5.a&p i muscle2010

Muscle Structure and Physiology

Marieb: Chap. 9

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Muscles

I. Introduction

1. Types of muscles: a) Skeletal = Voluntary Striated b) Cardiac = Involuntary Striated c) Visceral = Involuntary Smooth/Nonstriated

2. Properties: a) respond to stimulus with contraction =

excitability b) passively stretched = extensible c) return to original shape or length = elasticity

3. Functions: a) motion b) posture c) heat production d) stabilize joints

3Fig. 9.1

Copyright © 2010 Pearson Education, Inc.

Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium.

Bone

Perimysium

Endomysium(between individualmuscle fibers)

Muscle fiber

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiberin middle ofa fascicle

Blood vessel

Perimysium

Endomysium

Fascicle(a)

(b)

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II. Skeletal MuscleA. Extramuscular Tissue

1. Fascia - sheets of fibrous CT wrapped around muscle

2. Epimysium - fibrous CT, extension of fascia

3. Perimysium - separates muscle into individual bundles or fascicles

4. Endomysium - separates into individual muscle cells or fibers

5. Tendon - a) cordlike CT connected to

epimysium & bone b) aponeurosis - broad, flat

band connected to muscle or bone

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II. Skeletal muscle = Voluntary Striated

6. Blood Vessels a) outside perimysium but below or under epimysium =

larger vessels b) capillaries and small branches associated with

endomysium layer 7. Nervous Tissue

a) Travel with larger blood vessels b) branches to individual muscles or groups of muscles

8. Motor Unit - single motor neuron plus all muscle fibers it stimulates

1 neuron can stimulate 10 separate muscle fibers 150 separate muscle fibers 500 separate muscle fibers (Behavior?)

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9. Neuromuscular Junction (NMJ) Point of contact or

communication between neuron and muscle

10. Motor End Plate Specialized portion

of muscle membrane at a NMJ

Fig. 9.13


Figure 9.13 A motor unit consists of a motor neuron and all the muscle fibers it innervates.

Spinal cord

Motor neuroncell body

Muscle

Branching axonto motor unit

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motor neuronaxon

Axon terminals atneuromuscular junctions

Axons of motor neurons extend from the spinal cord to the muscle.There each axon divides into a number of axon terminals that formneuromuscular junctions with muscle fibers scattered throughoutthe muscle.

Branching axonterminals formneuromuscularjunctions, one permuscle fiber (photo-micrograph 330x).

(b)

(a)

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B. Ultrastructure of Skeletal Muscle

1. Sarcolemma - cell membrane 2. Sarcoplasm - cytoplasm 3. Multinucleated with many mitochondria 4. Sarcoplasmic Reticulum (SR)

Smooth ER SR usually in longitudinal direction If SR is perpendicular to surface, right angles,

known as Transverse SR or T-Tubules Triad = one T-Tubule plus 2 SR

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Fig. 9.5

5. Endomysium surrounding single muscle fiber

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5. Endomysium surrounds single muscle fiber/cell

Up to 30 cm long and 10-100um diameter Each muscle cell contains many myofibrils

Myofibril (1-2um) = bundles of myofilaments Myofilaments = individual muscle filaments

Thick = myosin Thin = actin or troponin or tropomyosin

Sarcomere = contractile unit, segment of a myofibril

6. Myofilament Information

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Fig. 9.2a/b


Figure 9.2a Microscopic anatomy of a skeletal muscle fiber.

Nuclei

Fiber

(a) Photomicrograph of portions of two isolated musclefibers (700x). Notice the obvious striations (alternatingdark and light bands).

Dark A band

Light I band


NucleusLight I bandDark A band

Sarcolemma

Mitochondrion

(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.

Myofibril

Figure 9.2b Microscopic anatomy of a skeletal muscle fiber.

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Figure 9.2c Microscopic anatomy of a skeletal muscle fiber.

I band I bandA bandSarcomere

H zoneThin (actin)filament

Thick (myosin)filament

Z disc Z disc

M line

(c) Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomere extends fromone Z disc to the next.


Figure 9.2d Microscopic anatomy of a skeletal muscle fiber.

Z disc Z discM line

Sarcomere

Thin (actin)filament

Thick(myosin)filament

Elastic (titin)filaments

(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.


Figure 9.2e Microscopic anatomy of a skeletal muscle fiber.

I bandthin

filamentsonly

Actinfilament

Myosinfilament

H zonethick

filamentsonly

M linethick filaments

linked byaccessoryproteins

Outer edgeof A band

thick and thinfilaments overlap

(e) Cross-sectional view of a sarcomere cut through in different locations.

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Fig. 9.3

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Fig. 9.3

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Fig. 9.4

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6. Myofilament Information a) Myosin - rod like tail terminating in two globular

heads (sites of cross bridges) Each thick filament = 200 myosin molecules with tails in

the center and heads facing outwards on each side (away from H zone)

b) Actin - thin filaments that contain active sites where myosin bridges attach

c) Regulatory proteins on actin Tropomyosin - spiral around actin and block actin sites to

bind myosin at rest Troponin - 3 sites: bind to actin, tropomyosin and calcium

d) Elastic (Titin) filaments - extend from Z disc to myosin to anchor myosin in place and help help muscle spring back after being stretched

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7. Anatomy of Relaxed muscle Sarcomere - region of a myofibril between two

successive Z discs/lines Z disc = anchors the thin actin filaments I band = extends from both sides of a Z disc,

contains only thin actin filaments A band = boundary of thicker myosin filaments

plus contain some thin actin from I band H zone of A band = less dense area where actin

ends but myosin continues M line of center of H zone = protein strands that

stabilize myosin together

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Fig. 9.2 c/d

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C. Sliding filament model of contraction Huxley 1954

1. Thin actin filaments slide past the thicker myosin due to activation of myosin cross bridges to sites on actin

2. Each cross bridge attaches and reattaches to move actin towards H zone

3. H zone disappears 4. I band shortens as Z discs move closer to each

other 5. A bands move closer to each other but do not

change length since length of myosin does not change, only actin sliding over myosin

19Fig. 9.6 - 1

M


Figure 9.6 Sliding filament model of contraction (1 of 2).

I IA

Z ZH

1 Fully relaxed sarcomere of a muscle fiber

20Fig. 9.6

21Fig. 9.6


Figure 9.6 Sliding filament model of contraction (2 of 2).

I IA

Z Z

2 Fully contracted sarcomere of a muscle fiber

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D. Physiology of Contraction 1. ATP = energy chemical for contraction

Hydrolysis of ATP ATP + H2O ADP + P + ENERGY Energy used by cell for active transport,

synthesis, and muscle contraction Dehydration Synthesis of ATP ADP + P + ENERGY ATP + H2O Energy supplied from cellular respiration

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D. Physiology of Contraction 2. At rest -

a) Calcium stored in the SR (not free in the sarcoplasm)

b) ATP attached to the myosin cross bridge heads in relaxed position (not power position)

c) troponin-tropomyosin attached to actin filament - blocking the actin site for the myosin head :. NO ACTIN-MYOSIN ATTACHMENTS AT THE CROSS BRIDGES - NO TENSION

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FIG. 9.11

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3. Nerve becomes active/excites a) conducts electrical charge down motor

neuron axon towards terminal branch endings at neuromuscular junction

b) in the ending of the neuron, vesicles are present that contain the neurotransmitter chemical - nerves that stimulate skeletal muscles contain acetylcholine (ACh)

c) electrical current dies out in nerve ending but vesicles move towards nerve membrane and release ACh contents into NMJ via exocytosis

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FIG. 9.9 modified

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Fig.9.11

modified

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4. ACh diffuses across the space towards muscle membrane of motor end plate (net diffusion) where there are receptors for ACh on the muscle membrane

ACh will change membrane permeability to sodium and muscle becomes electrically excited and active in this region

5. Excitation - Contraction coupling - the electrical changes on the muscle membrane lead to sliding of the myofilaments

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5. Excitation - Contraction Coupling a) electrical changes on surface of sarcolemma

spread down the T- tubules to the inner core of the muscle

b) remember - T-tubules meet the horizontal SR and form a triad

c) electrical changes of the SR cause Calcium (Ca2+) to be released into the sarcoplasm by opening Ca2+channels of the SR

Ca2+ is available to bind to the myofilaments 6. Calcium is free in the sacroplasm

a) Ca2+ activates the ATPase activity on the myosin head and powers the bending of the head

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Fig.9.11

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6b) Ca2+ also displaces the troponin - tropomyosin complex from the actin binding sites (see Figure 9.10 & 11)

c) actin-myosin cross bridge forms and flips backwards pulling the actin past the myosin towards the center of the sarcomere

d) TENSION develops e) one working stroke of head shortens

muscle 1% - usual muscle shortening 30 to 35% - therefore each myosin cross bridge attaches and reattaches several times during a contraction

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Figure 9.12 Cross Bridge Cycle

Actin

Cross bridge formation.

Cocking of myosin head. The power (working)stroke.

Cross bridgedetachment.

Ca2+

1

2

3

4

Myosinhead

Thickfilament

Thin filament

ADP

Myosin

P i

ADP

P iATPhydrolysis

ADP

P i

ATP

ATP

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7. Nerve stops firing and stops releasing ACh

As long as ACh is in gap - keeps stimulating muscle and Ca2+

ACh is destroyed in the space by an enzyme always present - acetylcholinesterase

Therefore - muscle membrane no longer electrically active and this cessation causes the free Ca2+ to be transported BACK into the SR spaces via an ATP-dependent active pump

NO FREE CALCIUM

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8. Without free Ca2+ - troponin-tropomyosin reoccupy the active site that binds myosin heads and cross bridges are broken and not reformed.

a) ATP is resynthesized and occupy the myosin binding sites

b) since cross bridges were broken - tension is lost and muscle returns to its original resting length

Myostenia gravis - problem with ACh receptor destruction

Rigor mortis - explain it????? 3-4 hours muscle stiffens, maximum by 12 hrs Dissipates over the next 2-3 days

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9. Creatine Phosphate Muscles cannot store adequate ATP for

continues muscle contractions At rest: ADP + P + Energy = ATP stored and on myosin ATP + creatine = creatine-phosphate + ADP

Exercise: ATP = ADP + P + Energy Creatine-phosphate + ADP =creatine + ATP

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E. Skeletal Muscle Physiology 1. Single Twitch Phenomenon (lab)

a) single stimulus - threshold Weakest stimulus that causes a muscle

contraction b) latent period (01. sec) - time between the

stimulation and actual mechanical contraction of muscle

chemical / electrical change / Calcium any tendon slack taken up first c) contraction phase - developing tension (.04s) d) relaxation phase (.05 sec) - calcium upt6ake

and breaking cross bridges

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e) fast muscle = 0.03 sec

(insect 0.003 sec) slow muscles =

seconds f) refractory period -

no response to second equal stimulus, but a response to a stronger stimulus

Fig. 9.14

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g) 1) slow oxidative muscle - posture, marathon red muscles (myoglobin) + mitochondria +

abundant capillaries = high aerobic activity but slow ATPase activity

2) fast glycolytic fibers - quick powerful movement - hitting baseball

White muscle low in myoglobin, more anaerobic and faster ATPase activity

Most muscle have a mixture of both Table 9.2 textbook

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2. All or None Phenomenon a) threshold - individual fibers respond maximally

when stimulated b) influenced by :

Temperature Products of metabolism Oxygen availability Fatigue

c) muscle contain many individual fibers Each fiber = all or none Entire muscle has graded response determined by the

number of fibers contracting Entire muscle has a maximum response (100% fibers)

40Copyright © 2010 Pearson Education, Inc.

Stimulus strength

Proportion of motor units excited

Strength of muscle contraction

Maximal contraction

Maximalstimulus

Thresholdstimulus

Figure 9.16 Relationship between stimulus intensity (graph at top) and muscle tension (tracing below).

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Fig. 9.16/17

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3. Summation - 2 consecutive stimuli with second response greater than the first

4. Staircase or Treppe - stimulate muscle second time after it relaxes and compare tension

Fig. 9.15b

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5. Tetanus - physiological response a) many rapid responses to prevent relaxation (2) b) fusion of twitches - continuous sustained

contraction (4) c) partial or incomplete tetanus - contractions do not

fuse (3) 6. Tonic Contraction

a) some cells contracted, others relaxed, interchange Posture Flaccid - less than normal tone to muscle

Fig. 9.15-9.16

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7. Isotonic a) same tone or tension b) change in length during contraction c) pick up a book

Fig. 9.18a

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8. Isometric a) same length b) develop or change tension c) try to left heavy object, push wall

Fig. 9.18b

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III. Cardiac muscle 1. Involuntary, striated muscle 2. Single nucleus, branched 3. Intercalated discs

Area of low resistance to electrical flow of current - network contraction response

4. Intrinsic rhythm - normal rate of pacemaker = 120+ per minute

Vagus nerve slows down intrinsic pacemaker to 60-80 beats per minute

5. Refractory period - rest No tetanus possible High rate = fibrillation

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Fig. 18.11

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6. Skeletal vs. cardac contraction Cardiac has longer tension due to calcium

involved from both SR and extracellular environment

Fig. 18.12

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IV. Smooth Muscle 1. Involuntary,

nonstriated muscle a) actin & myosin

poorer organization of fibers

b) fibers attached to membrane or lattice in sarcoplasm

c) SR less developed d) spindle shaped

Fig. 9.28a/b

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Figure 9.29 Sequence of events in excitation-contraction coupling of smooth muscle.

Activated myosin forms crossbridges with actin of the thinfilaments and shortening begins.

1

2

3

4

5

ATP

PiPi

Extracellular fluid (ECF)

ADP

Ca2+

Ca2+

Ca2+

Plasma membrane

Sarcoplasmicreticulum

Inactive calmodulin

Inactive kinase

Inactivemyosin molecule

Activated (phosphorylated)myosin molecule

Activated kinase

Activated calmodulin

Cytoplasm

Calcium ions (Ca2+)enter the cytosol fromthe ECF via voltage-dependent or voltage-independent Ca2+

channels, or fromthe scant SR.

Ca2+ binds to andactivates calmodulin.

Activated calmodulinactivates the myosinlight chain kinaseenzymes.

The activated kinase enzymescatalyze transfer of phosphateto myosin, activating the myosinATPases.

Thinfilament

Thickfilament

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2. Type of Contraction Slow contraction due to poor arrangement of

fibers Does not fatigue Tonic type of contraction Calcium involved from

SR Extracellular Movement is slower since there are no T-Tubules

See Fig. 9.29

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3. Visceral/single unit vs. Multiunit a) sheets lining blood vessels a) larger blood vessels

and walls of organs airways, eye muscles, arrector pili muscles

b) tight junctions between muscle cells

c) one nerve controls many different muscle cells, wave-like contraction

c) individual muscles with separate

motor nerves

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