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Muscular System: Histology and PhysiologyI. General Funct ional Characteristics of Muscle

A. Introduction1. More than 600 muscles in muscular system. 2. Each muscle has a function: moving a finger, or blinking an eyelid3. Muscular system makes up ~ 40% of the body weight.4. Muscle generally work as functional groups

a. synergistic - biceps brachii and brachialis flex elbow b. antagonistic - biceps brachii and tricepsc. Why? - Although at the same joint some are designed to be postural and other for rapid

powerful contraction.5. Naming of muscles (Criteria)

a. Shape: romboideus, trapezius, bicepsb. Location: pectoralis (chest) intercostal (ribs)c. Attachment: zygomat icus, sternocleidomastoidd. Size: maximus, minimus, brevis, longise. Orientation of fibers: rectus (straight), oblique (slanting)f. Relative position (lateral, medial, internal, external)g. Function: adductor, flexor, extensor, pronator

6. Muscle architecturea. Parallel - straplike, long excursion (contract over great distances)

(1) good endurance, not especially strong (2) - rectus abdominus, sartorius.

b. Convergent - fan shaped, force of contraction focused onto a single point of attatchment(1) stronger than parallel(2) deltoid and pectoralis major

c. Sphincteral - concentric arrangement around a body opening(1) act as a sphincter(2) orbicularis oris

d. Pennate - (feather like) many fibers per unit area(1) strong muscles, tire quickly(2) types

(a) unipennate(b) bipennate(c) multipennate.

B. Charcteristics of muscle1. contractility (shortens forcefully), 2. excitability (responds to stimuli by nerves and hormones.), 3. extensibility (can be stretched), 4. elasticity (recoils to resting length).

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C. Muscle Types1. skeletal, smooth and cardiac.

Features Skeletal Muscle Smooth Muscle Cardiac Muscle

Location Attached to bone walls of hollow organs, bloodvessels, eyes, glands and skin

heart

Cell shape very long, cylindircal (entirelength of short muscles)

Spindle shaped Cylindr ical and branched

Nucleus Multiple, peripherally located Single centrally located single centrally located

Special features Gap junctions join visceralsmooth muscle cells

Intercalated disks joincells

Control voluntary and involuntary(reflexs)

Involuntary Involuntary

Spontaneouscontraction

No Yes Yes

Function Body movement Food movement, empty urinarybladder, blood vessels, glands andduct contraction, movement of hair

pumps blood

II. Structure Skeletal Muscle:A. Skeletal muscle is composed of muscle fibers associated with smaller amounts of connective tissue,

blood vessels and nerves.B. Muscle fiber (General Features)

1. Each Muscle fibers is a skeletal muscle cell.2. Shape: cylindrical cell containing several nuclei.3. Develop from myoblasts (multinucleated cells).

a. Multilple nuclei within these cells come from the fusion of several cells4. Number of muscle fibers

a. stays relatively constant from the time of birth.b. Hypertrophy is due to an increase in the size of each muscle fiber.

(1) Addition of actin and myosin filaments.(2) Large muscles have large diameter

fibers, small muscles have smalldiameter fibers.

5. Muscle fibers can extend the full length of themuscle a. However, 3 or 4 fibers may be linked to

extend the full length of some longermuscles.

C. Connective Tissue1. External lamina

a. surrounds each muscle fiberb. Produced by the muscle fiber.

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c. Made of reticular fibers2. Endomysium

a. surrounds each muscle fiber.b. Loose connective tissue with reticular fibers.c. Found just outside of the external lamina

3. Fasciculi. a. Groups of muscle fibers.

4. Perimysium a. covers the each fasciculi,b. Made up of a heavier connective tissue layer

5. epimysiuma. Connective tissue sheath that covers the whole muscle.

6. facia a. Binds adjacent muscles together and binds overlying skin to muscle.b. The connective t issue of muscle if bound firmly to the connective tissue of tendons and

bones.

D. Structure of Muscle Fibers1. A muscle fiber is a single cell consisting of

a. sarcolemma- cell membraneb. sarcoplasm cytoplasm within muscle fibers

(1) contains many mitochondria, and glycogen granules.c. nuclei (several in skeletal muscle)d. myofibrils

(1) threadlike structure (1-3 um).(2) Composed of two kinds of protein filaments

(a) actin myofilaments (thin myofilaments)(b) myosin (thick myofilaments)

2. Myofibrils are organized into sarcomeres.a. Sarcomeres- the functional unit of the muscle

(1) Joined end to end to form myofibrils.(2) Sarcomeres extend from Z disk to Z disk.(3) Z disks are a filamentous network of

proteins that hold actin myofilaments.b. Six actin myofilaments (thin filaments)

surround a myosin myofilament (thickfilaments).

c. Myofibrils appear striated because of A bandsand I bands.(Slide)(1) I band -isotropic (light band)

(a) end of myosin filament to end tomysosin

(b) includes Z disk (c) Consists of only actin filaments

(2) A band - anisotropic (dark band)(a) length of myosin filaments

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(b) overlap with act in filaments within this band(3) H zone - at center of A band

(a) no overlap of actin within the H zone.(4) M line - middle of the H zone.

(a) filaments hold myosin molecules in place.

3. Myofilament structure.a. Actin myofilaments consist of :

(1) F (fibrous) actin stands (a) two st rands of F act in form a double helix(b) each F actin is composed of G (globular) actin monomers(c) each G actin has an active site for myosin to bind during muscle contraction.

(2) tropomyosin(a) winds along the groove

of the F actin helix(3) troponin composed of three

subunits. (a) actin binding site(b) tropomyosin binding site

i) troponin binds to theend of eachtropomyosinmolecule.

(c) calcium binding site(4) Tropomyosin and

troponin complexregulates the interactionbetween g actin andmyosin.

b. Myosin molecules(1) myosin myofilaments consist of:

(a) two heavy myosin molecules(a rodlike portion)with two globular heads(b) Four light myosin molecules which are attached to the globular heads.

(2) Each myosin myofilament consists of 300 myosin molecules w 150 heads at each end.(3) Heads of each myosin molecule are hinged

(a) they bend and straighten during contraction(b) have ATPase enzymatic activity(c) bind to the active sites on actin molecules forming a cross-bridge.

4. T tubules and sarcoplasmic reticuluma. Invaginat ions of the sorcolemma form T tubules (tube that extend through the cell).

(1) wrap around the sarcomeres.(2) lumen of T tubules contains extracellular fluid (which is high is Ca++). (3) Ca++ is necessary to trigger muscle contraction.

b. Sarcoplamic reticulum(1) is a specialized smooth endoplasmic reticulum(2) Ca++ is taken into and concentrated within the sarcoplasmic reticulum from the T tubule.

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c. terminal cisternae (1) an enlargement of the sarcoplasmic

ret iculum near the T tubuled. triad - T tubule and two terminal cisternae

III. Sliding Filament ModelA. Muscle contraction occurs when actin myofilaments

slide over myosin myofilaments resulting in theshortening of the sarcomere.1. Actin and myosin myofilaments do not change in

length during contraction.2. Actin and myosin myofilaments slide past one

another in a way that causes sarcomeres to shorten.B. The I band and H zones narrow during contractionC. The A band remains constant in length.

IV. Physiology of skeletal Muscle Fibers.A. Neuromuscular Junction - Axon meets muscle.

1. Motor neurons - Propagate action potentials from brainstem/spinal cord to muscle fibers.2. Neuromuscular junction -

a. synapse of motor neuron axon terminal and muscleb. near the center of the muscle fiber

3. Presynaptic terminal - axon terminal 4. Synaptic cleft - space between muscle fiber and axon terminal5. Postsynaptic membrane (motor end plate) - muscle membrane at the synapse.6. Synaptic vessicles - spherical sacs within the presynapitic terminal that carry the neurotransmitter

acetylcholine.7. Acetylcholine

a. Acetylcholine is the neurotransmitter released from the presynaptic terminalb. ACh binds to receptors of the postsynaptic membrane and changes membrane permeability to

Na+ which produces an action potential. c.

B. Synaptic Events1. Action potential reaches presynaptic terminal causing voltage-gated Ca++ channels to opened.2. Ca++ rushes into presynaptic terminal3. Ca++ causes synaptic vesicles containing

acetylcholine to fuse with the presynapticmembrane.

4. Acetylcholine is released into the synapse 5. Acetylcholine binds to ligand gated Na+

channels at the motor end plate.6. Na+ entering the cell causes a local

potential which reaches threshold andgenerates an act ion potential

7. Action potential sweeps across the musclefiber.

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C. Acetylcholinesterase1. Breaks down acetylcholine into choline and acetic acid2. Choline is recycled to the presynaptic terminal.

D. Excitation-Contraction Coupling1. Process by which an action potential causes contraction of a muscle.2. Action potentials causes voltage-gated Ca++ ion channels to open in T tubule system (triad - t

tubules, terminal cisternae).3. Ca++ ions are released from the sarcoplasmic

ret iculum.4. Ca++ ions bind to troponin bound to the actin

myofilament. 5. Ca++-Troponin complex causes t ropomyosin

to move deep into the actin groove 6. the active site on actin becomes exposed to the

myosin heads 7. A cross bridge is formed (Myosin head binds

to actin)8. Contraction: (shortening of sarcomere)

a. actin and myosin bind, b. myosin changes shapec. actin is pulled past the myosin.

E. Muscle Contraction1. Ca++ ions bind to troponin and active

sites on actin are exposed2. Myosin molecules attach tothe exposed

active sites on the actin myofilamentsand phosphate is released.

3. Energy stored in myosin head is used tomove myosin head (power stroke).Actin slides over the myosin and ADPis released.

4. ATP binds to the myosin head5. Energy from ATP break down is used

to move head into resting (cocked)position. Energy is stored in the myosinhead.

6. The movement is repeated as long asCa++ is bound to the troponin and there is adequateATP stores.

7. Relaxation occurs when calcium is taken up by thesarcoplasmic reticulum, ATP binds to myosin andtropomyosin moves back between act in and myosincovering the active site on myosin and preventingcross- bridge formation.

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F. Energy Requirements for Muscle Contraction1. One ATP molecule is required for each cycle of cross-bridge formation, movement and release.2. ATP is also required to transport Ca++ ions into the sarcoplasmic reticulum and to maintain

normal concentrations across the cell membrane.

V. Physiology of Skeletal MuscleA. Muscle twitch

1. A muscle twitch is the contraction of a single muscle fiber or a whole muscle in response to astimulus.

2. A muscle twitch has a lag, contraction and relaxation phase. (Table 10.2)a. Lag - time: from stimulus to beginning of contraction

(1) Action potential is propagated to presynaptic terminal(2) Ca diffuses into presynaptic terminal,

ACh released by exocytosis intosynapt ic cleft

(3) ACh binds to ACh receptor in postsynaptic sarcolemma.

(4) Activated ACh receptors causes ligandgated Na+ ion channels to open

(5) Na+ ions depolarize cell producing alocal potential that becomes an actionpotential

(6) ACh is degraded byacetylcholinesterase.

(7) Action potential is propogated to ttubules and sarcoplasmic reticulum

(8) Depolarization of t tubules causes voltage gated Ca+ ion channels to open.(9) Ca+ rushes out of the sarcoplasmic reticulum and into sacroplasm.(10) Ca+ bind to troponin(11) Troponin-tropomysin complex changes position and exposes the active sie on the

actin myofilaments.b. Contraction - time during contraction

(1) Cross bridge forms, move, release and reform between actin and myosin head.(2) Sarcomere is shortened(3) ATP is broken down as cross bridges are formed and myosin head is moved

c. Relaxation time during which relaxation occurs. (1) Ca+ ions are actively transported into the sarcoplasmic reticulum(2) troponin-tropomysosin complexes inhibit cross bridge formation.(3) muscle fibers lengthen passively.

B. Stimulus Strength and Muscle Contraction 1. all-or-none law of muscle contraction - For a given condition, a muscle fiber or motor unit

contracts with a consistent force in response to each action potential,a. Subthreshold action potential vs thresholdb. Motor unit - single motor neuron and all of the muscle fibers it enervates.

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2. multiple motor unit summation- For a whole muscle, a stimulusof increasing magnitude results ina graded contraction response ofincreased force as more motorunits are recruiteda. Subthreshold - no motor units

contractb. Threshold stimulus - activation

of a single motor unitc. Submaximal stimuli- activates

additional motor units.d. Maximal stimulus - all of the

motor units are activatede. Supramaximal stimulus - no

additional effect.3. Muscles performing delicate and precise movements have motor units with a small number of

muscle fibers. 4. Muscles performing more powerful movements have motor units with many muscle fibers.

C. Stimulus Frequency and Muscle Contraction1. The force of contraction increases with

increased stimulation frequency (multiplewave summation)

2. Incomplete tetanus is partial relaxationbetween contractions, and complete tetanusis no relaxation between contractions (actionpotentials are too rapid) .

3. Two reason for multiple wave summation:a. an increasing concentration of Ca++ ions

around the myofibrils and b. complete stretching of muscle elastic

elements (elasticity of connective tissue,etc.). Once stretched the total force canbe applied to the load to be lifted.

4. Treppe is an increase in the force of contraction during the first few contraction of a restedmuscle.a. Max stimulus at low frequency - increased

strength over first few contractions.b. Due to an increase in Ca++ ion levels around

the myofibrils and increased temperature(1) enzymes for muscle contraction respond

more effectively at higher temp.

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D. Types of Muscle Contractions1. Isometric contractions

a. - change in muscle tension but no change in muscle length. b. Eg. (Postural muscles)

2. Isotonic contraction a. - change in muscle length with no changes in muscle tension. b. (Arm and finger movement) c. Both isometric and isotonic contractions are involved in most body movements.d. Asynchronous contractions of motor units produce smooth, steady muscle contraction.

3. Concentric contractions cause muscle to shorten and tension to increase. a. Tension is great enough to overcome the opposing resistance.

4. Eccentric contractions tensions is maintained but muscles to increase in length due to opposingresistance.

E. Length versus tension1. Muscle contracts with less-than-maximum

force if its initial length is shorter orlonger than optimum.

F. Fatigue1. Fatigue is the decreases ability to do work2. Causes:

a. central nervous system, b. depletion of ATP in muscles, or c. depletion of acetylcholine in the

neuromuscular synapse.G. Physiological Contracture and Rigor Moris

1. Physiological contracture: inability ofmuscles to contract of relax due toextreme fatigue

2. rigor mortis (stiff muscles after death) result from inadequate amounts of ATP.H. Energy Source

1. Energy for muscle contraction comes from ATP.2. ATP comes from 3 energy sources

a. Creatine phosphate(1) ATP can be synthesized be reacting with creatine phosphate and is used to provide

energy for a short time during intense exercise. (2) ADP + creatine phosphate º creatine and ATP(3) During intense exercise creatine phosphate is quickly exhausted (10-15 sec.)

3. Anaerobic respirationa. anaerobic metabolism is used to provide energy for short amounts of time during intense

exercise. b. Anaerobic respiration produces ATP less efficiently but more rapidly that aerobic

metabolism. c. Lactic acid is a byproduct of anaerobic respiration.

4. Aerobic respirationa. Although ATP is produced more efficiently with aerobic respiration, it is produced more

slowly. b. Aerobic metabolism produces energy for muscle contractions under resting conditions or

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during exercise such as long-distance running. 5. Oxygen Debt

a. After anaerobic respiration, aerobic respiration is higher than normal, restoring creat inephosphate levels and converting lactic acid to glucose.

I. Slow and Fast Fibers1. Not all skeletal muscle have identical functional capabilities.2. Slow-twitch fibers

a. contract more slowly, have smaller diameters and are more resistant to fatigue than fasttwitch.

b. Break down ATP at a limited rate and have a increased ability for aerobic respiration becauseof a well-developed blood supply, many mitochondria, and increased myoglobin content.

3. Fast-twitch fibers a. Contract quickly, fatique rapidly and split ATP rapidly.b. Contain very little myoglobin, few mitochondria and large deposits of glycogen.c. Two types

(1) Fast-twitch fibers, fatigable fibers have large amounts of glycogen, a poor blood supply,fewer mitochondria, and litt le myoglobin.

(2) Fast-twitch fibers, fatigue-resistant fibers have a well-developed blood supply, more mitochondria, and myoglobin. (Found highly trained individuals)

J. Effects of Exercise1. Muscle increase (hypertrophy) or decrease (atrophy) in size results from a change in the size of

muscle fibers.2. Anaerobic exercise develops fast-twitch, fatigable fibers. Aerobic exercise develops slow-twitch

fibers and changes fast-twitch, fatiguable fibers into fast-twitch, fatigue-resistant fibers.K. Heat Production

1. Heat is produced as a by-product of chemical reactions in muscles. a. After exercise increased metabolism due to oxygen debt maintains an increased body temp.

2. Shivering produces heat to maintain body temperature.a. Increase heat production up to 18 times over resting muscle.