© 2007 McGraw-Hill Higher Education. All rights reserved. The Muscular System

© 2007 McGraw-Hill Higher Education. All rights reserved.

The Muscular System


Introduction


Skeletal muscle anatomy• Muscle

– Made up of fascicles (a bundle of muscle fibers)– Fiber (muscle cell or myocyte)

• Made up of myofibrils– Contain sarcomeres (functional unit)

» Myofilaments (actin and myosin)– Connective tissue

• Epimysium– Surrounds entire muscle

• Perimysium– Surrounds fascicles

• Sarcolemma– Surrounds fibers

• Satellite cells– Allow muscle to adapt to activity


Skeletal muscle anatomy

• Skeletal muscle cells are multi-nucleated– Unusual

• Satellite cells– Myoblasts

• Essential for growth of fibers

• Chemotaxic– Can migrate around

cell


• Connection of muscle to tendon– Allow transmission

of force to bones – Tendon

• connects to bone

– Myotendinous junction• Common spot

for sports injuries

Myotendinous junction


• Striated (striped) appearance

• A band– Anisotropic

• I band– Isotropic

• Means having the same optical properties in all directions

• H zone– Light

• Z line– Separates

sarcomeres

Sarcomeres


• Muscle– ~75% water– 20% protein– 5% salts, pigments

and substrates

• Protein– 12%

• myofibrillar

– 8%• Enzymes,

membrane proteins, transport channels

Contractile and regulatory proteins


• Major myofibrillar proteins– Actin and myosin

• Major contractile proteins

• Myosin– ATPase– “head”

attaches to actin

– Ratchet like action shortens sarcomere

Contractile proteins


– Troponin and tropomyosin• On actin• Ca2+ binds to troponin• Causes conformational

change in tropomyosin– Titin and nebulin

• Titin– Connects myosin to

Z disc• Nebulin

– Actin binding protein; may limit length

– M-line proteins• Keep actin and myosin

in correct spatial arrangement

– Desmin• Links Z-disks together

– α-actinin• Attaches actin filaments

ar Z disc– Spectrin and dystrophin

• Dystrophin involved in muscular dystrophy


• Thick filament– Hexameric protein

• One pair of heavy chains (MHC)

• Two pairs of light chains (LC)

• Muscle often characterized by MHC

– Type I– Type II

» a, dx, b• Light chains

– Modulate the contractile response

Contractile proteins


• Myosin– Light meromyosin

• Links all myosin molecules together

– Heavy meromyosin• Hinge and head• Head interacts

with actin when ATP is present

– ATPase• Close to actin

binding site in head region

Muscle organization


Muscle organization • Actin

– Made up of G (globular) actin sub-units

– (F) Filamentous actin• String of G actin

– Tropomyosin (Tm)• Filament that lies in

groove of actin molecule

• Blocks myosin binding site

– Troponin• On G actin• Ca2+ binding allows

myosin and actin to interact


Sliding filament theory • Huxley and Huxley

1954• Determined that the

following steps occur– Myosin and actin

interact– ATP hydrolyzed– Cross bridge stroke– Detachment of

myosin and actin


• A) no interaction– ATP is hydrolyzed to

“energize” myosin– Ca2+ release– Myosin attaches to

actin• B) crossbridge

movement– ADP and Pi released

• C) ATP binds– Breaks bond betw

actin and myosin• D) ATP hydrolyzed to

“re-energize” myosin

Sliding filament theory


• Capillarity– Several measures

• No. per fiber– ~2-4 caps/fiber– Problems?

• # Caps/fiber area– ~500-1000

caps/mm2

– Problems?• Cap length/fiber vol

– Best because» Capillaries are

not straight» Matches the

length of the capillary to the fiber volume

Capillaries and the microvascular unit



Microvascular unit

• Notice how tortuous or “non-straight” the capillaries are in these corrosion casts


• The force or tension that a muscle can exert is dependent upon it’s length– Actually, the

sarcomere length– Sarcomer has an

“optimal length”• This is where

maximal actin-myosin overlap occurs

Length-tension relationships


Types of contraction• Isometric

– Generates force, but no movement

• Isotonic– Generates force and

shortening occurs• Note: distance shortened is less

as load increases• However, note how training

changes the force-velocity relationship– Specifically, a trained

muscle can• Move a given load faster• Move a greater load at

the same speed compared to untrained


Excitability

• Muscles exhibit the property of excitability– They can be

activated by a nervous impulse

– They are polarized (i.e. charged)

• Resting membrane potential– Inside negative

relative to outside


• Greater # of negatively charged particles (ions) inside cell– Most are proteins– Also Na+, K+ and

Cl-

– Greater extracellular Na+ and intracellular K+

– Resting membrane potential is ~ -70 mV

• Na+/K+ ion pump• Membrane is

more permeable to K+ than Na+

Resting membrane potential


Action potential

• When cell is stimulated– Stimulus is subthreshold if there is no response– If threshold is reached, action potential

• Action potential– Rapid (1 ms) reversal of polarity


Action potential• Note that each ion has

it’s own equilibrium potential (where resting pot would be if only that ion existed)

• Note how action potential must be strong enough to reach threshold

• Also, Na+ influx is responsible for AP

• K+ efflux is responsible for repolarization

• Following AP, membrane is transiently more negative, hyperpolarization


Neuroanatomy made ridiculously simple

• Motor neuron– Cell body– Dendrites– Axon

• Myelin sheaths–Insulation–Increases the rate

of nerve transmission

• Nodes of Ranvier• Motor end plate

–ACh


Neuromuscular junction


NMJ

• Connection between nervous system and muscular system– Branch from a motor neuron and the fiber it

innervates– Small space (synapse)

• Ach is released from motor terminal or Motor end plate

–Ach is contained in vesicles–When action potential is reached

»Ca2+ enters the Motor end plate»Causes Ach to be released


AP transmission to muscle cell• AP then transmitted

through the T-tubules and sarcoplasmic reticulum– AP travels through T-

system– Calcium released from

Terminal citernae of SR

– This caused contraction

– Excitation-contraction coupling


Excitation contraction coupling• AP propagated across

NMJ– Down T-tubules– Stimulates Ca2+ release

from SR• Ryanodine receptor

– Ca2+ release channel

– Ca2+ binds to troponin– Causes shift in

tropomyosin– Allows actin-myosin

interaction– Contraction

• Cessation of AP– Ca2+ resequestered


Muscle fiber typing• Stain for ATPase

– Different ATPases in• Slow twitch• Fast twitch

– Incubate muscle samples at different pH

• Alkaline pH (10.3)– Dark-fast twitch– Light- slow twitch

• Acid pH (4.3)– Dark-slow twitch– Light-fast twitch

• Succinate dehydrogenase– Krebs cycle enzyme

» Darkest type I» Intermediate type IIa» Lightest type IIb


Muscle fiber typing


Classification schemes• How fast the respond

to a stimulus– FT, ST

• How fast they fatigue– Fatigue resistant,

fatiguable• Oxidative capacity• Color• Function• ATPase activity


Size principle• Recruitment of motor

units is based upon– The size of the motor

unit• Smaller motor units

more easily recruited

– Thus, they are recruited first

• Larger motor units are recruited when work needs are (and thus neural output) higher

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© 2007 McGraw-Hill Higher Education. All rights reserved. The Muscular System