Chapter 9



Chapter 9

Muscle and Muscle Tissue

I. Overview of Muscle Tissue

-over 600 muscles in body

-muscle about 1/2 of body mass (40% of body mass is skeletal muscle)

-can transform ATP into directed mechanical energy - muscle capable of exerting force

- skeletal muscles for mobility

-internal organ muscles force fluids throughout body

A. Muscle types

1. general characteristics

a. 3 types - smooth, skeletal, cardiac

b. muscle cells called muscle fibers

c. 3 types differ in: structure, location, function, how activated

2. skeletal

a. striated

b. voluntary

c. long/cylindrical

d. multinucleated

e. attach to bone or skin

f. contraction: fast/vigorous

g. fatigue possible

3. cardiac

a. striated

b. involuntary

c. intercalated discs

d. branching fibers

e. uninucleated

f. only in heart

g. contraction: slow/rhythmic

h. no fatigue

4. smooth

a. no striations

b. involuntary

c. uninucleated

d. visceral - lines hollow organs

e. contraction: slow/sustained

f. no fatigue

B. Functions

1. movement

a. locomotion/manipulation

b. respond to external environment

c. moves substances through body

2. posture - muscles function continuously

3. heat production

a. by-product of muscle metabolism and contractile activity (ATP broken down and

heat given off)

b. important in maintaining normal body temperature (mainly skeletal

muscles do this)

4. stabilizes joints

C. Functional characteristics of muscles

1. excitability (responsiveness) - ability to receive and respond to stimulus (chemical - hormones, neurotransmitters, etc.); this refers to generation and transmission of electrical current along plasma membrane - called action potential

2. contractility - ability to shorten

3. extensibility - ability to be stretched/extended

4. elasticity - ability of muscle fiber to resume resting length after contraction/stretch

II. Skeletal Muscles

A. Gross Anatomy of Skeletal Muscles

1. basic features

a. each muscle composed of 100's-1,000's of muscle fibers (cells)

b. each skeletal muscle is discrete organ - has bv; nerves; ct; etc.

2. connective tissue wrappings - several layers

a. endomysium - fine reticular ct sheath - surrounds each muscle fiber (cell);

invagination of perimysium

b. perimysium - collagenic sheath around each fascicle (bundle of muscle fibers/cells)

extension of epimysium

c. epimysium or deep fascia - dict - surrounds bundles of fascicles - make up muscle

d. function

-support/reinforcement of soft/fragile muscle

-provides access/exit routes for bv and nerves that serve muscle

3. nerve/blood supply

a. normal activity depends on nerve supply and rich blood supply (1 artery/several veins/1 nerve)

b. 1 muscle fiber - 1 nerve ending (100 muscle fibers – 100 nerve endings)

4. attachments

a. direct muscle attachment

-epimysium fused to periosteum of bone or perichondrium of cartilage

b. indirect muscle attachment

-tendon (fascia extends beyond muscle as rope like structure) or aponeurosis (sheet) anchors muscle to ct covering of bone/cartilage, to fascia of other muscle, or to skin of face

B. microscopic anatomy of skeletal muscle

1. basic features

a. cells cylindrical/multinucleated

b. sarcolemma - plasma membrane - p. 283

c. sarcoplasm - cytoplasm (has lots of glycosomes - granules of stored glycogen)

d. myofibrils - contractile elements of skeletal muscle - pp. 286, 283, 287

e. sarcoplasmic reticulum (SR) - smooth er - net over muscle - p. 287

f. T-tubules - modifications of sarcolemma - p. 287

g. myoglobin - O2 binding protein - not in other cells - stores O2 within muscle fiber

2. myofibrils (make up muscle cell)

a. characteristics

-100's to 1,000's per cell

-run parallel/full length of cell

-myofibrils contain contractile element of skeletal muscle – the sarcomere

-composed of chains of sarcomeres laid end to end

b. striations

-series of alternating dark bands (A)/light bands (I)

-A band

-light stripe in A band - H zone

-visible only in relaxed cell

-H zone bisected by dark line - M line

-I band

-dark area midline - Z disc (line)

c. sarcomere

-region of myofibril from one Z disc to next Z disc –

aligned end to end all the way down the muscle fiber

-smallest contractile unit of muscle fiber (cell)

d. myofilaments - what sarcomere is made of

-2 types of protein filaments

-actin - thin - extends across I band and partly into A band

-myosin - thick - extends entire length of A band

-Z disc - sheet of protein

-point of attachment of thin filaments

-connects one myofibril to another

e. ultrastructure and molecular composition of myofilaments

-myosin - thick filament

-rodlike tail - tails bundled to form smooth central portion of myosin

-each rodlike tail has 2 globular heads

-heads attach to tails with a hingelike area - allows heads to bend and straighten

- have actin binding sites on globular heads

-interact with specific sites on thin filaments forming cross bridges - these extend from the thick myosin filaments to the thin actin filaments.

-heads have ATP binding sites

-heads have ATPase enzyme - split ATP

-generate tension developed by muscle when contracts

-bundle of tails - heads on each end facing opposite directions

-elastic filament extends from Z disc to myosin and all the way to M line

-actin - thin filaments

-composed of 2 F actin filaments twisted together like strands of pearls

-F actin composed of smaller subunits - G actin (pearls on string)

-G actin has myosin binding sites - where myosin heads attach during muscle contraction

-tropomyosin - protein - coils around F actin - blocks actin's myosin binding sites so myosin can't bind

-troponin - protein - Ca+ ions bind to this

3. sarcoplasmic reticulum (SR) (smooth er) and T tubules

a. SR - system of membranous tubules - surround each myofibril - like sleeve

b. SR function - release/capture calcium ions

c. at H zones and A-I junctions - SR tubules fuse - form saclike channels called terminal cisternae

d. T tubules - long hollow tube at A-I junction – are perpendicular to myofibrils

-formed by sarcolemma

-T tubules run between terminal cisternae forming triads - (terminal cisternae, T tubules, terminal cisternae) p. 287

-1,000's in each muscle fiber (T system)

-conducts nerve stimulus deep within cell and sarcomeres

C. Sliding Filament Model of Contraction

1. sliding filament theory

a. relaxed state - thin and thick filaments overlap some

b. during contraction - thin filaments slide past thick ones overlapping great deal

2. contraction – refers only to activation of myosin's cross bridges (myosin heads attach to actin)

-relaxation - contraction ends; myosin heads detach (breaking cross bridges) and tension generated decreases and muscle fiber relaxes

3. when muscle fiber stimulated

a. myosin heads attach to active sites on actin forming cross bridges

b. sliding begins

4. myosin heads attach/detach (forming/breaking cross bridges) many times during contraction

a. occurs simultaneously to sarcomeres throughout cell

b. cells shorten

5. as muscle shortens, all of following occur:

-I bands shorten

-H zones disappear

-A bands move closer

-distance between Z discs shortens

D. Physiology of a Skeletal Muscle Fiber

1. requirements for muscle fiber contraction

a. fiber must be stimulated (activated) by nerve ending to change membrane potential

b. fiber must generate electrical current (action potential) along sarcolemma

c. action potential must be spread (propagated) along sarcolemma

d. intracellular Ca2+ must increase – final trigger for contraction

e. steps c and d link electrical signal to contraction and are called excitation- contraction coupling

2. the neuromuscular junction and the nerve stimulus

-somatic motor neurons or motor neurons of somatic (voluntary) nervous system stimulate/activate skeletal muscle

-motor neurons possess cell bodies - found in brain or spinal cord

-axons - long extensions of motor neuron that travel to muscle cells they serve

-each axon divides/branches as enters muscle – end of axon called axon terminal

-each branch forms neuromuscular junction (motor end plate) with single muscle fiber (cell) approximately midway along fiber

-synaptic cleft - fluid filled gap between axonal ending and muscle fiber

-synaptic vesicles in axonal endings contain acetylcholine

(ACh - neurotransmitter)

-nerve impulse reaches end of axon

-voltage gated calcium channels on nerve cell membrane open

-calcium flows into axonal ending

-causes some synaptic vesicles to fuse to membrane

- ACh released into synaptic cleft by exocytosis

-ACh attaches to ACh receptors on junctional folds of sarcolemma

-electrical events triggered

3. changes in plasma membrane – allows generation of action potential across sarcolemma

a. resting membrane potential

-inside of cell membrane negative

-outside of cell membrane positive

-cell membrane more permeable to K+ than Na+

-sodium/potassium pump maintains this potential

b. Summary of events in generation and propagation of actin potential in skeletal muscle fiber:

1. depolarization

-ACh causes ion channels in muscle cell membrane receptors to open allowing Na+ to diffuse into cell

- Na+ diffuses more rapidly into cell than K+ moves out

-causes inside of plasma membrane to become less negative and outside to become less positive - sarcolemma depolarized

2. propagation of action potential (electrical current)

-positive charge in one place on sarcolemma causes Na+ channels next to it to open

-Na+ gated channels open all along sarcolemma

-Na+ flows in continuing depolarization along sarcolemma

-action potential moves along behind depolarization - called propagation -

and propagation moves along length of sarcolemma

-action potential moves rapidly in all directions 3. repolarization - after depolarization passes, sarcolemma'a permeability changes again

-Na+ channels close and K+ channels open

-K+ rapidly leaves cell (called efflux)

-resting potential restored by sodium/potassium pump

-sarcolemma said to be repolarized

-refractory period - time during repolarization

-during refractory period cell insensitive to further stimulation

c. all or none response - once initiated, action potential unstoppable and results in full contraction of muscle cell

d. after binding to receptors, ACh quickly destroyed by acetylcholinesterase (AChE)

-ACh broken down into choline and acetic acid

-done to prevent continual muscle fiber contraction in absence of more nerve stimulation

4. excitation-contraction coupling

a. sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments

b. occurs during latent period (between action potential initiation and beginning of

mechanical activity (shortening)

c. electrical signal causes increase in intracellular Ca+ ions that allows filaments to slide

d. Steps of excitation-contraction coupling

1. action potential moves along sarcolemma and down T tubules

2. ap causes SR to release Ca+ ions into sarcoplasm

3. Ca+ ions bind to troponin; troponin changes shape moving tropomyosin out of way exposing binding sites on actin

4. myosin cross bridges (heads) attach and detach with actin, pulling actin filaments toward center of sarcomere; release of energy by ATP hydrolysis fuels cycling process

5. Ca+ ions decrease due to reuptake into SR

6. tropomyosin blockage restored; contraction ends/muscle fiber relaxes

E. Muscle Fiber Contraction

1. cross bridge active site binding requires Ca+ ions

2. in absence of Ca+ ions - myosin binding sites on actin blocked by tropomyosin molecules - muscle relaxed

3. Ca+ ions available

a. bind to troponin

b. troponin changes shape and moves tropomyosin out of way

c. binding sites "open"

4. events of contraction

a. cross bridge formation

-myosin head in high energy configuration position - "cocked"

-myosin head binds to active site on actin - cross bridge formed

b. power stroke

-myosin head attaches and bends, pulling actin towards center of sarcomere

-myosin head in low energy configuration

-at same time, ADP and Pi released from myosin head (ADP and Pi are produced from prior contraction)

c. cross bridge detachment

-cross bridge broken (myosin head detaches from actin) when new ATP binds to myosin head

-but each mysosin head will remain bound to actin until another ATP binds to it and pulls it back into its resting position

d. cocking of myosin head

-myosin head returns to original position as ATP splits (is hydrolyzed) into ADP and Pi - done by enzyme ATPase; provides energy to return myosin head to "cocked" position-back at beginning

5. general information

a. attachment/detachment occurs many times in course of contraction

b. cycle continues as long as Ca+ ions and ATP are present

c. only 1/2 of myosin heads working at same time

6. rigor mortis

a. cross bridge detachment ATP driven

b. we die, cells die, plasma membrane can't keep out Ca+ ions (membrane becomes permeable because ion pumps no longer working)

c. flood of Ca+ ions promotes cross bridge binding

d. dead - can't make ATP so cross bridge detachment can't occur, so muscles stay

contracted

e. rigor mortis begins 3-4 hours after death - peaks at 12 hours

f. rigor mortis goes away as actin/myosin break down - about 48-60 hours after peak

F. Contraction of a Skeletal Muscle

1. basic characteristics

a. relaxed - soft; contracted - hard/elastic structure

b. muscle cells - all or none response

c. muscles contract with varying degrees of force for different periods of time

d. muscle tension - force exerted by contracting muscle on object

e. load - opposing force exerted on muscle by weight of object to be moved

2. motor unit

a. definition: motor neuron and all muscle fibers it innervates

b. bundled axons enter muscle - each axon branches to form neuromuscular junction

with muscle fiber

c. fine motor control - small motor units

d. less precise - large motor unit

e. # of muscle fibers per unit - average of 150

3. muscle twitch

a. response of a motor unit to single action potential of its motor neuron (contraction of 1 muscle fiber)

-contracts quickly/relaxes

-strong or weak depending on # of motor units activated

b. myogram - machine that records mechanical contractile activity

c. 3 distinct phases in a twitch

-latent period - short - time after stimulation; excitation-contraction coupling occurring - muscle tension beginning to increase

-pd. of contraction - onset of contraction to peak of tension

-pd. of relaxation - no contractile force generated; muscle tension drops to zero

4. graded muscle responses

a. basic features

-variations in degree of muscle contraction - called graded response

-2 ways to grade muscle response

-change frequency of stimulation by increasing rapidity of stimulation to produce wave summation

-change strength of stimulation (make it stronger) by recruiting large # of motor units to produce multiple motor unit summation

b. muscle response to changes in stimulation frequency

-wave summation

-2 stimuli close together - 2nd contraction stronger than 1st

-1st not through contracting - 2nd contraction begins and "rides" on 1st - added on

-main function of wave summation - tetanus

- if stimulation continues - contractions become fused into a smooth, sustained contraction

c. muscle response to stronger stimuli (multiple motor unit summation)

-the more fibers activated, the stronger the contraction

5. Treppe - staircase effect

a. 1st muscle contraction weak

b. those occurring later - stronger

c. shows staircase pattern - treppe

d. why athletes warm up - contractions more efficient after warm-up pd.

6. muscle tone

a. relaxed muscles always slightly contracted

b. helps stabilize joints

c. helps to maintain posture

d. keeps muscles firm, healthy and ready

7. Isometric and Isotonic contractions

a. isotonic

-causes obvious movement

-muscle shortens - moves load

-tension exceeds load

b. isometric

-muscle tenses -doesn't shorten (actin filaments don't move - mysosin heads "spinning" their wheels at same actin binding site

-load greater than tension - load can't be moved

-used to maintain posture; hold joints stationary while others move

G. Muscle metabolism

1. providing energy for contraction

a. basic features

-ATP needed for contractile events (cross bridge attachment/detachment) and

Ca+ pump

-ATP use = ATP production, muscles can respond to stimuli long time

-muscles store little ATP; reserve exhausted quickly (6 seconds)

-ATP must be regenerated

-How? 1. interaction of ADP with creatine phosphate

2. aerobic respiration

3. anaerobic respiration (glycolysis)

b. coupled reaction with creatine phosphate

-ADP + creatine phosphate ATP + creatine

-lots of creatine hosphate stored in muscles

-quick energy -lasts 10-15 seconds

c. aerobic respiration

-glucose + O2 CO2 + H20 + ATP

-glucose is found in blood and glycogen that is broken down

-prolonged energy

d. anaerobic respiration and lactic acid formation

-used when muscles contract for long time

-O2 and glucose decrease

-aerobic respiration stops

-move into anaerobic respiration - lactic acid produced with small amts. of ATP

-medium length of time for energy

2. muscle fatigue

a. inability of muscles to contract

b. ATP use greater than ATP production

c. excessive lactic acid accumulation

-causes pH in muscles to drop which causes muscles to burn

d. ionic imbalances occur

-no ATP for Na+/K+ pump

-no ATP for cross bridge detachment

-muscles stay contracted

-this is muscle cramp

3. O2 debt

-volume of O2 required after exercise to oxidize lactic acid formed during exercise

4. heat production during muscle activity

-40% of energy from ATP is used for work - rest lost as heat

H. Muscle Fiber Type

1. 3 types of muscle fibers

a. slow oxidative fibers

b. fast glycolytic fibers

c. fast oxidative fibers

2. slow oxidative fibers

a. red

b. many capillaries

c. many mitochondria

d. slow activity of myosin ATPase (ATP can't be split fast) (slow myosin)

e. slow contraction

f. lots of myoglobin

g. low amount of glycogen

h. aerobic – oxygen dependent

i. fatigue resistant

j. high capacity to produce ATP by oxidative phosphorylation of ADP

k. movements requiring a small force

l. small in diameter and generate less force (muscles of the back - posture)

m. for endurance type of activities - marathon; posture

3. fast glycolytic fibers

a. white

b. few capillaries

c. few mitochondria

d. fast activity of myosin ATPase (ATP splits quickly by glycolysis – less efficient)

(fast myosin)

e. high glycolytic capacity (measure of maximum capacity of glycolysis

to produce ATP)

f. fast contraction

g. low amount of myoglobin

h. high amount of glycogen

i. anaerobic

j. fatigable

k. have high maximum shortening velocity

l. intermediate diameter

m. for short term intense or powerful movements - hitting a baseball, moving furniture

4. fast oxidative fibers – intermediate - between slow oxidative and fast glycolytic fibers

a. fast contraction

b. red to pink fibers

c. many capillaries

d. many mitochondria

e. fast activity of myosin ATPase (fast myosin)

f. lots of myoglobin

g. moderate amt of glycogen

h. intermediate amount of glycogen

i. aerobic – oxygen dependent (has some anaerobic glycolysis)

j. high oxidative capacity

j. moderately fatigue resistant

k. large diameter

l. shoulders, glutes

m. for sprinting; walking

5. born with mix of all 3 fiber types

a. can have more of one type than another

b. can help determine athletic capabilities

c. genetically controlled

d. can’t change one type into another or grow more of one type

e. what you have can be enhanced/modified by exercise

I. Effect of exercise on muscles

1. adaptations/benefits to exercise

a. capillaries increase in #

b. mitochondria in #

c. amt. of myoglobin increases

d. better metabolism

e. better neuromuscular coordination

f. increase endurance

g. increase strength

h. resistance to fatigue

i. better gastrointestinal motility

j. increase health/strength of skeleton

k. improve cardiovascular system

l. improve respiratory system

-increase amt. of O2 and nutrients delivered to cells

-better gas exchange

m. heart hypertrophies

-causes increase in stroke volume (amt. of blood ejected from left ventricle with each heart beat)

-heart more efficient

n. fatty deposits cleared from blood vessels

o. store more glycogen

p. can form more myofilaments and myofibrils (make muscles bigger)

q. connective tissue increases between cells ( makes muscles bigger)

r. causes split/tears in fibers and get subsequent growth when fibers repaired

(makes muscles bigger)

2. disease atrophy

a. muscle immobilized or paralyzed - degeneration/loss of mass

b. muscle tissue replaced by fibrous connective tissue - rehab impossible

III. Smooth Muscle

A. microscopic structure and arrangement of smooth muscle fibers

1. basic characteristics

a. smooth

b. spindle shaped cells

c. uninucleated

d. no striations

e. SR poorly developed

f. no T tubules

2. characteristics of myofilaments

a. more actin

b. no troponin - so always ready for contraction

c. no sarcomeres

d. intermediate filaments (non-contractile)

e. dense bodies = Z discs in skeletal muscle; anchor to sarcolemma

f. myosin has head along entire length

3. connective tissue

a. smooth muscle fibers organized into sheet

-2 layers perpendicular to each other

b. visceral

c. peristalsis

d. no structural neuromuscular junctions

e. diffuse junctions

-neurotransmitters released from varicosities (bulbous endings of nerve fibers) into wide synaptic cleft in general area of smooth muscle cells

B. Contraction of smooth muscle

1. mechanism and characteristics of contraction

a. whole sheet responds to stimulus in unison

b. gap junctions allow action potential to go cell to cell

c. some are pacemakers - set contractile pace for entire sheet of muscle

d. contraction same as in skeletal muscle except:

-Ca+ goes to myosin

-slower contractions

-energy efficient - maintains tension for prolonged pd. of time with little ATP usage

2. regulation of contraction

a. mostly same as in skeletal muscle

b. differences - some neural signals stimulate or inhibit group of muscle cells

3. special features of smooth muscle

a. basic features

-smooth muscle tone

-slow, prolonged contractile activity

-response to stretch different

-capable of shortening more than other muscle types

-has secretory functions

b. response to stretch - stretch-relaxation response

-allows organs to be filled

c. length-tension change

-after organ empties, goes back to original state

d. hyperplasia

-smooth muscle fiber can increase in # (uterus - puberty and pregnancy)

e. secretory function

-synthesize and secrete connective tissue proteins: collagen and elastin

C. Types of smooth muscle

1. single unit smooth muscle - more common

a. visceral muscle

b. cells contract as unit

c. gap junctions – allows cells to transmit action potential fiber to fiber

d. spontaneous action potential

2. multiunit smooth muscle

a. muscle fibers structurally independent of each other

b. closely regulated by nervous system

c. gap junctions rare

d. innervated by ans (involuntary)

e. susceptible to hormone controls

f. examples: large airways to lungs, large arteries, arrector pili muscles, muscles controlling pupil

IV. Developmental Aspects of Muscles

A. arise from embryonic mesoderm cells called myoblasts

B. Homeostatic Imbalance

1. muscular dystrophy

a. inherited

b. muscle destroying disease

-muscles increase in size b/c of fat and connective tissue deposits

-muscle fibers degenerate and atrophy

2. cardiac and skeletal muscle

a. can hypertrophy (fibers increase in size)

b. cardiac can divide at modest rate but most injured heart muscle repaired by scar tissue

c. skeletal muscles - ltd. regeneration of dead skeletal muscle – ability decreases with age

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