Muscular system - Lecture notes PDF

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MUSCULAR SYSTEM MUSCLE TYPES:



Three types of muscle tissue include:

1. Skeletal muscle

o

Skeletal muscle tissue is packaged into skeletal muscles: organs that are attached to bones and skin

o

Skeletal muscle fibers are longest of all muscle and have striations (stripes)

o

Also called voluntary muscle: can be consciously controlled

o

Contract rapidly; tire easily; powerful

2. Cardiac muscle

o

Cardiac muscle tissue is found only in heart

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Makes up bulk of heart walls

o

Striated

o

Involuntary: cannot be controlled consciously

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Contracts at steady rate due to heart’s own pacemaker, but nervous system can increase rate

3. Smooth muscle

o

Smooth muscle tissue: found in walls of hollow organs

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Examples: stomach, urinary bladder, and airways

o

Not striated

o

Involuntary: cannot be controlled consciously

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Can contract on its own without nervous system stimulation

MUSCLE TISSUE CHARACTERISTICS:

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All muscles share four main characteristics:

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Excitability (responsiveness): ability to receive and respond to stimuli

o

Contractility: ability to shorten forcibly when stimulated

o

Extensibility: ability to be stretched

o

Elasticity: ability to recoil to resting length

MUSCLE FUNCTIONS:

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Four important functions of muscle

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Produce Movement: responsible for all locomotion and manipulation

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Example: walking, digesting, pumping blood

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Maintain posture and body position

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Stabilize Joints

o

Generate heat as they contract

SKELETAL MUSCLE

SKELETAL MUSCLE ANATOMY:

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Skeletal muscle is an organ made up of different tissues with three features: nerve and blood supply, connective tissue sheaths, and attachments

o

Nerve and blood supply

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Each muscle receives a nerve, artery, and veins

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Consciously controlled skeletal muscle has nerves supplying every fiber to control activity

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Contracting muscle fibers require huge amounts of oxygen and nutrients

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o

o

Also need waste products removed quickly

Connective tissue sheaths

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Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue

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Support cells and reinforce whole muscle

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Sheaths from external to internal:

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Epimysium: dense irregular connective tissue entire muscle; may blend with fascia

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Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)

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Endomysium: fine areolar connective tissue surrounding each muscle fiber

Attachments

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Muscles span joints and attach to bones

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Muscles attach to bone in at least two places

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Insertion: attachment to movable bone

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Origin: attachment to immovable or less movable bone

Attachments can be direct or indirect

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Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage

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Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis.

MUSCLE FIBER MICROANATOMY:

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Fiber: a muscle cell

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Skeletal muscle fibers are long, cylindrical cells that contain multiple nuclei and other organelles

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Sarcolemma: muscle fiber plasma membrane

o

Sarcoplasm: muscle fiber cytoplasm

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Contains many glycosomes for glycogen storage, as well as myoglobin for O2 storage



o

Myoglobin stores oxygen and gives the red pigment to muscle

Modified organelles

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Myofibrils

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Sarcoplasmic

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T tubules

1. MYOFIBRILS:

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densely packed, rod-like elements

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Single muscle fiber can contain 1000s of myofibrils

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Accounts for ~80% of muscle cell volume

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Myofibril features:

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Striations

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Sarcomeres

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Myofilaments

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Molecular composition of myofilaments

o

Striations: stripes formed from repeating series of dark and light bands along length of each myofibril

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A bands: dark regions

 H zone: lighter region in middle of dark A band –

M line: line of protein (myomesin) that bisects H zone vertically

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I Band: lighter regions

 Z disc (line): coin-shaped sheet of proteins on midline of light I band

o

Sarcomere:

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Smallest contractile unit (functional unit) of muscle fiber

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Contains A band with half of an I band at each end

 

Consists of area between Z discs

Individual sarcomeres align end to end along myofibril, like boxcars of train

o

Myofilaments:

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Orderly arrangement of actin and myosin myofilaments within sarcomere

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o

Actin Myofilaments: thin filaments

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Extend across I band and partway in A band

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Anchored to Z discs

Myosin Myofilaments:

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thick filaments

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Extend length of A band

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Connected at M line

Sarcomere cross section shows hexagonal arrangement of one thick filament surrounded by six thin filaments

o

Molecular Composition of Myofilaments:

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Thick filaments: composed of protein myosin that contains two heavy and four light polypeptide chains

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Heavy Chains intertwine to form myosin tail

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Light Chains form myosin globular head

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During contraction, heads link thick and thin filaments together, forming cross bridges

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Myosins are offset from each other, resulting in staggered array of heads at different points along thick filament

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Thin filaments: composed of fibrous protein actin

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Actin is polypeptide made up of kidney-shaped G actin (globular) subunits

o

G actin subunits bears active sites for myosin head attachment during contraction

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G actin subunits link together to form long, Fibrous F actin (filamentous)

 

Two F actin strands twist together to form a thin filament

Tropomyosin and troponin: regulatory proteins bound to actin

2.Sarcoplasmic Reticulum

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network of smooth endoplasmic reticulum tubules surrounding each myofibril

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Most run longitudinally

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Terminal Cisterns form perpendicular cross channels at the A–I band junction

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SR functions in regulation of intracellular Ca2+levels

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Stores and release Ca2+

3. T-tubules (transverse tubules)

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Tube formed by protrusion of sarcolemma deep into cell interior

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Increase muscle fiber’s surface area greatly

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Lumen continuous with extracellular space

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Allow electrical nerve transmissions to reach deep into interior of each muscle fiber.

REVIEW OF MUSCLE ANATOMY:

Whole Muscle (surrounded by epimysium) Fascicle

(surrounded by perimysium) Muscle Fiber

(surrounded by endomysium)

Myofibrils

Sarcomeres

Myofilaments (actin & myosin)

SLIDING FILAMENT MODEL OF CONTRACTION

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Contraction: the activation of cross bridges to generate force

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In the relaxed state, thin and thick filaments overlap only slightly at ends of A band

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Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more

 

Neither thick nor thin filaments change length, just overlap more

When nervous system stimulates muscle fiber, myosin heads are allowed to bind to actin, forming cross bridges, which cause sliding (contraction) process to begin

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Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward the center of the sarcomere in a ratcheting action

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Causes shortening of muscle fiber

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Z discs are pulled toward M line

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I bands shorten

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Z discs become closer

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H zones disappear

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A bands move closer to each other

MUSCLE FIBER CONTRACTION:

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Membrane Potential:

o

A plasma membrane is more permeable to some particles than others depending on their size, solubility or charge (+) or (-)

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A voltage is electrical potential energy which is due to the separation of oppositely charged particles

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In reference to cells, the oppositely charged particles are ions, (Cl-, Na+, K+, etc.) and the barrier that separates them is the plasma membrane (sarcolemma in a fiber)

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In their resting state all cells have a voltage across their membrane, called the membrane potential

o

When there is a change, or depolarization, that is conducted along the membrane it is called an action potential

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Four steps must occur for skeletal muscle to contract:

1. Nerve Stimulation

2. An action potential, an electrical current, must be generated in sarcolemma

3. Action Potential must be propagated along sarcolemma

4. Intracellular Ca2+ levels must rise briefly

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Steps 1 and 2 occur at neuromuscular junction

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Steps 3 and 4 link electrical signals to contraction, so referred to as excitation-contraction coupling

1. Nerve stimulation:

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Skeletal muscles are stimulated by somatic motor neurons

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Axons (long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle

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Each axon divides into many branches as it enters muscle

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Axon branches end on muscle fiber, forming neuromuscular junction or motor end plate

o Each muscle fiber has one neuromuscular junction with one motor neuron

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Axon Terminal (end of axon) and muscle fiber are separated by gel-filled space called synaptic clef

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Stored within axon terminals are membrane-bound synaptic vesicles

o Synaptic vesicles contain neurotransmitter acetylcholine (ACh)

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Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors

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NMJ consists of axon terminals, synaptic cleft, and junctional folds

o Events at the neuromuscular junction

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Nerve impulse arrives at axon terminal, causing ACh to be released into synaptic clef

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ACh diffuses across cleft and binds with receptors on sarcolemma

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ACh binding leads to electrical events that ultimately generate an action potential through muscle fiber

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Clinical – Homeostatic imbalance:

o Many toxins, drugs, and diseases interfere with events at the neuromuscular junction

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Example: myasthenia gravis: disease characterized by drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness

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Involves shortage of ACh receptors because person’s ACh are attacked by own antibodies

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Suggests this is an autoimmune disease

2. Action potential across sarcolemma:

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Resting sarcolemma is polarized, meaning a voltage exists across membrane

o Inside of cell is negative compared to outside

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Action Potential is caused by changes in electrical charges

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Occurs in three steps:

a) End Plate Potential

b) Depolarization

c) Repolarization

a) End plate potential:

o ACh released from motor neuron binds to ACh receptors on sarcolemma

o Causes chemically gated ion channels (ligands) on sarcolemma to open

o Na+ diffuses into muscle fiber



Some K+ diffuses outward, but not much

o Because Na+ diffuses in, interior of sarcolemma becomes less negative (more positive)

o Results in local depolarization called end plate potential

b) Depolarization:

o generation and propagation of an action potential (AP)

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If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open

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Large influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction

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AP spreads across sarcolemma from one voltage-gated Na+ channel to next one in adjacent areas, causing that area to depolarize

c) Repolarization:

o Restoration of resting conditions

o Na+ voltage-gated channels close, and voltage-gated K+ channels open

o K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage

o Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete

o Ionic conditions of resting state are restored by Na+-K+

EXCITATION-CONTRACTION COUPLING:

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Excitation-contraction (E-C) coupling: events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)

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AP is propagated along sarcolemma and down into t-tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR (sarcoplasmic Reticulum)

o AP is brief and ends before contraction is seen

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AP is brief and ends before contraction is seen

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At low intracellular Ca 2+ concentration:

o

Tropomyosin blocks active sites on actin

o

Myosin heads cannot attach to actin

o

Muscle fiber remains relaxed

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Voltage-sensitive proteins in T tubules change shape, causing SR to release Ca2+ to cytosol

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At higher intracellular Ca2+ concentrations, Ca2+ binds to troponin

o

Troponin changes shape and moves tropomyosin away from myosin-binding sites

o

Myosin heads is then allowed to bind to actin, forming cross bridge

o

Cycling is initiated, causing sarcomere shortening and muscle contraction

o

When nervous stimulation ceases, Ca2+ is pumped back into SR, and contraction ends

CROSS BRIDGE CYCLING:

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Four steps of the cross bridge cycle

1. Cross Bridge Formation: high-energy myosin head attaches to actin thin filament active site

2. Working (power) Stroke: myosin head pivots and pulls thin filament toward M line

3. Cross Bridge Detachment: ATP attaches to myosin head, causing cross bridge to detach

4. Cocking of Myosin Head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state

o This energy will be used for power stroke in next cross bridge cycle



This process is repeated as long as:

o There are calcium ions present, allowing for crossbridge reattachment after each stroke.

o There is ATP to energize the myosin crossbridges



When the neural impulse stops:

o The calcium ions are no longer released from the sarcoplasmic reticulum

o ATP is used to actively transport the calcium ions back into the sarcoplasmic reticulum.

o Without calcium ions the crossbridges cannot reattach because the binding sites are no longer exposed

o The thin actin myofilaments will slide back into the relaxed position.

o The membrane potential is re-establish by ion pumps along the t-tubules and the sarcolemma.

REVIEW: Neural Impulse Motor Neuron Axon ↓ Chemical Messengers Released ↓ Sarcolemma depolarization (Action potential generated) ↓ A.P. continues to the T-tubules ↓ Sarcoplasmic Reticulum (releases calcium ions) ↓ Sarcoplasm ↓ Sarcomere ↓ Actin Myofilament (exposes myosin binding sites)

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Clinical – Homeostatic imbalance

o

Rigor Mortis

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3–4 hours after death, muscles begin to stiffen

o Peak rigidity occurs about 12 hours postmortem

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Intracellular calcium levels increase because ATP is no longer being synthesized, so calcium cannot be pumped back into SR

o Results in cross bridge formation



ATP is also needed for cross bridge detachment

o Results in myosin head staying bound to actin, causing constant state of contraction

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Muscles stay contracted until muscle proteins break down, causing myosin to release

WHOLE MUSCLE CONTRACTION:

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Same principles apply to contraction of both single fibers and whole muscles

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Contraction produces muscle tension, the force exerted on load or object to be moved

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Contraction may/may not shorten muscle

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o

Isometric Contraction: no shortening; muscle tension increases but does not exceed load

o

Isotonic Contraction: muscle shortens because muscle tension exceeds load

Force and duration of contraction vary in response to stimuli of different frequencies and intensities

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Each muscle is served by at least one motor nerve

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Motor nerve contains axons of up to hundreds of motor neurons

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Axons branch into terminals, each of which forms NMJ with single muscle fiber

Motor Unit consists of the motor neuron and a...