level

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myoglobin, ATP, & creatine phosphate decline

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Connective tissue & adipose tissue

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begin to replace some muscle tissue

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Exercise can help maintain

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a healthy muscular system

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According to the National Institute on Aging, exercise should include

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strength training, aerobics, & stretching

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Deep fascia

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is fascia that surrounds or penetrates muscles