lemastm/Teaching/BI234/Chapter 09... · Muscle Tissue Muscle – “little ... Involuntary controlVoluntary control Striated / uni-nucleated : ... Lines visceral organs Involuntary

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Chapter 9:

Muscle Tissue

Muscle – “little mouse”

Types of Muscle Tissue:

Chapter 9: Muscular System

Characteristics:

Composes heart wall

Involuntary control

Striated / uni-nucleated

Characteristics:

Attaches to skeleton

Voluntary control

Striated / multi-nucleated

Characteristics:

Lines visceral organs

Involuntary control

Non-striated / uni-nucleated

Skeletal Muscle Cardiac Muscle Smooth Muscle

Characteristics of Muscle:

1) Excitability: Ability to respond to stimulation

2) Contractility: Ability to shorten forcefully

3) Extensibility: Ability to stretch and still contract

4) Elasticity: Ability to resume resting length after contraction

Function of Muscle:

Prune Belly

Syndrome

4) Guard entrance / exit (e.g., lips / anus)

3) Support soft tissue (e.g., abdominal wall)

2) Maintain posture / body position

• Locomotion / manipulation (skeletal)

• Blood pressure (cardiac)

• Propulsion (smooth)

1) Produce movement

6) Store nutrients (e.g., glycogen)

5) Maintain body temperature (e.g., shivering)


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Gross Anatomy of Muscle: Connective Tissue Layers:

• Outside muscle covering

• Form tendons

1) Epimysium:

• Divides muscle into fascicles

Compartments

• Contains blood vessels / nerves

2) Perimysium:

• Surrounds individual muscle fibers

and ties them together

3) Endomysium:

Connective Components:

A) Tendons (cords)

B) Aponeuroses (sheets)

C) Fascia (wrap / bind body)

• Satellite cells (stem cells)

Endomysium

Perimysium

Epimysium

Marieb & Hoehn – Figure 9.1


Similar to Marieb & Hoehn – Figure 9.5

Microanatomy of Muscle:

Sarcolemma: Cell membrane

Transverse Tubules: Network of passageways through fiber

• Continuous with outside of cell

Sacroplasmic Reticulum: Specialized endoplasmic reticulum

• Contain calcium ions (Ca++) – [40,000x] compared to cytoplasm

Muscle Fiber (cell):

Functional Unit of Muscle

Sarcoplasma: Cytoplasm

Triad


Myofibrils: Cylindrical structures containing contractile fibers

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Myofibrils contain myofilaments (protein):

1) Actin (Thin filament)

2) Myosin (Thick filament)

Sarcomere: Repeating units of myofilaments

Myosin Actin

Sarcomere

Z line M line I Band

A Band

(~ 10,000 / myfilament)





Interactions between the thick and thin filaments of sarcomeres are

responsible for muscle contraction


Sliding Filament

Theory

Thick filaments:

• Composed of many myosin molecules

Tails: Attach molecules together

Heads: Bind with thin filament

Hinge


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Sliding Filament

Theory

Thick filaments:

• Composed of many myosin molecules

Tails: Attach molecules together

Heads: Bind with thin filament

Thin filaments:

• Composed of interwoven actin molecules

Tropomyosin: Cover active sites

Troponin: Bind tropomyosin to actin





Sliding Filament

Theory


Sarcoplasmic Reticulum

Ca++ Ca++ Ca++

Ca++ Ca++ Ca++ Ca++ Ca++

Ca++ Ca++ Ca++ Ca++

Neuromuscular Junction:

• Neuron Muscle fiber

• 1 connection / muscle fiber

Sarcolemma

Sarcoplasm

T-tubule

Synaptic

Knob

Neuron

Motor End Plate

Muscle Contraction Events:

1) Acetylcholine (ACh - neurotransmitter)

released from synaptic knob

ACh ACh ACh

2) ACh binds to receptors on motor end

plate; generates action potential

3) Action potential (AP - electrical impulse)

conducted along sarcolemma

Sarcomere of

myofibril


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Sarcoplasmic Reticulum

Ca++ Ca++ Ca++


Ca++ Ca++ Ca++ Ca++

Neuromuscular Junction:

• Neuron Muscle fiber

• 1 connection / muscle fiber

Sarcolemma

Sarcoplasm

T-tubule

Synaptic

Knob

Neuron

Motor End Plate

ACh ACh ACh

4) AP descends into muscle fiber via

T-tubules


5) AP triggers release of Ca++ from

sarcoplasmic reticulum

6) Ca++ initiates cross-bridging (actin / myosin) Sarcomere of

myofibril


Muscle Contraction Events:

1) Ca++ binds with troponin; exposes

active sites on actin

Cross-Bridging in Action:


Cross-bridging Events:

Troponin

Actin

Tropomyosin

Myosin

Head

Ca++

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2) Myosin head (cocked) binds with

active site

3) Myosin head pivots – pulls actin

forward






active site


forward

4) ATP binds to myosin head; head

detaches and re-cocks




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ATP

ATP ATP

ADP

P

Re-cock




active site


forward



5) Myosin head binds to active site;

Process repeated




ATP

ATP

ADP

P

Re-cock

ATP

ADP

P

Re-cock ATP

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active site


forward



5) Myosin head binds to active site;

Process repeated

6) Process ends when APs cease

• Ca++ returned to sarcoplasmic

reticulum (active transport)

• ACh broken down by

acetylcholinesterase (AChE)




Martini & Nath – Table 10.1

• Rigor Mortis • Tetanus


What Regulates Muscle Tension Production?

A) Single Fiber:

• Tension Production = # of cross-bridge attachments

Muscle Tension: Force exerted on an object by a contacting muscle

Muscle Mechanics:

• All-or-none response (muscle “on” or “off”)

Too contracted = no room for movement; poor

cross-bridge formation

Too stretched = no cross-bridge

formation

Resting length = # of cross-bridges;

distance to slide

(maximal muscle force;

normal resting length)


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Muscle Mechanics:

B) Whole Muscle:

• Tension Production = 1) Frequency of stimulation

2) # of muscle fibers activated




Muscle Mechanics:

B) Whole Muscle – Frequency of stimulation:

Twitch = Single stimulus-contraction-relaxation sequence

Ten

sio

n

Time

Stim

ulu

s

Latent Period:

Time between stimulus

and tension development

LP

Ca++ release;

cross-bridge formation

Ca++ uptake;

cross-bridge detachment

CP

Contraction Phase:

Period where tension

rises to peak level

RP

Relaxation Phase:

Period where tension

falls to resting level




Muscle Mechanics:


Twitch = Single stimulus-contraction-relaxation sequence

Martini & Nath – Figure 10.15

Twitches alone are not

a useful contraction

Variation exists in the

duration of twitches

among muscles


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Muscle Mechanics:


Incomplete Tetanus: Rapid cycles of contraction & relaxation Ten

sio

n

Time

Stim

ulu

s

Maximum

Tension

Summation:

Addition of twitches to

produce a more powerful

contraction




Muscle Mechanics:


Ten

sio

n

Time

Complete Tetanus: Rapid stimulation erases relaxation phase

Stim

ulu

s

Maximum

Tension

SR can not reclaim Ca++

(stimulation too rapid)

Most normal muscle contraction

involves complete tetanus




Muscle Mechanics:

B) Whole Muscle – Number of muscle fibers activated:

Motor Unit: A single motor neuron and all the muscle fibers innervated by it

All-or-none

response

Recruitment:

Addition of motor units to produce

smooth, steady muscle tension (small large motor units)

Muscle Tone:

Resting tension maintained in muscle

Motor unit size dictates control:

• Fine Control = 1-5 fibers / MU (e.g., eye)

• Gross Control = 1000’s fibers / MU (e.g., leg)



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Types of Muscle Contractions:

Isometric Contraction:

(e.g., pushing against wall / standing)

Tension maximized; no muscle shortening

Isotonic Contraction:

Tension stabilized; muscle shortens

(e.g., lifting / walking)


Muscle Elongation (following contraction):

• Passive process:

1) Elastic rebound (fibers / organelles)

2) Antagonistic muscles (e.g., biceps brachii vs. triceps brachii)


• Muscle fibers use ATP for contraction (~ 600 trillion / second)

How does a fiber replenish ATP?

1) Creatine phosphate (CP) reserves: high energy

• Nitrogen-containing compound bound to phosphate

ATP

ADP CP ADP

ATP

Converted to

creatine

Creatine

kinase

• Limited supply

• Replaced by aerobic / anaerobic respiration

• Only enough reserve to sustain short contraction periods (~ 5 sec)

• Must be replenished to sustain contraction

Muscle Energetics:


ATP ADP + P

Creatine

Phosphate

15 seconds


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ATP

ADP ADP

ATP

• Only enough reserve to sustain short contraction (~ 2 sec)


Muscle Energetics:

2) Anaerobic Metabolism (Glycolysis):

Glucose

Pyruvate

Lactate

(Glycogen) • Primary source of ATP during times of peak activity

• Inefficient (2 ATP / glucose)

• Acid buildup leads rapidly to muscle fatigue


ATP ADP + P

Creatine

Phosphate

15 seconds

Anaerobic

Respiration

1 - 2 minutes




ATP

ADP ADP

ATP

• Only enough reserve to sustain short contraction (~ 2 sec)


Muscle Energetics:

Glucose

Pyruvate

CO2 + H2O

(Glycogen)

3) Aerobic Metabolism (Aerobic Respiration):

• Primary source of ATP during periods of rest (95% of ATP)

• Efficient (36 ATP / glucose)

• Can use multiple energy sources (carbohydrates, lipids, proteins)


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ATP ADP + P

Creatine

Phosphate

15 seconds

Anaerobic

Respiration

1 - 2 minutes Weeks

Aerobic

Respiration


Energy Use at Different Levels of Activity:

• Aerobic metabolism predominates

• Fuel = Fatty acids

• ATP surplus: 1) Glycogen reserves

2) Recharge CP

At Rest:



• Aerobic metabolism predominates

• Fuel = Glycogen

• ATP required for muscle contraction

At Moderate Activity:


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• Anaerobic metabolism predominates

• Fuel = Glycogen

• ATP required for muscle contraction

• ATP acquired from CP (initially)

At Peak Activity:


Muscle Fatigue: Fibers lose ability to contract despite neural stimulation

Decoupling of electrical

signaling and muscle contraction

Low Intensity Activity:

Fatigue = Energy reserves depleted

Energy reserves include

glycogen, lipids, proteins

Muscle Recovery: Return of fibers to pre-activity conditions

A) Lactate removed (blood liver) or recycled (glycogen)

B) Aerobic respiration replenishes energy reserves

• Oxygen debt: Amount of oxygen needed to restore body to

pre-activity conditions

• Muscle fibers, liver, sweat glands

C) Excess heat is lost via blood flow to skin

Process can take days to weeks…

High Intensity Activity:

Fatigue = Ionic imbalances


Muscle Performance: Power:

Maximum amount of tension

produced by a muscle

Endurance: Amount of time a muscle

can perform an activity

Factors Determining Muscle Performance:

1) Muscle Fiber Type:

vs.

• Contract in 0.01 s or less (after stimulation)

• Large diameter (densely packed myofibrils)

• Rely on anaerobic respiration

• Large glycogen reserves

• Fatigue rapidly

A) Fast Fibers (most common)

• 3x slower contract; ½ the diameter

• Powerful ( sarcomere # = tension)

• Sustained contractions ( endurance)

• Rely on aerobic metabolism:

• Extensive capillaries ( O2 supply)

• Myoglobin: Binds O2 ( O2 storage)

• Large # of mitochondria ( O2 usage)

B) Slow Fibers


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The distribution of muscle fibers impacts muscle performance:

White Muscle:

Muscle dominated by fast fibers (e.g., turkey breast)

Red Muscle:

Muscle dominated by slow fibers (e.g., turkey leg)

(Genetically determined)

Human muscles a mixture of the

two fiber types

Fast Fibers Slow Fibers

Marathoners 18% 82%

Avg. Human 55% 45%

Sprinters 64% 37%


Physical Conditioning:

1) Anaerobic Endurance

• Sustained, powerful muscle contractions (e.g., weight lifting)

• Training = Frequent, brief, intense workouts

• Muscle Hypertrophy: Enlargement of muscle

2) Aerobic Endurance

• Continual contraction of muscle over time (e.g., jogging)

• Training = Sustained low levels of muscle activity

• muscle fiber size, not overall #

• mitochondria; capillaries


Cardiac Muscle: Skeletal Muscle: Smooth Muscle: Property

Filament

Organization

Control

Mechanism

Calcium

Source

Contraction

Energy Source

Sarcomeres along

myofibrils

Sarcomeres along

myofibrils Scattered in

sarcoplasm

Neural Automaticity (pacemaker cells)

Automaticity,

neural, hormonal

Sarcoplasmic

reticulum

SR / across

sarcolemma

Across

sarcolemma

Rapid onset;

tetanus can occur;

rapid fatigue

Slower onset;

no tetanus;

fatigue-resistant

Slow onset;

tetanus can occur;

fatigue-resistant

Aerobic / Anaerobic

metabolism

Aerobic

metabolism

Aerobic

metabolism

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lemastm/Teaching/BI234/Chapter 09... · Muscle Tissue Muscle – “little ... Involuntary controlVoluntary control Striated / uni-nucleated : ... Lines visceral organs Involuntary