Vertebrate Physiology - Ch 8

Muscle cells

Excitabe, specialized cells that produce force and do work

What do muscle cells do?

Can shorten and develop tension;

Convert chemical energy of ATP into mechanical energy

What are the three types of muscle cells?

Skeletal, cardiac, and smooth muscle

Skeletal muscles are

Striated and voluntary

Cardiac muscles are

Striated and involuntary

Smooth muscles are

Unstriated and involuntary

What does the contraction of muscles allow?

Purposeful movement; manipulation of objects; propulsion of contents through organs; emptying contents of select organs; production of heat and sound

What system do skeleal muscles make up

Muscular system

Describe skeletal muscle cells (fibers)

Large (10-100 um diameter), elongated, cylindrical

How are skeletal muscle fibers formed?

Fusion of myoblasts during embryonic development

How do muscle cells arrange themselves?

Lie parallele to each other and bundled by connective tissue;

Extend full length of muscle

Myofibrils

Specialized contractile elements

Myofibrils make up __% of muscle volume

90%

Describe myofibrils

Cylindrical intracellular organelles (1um diameter) extending entire length of muscle fiber

Greater density of myofibrils →

Greater force generated

Muscle fibers without many myofibrils can’t generate much tension but can

Contract at higher frequency for longer time

Myofibrils have regular arrangements of what kinds of filaments?

Thick and thin

Describe the size of thick filaments

12-18 um in diamater and 1.6 um in length

Describe the composition of thick filaments

Composed of motor protein myosin:

→ Tails intertwined → globular heads project out at one end

What does each globular head of a motor protein myosin have?

Actin binding site and ATPase activity

Describe the size of thin filaments

5-8 um diameter and 1 um length

Describe the composition of thin tilaments

Composed of actin, tropomyosin, and troponin

What do the actin of thin filaments have?

Sites for attachment to myosin

What does the tropomyosin of thin filaments do?

Forms strands that cover myosin binding sites (muscle relaxed)

Troponin

Protein complex with three units that bind to various molecules

What do the subunits of troponin bind to?

Tropomyosin, actin, and Ca2+

Describe the striations of skeletal muscles

Alternating dark bands and light bands

Give the term used to describe the dark bands of skeletal muscles

A bands

Give the term used to describe the light bands of skeletal muscles

I bands

Describe a band

Stacked set of thick filaments and portions of thin filaments that overlap them

H zone

Lighter area in middle of A band;

Only thick filaments with no overlapping thin filaments

M line

Center of A band;

Holds thick filaments together using protein myomesin

I band

Thin filaments where they don’t overlap with thick filaments

Z line

Center of I band;

Flat cytoskeltal disc where thin filaments connect

Sarcomere

Area between two Z lines;

Functional unit of skeletal muscle

Sliding mechanism

Contraction → thin filaments slide toward center of A band → sarcomere shortens

Four steps of cross-bridge cycling

1) Binding: myosin cross bridge binds to actin molecule

2) Power stroke: cross bridge bens and pulls thin filament inward

3) Detachment: cross bridge detaches and returns to original conformation

4) Binding: cross bridge binds to new site on actin → cycle repeats

Power strokes of all cross bridges extended from thick filament are directed toward ___

Center of thick filament

All six thin filaments surrounding each thick filament pulled ___ through cross-bridge cycling during ____

Inward simultaneously; muscle contraction

What stimulates skeletal muscles to contract?

Release of ACh at neuromuscular junctions

Surface membrane dips deeply into muscle fiber to form ____

Transverse tubule (T-tubule)

How do action potentials enter the interior of muscle fibers?

Along T-tubules

Where is Ca2+ stored?

Terminal cisternae of sarcoplasmic reticulum

What does the action potential in the T-tuble trigger?

Release of Ca2+ from sarcoplasmic reticulum into cytosol

What are the effects of elevated cytosolic Ca2+?

Increased binging of Ca2+ to troponin → formation of cross-bridges

Where does Ca2+ go during relaxation?

Pumped back into sarcoplasmic reticulum by Ca2+ ATPase

Describe how ATP powers cross-bridge cycling (three steps)

1) Myosin ATPas on thick filaments splits ATP into ADP and inorganic phosphate (Pi)

2) ADP and Pi reman attached to myosin and energize it

3) During/after power stroke, ADP and Pi released

Does contractile activity or action potential last longer?

Contractile activity

How long does a single action potential last?

1-2 msec

What does a single action potential generate?

Muscle contraction (twitch)

How long is the average contraction time?

50 msec → continues until completion of Ca2+ release

When does relaxation occur?

As Ca2+ pumped back into sarcoplasmic reticulum

Does contraction or relaxation time last longer?

Relaxation time

What is the total time is takes for a muscle to twitch?

100 msec

What is the resting potential of a skeletal muscle fiber

-90 mV

What is the resting potential of a neuron?

-70 mV

How many motor neurons supply each vertebrate muscle fiber?

One

What do motor neurons do in space?

Branch and innervate many muscle fibers

What forms a motor unit?

All muscle fibers innervated by single motor neuron → contract simultaneously

What produces a stronger muscle contraction?

Activation of more motor units (motor unit recruitment)

If a muscle has few muscle fibers per motor unit, what is it used for?

Precise, delicate movement

If a muscle has many muscle fibers per motor unit, what is it used for?

Powerful, less controlled movement

Asynchronous recruitment of motor units coordinated by brain to ___

Prevent fatigue

Twitch summation

Muscle fiber stimulated before relaxing from last stimulus → second contraction added to first

Is the duration of the muscle contraction or the action potential longer?

Muscle contractions

Factors that contribute to twitch summation

Sustained elevation of cytosolic Ca2+;

More times to stretch tendons → less time for them to recoil to resting position

What creates a smooth sustained contraction (tetanus)?

Muscle fiber stimulated rapidly → no chance to relax between stimuli

How are graded muscle contractions produced?

Control number of motor units stimulated and frequency of stimulation

Unfushed tetanic contractions and asynchronous motor units used in____

Normal physiological motor control

Length-tension relationship

Every muscle has optimal length (lo) at which max force acheived by tetanic contraction

Length-tension relationship in terms of sliding-filament mechanism

Mex tension achieved when max number of cross-bridge sites accessible for actin binding

Are muscles usually stretched to their optimal length by their normal attachment to the skeleton?

Yes

What must happen for muscles to shorten during contraction?

Tension must exceed force that opposes movement

Isotonic contraction

Muscle shortens; tension remains constant; work done

Equation for work done

Work = force x distance

Isometric contraction

Muscle stopped from shortening; tension developes at constant muscle length; no work done

Load-velocity relationship

Greater load → lower velocity of shortening

Velocity ____ when load exceeds tension (isometric contraction)

Falls to zero

Eccentric contraction

Muscle lengthens during contraction → stretched by external force

Lever

Rigid structure capale of moving around pivot point (fulcrum)

____ are levers and ___ are fulcrums

Bones; joints

Power arm

Part of lever between fulcrum and point where upward force appleid

Load arm

Pary ot lever between fulcrum and downward force exerted by load

_____ amplfies movement of biceps into larger, rapid movements of hand

Lever system of elbow joint

Muscle structure in vertebrates

Tendons attach muscle to bone

What do flexors do?

Bend limb

What do extensors do?

Straighten limb

What’s the problem with ATP need for muscle contractions

ATP storage limited

Creatine phosphate

Energy storage molecule containing high-energy phosphate for donation to ADP

Where does glycolysis take place?

Muscle cytoplasm

What’s special about glycolysis?

Can form ATP without oxygen → fueled by glucose

Does glycolysis have a high or lower ATP yield? Is it faster or slower than oxidative phosphorylation?

Low → 2 ATP per glucose; faster

When is glycolysis used?

During high-intensity activity → lactate production and acidosis

Where does oxidative phosphorylation occur?

Mitochondria

Where does oxidative phosphorylation get its oxygen from?

Myoglobin in muscle fibers → fueled by fatty acids or glucose via pyruvate

How much ATP produced from oxidative phosphorylation?

30 ATP per glucose

When is oxidative phosphorylation used?

Light to moderate (aerobic) activity

Fatigue

Inability to maintain muscle tension

What causes muscle fatigue?

Increase in ADP and Pi; accumulated lactated and extracellular K+; depletion of glycogen reserves

What causes central fatigue?

Decrease in CNS stimulation of motor neurons