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
