Muscle Physiology

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Dr. Ebneshahidi

Skeletal Muscle

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Figure 9.2 (a)

Functions of the muscular system

1. Locomotion – body movements are due to skeletal musclecontraction.

2. Vasoconstriction and vasodilatation - constriction and dilationof blood vessel walls are the results of smooth muscle contraction.

3. Peristalsis – wavelike motion along the digestive tract isproduced by the smooth muscle.

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produced by the smooth muscle.

4. Cardiac motion – heart chambers pump blood to the lungs andto the body because of cardiac muscle contraction.

5. Posture maintenance - contraction of skeletal musclesmaintains body posture and muscle tone.

6. Heat generation – about 75% of ATP energy used in musclecontraction is released as heat.

Comparison of the three types of muscle

Striation: only present in skeletal and cardiac muscles. Absent insmooth muscle.

Nucleus: smooth and cardiac muscles are uninculcated (onenucleus per cell) skeletal muscle is multinucleated (several nucleiper cell).

Transverse tubule (T tubule): well developed in skeletal and

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cardiac muscles to transport calcium. Absent in smooth muscle.

Intercalated disk: specialized intercellular junction that onlyoccurs in cardiac muscle.

Control: skeletal muscle is always under voluntary control‚ withsome exceptions (the tongue and pili arrector muscles in thedermis). Smooth and cardiac muscles are under involuntarycontrol.

Innervation: motor unit

a) a motor nerve and a myofibril from a neuromuscularjunction where gap (called synapse) occurs between the twostructures. At the end of motor nerve‚ neurotransmitter (i.e.acetylcholine) is stored in synaptic vesicles which will release theneurotransmitter using exocytosis upon the stimulation of a nerveimpulse. Across the synapse the surface the of myofibril containsreceptors that can bind with the neurotransmitter.

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Neuromuscular junction

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Skeletal muscle fiber

1. Each skeletal muscle fiberis a single muscle cell whichis the unit of contraction.

2. Muscle fibers arecylindrical cells with manynuclei.

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3. The cell membrane iscalled sarcolemma thecytoplasm is calledsarcoplasm.

4. The sarcoplasm containsabundant parallel thread likemyofibrils that run in parallelfashion.

5. The myofibrils contain 2kinds of protein filaments.

a. Thick filaments –composed of myosin.

b. Thin filaments – composed

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b. Thin filaments – composedof Actin, troponin, andtropomyosin.

c. striations are produced byalternating light and darkfilaments.

Arrangement of the Filaments in a Sarcomere

Longitudinal section within one sarcomere

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Ultrastructure of Myofilaments: Thick Filaments

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Ultrastructure of Myofilaments: Thin Filaments

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Striation pattern of skeletal muscles: 2 parts

1. The I bands (The light bands).

Extends from the edge of one stack of thick filaments to the edgeof next stack of thick filaments.

The I band is composed of thin actin filaments.

2. The A bands (The dark bands) – composed of thick myosinfilaments overlapping thin filaments (actin).

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filaments overlapping thin filaments (actin).

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- myosin filaments are heldtogether by Z lines (notattached).

- A band consist of a regionwhere the thick and thinfilaments overlap and aregion called central region

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(H zone) consisting of onlythick filaments. In the centerof A band is a dark bandcalled the M line.

Sarcomere: The segment ofmyofibrils that extends from

one Z line to the next Z line.

Cross bridge Attachment:The activated myosin headsare attracted to the exposedbinding sites on actin andcross bridge attachmentoccurs.

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Power stroke: The slidingaction which occurs at thesame time for thousands ofactin and myosin moleculesis referred to as the power

stroke.

Muscle Response:

All – or – none response.

a. if a muscle fiber contracts at all it will contract completely.

b. motor units respond in an all – or – none manner.

Threshold stimulus: is the minimal stimulus needed to elicit amuscular contraction.

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muscular contraction.

Twitch: single short contraction reflecting stimulation of somemotor units in a muscle.

Latent period: is the time between stimulus and respondingmuscle contraction.

Refractory period: During his period immediately followingcontraction a muscle can not respond.

The muscle twitch

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Summation

Summation: A rapid series of stimuli may produce summation oftwitches and a sustained contraction.- Forceful sustained contraction without relaxation is a tetaniccontraction.- Tetany is the result of low Ca2+ concentrations.

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Types of Contractions:

Isotonic: when a muscle contracts and its ends are pulled closertogether.

Isometric: when a muscle contracts but attachments do not move.

Isokinetic: when the force a muscle generates is less than thatrequired to move or lift an object the contraction is calledisokinetic.

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isokinetic.

Fast & Slow Muscles

a. white or fast skeletal muscle fibers have few mitochondriareduced ability to carry on aerobic respiration and tend to fatiguerapidly (ex. extra ocular muscles). Designed for speed & fatigueeasily.

b. Red or slow skeletal muscle fibers have many mitochondriaare designed for enduration and can contract for long periods oftime (ex. Solues).

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time (ex. Solues).

Muscle Fatigue:

- a fatigued muscle loses its ability to contract.

- muscle fatigue is due to accumulation of lactic acid and ATPexhaustion.

Oxygen debt:

a. During rest or moderate exercise O2 is sufficientto support aerobic respiration (using may ATPmolecules).

b. During strenuous exercise O2 deficiency maydevelop and lactic acid may accumulate as a result

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develop and lactic acid may accumulate as a resultof anaerobic respiration.

c. The amount of O2 needed to convert accumulatedlactic acid to glucose and restore supplies of ATPand creatine phosphate is called oxygen dept.

Role of Ca+ in muscle contraction:

1. promotes neurotransmitter release.

2. Triggers Ca+ release from SR.

3. Triggers sliding of myofilaments and ATPase activity.

4. promotes glycogen breakdown & ATP synthesis.

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Sliding Filament Theory

1. A myofiber together withall of its myofibrils shortensby movement of the insertiontowards the origin of themuscle.

2. Shortening of themyofibrils is caused by

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myofibrils is caused byshortening of the sareomere(The distance between Zlines is reduced).

3. Shortening of thesarcomere is accomplishedby each filament remains thesame during contraction.

4. sliding is produced by powerstrokes of myosin cross bridgeswhich pull the thin actin over thethick myosin.

5. The A band remains the samelength during contraction but arepulled toward the origin of themuscle.

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muscle.

6. Adjacent A bands are pulled closertogether as the I bands between themshorten.

7. The H band shorten duringcontraction as the thin filaments onthe sides of the sarcomeres are pulledtowards the middle.

Major Events of muscle contraction

1. The distal end of a motor neuron releases Acetylcholine.2. Acetylcholine diffuse across the gap at the neuromuscular junction.3. The sarcolemma is stimulated and a muscle impulse travels over thesurface of the muscle fiber and deep into the fiber through the transversetubules and reaches the sarcoplasmic reticulum.4. Ca2+ ions diffuse from the sarcoplasmic reticulum into the sarcoplasmbind to troponin molecules.

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Neuromuscular Junction:

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5. Tropomyosin molecules move and expose specific sites on actinfilament.6. Actin and myosin filaments form linkages.7. Actin filaments are pulled inward by myosin cross – bridges.8. muscle fiber shortens as a contraction occurs.

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Muscle contraction: Role of Ca+

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Sliding of actin filament over myosin

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Major events of muscle relaxation

1. Acetylcholinestrase decomposes acetylcholine and the muscle fibermembrane is no longer stimulated.2. Ca2+ ions are actively transported into the sarcoplasmic reticulum.3. ATP causes linkage between actin and myosin filaments to break.4. Cross–bridges re–open.5.Troponin & tropomysin molecules inhibit the interaction betweenmyosin and actin filaments.6. Muscle fiber remain relaxed yet ready until stimulated again.

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6. Muscle fiber remain relaxed yet ready until stimulated again.

1. Smooth muscles contain filaments of actin and myosin.2. Lack transverse tubules and S.R. is not well developed.3. Display rhythmicity (spontaneous repeated contractions) responsiblefor peristalsis (alternate contraction and relaxation) .4. Lack troponin (protein that binds to Ca2+) instead calmodulin binds toCa2+.5. Both Acetylcholine & Norepinephrine are neurotransmitters forsmooth muscles.

Smooth Muscle Contraction

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smooth muscles.6. Hormones and stretching affect smooth muscle contractions.7. Can contract for a long period of time.

Cardiac muscle

a) unique arrangement of actin and myosin filaments produces

the cross - striations (an optical illusion under the microscope)‚and rapid contraction with powerful forces involved.

b) muscle cells are joined by intercalated disks‚ and allowmuscle groups to form branching networks - both features arenecessary for cardiac muscle to function as a unit(″syncytium″).

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(″syncytium″).

c) SR and T tubules are well developed‚ so a large amount ofcalcium can be released rapidly through the T tubules.

d) contains more mitochondria in each muscle cell thanskeletal and smooth muscles‚ providing more ATP energy forcontinuous contraction.

Cardiac Muscle

self- exciting muscle fibers form ″pacemakers″ which initiate spontaneous nerve impulses for autorthymic contraction. Thesepacemakers can be influenced by the autonomic nervoussystem and hormones.

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Cardiac Muscle:

1. Contracts for a longer time than skeletal musclebecause transverse tubules supply extra Ca+2 ions.

2. intercalated disc connects the ends of adjacentmuscles and hold cells together as a unit (syncytium).

3. Fibers contracts as a unit .

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3. Fibers contracts as a unit .

4. Muscle fibers are self – exiting, rhythmic, and remainrefractory until a contraction is completed.

5. No Tetanic contractions.

Electromyogram (EMG)

a) Latent period – chemical reactions and physical changes thatoccur preceding the actual contraction of a skeletal muscle.

b) Period of contraction – actin causing the shortening ofsacromere and the contraction of muscle.

c) period of relaxation- actin returns to its original positioncausing the lengthening of sarcomeres and the relaxation ofmuscle.

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muscle.

Clinical Terms:

Convulsion: series of involuntary contractions of variousvoluntary muscles.

Fibrosis: Degenerative disease in which connective tissuereplaces skeletal muscle tissue.

Myalgia: pain resulting from any muscular disorder.

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Myasthenia gravis: an autoimmune chronic disease characterizedby muscles that are weak and easily fatigue. it results from theimmunes systems attack on neuromuscular junctions.

Paresis: partial or slight paralysis of the muscle.

Muscular dystrophy: progressive muscle weakness and atrophycaused by deficient dystrophin protein.

Clinical Terms

Myopathy: Any muscular disease.

Paralysis: loss of ability to move a body part.

Myotonia: prolonged muscular spasm.

Myositis: inflammation of skeletal muscle tissue.

Spasm: A sudden involuntary smooth or skeletal muscle twitch

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Spasm: A sudden involuntary smooth or skeletal muscle twitchcan range from mild to very painful irritation.

Tics: spasm of eye–lid or facial muscles.

Cramp: a prolonged spasm that cause a muscle to become tautand painful.