Muscle Physiology
Lab #9
Skeletal Muscle Organization
• Muscle fibers (cells) – elongate cells, parallel
arrangement– sarcolemma – cell membrane – sarcoplasmic reticulum (SR)
• Internal membraneous network• Stores Ca2+
– transverse tubules• Connect SR to sarcolemma
– myofibrils – protein bundles composed of:
• thick filaments• thin filaments
Myofibril Structure
• Composed of sarcomeres – smallest functional unit of
muscle – repeating units of thin and
thick protein filaments• Thick Filament = Myosin • Thin Filament = Actin,
Troponin, Tropomyosin
Thick Filament Structure
• Bundles of several hundred myosin molecules – intertwining tails + globular heads
• heads contain: – actin binding sites
– ATP-hydrolyzing sites
• project outward towards actin
• form crossbridges– bonds with actin
– Important during contraction
Thin Filament Structure
• Actin– primary structural protein – spherical protein subunits connected
in long, double strand – Contains myosin binding site
• Tropomyosin – threadlike proteins – normally cover myosin binding sites
• Troponin – Ca2+ Binding Protein – holds tropomyosin in place
Skeletal Muscle Contraction
• Somatic motor neuron induces action potential in muscle fiber
• AP travels down sarcolemma• Induces depolarization in the
T-tubules• Depolarization conducted to
the SR• Voltage-gated Ca2+ channels
open in the SR – releasing Ca2+ into the cytosol
Skeletal Muscle Contraction
• Ca2+ binds to troponin• Troponin undergoes shape
change – pulls on tropomyosin
• tropomyosin shifts position– uncovers myosin binding
sites on the actin filaments
Skeletal Muscle Contraction
• myosin head binds to actin• crossbridge bends
– power stroke – pulls thin filaments toward center
of the sarcomere
• crossbridge link breaks• myosin head returns to original
configuration• binds to next actin molecule
Skeletal Muscle Contraction
• Movement of thin filaments over thick
• sarcomere shortening • length of the filaments do
not change
Muscle Relaxation
• Ca2+ pumped back into the SR by active carrier-mediated transport– troponin releases Ca2+
– tropomyosin covers myosin binding sites on the actin molecules
• Membrane-bound enzyme (acetylcholinesterase)– breaks down ACh released at the NMJ
All or None
• Individual muscle fibers respond to a single stimulus in an all or none fashion– undergo action potential– action potential triggers contraction– subthreshold stimulus = no contraction– superthreshold stimulus = maximal contraction
Motor Units
• Multiple muscle fibers are enervated by a single motor neuron
• Motor Unit– motor neuron + all muscle
fibers it innervates
– muscle fibers in a motor unit contract as a single unit
Motor Unit Recruitment
• Individual motor units contract in an all-or-none fashion
• Differences in contractile strength are due to differences in the number of contracting motor units
• Motor Unit Recruitment– increasing the number of contracting motor units to
increase the overall strength of contraction
Muscle Mechanics
• Twitch
– single contraction and relaxation of muscle in response to a single action potential
• Tetany (Tetanus)
– sustained tension exerted by a muscle due to continuous contraction
Twitch Properties• Latent Period
– Time btw AP production and development of tension.
– Excitation-contraction coupling
• Contraction Time– Time btw beginning of contraction
and peak tension
– Shortening of muscle• Relaxation Time
– Time btw peak tension and return to resting tension
– Ca2+ re-uptake and stretching of muscle back to resting length
Latent Period
Contraction Time
Relaxation Time
Temporal Summation
• AP duration much shorter than contraction duration– several APs can occur in a muscle
fiber during the course of a contraction
• Multiple APs can have a summation effect on tension generation– induce release of Ca2+ from SR
before muscle is fully relaxed
– increase tension
Muscle contraction
Action potentials
Tetany
• High rate of AP generation in fibers
• no time for muscle to relax
• sustained level of tension
Experiments:Frog gastrocnemius recordings
• Follow instructions in manual CAREFULLY!!!– Threshold– Recruitment in muscle organs– Twitch time measures
• Latent period, contraction time, relaxation time
– Repeated stimulation and stimulus frequency
Electromyograms (EMGs)
• Muscles undergo action potentials
• Electrical signals conducted through body fluids
• Can be measured from the surface of the body
• EMGs can indicate muscle activity
Types of Muscle Contractions
• Isotonic Contractions– Muscle shortens in length– Generated same strength for
a given load throughout the contraction process
• Isometric Contractions– Muscle does not shorten,
remains at the same length– Load = strength of
contraction
Experiment: EMGs
• Motor unit recruitment on forearm
• Isotonic vs. Isometric contractions