Skeletal muscle physiology
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Skeletal Muscle Physiology. Muscular System Functions. Body movement (Locomotion) Maintenance of posture Respiration Diaphragm and intercostal contractions Communication (Verbal and Facial) Constriction of organs and vessels Peristalsis of intestinal tract

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Skeletal Muscle Physiology

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Skeletal muscle physiology

Skeletal Muscle Physiology

Muscular system functions

Muscular System Functions

  • Body movement (Locomotion)

  • Maintenance of posture

  • Respiration

    • Diaphragm and intercostal contractions

  • Communication (Verbal and Facial)

  • Constriction of organs and vessels

    • Peristalsis of intestinal tract

    • Vasoconstriction of b.v. and other structures (pupils)

  • Heart beat

  • Production of body heat (Thermogenesis)

Properties of muscle

Properties of Muscle

  • Excitability: capacity of muscle to respond to a stimulus

  • Contractility: ability of a muscle to shorten and generate pulling force

  • Extensibility: muscle can be stretched back to its original length

  • Elasticity: ability of muscle to recoil to original resting length after stretched

Types of muscle

Types of Muscle

  • Skeletal

    • Attached to bones

    • Makes up 40% of body weight

    • Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement

    • Voluntary in action; controlled by somatic motor neurons

  • Smooth

    • In the walls of hollow organs, blood vessels, eye, glands, uterus, skin

    • Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow,

    • In some locations, autorhythmic

    • Controlled involuntarily by endocrine and autonomic nervous systems

  • Cardiac

    • Heart: major source of movement of blood

    • Autorhythmic

    • Controlled involuntarily by endocrine and autonomic nervous systems

Connective tissue sheaths

Connective Tissue Sheaths

  • Connective Tissue of a Muscle

    • Epimysium. Dense regular c.t. surrounding entire muscle

      • Separates muscle from surrounding tissues and organs

      • Connected to the deep fascia

    • Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle

      • Contains b.v and nerves

    • Endomysium. Loose connective tissue that surrounds individual muscle fibers

      • Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue

  • Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Nerve and blood vessel supply

Nerve and Blood Vessel Supply

  • Motor neurons

    • stimulate muscle fibers to contract

    • Neuron axons branch so that each muscle fiber (muscle cell) is innervated

    • Form a neuromuscular junction (= myoneural junction)

  • Capillary beds surround muscle fibers

    • Muscles require large amts of energy

    • Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Muscle tissue types

Muscle Tissue Types

Skeletal muscle

Skeletal Muscle

  • Long cylindrical cells

  • Many nuclei per cell

  • Striated

  • Voluntary

  • Rapid contractions

Basic features of a skeletal muscle

Basic Features of a Skeletal Muscle

  • Muscle attachments

    • Most skeletal muscles run from one bone to another

    • One bone will move – other bone remains fixed

      • Origin – less movable attach- ment

      • Insertion – more movable attach- ment

Basic features of a skeletal muscle1

Basic Features of a Skeletal Muscle

  • Muscle attachments (continued)

    • Muscles attach to origins and insertions by connective tissue

      • Fleshy attachments – connective tissue fibers are short

      • Indirect attachments – connective tissue forms a tendon or aponeurosis

    • Bone markings present where tendons meet bones

      • Tubercles, trochanters, and crests

Skeletal muscle structure

Skeletal Muscle Structure

  • Composed of muscle cells (fibers), connective tissue, blood vessels, nerves

  • Fibers are long, cylindrical, and multinucleated

  • Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length

  • Develop from myoblasts; numbers remain constant

  • Striated appearance

  • Nuclei are peripherally located

Muscle attachments

Muscle Attachments

Antagonistic muscles

Antagonistic Muscles

Microanatomy of skeletal muscle

Microanatomy of Skeletal Muscle

Muscle fiber anatomy

Muscle Fiber Anatomy

  • Sarcolemma - cell membrane

    • Surrounds the sarcoplasm(cytoplasm of fiber)

      • Contains many of the same organelles seen in other cells

      • An abundance of the oxygen-binding protein myoglobin

    • Punctuated by openings called the transverse tubules (T-tubules)

      • Narrow tubes that extend into the sarcoplasm at right angles to the surface

      • Filled with extracellular fluid

  • Myofibrils -cylindrical structures within muscle fiber

    • Are bundles of protein filaments (=myofilaments)

      • Two types of myofilaments

        • Actin filaments (thin filaments)

        • Myosin filaments (thick filaments)

    • At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma

    • When myofibril shortens, muscle shortens (contracts)

Sarcoplasmic reticulum sr

Sarcoplasmic Reticulum (SR)

  • SR is an elaborate, smooth endoplasmic reticulum

    • runs longitudinally and surrounds each myofibril

    • Form chambers called terminal cisternae on either side of the T-tubules

  • A single T-tubule and the 2 terminal cisternae form a triad

  • SR stores Ca++ when muscle not contracting

    • When stimulated, calcium released into sarcoplasm

    • SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

Sarcoplasmic reticulum sr1

Sarcoplasmic Reticulum (SR)

Parts of a muscle

Parts of a Muscle

Sarcomeres z disk to z disk

Sarcomeres: Z Disk to Z Disk

  • Sarcomere - repeating functional units of a myofibril

    • About 10,000 sarcomeres per myofibril, end to end

    • Each is about 2 µm long

  • Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.

    • A bands: a dark band; full length of thick (myosin) filament

    • M line - protein to which myosins attach

    • H zone - thick but NO thin filaments

    • I bands: a light band; from Z disks to ends of thick filaments

      • Thin but NO thick filaments

      • Extends from A band of one sarcomere to A band of the next sarcomere

    • Z disk: filamentous network of protein. Serves as attachment for actin myofilaments

    • Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

Structure of actin and myosin

Structure of Actin and Myosin

Myosin thick myofilament

Myosin (Thick) Myofilament

  • Many elongated myosin molecules shaped like golf clubs.

  • Single filament contains roughly 300 myosin molecules

  • Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.

  • Myosin heads

    • Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)

    • Attached to the rod portion by a hinge region that can bend and straighten during contraction.

    • Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Actin thin myofilaments

Actin (Thin) Myofilaments

  • Thin Filament: composed of 3 major proteins

    • F (fibrous) actin

    • Tropomyosin

    • Troponin

  • Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.

    • Composed of G actin monomers each of which has a myosin-binding site (see yellow dot)

    • Actin site can bind myosin during muscle contraction.

  • Tropomyosin: an elongated protein winds along the groove of the F actin double helix.

  • Troponin is composed of three subunits:

    • Tn-A : binds to actin

    • Tn-T :binds to tropomyosin,

    • Tn-C :binds to calcium ions.

Now putting it all together to perform the function of muscle contraction

Now, putting it all together to perform the function of muscle: Contraction

Skeletal muscle physiology

Z line

Z line

Skeletal muscle physiology

H Band

Sarcomere relaxed

Sarcomere Relaxed

Sarcomere partially contracted

Sarcomere Partially Contracted

Sarcomere completely contracted

Sarcomere Completely Contracted

Skeletal muscle physiology


Binding Site



Skeletal muscle physiology


Excitation contraction coupling

Excitation-Contraction Coupling

  • Muscle contraction

    • Alpha motor neurons release Ach

    • ACh produces large EPSP in muscle fibers (via nicotinic Ach receptors

    • EPSP evokes action potential

    • Action potential (excitation) triggers Ca2+ release, leads to fiber contraction

    • Relaxation, Ca2+ levels lowered by organelle reuptake

Excitation contraction coupling1

Excitation-Contraction Coupling

Excitation contraction coupling2

Excitation-Contraction Coupling

Sliding filament model of contraction

Sliding Filament Model of Contraction

  • Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

  • In the relaxed state, thin and thick filaments overlap only slightly

  • Upon stimulation, myosin heads bind to actin and sliding begins

Skeletal muscle physiology

How striated muscle works: The Sliding Filament Model

The lever movement drives displacement of the actin filament relative to the myosin

head (~5 nm), and by deforming internal elastic structures, produces force (~5 pN).

Thick and thin filaments interdigitate and “slide” relative to each other.

Neuromuscular junction

Neuromuscular Junction

Neuromuscular junction1

Neuromuscular Junction

  • Region where the motor neuron stimulates the muscle fiber

  • The neuromuscular junction is formed by :

    1. End of motor neuron axon (axon terminal)

    • Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine(ACh)

      2. The motor end plate of a muscle

    • A specific part of the sarcolemma that contains ACh receptors

  • Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

  • Neuromuscular junction2

    Neuromuscular Junction

    Motor unit the nerve muscle functional unit

    Motor Unit: The Nerve-Muscle Functional Unit

    • A motor unit is a motor neuron and all the muscle fibers it supplies

    • The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500)

    • Muscles that control fine movements (fingers, eyes) have small motor units

    • Large weight-bearing muscles (thighs, hips) have large motor units

    Motor unit the nerve muscle functional unit1

    Motor Unit: The Nerve-Muscle Functional Unit

    • Muscle fibers from a motor unit are spread throughout the muscle

      • Not confined to one fascicle

    • Therefore, contraction of a single motor unit causes weak contraction of the entire muscle

    • Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

    Motor unit all the muscle cells controlled by one nerve cell

    Motor UnitAll the muscle cells controlled by one nerve cell

    Acetylcholine opens na channel

    Acetylcholine Opens Na+ Channel

    Muscle contraction summary

    Muscle Contraction Summary

    • Nerve impulse reaches myoneural junction

    • Acetylcholine is released from motor neuron

    • Ach binds with receptors in the muscle membrane to allow sodium to enter

    • Sodium influx will generate an action potential in the sarcolemma

    Muscle contraction cont d

    Muscle Contraction (Cont’d)

    • Action potential travels down T tubule

    • Sarcoplamic reticulum releases calcium

    • Calcium binds with troponin to move the troponin, tropomyosin complex

    • Binding sites in the actin filament are exposed

    Muscle contraction cont d1

    Muscle Contraction (cont’d)

    • Myosin head attach to binding sites and create a power stroke

    • ATP detaches myosin heads and energizes them for another contaction

    • When action potentials cease the muscle stop contracting

    Contraction speed

    Contraction Speed

    Skeletal muscle physiology

    Myosin is a Molecular Motor

    Myosin is a hexamer:

    2 myosin heavy chains

    4 myosin light chains

    Coiled coil of two a helices

    2 nm

    C terminus

    Myosin head: retains all of the motor functions of myosin,

    i.e. the ability to produce movement and force.


    binding site

    Myosin S1 fragment

    crystal structure

    Ruegg et al., (2002)

    News Physiol Sci 17:213-218.

    NH2-terminal catalytic

    (motor) domain

    neck region/lever arm

    Skeletal muscle physiology

    Chemomechanical coupling– conversion of chemical energy

    (ATP about 7 kcal x mole-1) into force/movement.

    • ATP is unstable thermodynamically

    • Two most energetically favorable steps:

    • 1. ATP binding to myosin

    • 2. Phosphate release from myosin

    • Rate of cycling determined by M·ATPase activity and external load

    Adapted from Goldman & Brenner (1987) Ann Rev Physiol 49:629-636.

    Skeletal muscle physiology

    Shortening Velocity Vependent on ATPase Activity

    Different myosin heavy chains (MHCs) have different ATPase activities.

    There are at least 7 separate skeletal muscle MHC genes…arranged in series

    on chromosome 17.

    Two cardiac MHC genes located in tandem on chromosome 14.

    The slow b cardiac MHC is the predominant gene expressed in slow fibers

    of mammals.

    Goldspink (1999) J Anat 194:323-334.

    Skeletal muscle physiology

    Power Output: The Most Physiologically Relevant

    Marker of Performance

    Power = work / time

    = force x distance / time

    = force x velocity

    Peak power obtained at intermediate loads and intermediate


    Figure from Berne and Levy, Physiology

    Mosby—Year Book, Inc., 1993.

    Skeletal muscle physiology

    Most likely to cause

    muscle injury

    Three Potential Actions During Muscle Contraction:

    Biceps muscle shortens

    during contraction

    • shortening

    (Isotonic: shortening against fixed load, speed dependent on M·ATPase activity and load)

    • isometric

    • lengthening

    Biceps muscle lengthens

    during contraction

    Motor unit ratios

    Motor Unit Ratios

    • Back muscles

      • 1:100

    • Finger muscles

      • 1:10

    • Eye muscles

      • 1:1

    Skeletal muscle physiology



    • To increase force:

    • Recruit more M.U.s

    • Increase freq.

    • (force –frequency)

    Recall The Motor Unit:

    motor neuron and the muscle fibers it innervates

    • The smallest amount of

      muscle that can be activated


    • Gradation of force in skeletal

    • muscle is coordinated largely

      by the nervous system.

    • Recruitment of motor units

    • is the most important means

    • of controlling muscle tension.

    • Since all fibers in the motor

    • unit contract simultaneously,

    • pressures for gene expression

    • (e.g. frequency of stimulation,

    • load) are identical in all fibers

    • of a motor unit.

    Skeletal muscle physiology

    Physiological profiles of motor units:

    all fibers in a motor unit are of the same fiber type

    • Slow motor units contain slow fibers:

    • Myosin with long cycle time and therefore uses ATP at a slow rate.

    • Many mitochondria, so large capacity to replenish ATP.

    • Economical maintenance of force during isometric contractions and efficient performance of repetitive slow isotonic contractions.

    • Fast motor units contain fast fibers:

    • Myosin with rapid cycling rates.

    • For higher power or when isometric force produced by slow motor units is insufficient.

    • Type 2A fibers are fast and adapted for producing sustained power.

    • Type 2X fibers are faster, but non-oxidative and fatigue rapidly.

    • 2X/2D not 2B.

    Modified from Burke and Tsairis, Ann NY Acad Sci 228:145-159, 1974.

    Skeletal muscle physiology

    Increased use: strength training

    Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns.

    Long term gains almost solely the result of hypertrophy i.e. increased size.

    Skeletal muscle physiology

    The PI(3)K/Akt(PKB)/mTOR pathway is a

    crucial regulator of skeletal muscle


    • Application of IGF-I to C2C12 myotube cultures induced both increased width and phosphor-ylation of downstream targets of Akt (p70S6 kinase, p70S6K; PHAS-1/4E-BP1; GSK3) but did NOT activate the calcineurin pathway.

    • Treatment with rapamycin almost completely prevented increase in width of C2C12 myotubes.

    • Treatment with cyclosporin or FK506 does not prevent myotube growth in vitro or compensatory hypertrophy in vivo

    • Recovery of muscle weight after following reloading is blocked by rapamycin but not cyclosporin.

    Rommel et al. (2001) Nature Cell Biology 3, 1009.

    Skeletal muscle physiology

    Performance Declines with Aging

    --despite maintenance of physical activity




    Performance (% of peak)





    Basketball (rebounds/game)








    Age (years)

    D.H. Moore (1975) Nature 253:264-265.

    NBA Register, 1992-1993 Edition

    Skeletal muscle physiology

    Number of motor units declines during aging

    - extensor digitorum brevis muscle of humans



    Individual fiber atrophy

    (which may be at least

    partially preventable and

    reversible through exercise).

    Loss of fibers

    (which as yet appears


    Campbell et al., (1973) J Neurol Neurosurg Psych 36:74-182.

    Skeletal muscle physiology

    Motor unit remodeling with aging









    • Fewer motor units

    • More fibers/motor unit

    Skeletal muscle physiology

    • Mean Motor Unit Forces:

      • FF motor units get smaller in old age and decrease in number

      • S motor units get bigger with no change in number

      • Decreased rate of force generation and POWER!!








    Maximum Isometric Force (mN)










    Kadhiresan et al., (1996)

    J Physiol 493:543-552.

    Motor Unit Classification

    Skeletal muscle physiology

    Muscle injury may play a role in the development of

    atrophy with aging.

    • Muscles in old animals are more susceptible to contraction-

    • induced injury than those in young or adult animals.

    • Muscles in old animals show delayed and impaired recovery

    • following contraction-induced injury.

    • Following severe injury, muscles in old animals display

    • prolonged, possibly irreversible, structural and functional

    • deficits.

    Disorders of muscle tissue

    Disorders of Muscle Tissue

    • Muscle tissues experience few disorders

      • Heart muscle is the exception

      • Skeletal muscle – remarkably resistant to infection

      • Smooth muscle – problems stem from external irritants

    Disorders of muscle tissue1

    Disorders of Muscle Tissue

    • Muscular dystrophy – a group of inherited muscle destroying disease

      • Affected muscles enlarge with fat and connective tissue

      • Muscles degenerate

        • Types of muscular dystrophy

          • Duchenne muscular dystrophy

          • Myotonic dystrophy

    Disorders of muscle tissue2

    Disorders of Muscle Tissue

    • Myofascial pain syndrome – pain is caused by tightened bands of muscle fibers

    • Fibromyalgia – a mysterious chronic-pain syndrome

      • Affects mostly women

      • Symptoms – fatigue, sleep abnormalities, severe musculoskeletal pain, and headache

    Skeletal muscle physiology

    Muscular Dystrophy:

    A frequently fatal disease of muscle deterioration

    • Muscular dystrophies have in the past been classified based on subjective and sometimes

    • subtle differences in clinical presentation, such as age of onset, involvement of particular

    • muscles, rate of progression of pathology, mode of inheritance.

    • Since the discovery of dystrophin, numerous genetic disease loci have been linked to protein

    • products and to cellular phenotypes, generating models for studying the pathogenesis of the

    • dystrophies.

    • Proteins localized in the nucleus, cytosol, cytoskeleton, sarcolemma, and ECM.

    Cohn and Campbell (2000) Muscle Nerve 23:1459-1471.

    Skeletal muscle physiology

    Dystrophin function:

    transmission of force to extracellular matrix



    dystroglycan (a and b)

    sarcoglycans (a, b, g, d)

    syntrophins (a, b1)

    dystrobrevins (a, b)


    laminin-a2 (merosin)

    (Some components of

    the dystrophin glycoprotein

    complex are relatively

    recent discoveries, so one

    cannot assume that all

    players are yet known.)

    Cohn and Campbell (2000) Muscle Nerve 23:1459-1471.

    Oxidative and glycolytic fibers

    Oxidative and Glycolytic Fibers

    Skeletal muscle physiology



    Creatine + ATP

    Creatine phosphate + ADP


    • Molecule capable of storing ATP energy

    Creatine phosphate

    Creatine + ATP

    Creatine Phosphate

    • Molecule with stored ATP energy

    Creatine phosphate+ ADP

    Muscle fatigue

    Muscle Fatigue

    • Lack of oxygen causes ATP deficit

    • Lactic acid builds up from anaerobic respiration

    Muscle fatigue1

    Muscle Fatigue

    Muscle atrophy

    Muscle Atrophy

    • Weakening and shrinking of a muscle

    • May be caused

      • Immobilization

      • Loss of neural stimulation

    Muscle hypertrophy

    Muscle Hypertrophy

    • Enlargement of a muscle

    • More capillaries

    • More mitochondria

    • Caused by

      • Strenuous exercise

      • Steroid hormones

    Steroid hormones

    Steroid Hormones

    • Stimulate muscle growth and hypertrophy

    Muscle tonus

    Muscle Tonus

    • Tightness of a muscle

    • Some fibers always contracted



    • Sustained contraction of a muscle

    • Result of a rapid succession of nerve impulses



    Refractory period

    Refractory Period

    • Brief period of time in which muscle cells will not respond to a stimulus



    Refractory periods

    Refractory Periods

    Skeletal Muscle

    Cardiac Muscle

    Isometric contraction

    Isometric Contraction

    • Produces no movement

    • Used in

      • Standing

      • Sitting

      • Posture

    Isotonic contraction

    Isotonic Contraction

    • Produces movement

    • Used in

      • Walking

      • Moving any part of the body

    Muscle spindle

    Muscle Spindle

    Muscle spindle responses

    Muscle Spindle Responses

    Alpha gamma coactivation

    Alpha / Gamma Coactivation

    Golgi tendon organs

    Golgi Tendon Organs

    Developmental aspects regeneration

    Developmental Aspects: Regeneration

    • Cardiac and skeletal muscle become amitotic, but can lengthen and thicken

    • Myoblast-like satellite cells show very limited regenerative ability

    • Cardiac cells lack satellite cells

    • Smooth muscle has good regenerative ability

    • There is a biological basis for greater strength in men than in women

    • Women’s skeletal muscle makes up 36% of their body mass

    • Men’s skeletal muscle makes up 42% of their body mass

    Developmental aspects male and female

    Developmental Aspects:Male and Female

    • These differences are due primarily to the male sex hormone testosterone

    • With more muscle mass, men are generally stronger than women

    • Body strength per unit muscle mass, however, is the same in both sexes

    Developmental aspects age related

    Developmental Aspects: Age Related

    • With age, connective tissue increases and muscle fibers decrease

    • Muscles become stringier and more sinewy

    • By age 80, 50% of muscle mass is lost (sarcopenia)

    • Decreased density of capillaries in muscle

    • Reduced stamina

    • Increased recovery time

    • Regular exercise reverses sarcopenia

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