BIO 210 CH.11 PHYS.OF THE MUSCULAR SYSTEM:
SUPPLEMENT 1
I.
GENERAL FUNCTIONS
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A.
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B.
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C.
The Muscular System has 3 General Functions
MOVEMENT
Movement is the Primary Function of the Muscular System; When Skeletal Muscles Contract, This
Usually Produces Movement(Bones and Joints Assist)
HEAT PRODUCTION
Skeletal Muscles Produce the Majority of the Body’s Heat Because They are So Active and So
Numerous (Heat Produced in Cells By Nutrient Catabolism)
POSTURE
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Skeletal Muscles Are Responsible For Posture
Posture = Body Position
Good Posture = The Position That Best Favors Function (Places the Least Strain on Body
Structures)
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Good Posture is Important for Optimal Body Function
II.
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A.
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1.
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2.
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3.
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FUNCTION OF SKELETAL MUSCLE TISSUE
Skeletal Muscle Tissue is the Major Tissue That Composes Skeletal Muscles
CHARACTERISTICS OF SKELETAL MUSCLE CELLS (FIBERS)
Skeletal Muscle Tissue is Composed of Skeletal Muscle Fibers (Cells) Arranged in Fascicles
Characteristics of SMF’s = Special Properties of SMF’s That Make These Cells unique and Able to
Function As They Do
EXCITABILITY (IRRITABILITY)
The Ability to Be Stimulated
Important Because Before Contraction of Skeletal Muscles Can Occur, Excitation Must Occur (SMF’s
Must Be Stimulated by A Nerve Impulse)
CONTRACTILITY
The Ability to Contract (Shorten)
Important Because Contraction Usually Produces Movement
EXTENSIBILITY
The Ability to Extend (Stretch Out)
Important Because Extensibility Allows SMF’s to Return to Resting Length After Contraction
B.
OVERVIEW OF THE SKELETAL MUSCLE FIBER
1.
SARCOLEMMA
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Plasma Membrane of a SMF
2.
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SARCOPLASM
Cytoplasm of a SMF
3.
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SARCOPLASMIC RETICULUM
Endoplasmic Reticulum of a SMF
Similar, not Identical to ER
SR Contains Calcium Pumps That Pump Ca++ From Sarcoplasm into SR for Storage
4.
MANY MITOCHONDRIA
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5.
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SEVERAL NUCLEI
Each SMF Contains Several Nuclei, Located Near Sarcolemma
Unusual
Reason: SMF’s Complex in Structure & Function
6.
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Each SMF Contains Many Mitochondria
Reason: Active in Catabolism
MYOFIBRILS
Each SMF Contains Numerous Myofibrils
Microscopic Threadlike Fibers that Extend the Length of the Muscle Fiber
7.
MYOFILAMENTS
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Myofibrils Composed of Myofilaments
UltraMicroscopic Threadlike Fibers
2 Types
a. THICK: MYOSIN
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Composed of One Kind of Protein Molecule - Myosin
Shape: Golf Clubs
b. THIN: ACTIN, TROPOMYOSIN, TROPONIN
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8.
Composed of Three Kinds of Protein Molecules - Actin, Tropomyosin, Troponin
Shapes:
1) Actin: 2 Twisted Strands of Beads
2) Tropomyosin: Long and Narrow
3) Troponin: Oval
Myosin (Cross Bridges) Are Chemically Attracted to the Active Sites of Actin
When SMF Resting:
Tropomyosin Covers the Active Sites of Actin and Troponin Holds Tropomyosin in Place
SARCOMERES
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Microscopic Contracting Units of SMF’s
Each Myofibril Composed of Numerous Sarcomeres Lined Up End to End (Extend the Length
of Myofibrils)
Each Sarcomere Consists of Thick and Thin Myofilaments in a Specific Arrangement
(Myofibrils Composed of Myofilaments; However Myofilaments Are Arranged into
sarcomeres – Contracting Units)
Arrangement of Thick and Thin Myofilaments in Sarcomere
1) Thick Myofilaments Stacked Toward Center of Sarcomere
2) Thin Myofilaments Stacked Toward Edges of Sarcomere
3) Thick and Thin Myofilaments Alternate
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9.
Myofibrils and Myofilaments are Part of the Cytoskeleton of a SMF
The Arrangement of Myofilaments in Sarcomeres Cross-Striations
T TUBULES
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10.
Transverse Tubules
Inward Extensions of Sarcolemma that Extend Transversely Across the Sarcoplasm
TRIAD
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“Three Structures”
T Tubule Sandwiched Between Sarcoplasmic Reticulum on Either Side
Allows NI’s Traveling Along Sarcolemma toMove Inside the SMF and Easily Stimulate
Sarcoplasmic Reticulum
C.
THE MECHANISM OF CONTRACTION
1.
EXCITATION: SMF STIMULATED BY NI
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NI Travels Along the plasma Membrane of Neuron
When NI Reaches the End of the Neuron’s P. Membrane, Neurotransmitters are Released and Cross
the Synapse
Neurotransmitters Bind to Receptors in Sarcolemma, NI Begins Along Sarcolemma
NI Travels Along Sarcolemma, Moves Inward at T Tubules, Stimulates Sarcoplasmic Reticulum
2.
CONTRACTION (SLIDING FILAMENT THEORY)
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Ca++ Ions Released From SR
Ca++ Ions Bind to Troponin - Exposes Actin’s Active Sites Myosin Cross Bridges Become Energized
(Get Energy From ATP)
 Myosin Cross Bridges Bind to Actin’s Active Sites, Bend Upward, and Pull the Thin Myofilaments
Toward the Center of Each Sarcomere (Shortens Sarcomeres)
 Myosin Cross Bridges Release From Actin, Get Energized, Bind to the Next Active Sites, And Pull
Again (Shortens Sarcomeres)
 Previous Step Repeats (As Long As Actin’s Active Sites Are Exposed and ATP is Available)
*End Result: Sarcomeres Shorten, SMF Shortens
3.
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RELAXATION
SR Pumps (by Active Transport) Ca++ Back into Sacs for Storage à Ca++ Stripped From Troponin
Troponin Reattaches to Tropomyosin - Actin’s Active Sites Become Covered
Contraction Automatically Shuts Off (Actin’s Active Sites Covered)
Sarcomeres Extend, SMF Extends (D/T Extensibility)
D.
THE ALL-OR-NONE PRINCIPLE
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SMF’s Follow the All-Or-None Principle
All-Or-None Principle
 When Stimulated, SMF’s Either Contract With All the Force Possible Under Existing
Conditions Or They Don’t Contract At All
 Explanation of All-Or-None Principle
1) When Stimulated, SMF’s Either Contract Or They Don’t Contract (Depends upon the
Strength/Frequencyof the Nerve Impulse)
2) If SMF’s Contract, They Always Contract as Forcefully as Possible;However, Existing
Conditions Determine the Actual Strength of Contraction
III.
FUNCTION OF SKELETAL MUSCLE ORGANS
A. MOTOR UNIT
 Definition: A Motor Neuron + the SMF’s to Which It’s Attached
 The Number of SMF’s to Which a Motor Neuron Attaches Varies
 Generalizations
 The Fewer the Number of SMF’s to Which a Motor Neuron Attaches, the More Precise the
Movements (i.e., Muscles Responsible For Finger Movements)
 The Larger the Number of SMF’s to Which a Motor Neuron Attaches, the More
Powerful the Contraction (i.e., Abdominal Muscles)
B. BASIC PRINCIPLES OF CONTRACTION
1. SKELETAL MUSCLES CONTRACT ONLY IF STIMULATED
2. SKELETAL MUSCLES PRODUCE MOVEMENTS BY PULLING ON BONES
3. SKELETAL MUSCLES USUALLY LIE PROXIMAL TO THE BODY PART THEY MOVE
4. SKELETAL MUSCLES USUALLY WORK IN GROUPS RATHER THAN ALONE
5. SKELETAL MUSCLE CONTRACTIONS ARE OF SEVERAL TYPES
a. TONIC CONTRACTION (MUSCLE TONE)
 Continued, Partial Contraction
 Explanation:
 Continued as Long as One is Awake and Conscious
 Partial Contraction: Only a Few of the Total SMF’s in a Muscle Contract at the Same
Time
 SMF’s Contract in Relays
 Controlled by Negative Feedback
 Results in Firmness (Tone)in a Muscle Rather Than Recognizable Movement
 Tonic Contractions Are Important in Maintaining Posture
b. ISOTONIC CONTRACTION
 Contraction in Which the Tone Within a Muscle Remains the Same, But the Length of the
Muscle Changes (Muscle Shortens) Occurs When Little/No Load Placed On a Muscle (All the
Energy of Contraction is Used to Shorten Sarcomeres)
c. ISOMETRIC CONTRACTION
 Contraction in Which the Length of a Muscle Remains the Same, But the Tone Within the
Muscle Changes (Tone Increases)
 Occurs When Too Much Load is Placed On a Muscle (All the Energy Of Contraction is Used
to Try and Move the Load)
*NOTE: Most Body Movements Are a Combination of Both Isotonic and Isometric Contractions
*NOTE: Remaining Contractions Are Experimental Contractions (Can Be Induced In the Lab, But Don’t
Occur in the Body the Way They Occur in the Lab); However, Principles Have Been Developed From
Them That Can Be Applied to Muscles in the Body
d. TWITCH CONTRACTION
 Quick Jerk of a Muscle That Occurs When An Entire Muscle (All Motor Units Are)
Stimulated Simultaneously
 Similar Situation Occurs in the Body Under Abnormal Conditions – Seen as Muscle
Twitches/Spasms/Cramps (May Be D/T Muscle Irritation/Inflammation, Fluid/Electrolyte
Imbalances, Overactivity of the Nervous System,etc.)
e. TREPPE (STAIRCASE PHENOMENON)
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f.
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A Gradual, Steplike Increase in the Strength of Muscle Contraction
Occurs When an Entire Muscle is Repeatedly Stimulated at Brief, Constant Intervals
Generalization That Developed From Treppe:
A Muscle Contracts More Forcefully After It has Contracted a Few Times
Application: Athletes “Warm Up” Before Beginning an Athletic Event
If Treppe Continues:
 Strength of Muscle Contraction Gradually Decreases
 Muscle Exhibits Fatigue (Physiological in Lab, Psychological in Body)
TETANIC CONTRACTION (TETANUS)
Smooth, Sustained Contraction That Occurs When An Entire Muscle is Repeatedly Stimulated
in Rapid Succession (Muscle Doesn’t Have Time to Relax Before the Next Contraction,
Creates a Series of Twitches Close Together)
This Type of Contraction is Exhibited By Most Skeletal Muscles Most of the Time, However
is Carefully Co-ordinated
6. SKELETAL MUSCLES CONTRACT ACCORDING TO THE GRADED STRENGTH PRINCIPLE
a. DEFINITION – GRADED STRENGTH PRINCIPLE
 Skeletal Muscles Contract With Varying Degrees of Strength At Different Times
b. FACTORS THAT AFFECT GRADED STRENGTH PRINCIPLE
1. AMOUNT OF LOAD IMPOSED ON THE MUSCLE (STRETCH REFLEX)
 Within Limits, the More Load That’s Placed On a Muscle, The Stronger the
Contraction D/T Stretch Reflex (Neg. FB)
2. NUMBER OF SKELETAL MUSCLE FIBERS CONTACTING SIMUTANEOUSLY
 The More SMF’s That Contract At the Same Time the Stronger The Contraction
 Depends On:
a. NUMBER OF MOTOR UNITS RECRUITED (Activated) Which in Turn Depends On:
b. INTENSITY AND FREQUENCY OF STIMULATION (How Strong/How Often the
Nerve Impulses)
*The More Intense and Frequent the Stimulationà Means More Motor Units That Are Activated Means More SMF’s Contracting At The Same Time - Means a Stronger Contraction
3. METABOLIC CONDITION OF SKELETAL MUSCLE FIBERS
 ATP Available to SMF’s
 Since a Skeletal Muscle is Composed of SMF’s, Amount of ATP Available to SMF’s
Affects Strength of Contraction
4. INITIAL LENGTH OF SKELETAL MUSCLE FIBERS (LENGTH/TENSION RELATIONSHIP)
 SMF’s Must Be of Optimal Length At the Beginning of The Contraction to Produce The
Strongest Contraction