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Myology

Bio 250Human Anatomy & Physiology

Types of Muscle Tissue

Skeletal, striated, voluntaryVisceral, smooth, involuntaryCardiac heart muscle involuntaryCardiac, heart muscle, involuntary

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Functions of Muscle

Motion: e.g.walking, running, writing, chewing, swallowing or breathingHeat production: muscles make up p pnearly half of body and are in a constant state of activity producing heat as by-productPosture and body support: muscles give form and stability to the body

Skeletal Muscles are Organs

composed of muscle, nerve, vascular and various types of connective tissuesConnective tissue coverings include:

Epimysium (surrounds entire muscle)Perimysium (surrounds fasciculi)Endomysium (surrounds each muscle cell i.e. muscle fiber)

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Skeletal Muscle Cells (1-40 mm)

Each FIBERrepresents a single muscle cellCells are long and thread-likeMulti-nucleate (formed by fusion of myoblasts)

Skeletal Muscle Cells

Sarcoplasm contains mitochondria, sarcoplasmic reticulum, and myofibrils composed of actin and myosinStriations are the result of the regular arrangement of actin and myosin filaments

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Skeletal Muscle Cells

Transverse tubulesextend from sarcolemma to terminal cisternae which connect to the sarcoplasmic reticulumNote triadUnit of contraction called a sarcomere

Review of Skeletal Muscle Structure

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Neuromuscular JunctionMuscle fibers stimulated to contract by motor neuronsMotor end plate of a muscle fiber lies on one side of the myoneuralside of the myoneural junctionIn response to a nerve impulse, the end of the motor nerve secretes a neurotransmitter which binds to receptors sites and stimulates the sarcolemmaEventually causes muscle contraction

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Review of Neuromuscular Junction

Motor Units

All or none responseWhen a muscle fiber is stimulated, it either contracts maximally or not at allO t d thOne motor neuron and the muscle fibers associated with it constitute a motor unitWhen a motor unit is stimulated, it either contracts maximally or not at allFine movements can be produced in muscles whose motor units contain small numbers of muscle fibers

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Skeletal Muscle Contraction

Structural and functional unit of contraction called SARCOMEREMuscle fiber contraction results from a sliding movement within myofibrilsyActin and myosin filaments slide past each otherSacromeres shorten and muscle fiber as a whole shortens

Stimulus for Muscle Contraction

Muscle fiber is stimulated by acetylcholine at myoneural (neuromuscular) junctionAcetylcholine binds to receptors on the sarcolemma and initiates an action potentialAction potential travels along the sarcolemma and enters sarcoplasm at T-tubulesT-tubules stimulate terminal cisternae to release stored Calcium ions

Role of Calcium Ions

Calcium ions diffuse throughout sarcoplasmIons expose myosin binding sites on the actin moleculeMyosin cross bridges form with actinMyosin cross bridges form with actinActivated myosin breaks down ATPEnergy released from ATP used to slide Actin and Myosin filaments togetherSarcomere and muscle fiber shortenThis is called CONTRACTIONNote that ATP is needed to release myosin heads from actin in order for relaxation to occur

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Review of Muscle Contraction

What happens to ADP?

ADP is rebuilt within the mitochondriaDuring periods of high muscle activity, CREATINE PHOSPHATE is used toCREATINE PHOSPHATE is used to restore some of the ADP to ATPActivity: ADP + CP -> ATP + CDuring rest periods, creatine is recharged to creatine phosphateAt rest: C + ATP -> CP + ADP

What happens to acetylcholine?

An enzyme in the synaptic cleft called ACETYLCHOLINESTERASE destroys acetylcholine stopping the stimulation fof the receptors on the sarcolemma

Acetylcholine ->acetate + cholineThis insures that one nerve stimulus causes one muscle fiber contractionMotor nerve rebuilds acetylcholine from acetate acid and choline

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Energy for Muscle Contraction

One muscle fiber (cell) has:Six trillion myosin moleculesEach myosin bridge breaks down 400 ATP y gmolecules per secondEach muscle fiber requires about 600 trillion ATP molecules per second to contract

Energy reserves include: stored ATP, CP and glycogen

Oxygen Supply and Respiration

Anaerobic respirationresults in a gain of only a few ATP molecules (2 ATP per glucose)Aerobic respiration resultsAerobic respiration results in gain of many ATP molecules (36 additional ATP per glucose)Oxygen is carried from lungs to cells by hemoglobin within RBC’sMyoglobin in muscle cells can store some oxygen during periods of rest

Oxygen DebtAt rest or moderate exercise, oxygen is supplied sufficiently to support aerobic respiration (fatty acids primary fuel)During strenuous exercise glucose is the primary fuel from stored glycogen in muscle.During strenuous exercise, an oxygen deficiency may develop:

Anaerobic respiration results in accumulation of lactic acidAmount of oxygen needed to convert lactic acid back to glucose and restore supplies of ATP and CP and recharge myoglobin is called OXYGEN DEBT.

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Muscle Fatigue

Fatigued muscles lose ability to contractUsually due to accumulation of lactic acidAthletes usually produce less lactic acid because of increased ability to supply oxygen and nutrients toincreased ability to supply oxygen and nutrients to musclesPhysiological contracture results from lack of ATP needed to break myosin bridges and “pump” Ca++ back into the sarcoplasmic reticulumRigor mortis- contraction of all the muscles of the body within a few hours of death. Lasts 15-72 hours depending on conditions like temperature. Lysosomal enzymes eventually break bridges.

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Muscular Responses

Threshold stimulus vs sub-threshold stimulusAll or none responseAll or none response

Muscle fibers follow A or N responseMotor units follow A or N responseMuscles DO NOT follow A or N response

Recruitment of Motor Units

Low intensity of stimulation of muscle stimulates small numbers of motor unitsHigh intensity of stimulation causesHigh intensity of stimulation causes additional motor units to be RECRUITEDMuscles have graded responses

Twitch contraction

Myogram is a recording of muscular contractionTwitch includes:

latent periodcontraction phaserelaxation phase

Refractory period is time following contraction when muscle remains unresponsive to stimulation

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Staircase Effect (Treppe)

An inactive muscle undergoes a series of contractions of increasing strength when subjected tostrength when subjected to a series of stimuliAppears to be due to two primary causes:

Failure to remove Calcium ions from the sacroplasm completelyIncreasing temperature within the muscle

Sustained Contractions

A rapid series of stimuli may produce a summation of twitches resulting in a sustained contraction called TETANYWhen contractions fuse, the strength of contraction may increase due to recruitment of fibers and warming of the muscleEven at rest, some motor units are always contracted giving muscle its firmness called TONUS or muscle tone

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