Mammalian Phys. Muscles

Motor Unit Recruitment

Asynchronous recruitment of motor units helps delay or prevent fatigue.

Relaxed steps:

1. no excitation 2. no cross bridge binding 3. muscle fiber is relaxed

Splitting of ATP by myosin ATPase

= energy for power stroke of cross bridge

Two primary factors that make whole-muscle tension

# of muscle fibers contracting within a muscle -Tension developed by each contracting fiber

Muscle Fatigue

- Exercising muscle can no longer respond to stimulation - Defense mechanism - Central Fatigue-CNS neurons no longer activate the motor units of muscle

Cross-bridges

- Project from each thick filament in six directions toward the surrounding thin filaments

Types of Skeletal Muscle Fibers

- Slow-oxidative (type I) fibers - Fast-oxidative (type IIa) fibers - Fast-glycolytic (type IIx) fibers

Twitch

-Brief, weak contraction -Due to single action potential - Too short and too weak to be useful - Normally does not take place in body

alternate pathways for forming new ATP

-Creatine phosphate - Oxidative phosphorylation - Glycolysis - Lactate production

Factors influencing extent of tension development

-Frequency of stimulation - Length of fiber at onset of contraction - Extent of fatigue - Thickness of fiber

Tetanus

-High frequency stimulation; muscle does not have a chance to relax between stimuli -Contraction is usually three to four times stronger than a single twitch

Golgi tendon organs

-Located in muscle tendons - Respond to changes in the muscle's tension

Two factors influenced contractions

-Number of muscle fiber contracting -Tension developed by each contracting fiber

Four steps in the excitation, contraction, and relaxation processes require ATP:

-Splitting of ATP by myosin -Binding of ATP to myosin breaks cross bridge - Active transport of Ca2+ back into lateral sacs of the SR during relaxation -Activity of Na+-K+ pump during action potential production

skeletal muscle (classification, location, function)

-Striated, voluntary muscle -bundles of long, thick, cylindrical, striated, contractile, multinucleate cells that extend the length of the muscle -Attached to bones -Movement of body in relation to external environment

Energy Sources for Contraction

-Transfer of high-energy phosphate from creatine phosphate to ADP -Oxidative phosphorylation (citric acid cycle and electron transport system -Glycolysis - Supports anaerobic or high-intensity exercise

Skeletal Muscle Fibers

-lie parallel to one another -Bundled together by connective tissue

Functions of controlled muscle contractions

-purposeful movement of the whole/parts of the body -manipulation of external objects -pushing forward of contents through hollow internal organs -emptying of contents of certain organs to external environment

Cardiac muscle

-striated, involuntary muscle -Interlinked, slender, cylindrical, striated, branched, contractile cells connected -Wall of heart -Pumping of blood

Smooth muscle

-unstriated, involuntary muscle -Loose network of short, slender, spindle-shaped, unstriated, contractile cells that are arranged in sheets -Walls of hollow organs and tubes (stomach and blood vessels) -Movement of contents within hollow organs

Excited

1. Muscle fiber is excited and Ca2+ is released 2. Ca2+ binds with troponin (exposing cross-bridge) 3. Cross-bridge binding occurs 4. Binding of actin and myosin cross bridge triggers powerstroke--pulling inward

Fast versus slow fibers

Fast fibers have higher myosin ATPase (ATP-splitting) activity than slow fibers

Oxidative versus glycolytic fibers

Fiber types differ in ATP-synthesizing ability

Titin

Giant, highly elastic protein - Largest protein in body - Extends in both directions of thick filament.

Genetic endowment of muscle fiber types

Largely determined by the type of activity for which the muscle is specialized

Troponin

Made of three polypeptide units • One binds to tropomyosin • One binds to actin • One can bind with Ca2+

Pain Reflexes

Skeletal muscle reflexes can be triggered by painful stimulation of the skin

Tropomyosin

Thread-like molecules -lie END-to-END alongside groove of actin spiral.

power stroke

action of myosin pulling actin inward (toward the M line)

Involuntary

cardiac and smooth muscle

Muscle spindle structure

collections of specialized muscle fibers known as intrafusal fibers

Actin and myosin are often called...

contractile proteins

Crossed extensor reflex

ensures the opposite limb is in a position to bear the weight of the body as the injured limb is withdrawn from the stimulus

Work

force multiplied by distance

Single-unit smooth muscle cells form

functional syncytia

Gamma motor neurons

initiate intrafusal fibers

Isotonic (i.e., constant tension)

load remains constant as the muscle changes

Isometric (i.e., constant length)

muscle length remains constant as tension increases

Force

muscle tension required to overcome the load (the weight of the object)

Muscle fibers are composed of

myofibrils

Active transport of Ca2+ back into SR during relaxation =

need ATP

Stretch reflex

negative-feedback mechanism--sense and resist changes in muscle length

Multiunit smooth muscle is

neurogenic

Motor activity can be classified as

reflex, voluntary, or rhythmic

I band

remaining portion of thin filaments that do not project into A band

velocity

shortening is related to the load

Alpha-gamma coactivation

simultaneous stimulation of gamma and alpha motor-neuron system

Striated

skeletal and cardiac muscle

Voluntary

skeletal muscle

Unstriated

smooth muscle

number of fibers contracting within a muscle depends on

the extent of motor unit recruitment

Frequency of stimulation can influence

the tension developed by each muscle fiber

A band

thick filaments along with portions of thin filaments that overlap

Sliding filament mechanism

thin filaments from opposite sides of each sarcomere sliding closer together between the thick filaments

Withdrawal reflex

used to withdraw from a painful stimulus

Muscle Contractions

vary in strength

Isokinetic (i.e., constant motion)

velocity remains constant as the muscle fibers shorten

Lever Systems

Bones (levers), muscles (force), and joints (fulcrums) interact to form lever systems - rigid structure capable of moving around a pivot point known as a fulcrum

Twitch summation

Due to sustained elevation of cytosolic calcium

Sarcoplasmic Reticulum

Modified ER - Network compartments surrounding each myofibril -Not continuous but encircles myofibrils

Eccentric contractions

Muscle lengthens

Concentric contractions

Muscle shortens

4 types of muscles

Striated Unstriated Voluntary Involuntary

Transverse Tubules

T tubules -Run perpendicularly -continuous with surface membrane -Action potential -->T tubule triggers release of Ca2+ from SR into cytosol

Myosin forms

THICK filaments -two identical subunits, each shaped somewhat like a golf club -Highly abundant

Actin forms

THIN filaments - Two other proteins, tropomyosin and troponin, lie across the surface -Highly abundant

Ca2+ binds to_____ unblocking_____ =

Troponin, Tropomyosin, Muscle contraction