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