Muscle Physiology

OutlineSkeletal Muscle Structure Muscle Contraction: Cell EventsMuscle Contraction: Mechanical EventsMuscle MetabolismTypes of Skeletal Muscle FibersVI. Smooth and Cardiac Muscles

1- Skeletal Muscle Structure

Muscle = group of fasciclesMuscle fibers extend length of muscle from tendon to tendon

Motor unitsMotor unit: Composed of one motor neuron and all the muscle fibers that it innervates

There are many motor units in a muscle

The number of fibers innervated by a single motor neuron varies (from a few to thousand)

The fewer the number of fibers per neuron the finer the movement (more brain power)

Which body part will have the largest motor units? The smallest?

Components of a muscle fiber

Muscle fiber componentsSarcolemma: muscle cell membraneSarcoplasma: muscle cell cytoplasmMotor end plate: contact surface with axon terminalT tubule: cell membrane extension into the sarcoplasm (to reach the myofibrils)Cisternae: areas of the ER dedicated to Ca++ storage (located on each side of the T-tubules)Myofibrils: organized into sarcomeresFigure 12.2 (2 of 2)

The sarcomereThe myofibrils are organized into a repetitive pattern, the sarcomereMyosin: thick filamentActin: thin filamentBands formed by pattern: A and I and H bandsZ line: area of attachment of the actin fibersM line: Myosin fiber centers

The sarcomereFigure 12.5d

Myosin structure Many myosin molecules per filament, golf club shapeLong tail topped by a thickening: the head forms crossbridges with the thin filamentPresence of the enzyme, ATPase in the head release energy for contraction

Actin structureFormed by 3 different proteins: - globular (G) actins: bind to myosin heads - tropomyosin: long, fibrous molecule, extending over actin, and preventing interaction between actin and myosin - troponin: binds reversibly to calcium and able to move tropomyosin away from the actin active siteFigure 12.4

2- Muscle contraction: Cell eventsFigure 11.13

Synaptic eventsThe AP reaches the axonal bulbVoltage-gated calcium channels openThe influx of calcium in the bulb activates enzymes the vesicles containing the neurotransmitter molecule dock and release the neurotransmitter in the synapseThe neurotransmitter for skeletal muscles is always acetylcholineThe receptors on the muscle fiber are cholinergic receptorsThese receptors are nicotinic (fast) acting receptors

2- The Mechanism of Force Generation in Muscle

Figure 12.7

http://www.blackwellpublishing.com/matthews/myosin.html

http://www.ebsa.org/npbsn41/intro_muscle.html

Figure 12.6

Muscle relaxationAch is removed from the receptors by acetylcholinesteraseLigand-gated Na+channels closeNa/K pumps reestablish the RMPCa++ ions leave troponin and are brought back into the cisternae (this process needs energy)Tropomyosin moves back over the actin active siteThe myosin heads release their binding to actinThe filaments passively move back into resting position

ApplicationsMyasthenia gravis: autoimmune disease where antibodies against the Ach receptors are produced. Which consequences do you expect?

Muscular dystrophy: some proteins forming the muscle fibers are abnormal. Which consequences do you expect?

Curare binds to the Ach receptor without activating them. What are the effect of curare on the skeletal muscle?

The botulism toxin prevents the release of the neurotransmitter into the synapse. What will be the consequence?

Nerve gas inhibits acetylcholinerestase present in the synapse. What will be the consequence?

Rigor mortis: why does the body stiffen shortly after death?

3- Muscle contraction: Mechanical events1 stimulation 1 twitch

Muscle twitch: 3 phases: - latent phase - contraction phase - relaxation phase

do not confuse the AP and the twitch!!!

Figure 12.16

Events during the twitchLatent phase: Stimulus to beginning contraction: AP to myosin binding to actin active siteContraction phase: beginning to end of muscle tension myosin heads slide along the actin filamentsRelaxation phase: peak tension to no tension Ca++ ions moved back into the cisternae, tropomyosin moves back over actin, myosin head release actin and the filaments move back into resting position

Figure 12.18

Isometric/isotonic contractionsIsometric: muscle contraction without movement no muscle shortening

Isotonic: muscle contraction with movement muscle shortens

Effect of consecutive stimuli: TreppeTreppe: gradual increase in contraction intensity during sequential stimulation

Might be due to calcium ions accumulating in the cytoplasm with each stimulation Figure 12.15

Summation and tetanusSummation: Rapid sequence of stimuli muscle twitches fuse into each other, each subsequent one being stronger that its precedent (due to Ca++?)Tetanus: very rapid sequence of stimuli: no relaxationFigure 12.17

RecruitmentAn increase in force is made possible by recruiting more motor unitsMuscles have various sizes of motor units allows them to adjust the size of the effort to be madeActivating motor units alternatively allows the muscle to sustain contraction with minimal fatigue

Figure 12.19

IV- Muscle metabolismMuscle fibers use ATP (only first few seconds) for contractionATP must then be generated by the muscle cell: - from creatine phosphate, first - from glucose and glycogen - from fatty-acids

ATP formation from the above compound is possible if oxygen is present (oxidative phosphorylation: 36 ATP per glucose)Oxygen is delivered to the muscle by myoglobin, a molecule with high affinity to oxygen and related to hemoglobin

If the effort is strong and sustained, the muscle might not have enough oxygen delivered to it by myoglobin anaerobic glycolysis with only 2 ATP formed per glucose and synthesis of lactic acid

Consequence of anaerobic metabolism?

Figure 12.11

Muscle fatigueMuscle fatigue: a decline in the ability of the muscle to sustain the strength of contractionCauses: - rapid build-up of lactic acid - decrease in oxygen supply - decrease in energy supply (glucose, glycogen, fatty-acids)Decreased neurotransmitter at the synapse - psychological causes

Effects of exercise on the muscleAerobic exercises: long sustained exercises promote increased oxidative capacity of the muscle fiber increased blood vessel supply, increased mitochondria

High intensity, short burst exercise: increased glycolytic activity increased synthesis of glycolytic enzymes, increased synthesis of myofibrils (increased muscle size)

V- Types of Muscle FibersVarious muscles contract at different speed composed of different types of muscle fibersFigure 12.23

Basis for classificationVelocity of contraction: slow vs fastEnergy source: oxidative vs glycolytic

Oxydative Glycolytic Primary energy through oxidative phosphorylationMany mitochondriaMyoglobin (red)Small diameterResistant to fatigue

Primary energy through anaerobic glycolysisFewer mitochondriaMany glycolytic enzymesHigh glycogen storesUse little oxygenanaerobicLarge diameterQuick to fatigue

Which types of meat are chicken breast and duck breast?

Why the difference?

VI- Smooth and Cardiac Muscles

SkeletalCardiacSmoothAppearanceControlvoluntaryunvoluntaryUnvoluntaryNeural inputsomaticANSANSHormone0EpiEpi/othersCa++ protTroponinTroponinCalmodulinGap junctionsNoYesYesPacemakerNoYesNo

ReadingsChp. 12, p. 323-359

Documents

Muscle Physiology