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Chapter 10Muscle Tissue

Alternating contraction and relaxation of cells

Chemical energy changed into mechanical energy

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General Functionsof Skeletal Muscles

Movement– Reflex

– Voluntary

Posture and body position

Support soft tissues

Guard entrances and exits

Maintain body temperature

Store nutrient reserves

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Skeletal Muscle

Anatomy

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The Muscular System

Skeletal muscle major groupings

How movements occur at specific joints

Learn the origin, insertion, function and innervation of all major muscles

Important to allied health care and physical rehabilitation students

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Muscle Attachment Sites:Origin and Insertion

Skeletal muscles shorten & pull on the bones they are attached to

Origin is the bone that does not move when muscle shortens (normally proximal)

Insertion is the movable bone (some 2 joint muscles)

Fleshy portion of the muscle in between attachment sites = belly

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Fascicle Arrangements

A contracting muscle shortens to about 70% of its length

Fascicular arrangement represents a compromise between force of contraction (power) and range of motion– muscles with longer fibers have a greater range of

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Coordination Within Muscle Groups

Most movement is the result of several muscle working at the same time

Most muscles are arranged in opposing pairs at joints– prime mover or agonist contracts to cause the

desired action

– antagonist stretches and yields to prime mover

– synergists contract to stabilize nearby joints

– fixators stabilize the origin of the prime mover• scapula held steady so deltoid can raise arm

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How Skeletal Muscle are Named

Direction the muscle fibers run

Size, shape, action, number of origins or locations

Examples from Table 11.2– triceps brachii -- 3 sites of origin

– quadratus femoris -- square shape

– serratus anterior -- saw-toothed edge

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Muscles of Facial Expression

Arise from skull & insert onto skin

Encircle eyes, nose & mouth

Express emotions

Facial Nerve (VII)

Bell’s palsy = facial paralysis due

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Muscles of Facial Expression

Orbicularis oculi closes the eye

Orbicularis oris puckers the mouth

Buccinator forms the muscular portion of the cheek & assists in whistling, blowing, sucking & chewing

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Muscles that Move the Mandible

Masseter, temporalis

Arise from skull & insert on mandible

Cranial nerve V (trigeminal nerve)

Protracts, elevates or retracts mandible– Temporalis & Masseter

elevate the mandible (biting)

– temporalis retracts

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Muscles that Move the Head

Sternocleidomastoid muscle– arises from sternum & clavicle & inserts onto mastoid process

of skull– innervated by cranial nerve XI (spinal accessory)– contraction of both flexes the cervical vertebrae & extends

head – contraction of one, laterally flexes the neck and rotates face

in opposite direction

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Muscles of Abdominal Wall

Notice 4 layers of muscle in the abdominal wall

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Muscles of Abdominal Wall

4 pairs of sheetlike muscles– rectus abdominis = vertically oriented

– external & internal obliques and transverses abdominis• wrap around body to form anterior body wall

• form rectus sheath and linea alba

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Transverse Section of Body Wall

Rectus sheath formed from connective tissue aponeuroses of other abdominal muscles as they insert in the midline connective tissue called the linea alba

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Axial Muscles that Move the Arm

Pectoralis major & Latissimus dorsi extend from body wall to humerus.

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Muscles that Move the Arm

Deltoid arises from acromion & spine of scapula & inserts on arm – abducts, flexes & extends arm

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Flexors of the Forearm (elbow)

Cross anterior surface of elbow joint & form flexor muscle compartment

Biceps brachii– scapula to radial tuberosity

– flexes shoulder and elbow & supinates hand

Brachialis– humerus to ulna

– flexion of elbow

Brachioradialis– humerus to radius

– flexes elbow

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Extensors of the Forearm (elbow)

Cross posterior surface of elbow joint & forms extensor muscle compartment

Triceps brachii– long head arises scapula

– medial & lateral heads from humerus

– inserts on ulna

– extends elbow & shoulder joints

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Muscles that Move the Vertebrae

Quite complex due to overlap

Erector spinae fibers run longitudinally– 3 groupings

• spinalis

• iliocostalis

• longissimus

– extend vertebral column

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Muscles Crossing the Hip Joint

Quadriceps femoris has 4 heads– Rectus femoris crosses hip

– 3 heads arise from femur

– all act to extend the knee

Adductor muscles – bring legs together

– cross hip joint medially

– see next picture

Pulled groin muscle– result of quick sprint activity

– stretching or tearing of iliopsoas or adductor muscle

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Adductor Muscles of the Thigh

Adductor group of muscle extends from pelvis to linea aspera on posterior surface of femur– adductor longus

– gracilis

– adductor magnus (hip extensor)

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Muscles of the Butt & Thigh

Gluteus muscles– maximus, medius

– maximus extends hip

– medius & minimus abduct

Deeper muscles laterally rotate femur

Hamstring muscles– semimembranosus

(medial)

– semitendinosus (medial)

– biceps femoris (lateral)

– extend hip & flex knee

Pulled hamstring– tear of origin of

muscles from ischial

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Muscles of the Calf (posterior leg)

3 muscles insert onto calcaneus– gastrocnemius arises femur

• flexes knee and ankle

– soleus arise from leg• flexes ankle

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Muscles of the Leg and Foot

Anterior compartment of leg– extensors of ankle & toes

• tibialis anterior

– tendons pass under retinaculum

Shinsplits syndrome– pain or soreness on anterior

tibia

– running on hard surfaces

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Functional Anatomy

Connective tissue components

Muscle fibers (cells)

Nerves: – (Containing motor neurons) convey impulses for

muscular contraction

Blood supply: – Provides nutrients and oxygen for contraction

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Functional Anatomy Connective Tissue

Superficial fascia: loose CT & fat underlying the skin

Deep fascia: dense irregular CT around muscle

Connective tissue components of muscle include– Epimysium: surrounds the whole muscle

– Perimysium: surrounds bundles (fascicles) of 10-100 muscle cells

• Blood vessels

• nerves

– Endomysium: separates individual muscle cells• Capillary network

• Satellite cells

• Nerve fibers

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Functional Anatomy Connective Tissue

All the CT layers extend beyond the muscle belly to form:– Tendon: strong tough cord of connective tissue

that extends from muscle to bone

– Aponeurosis: a strong sheath of connective tissue that extends from muscle to muscle

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Skeletal Muscle Fibers

Muscle Fibers: highly specialized skeletal muscle cells – Myoblasts

– Multinucleated

– Fascicles: groups of skeletal muscle fibers

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Skeletal Muscle Fibers

Every mature muscle cell developed from 100 myoblasts that fuse together in the fetus (multinucleated)

Mature muscle cells can not divide

Muscle growth is a result of cellular enlargement & not cell division

Satellite cells retain the ability to regenerate new cells

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Skeletal Muscle Fibers

Sarcolemma = plasma membrane

Sarcoplasm = cytoplasm– Myofibrils, myofilaments

– Myoglobin: red-colored, oxygen-binding protein

Transverse tubules: invaginations of the sarcolemma into the center of the cell– filled with extracellular fluid

– carry muscle action potentials down into cell

Mitochondria lie in rows throughout the cell– near the muscle proteins that use ATP during contraction

Sarcoplasmic reticulum: storage site for calcium

Terminal cisternae

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Skeletal Muscle FibersMyofibrils

Thin myofilaments – Composed of actin, tropomyosin, and troponin

– Tropomyosin and troponin are regulatory proteins

Thick myofilaments – Consist mostly of myosin

– Projecting myosin heads are called cross bridgesand contain actin and ATP-binding sites

– Elastic filaments help stabilize the position of thick filaments

Actin and myosin are the two contractile proteins in muscle

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Skeletal Muscle FibersSarcomeres

Thick and thin filaments overlap each other in a pattern that creates striations – light I bands

– dark A bands

The I band region contains only thin filaments

They are arranged in compartments called sarcomeres, separated by Z discs– H zone

– Zone of overlap

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Sarcomere

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Sarcomeres

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Sarcomeres

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The Proteins of Muscle

Myofibrils are built of 3 kinds of protein– contractile proteins

• myosin and actin

– regulatory proteins which turn contraction on & off• troponin and tropomyosin

– structural proteins which provide proper alignment, elasticity and extensibility• titin, myomesin, nebulin and dystrophin

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Myosin

Myosin heads contain:• ATPase• Dynamic binding

sites for• ATP• Actin

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Actin

Dynamic binding sites for:

• myosin• Tropomyosin

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Regulatory Proteins

Troponin binds:• Tropomyosin• Ca+

Tropomyosin binds:• troponin• Actin

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Structural Proteins

Titin: anchors thick filament to M line and Z disc

Myomesin: anchors thick filaments at the M line

Nebulin: strengthens thin filaments

Dystrophin: anchors thin filaments to sarcolemma

NebulinDystrophin

Sarcolemma

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Control of Skeletal Muscle Activity

A motor neuron transmits a nerve impulse (action potential) to skeletal muscle where the nerve impulse serves as a stimulus for contraction

Motor unit: a motor neuron and the muscle fibers it stimulates– A single motor unit may innervate as few as 10 or as many as 2000

muscle fibers, with an average of 150 fibers being innervated by each motor neuron

Neuromuscular junction (synapse): site where axon terminal of a motor neuron meets the muscle fiber sarcolemma (plasma membrane)– Separated by a gap called the neuromuscular cleft– Motor end plate: pocket formed around motor neuron by

sarcolemma– Acetylcholine is released from synaptic vesicles of the motor

neuron• Triggers a muscle action potential

Neuromuscular Junction (NMJ) or Synapse

NMJ = myoneural junction– end of axon nears the surface of a muscle fiber at its motor

end plate region (remain separated by synaptic cleft or gap)

Structures of NMJ Region

Synaptic end bulbs are swellings of axon terminals

End bulbs contain synaptic vesicles filled with acetylcholine (ACh)

Motor end plate membrane contains 30 million ACh receptors

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The Motor Unit

All or none principle: when they are sufficiently stimulated, muscle fibers contract with all the force possible under the existing conditions

Threshold stimulus: the minimal level of stimulation required to cause a fiber to contract

Total strength of a contraction depends on how many motor units are activated & how large the motor units are

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Muscle Control

Fine muscle control: a relatively few muscle fibers stimulated by one motor neuron, resulting in finer control over the amount of tension in the whole muscle– Tend to have the most excitable neurons– often are activated first

Course muscle control: many muscle fibers stimulated by one motor unit, resulting in less control over the amount of tension in the whole muscle but greater power– Less excitable neurons– Activated when stronger contractions

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Skeletal Muscle

Physiology

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How Does Contraction Begin?

Nerve impulse reaches an axon terminal & synaptic vesicles release Ach

ACh diffuses to receptors on the sarcolemma & Na+ channels open and Na+

rushes into the cellA muscle action potential spreads over

sarcolemma and down transverse tubulesSR releases Ca+2 into the sarcoplasmCa+2 binds to troponin & causes troponin-

tropomyosin complex to move & reveal myosin binding sites on actin--the contraction cycle begins

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Excitation - Contraction Coupling

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Contraction Cycle

Sliding Filament Model: Repeating sequence of events that cause the thick & thin filaments to move past each other

4 steps to contraction cycle– ATP hydrolysis– attachment of myosin to actin to form

crossbridges– power stroke– detachment of myosin from actin

Cycle keeps repeating as long as there is ATP available & high Ca+2 level near thin filament

Cross-bridge Formation

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Relaxation

Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft– Sodium potassium pumps remove sodium

Ca+2 release channels closeActive transport pumps Ca+2 back into

storage in the sarcoplasmic reticulumCalcium-binding protein (calsequestrin)

helps hold Ca+2 in SR – Ca+2 concentration 10,000 times higher than in

cytosol

Tropomyosin-troponin complex covers binding site on the actin

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Rigor Mortis

Rigor mortis is a state of muscular rigidity that begins 3-4 hours after death and lasts about 24 hours

After death, Ca+2 ions leak out of the SR and allow myosin heads to bind to actin

Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells

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Tension Production

Contraction: a shortening or increase in tension– Tension: a stretching or pulling force

Active tension (internal): tension generated by contractile elements (thick and thin filaments)– All or none response

Passive tension (external): tension generated by elastic elements. It is not related to muscular contraction– Elastic elements: elastic filaments (titin), connective

tissue coverings, and tendons– Elastic elements stretch slightly before they relay the

tension generated by the sliding filaments to the whole muscle

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Tension Produced by Muscle FibersLength Tension Relationship

Optimal overlap of thick & thin filaments– produces greatest number of crossbridges and the

greatest amount of tension

As stretch muscle (past optimal length)– fewer cross bridges exist & less force is produced

If muscle is overly shortened (less than optimal)– fewer cross bridges exist & less force is produced

– thick filaments crumpled by Z discs

Normally– resting muscle length remains between 70 to 130% of the

optimum

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Tension Produced by Muscle Fibers Twitch

Twitch: a brief contraction of all the muscle fibers in a motor unit in response to a single action potential

– Latent period: time during which impulse is traveling along sarcolemma and down T tubules to SR

• Ca is being released• No change in tension

– Contraction period: tension increases

• Cross bridges are swiveling

– Relaxation period: muscle relaxes and tends to return to its original length

• Tension decreases

– Refractory period: period when muscle cannot respond

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Tension Produced by Muscle Fibers

Wave Summation: the increased strength of a contraction resulting from the application of a second stimulus before the muscle has completely relaxed

– Tetanus: when a muscle fiber is stimulated so rapidly it does not relax between stimuli

• Incomplete (unfused) tetanus: a sustained muscle contraction that permits partial relaxation between stimuli

• Complete (fused) tetanus: a sustained contraction that lacks even partial relaxation between stimuli

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Tension Produced by Skeletal MusclesMotor Units

Recruitment: process of increasing the number of active motor units– low levels of stimulation -- relatively small

number of motor neurons are activated, so relatively few motor units are stimulated

– higher levels of stimulation -- more motor neurons stimulated resulting in the recruitment of more motor units

– A muscle is contracting at maximal intensity when all motor units are activated simultaneously

Asynchronous motor unit summation: motor units are activated on a rotating basis– Some are resting while others are contracting– Prevents fatigue

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Tension Produced by Skeletal Muscles Muscle Tone

Involuntary contraction of a small number of motor units (alternately active and inactive in a constantly shifting pattern)– keeps muscles firm even though relaxed

– does not produce movement

Hypotonia: decreased or lost muscle tone– Such muscles are said to be flaccid

Hypertonia: increased muscle tone and may be expressed as either spasticity(stiffness) or rigidity

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Tension Produced by Skeletal Muscles Contractions

Load: the weight of an object to be moved

Isotonic: tension or force generated by muscle is greater than the load and the muscle changes length– Concentric contraction– Eccentric contraction

Isometric: load is greater than the tension or force generated by the muscle and the muscle does not change length

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Tension Produced by Skeletal Muscles Isotonic and Isometric Contraction

Isotonic contractions = a load is moved – concentric contraction = a muscle shortens to produce

force and movement– eccentric contractions = a muscle lengthens while

maintaining force and movement

Isometric contraction = no movement occurs– tension is generated without muscle shortening– maintaining posture & supports objects in a fixed position

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Energy Use and Muscular ActivityProduction of ATP in Muscle Fibers

Muscle uses ATP at a great rate when activeEnergy Reserves

– Stored ATP in cell• Sarcoplasmic ATP only lasts for few seconds

– Creatine phosphate– Stored Glycogen

Generation of ATP– Glycolysis (anaerobic cellular respiration)– Aerobic cellular respiration

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Energy Use and Muscular ActivityGlycolysis

ATP produced from glucose breakdown into pyruvic acid during glycolysis – if no O2 present

• pyruvic converted to lactic acid which diffuses into the blood

Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity

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Energy Use and Muscular ActivityAerobic Cellular Respiration

ATP for any activity lasting over 30 seconds – if sufficient oxygen is available, pyruvic acid

enters the mitochondria to generate ATP, water and heat

– fatty acids and amino acids can also be used by the mitochondria

Provides 90% of ATP energy if activity lasts more than 10 minutes

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Energy Use and Level of Activity Muscle Fatigue

Muscle fatigue: physiological inability to contract– Glycogen stores are exhausted and ATP

regeneration does not match ATP use– Levels of lactic acid increase and pH

drops adversely affecting enzymes• Thereby slowing reactions

Contractures: state of continuous contraction

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Energy Use and Level of ActivityRecovery Period

Muscle tissue has two sources of oxygen.– diffuses in from the blood– released by myoglobin inside muscle fibers

Aerobic system requires O2 to produce ATP needed for prolonged activity– increased breathing effort during exercise

Recovery oxygen uptake– Oxygen Debt: elevated oxygen use after

exercise– lactic acid is converted back to pyruvic acid

elevated body temperature

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Muscle PerformanceVariations in Skeletal Muscle Fibers

Myoglobin, mitochondria and capillaries– red muscle fibers

• more myoglobin, an oxygen-storing reddish pigment • more capillaries and mitochondria

– white muscle fibers• less myoglobin and less capillaries give fibers their

pale color

Contraction and relaxation speeds vary– how fast myosin ATPase hydrolyzes ATP

Resistance to fatigue– different metabolic reactions used to generate

ATP

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Slow fibers (Slow oxidative) – slow-twitch, Type I fibers– red in color: lots of mitochondria, myoglobin & blood vessels– prolonged, sustained contractions for maintaining posture

Intermediate fibers (Fast oxidative-glycolytic)– fast-twitch A, Type II-A fibers– red in color: lots of mitochondria, myoglobin & blood vessels– split ATP at very fast rate; used for walking and sprinting

Fast fibers (Fast glycolytic) – fast-twitch B, Type II-B fibers– white in color: few mitochondria & BV, low myoglobin– anaerobic movements for short duration; used for weight-

lifting

Types of Muscle Fibers

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Muscle PerformanceMuscle Fiber Types

Fast fibers Slow fibers

Characteristic Type IIb Type IIa Type I

Number of mitochondria Low High/moderate High

Resistance to fatigue Low High/moderate High

Predominant energy system Anaerobic Combination Aerobic

ATPase activity Highest High Low

Vmax (speed of shortening) Highest Intermediate Low

Efficiency Low Moderate High

Specific tension High High Moderate

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Fiber Types and Performance

Power athletes – Sprinters– Possess high percentage of fast fibers

Endurance athletes – Distance runners– Have high percentage of slow fibers

Others– Weight lifters and nonathletes– Have about 50% slow and 50% fast fibers

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Muscle Performance Atrophy and Hypertrophy

Atrophy– wasting away of muscles– caused by disuse (disuse atrophy) or severing of

the nerve supply (denervation atrophy)– the transition to connective tissue can not be

reversed

Hypertrophy– increase in the diameter of muscle fibers – resulting from very forceful, repetitive

muscular activity and an increase in myofibrils, SR & mitochondria

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Muscle PerformancePhysical Conditioning

Strength training– Contracting muscles against heavy resistance

– Results in increased numbers of myofilaments in each muscle fiber

– Increases mass of muscle (hypertrophy)

– Anerobic conditioning

Endurance training– Results in increased number of blood vessels in

a muscle without significantly increasing its size

– Can also increase number of mitochondria in muscle fibers

– Aerobic conditioning

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Cardiac Muscle

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Cardiac Muscle TissueStructural Characteristics

Found only in the heart It is striated and involuntary The fibers are quadrangular and usually contain a single

centrally placed nucleus Compared to skeletal muscle, cardiac muscle tissue has

– More sarcoplasm– More mitochondria– Less well-developed sarcoplasmic reticulum– Larger transverse tubules located at Z discs– Myofilaments are not arranged in discrete myofibrils

The fibers branch freely and are connected via gap junctions Intercalated discs provide strength and aid in conduction of

muscle action potentials by way of gap junctions located in the discs

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Cardiac MuscleStructural Characteristics

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Cardiac Muscle TissueFunctional Characteristics

Unlike skeletal muscle tissue, cardiac muscle tissue contracts and relaxes rapidly, continuously, and rhythmically– ATP is generated aerobically in large, numerous

mitochondria

Cardiac muscle can contract without extrinsic (outside) stimulation and can remain contracted longer than skeletal muscle tissue

Cardiac muscle has a long refractory period that allows time for the heart to relax between beats and which prevents tetanus

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Smooth Muscle

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Smooth Muscle TissueStructural Characteristics

Smooth muscle tissue is non-striated and involuntary

Smooth muscle fibers are considerably smaller than skeletal muscle fibers and are thickest at the center, tapering at the ends– Each fiber contains a single, oval, centrally located

nucleus and thick and thin myofilaments, although the arrangement of the myofilaments is not in orderly sarcomeres as in skeletal muscle tissue

Smooth muscle fibers contain intermediate filaments and dense bodies (which function as Z discs)

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Smooth MuscleStructural Characteristics

Varicosities: expanded portions of axon terminals that release neurotransmitter

No striations

Dense bodies: structures that function as Z discs

No transverse tubules

Sarcoplasmic reticulum is not well developed

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Smooth MuscleFunctional Characteristics

Thick & thin myofilaments not orderly arranged so lacks sarcomeres

Sliding of thick & thin filaments generates tension

Transferred to intermediate filaments & dense bodies attached to sarcolemma

Muscle fiber contracts and twists into a helix as it shortens -- relaxes by untwisting

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Smooth MuscleFunctional Characteristics

Single unit (visceral) smooth muscle: fibers are connected by gap junctions, forming large networks

Multiunit smooth muscle: fibers act independently– Each fiber is

directly stimulated by axon terminal of a motor neuron branch

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Smooth MuscleFunctional Characteristics

Contraction starts slowly & lasts longer– no transverse tubules & very little SR

– Ca+2 must flows in from outside

Calmodulin replaces troponin– Ca+2 binds to calmodulin turning on an enzyme

(myosin light chain kinase) that phosphorylates the myosin head so that contraction can occur

– enzyme works slowly, slowing contraction

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Smooth Muscle Tone

Ca+2 moves slowly out of the cell– delaying relaxation and providing for

state of continued partial contraction

– sustained long-term

Useful for maintaining blood pressure or a steady pressure on the contents of GI tract

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Regulation of Contraction

Regulation of contraction due to – nerve signals from autonomic nervous system– changes in local conditions (pH, O2, CO2,

temperature & ionic concentrations)– hormones (epinephrine -- relaxes muscle in

airways & some blood vessels)

Stress-relaxation response– when stretched, initially contracts & then

tension decreases to what is needed– stretch hollow organs as they fill & yet

pressure remains fairly constant– when empties, muscle rebounds & walls firm up

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Homeostasis

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Regeneration of Muscle Tissue

Skeletal muscle fibers cannot divide and have limited powers of regeneration– Growth after the first year is due to

enlargement of existing cells, rather than an increase in the number of fibers (although new individual cells may be derived from satellite cells)

Cardiac muscle fibers cannot divide or regenerate

Smooth muscle fibers have limited capacity for division and regeneration

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Aging and Muscle Tissue

Aging– Beginning at about 30 years of age there is a progressive

loss of skeletal muscle, which is replaced by fat

– There is also a decrease in maximal strength and a slowing of muscle reflexes

Developmental anatomy of the muscular system– With few exceptions, muscle develop from mesoderm

– Skeletal muscles of the head and extremities develop from general mesoderm; The remainder of the skeletal muscles develop from the mesoderm of somites

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Abnormal Contractions

Spasm: involuntary contraction of single muscle Cramp: a painful spasm Tic: involuntary twitching of muscles normally

under voluntary control--eyelid or facial muscles

Tremor: rhythmic, involuntary contraction of opposing muscle groups

Fasciculation: involuntary, brief twitch of a motor unit visible under the skin

Paralysis: loss of voluntary movement– Flaccid paralysis: paralysis with loss of muscle tone– Spastic paralysis: paralysis with rigidity