Describe how the structures of muscle fibers allow for the function of muscle tissue.
Assess metabolic pathways for generation of energy for muscle movements.
Compare isometric, concentric, and isotonic skeletal muscle contractions.
Describe the sliding filament theory of muscle contraction, including the sequence of molecular interactions leading to contraction.
Explain how excitation and contraction of myocytes are coupled, beginning with motor neuron stimulation.
Determine the work relationship between whole muscle contractions, contractile force and tension.
Muscle Tissue
Muscle is a tissue built of specialized contractile cells (called myocytes or muscle fiber).
There are two main categories of muscle:
Striated muscle tissue
Has the appearance of alternating light and dark bands, which are collections of contractile proteins called myosin and actin arranged into repeating units (sarcomeres)
With vertebrates – striated muscle are:
Skeletal muscle – locomotion and posture
Cardiac muscle – pump blood/lymph through heart
Smooth muscle tissue
Also uses contractile proteins actin and myosin, but they are not organized into sarcomeres
In vertebrates - smooth muscle lines hallow and tubular organs
Smooth Muscle
In vertebrates:
Locations:
Gastrointestinal tract
Respiratory tract
Reproductive tract
Urinary tract
Eyes
Base of hairs or feathers
Functions for hollow organs:
Changing size and volume
Example: uterus and bladder
Propelling material along a tube
Example: partially digested food thru small intestines
Maintaining tension for long periods of time
Example: sphincters of the bladder or anus
Cardiac Muscle
Only muscle that makes up the heart
Like skeletal muscle:
Has striations with actin and myosin arranged in sarcomeres
Have a single nucleus
Uniqueness:
Adjacent heart myocytes connected by intercalated discs containing gap junctions
Gap junctions – allow for ions to pass quickly from 1 myocyte to the next
Electric coupling ensures all cells connected by these gap junctions will contract at the same time
Specialized pacemaker cells – maintain the rhythm for the contractile activity of the heart
Skeletal Muscle Fibers (Muscle Cells) Structure
Multinucleated cells that originate from myoblasts
Surrounded by a sarcolemma (plasma membrane)
Muscle fibers contain 100s of myofibrils
Repeating bands: dark A bands and lighter I bands
Contracting unit of muscle fibers is the sarcomere
Thin filaments made mostly from actin proteins with regulatory proteins troponin (TN) and tropomyosin (TM)
Anchored to Z discs by nebulin protein
Actin extends from Z discs partway into the A band
TN and TM control access of actin from the myosin cross-bridges
Thick filaments are made of myosin proteins
Contain crossbridges or myosin heads with binding sites for actin and ATP
Titan protein maintains thick filaments at the center of the sarcomere
Skeletal Muscle Contractions
Cross-bridges of the thick filaments draw the thin filaments toward the center of each sarcomere.
Muscle contractions require myosin filaments to pull actin filaments inward, thereby shortening the muscle fiber
Requirements:
Ca^{2+} binding to troponin >> troponin pulls tropomyosin out of the way and exposes actin binding sites to myosin head (cross-bridge)
Ca^{2+} necessary to expose actin binding sites to myosin
ATP binding necessary for detachment of myosin from actin
ATP hydrolysis to ADP and Pi provides energy for power stroke
Skeletal Muscle Contractions - Key Points
Before ATP binding, myosin and actin are in the rigor conformation state
ATP binding allows myosin head to release actin
Once ATP hydrolysis occurs, myosin-ADP-Pi requires Ca^{2+} to have exposed the actin binding sites for the myosin heads
Release of Pi allows the chemical energy to drive the power stroke
ADP release returns the rigor conformation
Smooth Muscle Contractions
Smooth muscle actin and myosin filaments are arranged around the cells’ periphery
Structure of myofilaments:
Greater proportion of actin to myosin with actin filaments attached to dense bodies
Thick filaments have cross-bridges (myosin heads) along the entire length
Contractions:
Cross-bridge actions cause cell to shorten and plum in the center
Smooth muscle myosin ATPase occurs at a slower rate compared to skeletal muscle myosin ATPase.
Results in:
Slower speed of contraction
Longer contraction time
Cellular Protein Role in Animal Physiology
Membrane proteins can also be ligand-gated
Some cone snail toxins specifically target these ligand-gated channels
Skeletal Muscle Contractions Are Voluntary
Depend on neural excitation
Each muscle fiber is innervated by a motor neuron
Motor neurons release a chemical neurotransmitter called acetylcholine into the neuromuscular junction between the motor neuron and the muscle fiber’s sarcolemma
Influx of positively charged Na^+ ions thru the ligand gated channels depolarizes the sarcolemma = excitation
Excitation leads to activation of the contractile machinery with release of Ca^{2+} and a subsequent contraction.
Skeletal Muscle Fibers (Muscle Cells) Structure
Multinucleated cells that originate from myoblasts
Surrounded by a sarcolemma (plasma membrane)
Transverse tubules are continuous with the sarcolemma and transverse the long axis of the muscle fiber
T-tubules are closely associated with the sarcoplasmic reticulum that envelops myofibrils and contain high amounts of Ca^{2+}
Ca^{2+} release from the sarcoplasmic reticulum depends on two proteins: DHPR and RyR calcium channels
T-tubules have dihydropyridine receptors (DHPR) that are voltage-gated and change conformation when the t-tubule membrane depolarizes
The sarcoplasmic reticulum has ryanodine receptors (RyR) that touch DHPR and open
Skeletal Muscle Contractions Are Voluntary (DHPR and RyR)
Before t-tubules’ dihydropyridine voltage-gated receptors (DHPR) are stimulated: Muscle cell at rest: rigor conformation = calcium
After t-tubules’ dihydropyridine voltage-gated receptors (DHPR) are stimulated: Muscle cell with exposed actin biding sites for myosin head cross-bridges = calcium
After depolarization is over (motor neuron is no longer releasing acetylcholine to stimulate the ligand- gated channels):
DHPR and RyR reclose
ATP-dependent Ca^{2+} pumps decrease cytosol calcium levels. Muscle cell returning to rest and rigor conformation = calcium
Muscle Energetics
Vertebrate muscles have 3 sources for ATP:
Phosphagen creatine phosphate
Anaerobic glycolysis
Aerobic catabolism
Phosphagen creatine phosphate – rapid production, short lived (~10 seconds)
Anaerobic glycolysis – moderate production; will eventually be exhausted with a state of all-out-exertion
Aerobic catabolism – lowest rate of production; can be maintained on a relatively sustained basis
Vertebrate muscles fibers can be grouped based upon:
Myosin ATPase activity
Metabolic feature after creatine phosphate is used up
Slow oxidative (SO) fibers - Type I
Rich in myoglobin and mitochondria, appear red, and slow to fatigue
Slow ATPase activity
Make ATP principally by oxidative phosphorylation
Fast glycolytic (FG) fibers - Type II
Little myoglobin, rich in glycogen, appear white, fatigue quickly
Fast ATPase activity
Make ATP principally by anaerobic glycolysis
Fast oxidative glycolytic (FOG) fibers - Type II
Rich in mitochondria and myoglobin, intermediate resistance to fatigue
Predominantly use oxidative phosphorylation to make ATP
Whole Skeletal Muscle
Skeletal muscle is made up of 100s of muscle fibers that work in concert for coordinated movements
Connective tissues surround the individual fibers and organize them into functional bundles
The connective tissue is continuous with tendons that connect muscles to bones
The organization of the sarcomeres results in pulling motions only
Whole muscles work in antagonist pairs to pull opposite sides of skeletal joints
Gastrocnemius (calf muscle) pulls on the heel bone to extend the foot downward
Shortening of the cranial tibial muscle allows for flexing of the foot upward and re-lengthening the calf muscle.
Muscle Contraction – Force Generated by Cross Bridge Activity
Isometric contraction
Sarcomere may shorten slightly but maintains tension
Whole muscle stays the same length
Purpose: help animals hold a steady, unmoving posture
Ex: a stalking cat that is poised midstep
Ex: tightening the bicep muscle without allowing your elbow joint to move
Muscles exert its force on a load where the force of the muscle opposes the force of the load
Concentric contraction
Force is generated, whole muscle shortens
Ex: tightening the bicep muscle to pick up a weight thereby reducing the angle of the elbow joint
Maximum tension developed when muscle fibers have the optimal overlap of thin filaments with myosin cross-bridges
Eccentric contraction
Force is generated where the whole muscle lengthens
The contractile activity is resisting stretch imposed by external forces
Ex: Slowly lengthening a tightened bicep (with the triceps being the antagonists) to lower a weight down and extend the elbow joint
NOTE: eccentric contractions and the damage caused with the cellular processes needed to repair the damage is primarily responsible for delayed muscle soreness
Muscle Contraction - Overall Work Performed
Muscles do work when they change the position of objects with mass
Amount of work = force x distance
Work = Force * Distance
Gives insight into the amount of energy that is transferred from muscle to the load
Muscles will consume energy as they produce force, even if they are not moving a load
Greatest amount of work will be performed when the sarcomeres are at their optimal length prior to contracting
If a muscle contracts and no load is present, no work is done
If the force to move an object is greater than the muscle can perform, the distance = 0 and no mechanical work occurs
Example: Tetanus (an acute, infectious disease caused by spores of the bacterium Clostridium tetani)
Tetanus toxin prevents inhibition of motor neurons firing so the motor neurons continue to fire and cause the muscles to sustain a contraction