Chapter 9: Muscles and Muscle Tissues

Excitability

Ability to receive and respond to stimuli

Contractility

Ability to shorten forcibly when stimulated

Extensibility

Ability to be stretched

Elasticity

Ability to recoil to resting length

Cardiac Muscle

• Found only in heart

• Cells are striated, branched, and connected by intercalated discs

• Involuntary (cannot be controlled consciously)

Smooth Muscle

• Found in walls of hollow organs (viscera)

• Muscle fibers (cells) are not striated and are spindle shaped

• Involuntary

• Involved in many “housekeeping” functions of the body

• Ex: Digestion, blood circulation

Skeletal Muscle

• Muscle fibers (cells) are the longest of all muscle cells, have striations, and are multinucleate

• Voluntary (can be continuously controlled)

• Contract rapidly; tire easily; powerful

Skeletal Muscle Fibers

• Are formed by fusion of many myoblasts during development

• Contains abundant amounts of…

  • Mitochondria

  • Glycosomes: micro-organelles that store oxygen

  • Myoglobin: protein that stores O2 (gives meat its red pigment)

  • Sarcolmma: muscle fiber plasma membrane

  • Sarcoplasm: muscle fiber cytoplasm

Glycosomes

Mirco-organelles that store oxygen and release it during muscle activity

Myoglobin

Protein that stores O2 (gives meat its red pigment)

Sarcolemma

Muscle fiber plasma membrane

Sarcoplasm

Muscle fiber cytoplasm

Sarcomere

• Functional unit of muscle fiber, smallest contractile unit

• Consists of area between Z discs

• Individual ______ align end to end along myofibril like boxcars of train

A bands

Dark regions

I bands

Lighter regions

M line

Bisects A band vertically

Z discs (line)

Sheet of proteins on midline of I band

Elastic Filament

• Composed of protein titin

• Holds thin filaments in place; helps recoil after stretch; resists excessive stretching

Dystrophin

Links thin filaments to proteins of sacrolemma

Duchenne Muscular Dystrophy

• Most common form, generally appears during childhood

• Caused by a defective gene for dystrophin

Sacroplasmic Recticulum

• Network of smooth ER tubules surrounding myofibril

• Functions in regulation of intracellular Ca2+ levels

T-tubules

• Tube formed by protrusion of sarcolemma deep into the cell interior

• Lumen continuous with extracellular space and fluids

Triad

Area formed by a T-tubule with a terminal cistern on either side

Triad Relationship

• T-tubules and ST membranes linked together by integral proteins

• T-tubules→voltage sensors proteins

• SR→gated Ca2+ channels

Sliding Filament Model of Contraction

States that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more

Ligand (chemically) Gated Ion Channels

Opened by chemical messengers such as neurotransmitters (ex: ACh receptors on muscle cells)

Voltage-gated Ion Channels

Open or close in response to voltage changes in membrane potential

4 Steps Must Occur For Skeletal Muscle Fiber To Contract

• Events a neuromuscular junction

• Muscle fiber excitation

• Excitation-contraction coupling

• Cross bridge cycling

Events At Neuromuscular Junction (step 1)

• Action potential arrives at axon terminal

• Causes release of ACh neurotransmitter into synaptic cleft

• ACh binds to ligand-gated Na+ channels on sarcolemma

• Causes channels to open, allowing Na+ to enter into muscle fiber cell

• Acetylcholinesterase degrades Ach

• Ends signal

Muscle Fiber Excitation (step 2)

• The resting sarcolemma is polarized, the inside of the cell is negative compared to the outside

• Muscle fiber excitation is caused by changes in the electrical charges across cell membrane

• Occurs in 3 steps:

  • generation of end plate potential

  • depolarization

  • repolarization

Refractory Period

Muscle fiber cannot be stimulated for specific amount of time, until repolarization is complete

Excitation-contraction (E-C) Coupling (step 3)

• Events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)

• AP is proopagated along sarcolemma and down into T-tubulesCa2+ is eventually returned to SR by Ca2+ pump (requires ATP)

Cross Bride Cycling

• At low intracellular Ca2+ concentration:

  • tropomyosin blocks active sites on actin

  • myosin heads cannot attach to actin

  • muscle fiber remains relaxed

• At higher intracellular Ca2+ concentrations, Ca2+ binds to troponin

• Troponin changes shape and moves tropomyosin away from myosin-binding sites allowing myosin heads to bind to actin, forming cross-bridge

• 4 steps of the cross-bridge cycle

  • cross-bridge formation

  • working (power) stroke

  • cross bridge of myosin head

Isometric Contraction

• No muscle shortening

  • Muscle tension increases but does not exceed load

  • Cross bridges generate force, but myofibrils do not shorten

Isotonic Contraction

• Muscle shortens

  • Muscle tension exceeds the load

Motor Unit

Nerve muscle functional unit

Muscle Twitch

Simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron

latent Period

Events of excitation-contraction coupling (no muscle tension seen)

Period Of Contraction

Cross brdige formation (tension increases)

Period Of Relaxation

Ca2+ reentry into SR (tension declines to zero)

Graded Muscle Responses

Is graded by control os skeletal movement

Temporal Summation

Results if two stimuli are received by a muscle in rapid succession

Incomplete (unfused) Tentanus

Muscles progress to sustained, quivering contraction

Fused (complete) Tetanus

Contractions “fuse” into one smooth sustained contraction plateau

Recruitment (or multiple motor unit summation)

Results as stimulus recruits additional motor units within a muscle