Muscles & Muscle TissueMuscles & Muscle Tissue

A. Functions1. provide energy for movement2. maintain posture3. thermogenesis4. maintain hollow organ volume

Muscles & Muscle TissueB. characteristics

1. excitability2. contractility3. extensibility4. elasticity

Muscles & Muscle TissueC. Three types

1. SkeletalA) long, cylindrical cells, multi-nucleated, non-branching, and voluntary

B) striated – caused by special arrangement of myofilaments

C) stimulus from nervous system

Muscles & Muscle Tissue2. Cardiac

A) only in heartB) striated, mononucleated, branching,

and involuntaryC) intercalated discs – gap junctions

between cells; distinguishing characteristic

D) stimulus is intrinsic

Muscles & Muscle Tissue3. Smooth

A) found lining the digestive, respiratory & reproductive tracts; also surrounding blood vessels

B) mononucleated, no striations (distinguishing characteristic), and involuntary

C) stimulus from nervous system, some hormones or on its own (stretch)

Muscles & Muscle TissueD. Structure of Skeletal Muscle

1. Whole muscle – bundle of fasciclesA) epimysium – CT layer surrounding whole muscle

2. Fascicle – bundle of muscle fibers (cells)A) perimysium – CT layer surrounding and tying together fascicles

Muscles & Muscle Tissue3. Muscle fiber – bundle of myofibrils

A) endomysium – CT layer surrounding and tying together muscle fibers

B) sarcolemmaC) sarcoplasmD) transverse-tubules (T-tubules) –

inward projections of the sarcolemma; unite with the sarcoplasmic reticulum

Muscles & Muscle Tissue4. Myofibrils – composed of 2 types of

myofilamentsA) thin – composed of 3 protein fibers

1) actin – contractile protein; contains myosin binding sites

2) troponin – regulatory protein

Muscles & Muscle Tissue3) tropomyosin – regulatory protein;

combines with troponin to form the troponin-tropomyosin complexa) when the muscle is relaxed, the

troponin-tropomyosin complex blocks the myosin binding sites on the actin

Muscles & Muscle TissueB) thick – composed of 1 protein fiber

1) myosin – contractile protein; golf-club shapeda) head – will bind to the myosin binding sites on the actin during contraction

b) tail – intertwined to hold the myosin fibers together

Muscles & Muscle Tissue5. Sarcomere – specialized arrangement of

myofilamentsA) functional unit of muscle

Muscles & Muscle Tissue6. Sarcoplasmic reticulum (SR) – fluid

filled tubes surrounding each myofibrilA) storage site of calcium (Ca++)B) adjacent terminal cisternae unite

with the T-tubules to form the triad

Muscles & Muscle Tissue1) the terminal cisternae are enlarged

portions of the SR surrounding each T-tubule

2) allow an impulse to be transmitted from the T-tubules to the SR

Muscles & Muscle TissueE. Skeletal Muscle Contraction

1. Involves a motor unitA) a single muscle may have many

motor units2. Three steps

A) Nerve-Muscle Communication1) Occurs at the neuromuscular

junctiona) motor-end plateb) synaptic cleft

Muscles & Muscle Tissue2) Process:

a) an impulse travels down the motor neuron

b) ACh is released from the neuron into the synaptic cleft

c) ACh binds to receptors on the motor-end plate causing chemical-gated Na+ channels to open

Muscles & Muscle Tissued) Na+ moves into the muscle fiber causing

depolarization of the motor-end platee) depolarization of the motor-end plate

causes an opening of voltage-gated Na+ channels in the sarcolemma

f) this causes an action potential to be transmitted along the length of the sarcolemma

Muscles & Muscle TissueB) Excitation-Contraction Coupling

1) involves T-tubules and sarcoplasmic reticulum (SR)

2) process:a) AP travels down the sarcolemma, also travels down the T-tubules

b) AP is then transferred to the SR at its intersection with the T-tubule

Muscles & Muscle Tissuec) the AP causes an opening of Ca++

release channels in the SRd) Ca++ floods the sarcoplasm

surrounding the thin & thick filamentse) Ca++ binds to troponin causing a

shifting of the troponin-tropomyosin complex exposing the myosin binding sites on the actin

Muscles & Muscle TissueC) Sliding Filament Mechanism

1) involves the thin and thick filaments2) process:

a) ATP is split by ATPase on the myosin head resulting in an “energized” myosin head

i) this occurs at the end of the previous contraction

ii) ADP & P stay attached to the myosin head

Muscles & Muscle Tissueb) the energized myosin head binds to

the exposed binding site on the actinc) using the energy from ATP, the myosin

head swivels inward pulling the thin filament towards the center of the sarcomere = power strokei) the ADP & P are released

Muscles & Muscle Tissued) ATP binds to the myosin head causing

it to break away from the binding sitee) the ATP is split by ATPase, re-

energizing the myosin head f) the process repeats and will continue

as long as ATP and Ca++ are presentMuscle Contraction

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1 Exposed binding sites on actin allow the muscle contraction cycle to occur

Ca+2 Ca+2 Ca+2

ADP + P ADP + P

Contraction cycle

2 Cross-bridge binds actin to myosin

ADP + P ADP + P

P PADP ADP

3

ADP + P

Cross-bridge pulls actin filament (power stroke), ADP and P released from myosin

ATP ATP ATP

ATP

4 New ATP binds to myosin, causing linkage to release

ADP + P ADP + P

5 ATP splits, whichprovides power to“cock” the myosincross-bridge

Muscles & Muscle TissueF. Relaxation – 2 mechanisms

1. AcetylcholinesteraseA) breaks down ACh in the synaptic cleft

2. Ca++ active transport pumpsA) found in the walls of the sarcoplasmic reticulum

B) pump Ca++ back into SR

Muscles & Muscle TissueG. Muscle Metabolism

1. Muscle stores enough ATP for about 4-6 sec of work

2. Three processes provide muscle cells with more ATP

Muscles & Muscle TissueA) phosphagen system

1) creatine kinase transfers a phosphate group from creatine phosphate (CP) to an ADP molecule creating 1 ATP for each CP

2) allows for about 10-15 seconds of energy for maximum activity

Muscles & Muscle TissueB) fermentation (anaerobic) – no oxygen

needed; occurs in cytoplasm1) incomplete oxidation of glucose2) produces 2 pyruvic acid & 2 ATP

from 1 glucose molecule3) pyruvic acid is converted to lactic

acid4) allows for about 30-40 seconds of

max activity

Muscles & Muscle TissueC) cellular (aerobic) respiration – requires

oxygen; occurs in mitochondria1) complete oxidation of glucose2) produces 6 CO2, 6 H2O, & 32 ATP from

one glucose molecule3) can also use fatty acids & amino acids4) amount of energy produced depends

on fitness of individual

Muscles & Muscle TissueH. Muscle fatigue

1. Can be caused by a number of factorsA) inadequate O2

B) glucose/glycogen depletionC) ACh depletionD) lactic acid accumulation

2. Oxygen debt – the amount of O2 necessary to restore the muscle to normal state

Muscles & Muscle TissueI. Muscle Contractions

1. Muscle twitch – response of the motor unit to a single impulseA) latent periodB) contractile periodC) relaxation period

2. Graded muscle responsesA) the way muscles normally functionB) creates smooth contractions of

varying strength

Muscles & Muscle TissueC) 2 primary graded responses

1) wave summation – increases the strength of contraction by increasing the frequency of the stimulusa) tetanus

2) multiple motor unit summation (recruitment) – increases the strength of contraction by activating more motor units

Muscles & Muscle TissueD) treppe

1) stronger contractions resulting from no increase in stimulation

2) possibly due to increased Ca++ availability and temperature

3) basis behind “warming up” before exercise

Muscles & Muscle TissueE) muscle tone

1) slight contraction seen in “resting” muscles

2) keeps muscles firm, healthy, and ready to respond

F) types of contractions1) isometric contraction

a) muscle does not change in lengthb) tension increases

Muscles & Muscle Tissue2) isotonic contraction

a) muscle length changesb) tension remains constantc) 2 types

1) concentrica) muscle shortens as it contracts

2) eccentrica) muscle lengthens as it contracts

Muscles & Muscle TissueJ. Muscle Fiber Types

1. Slow (Red) Oxidative FibersA) rely on aerobic metabolism

1) more myoglobin2) more capillaries3) more mitochondria

B) long, slow contractionsC) fatigue resistant

Muscles & Muscle Tissue2. Fast (White) Glycolytic Fibers

A) rely on anaerobic metabolism1) less myoglobin2) fewer capillaries3) fewer mitochondria

B) rapid, powerful contractionsC) fatigue easily

Muscles & Muscle Tissue3. Fast (Pink) Oxidative Fibers

A) similar to red fibers except:1) faster contractions2) can use anaerobic metabolism3) fatigues more easily than red fibers but not as easily as white fibers

Muscles & Muscle TissueK. Benefits of Exercise

1. increases in fiber size and strength2. increased muscle tone3. increase in blood supply, therefore

increased RBCs4. increased cardiovascular & respiratory

function5. lowers BP

Muscles & Muscle TissueL. Smooth Muscle Contraction

1. Same principle as skeletal muscle but a slower and longer sustained contraction

2. Differences include:A) no T-tubules or sarcomeresB) Ca++ from SR & ECF C) no troponin-tropomyosin complex

Muscles & Muscle Tissue1) calmodulin instead of troponin2) myosin light chain kinase instead of

ATPaseD) contracts in response to

norepinephrine as well as AChE) contracts in response to certain

hormones (ex. oxytocin)F) contracts in response to stretch

Muscles & Muscle TissueM. Cardiac Muscle Contraction

1. Again, same contractile principle as skeletal except:A) contracts continuouslyB) contracts as a unitC) stimulus is intrinsic, but also neural

and hormonal controlD) Ca++ from SR & ECFE) cannot undergo tetanus

Muscles & Muscle TissueN. Muscular Disorders

1. Myasthenia gravis – characterized by drooping upper eyelids, difficulty swallowing & talking, and generalized muscle weaknessA) results from loss of ACh receptors

2. Rigor mortis – muscle stiffness following deathA) results from lack of ATP to break the

myosin cross-bridges

Muscles & Muscle Tissue3. Atrophy – loss of muscle mass

A) results from immobilization or loss of neural stimulation

B) occurs to a small extent from non-use of healthy muscles

4. Muscular dystrophy – a group of inherited muscle destroying diseasesA) muscles enlarge due to fat and CT

deposits while muscles fibers atrophy