I need help with Anatomy Class

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PowerPoint® Lecture Slides prepared by Barbara Heard, Atlantic Cape Community College

C H A P T E R

Muscles and Muscle Tissue: Part A

Muscle Tissue

Nearly half of body's mass
Transforms chemical energy (ATP) to directed mechanical energy  exerts force
Three types
Skeletal
Cardiac
Smooth
Myo, mys, and sarco - prefixes for muscle

Types of Muscle Tissue

Skeletal muscles
Organs attached to bones and skin
Elongated cells called muscle fibers
Striated (striped)
Voluntary (i.e., conscious control)
Contract rapidly; tire easily; powerful
Require nervous system stimulation

Types of Muscle Tissue

Cardiac muscle
Only in heart; bulk of heart walls
Striated
Can contract without nervous system stimulation
Involuntary

Types of Muscle Tissue

Smooth muscle
In walls of hollow organs, e.g., stomach, urinary bladder, and airways
Not striated
Can contract without nervous system stimulation
Involuntary

Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4)

Special Characteristics of Muscle Tissue

Excitability (responsiveness): ability to receive and respond to stimuli
Contractility: ability to shorten forcibly when stimulated
Extensibility: ability to be stretched
Elasticity: ability to recoil to resting length

Muscle Functions

Four important functions
Movement of bones or fluids (e.g., blood)
Maintaining posture and body position
Stabilizing joints
Heat generation (especially skeletal muscle)
Additional functions
Protects organs, forms valves, controls pupil size, causes "goosebumps"

Skeletal Muscle

Connective tissue sheaths of skeletal muscle
Support cells; reinforce whole muscle
External to internal
Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia
Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)
Endomysium: fine areolar connective tissue surrounding each muscle fiber

Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium.

Bone

Tendon

Epimysium

Perimysium

Endomysium

Muscle fiber

in middle of

a fascicle

Blood vessel

Perimysium

wrapping a fascicle

Endomysium

(between individual

muscle fibers)

Muscle

fiber

Perimysium

Fascicle

Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3)

Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3)

Table 9.1 Structure and Organizational Levels of Skeletal Muscle (3 of 3)

Microscopic Anatomy of a Skeletal Muscle Fiber

Long, cylindrical cell
10 to 100 µm in diameter; up to 30 cm long
Multiple peripheral nuclei
Sarcolemma = plasma membrane
Sarcoplasm = cytoplasm
Glycosomes for glycogen storage, myoglobin for O2 storage
Modified structures: myofibrils, sarcoplasmic reticulum, and T tubules

Myofibrils

Densely packed, rodlike elements
~80% of cell volume
Contain sarcomeres - contractile units
Sarcomeres contain myofilaments
Exhibit striations - perfectly aligned repeating series of dark A bands and light I bands

Figure 9.2b Microscopic anatomy of a skeletal muscle fiber.

Diagram of part of a muscle fiber showing

the myofibrils. One myofibril extends from the cut end of the fiber.

Sarcolemma

Mitochondrion

Myofibril

Nucleus

Light

I band

Dark

A band

Striations

H zone: lighter region in midsection of dark A band where filaments do not overlap
M line: line of protein myomesin bisects H zone
Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors thin filaments and connects myofibrils to one another
Thick filaments: run entire length of an A band
Thin filaments: run length of I band and partway into A band
Sarcomere: region between two successive Z discs

Sarcomere

Smallest contractile unit (functional unit) of muscle fiber
Align along myofibril like boxcars of train
Contains A band with ½ I band at each end
Composed of thick and thin myofilaments made of contractile proteins

Figure 9.2c Microscopic anatomy of a skeletal muscle fiber.

Small part of one

myofibril

enlarged to show

the myofilaments

responsible for the

banding pattern.

Each sarcomere

extends from one Z

disc to the next.

Thin (actin)

filament

Z disc

H zone

Z disc

Thick

(myosin)

filament

I band

A band

I band

M line

Sarcomere

Figure 9.2d Microscopic anatomy of a skeletal muscle fiber.

Enlargement of

one sarcomere (sectioned length-

wise). Notice the

myosin heads on

the thick filaments.

Z disc

Sarcomere

M line

Z disc

Thin

(actin)

filament

Elastic

(titin)

filaments

Thick

(myosin)

filament

Myofibril Banding Pattern

Orderly arrangement of actin and myosin myofilaments within sarcomere
Actin myofilaments = thin filaments
Extend across I band and partway in A band
Anchored to Z discs
Myosin myofilaments = thick filaments
Extend length of A band
Connected at M line

Ultrastructure of Thick Filament

Composed of protein myosin
Each composed of 2 heavy and four light polypeptide chains
Myosin tails contain 2 interwoven, heavy polypeptide chains
Myosin heads contain 2 smaller, light polypeptide chains that act as cross bridges during contraction
Binding sites for actin of thin filaments
Binding sites for ATP
ATPase enzymes

Ultrastructure of Thin Filament

Twisted double strand of fibrous protein
F actin
F actin consists of G (globular) actin subunits
G actin bears active sites for myosin head attachment during contraction
Tropomyosin and troponin - regulatory proteins bound to actin

Longitudinal section of filaments within one

sarcomere of a myofibril

Thick filament

Thin filament

In the center of the sarcomere, the thick filaments

lack myosin heads. Myosin heads are present only

in areas of myosin-actin overlap.

Thick filament.

Thin filament

Each thick filament consists of many myosin

molecules whose heads protrude at opposite�ends of the filament.

A thin filament consists of two strands of actin

subunits twisted into a helix plus two types of

regulatory proteins (troponin and tropomyosin).

Portion of a thick filament

Portion of a thin filament

Myosin head

Tropomyosin

Troponin

Actin

Actin-binding sites

ATP-

binding

site

Heads

Tail

Flexible hinge region

Myosin molecule

Actin subunits

Active sites

for myosin

attachment

Figure 9.3 Composition of thick and thin filaments.

Structure of Myofibril

Elastic filament
Composed of protein titin
Holds thick filaments in place; helps recoil after stretch; resists excessive stretching
Dystrophin
Links thin filaments to proteins of sarcolemma
Nebulin, myomesin, C proteins bind filaments or sarcomeres together; maintain alignment

Sarcoplasmic Reticulum (SR)

Network of smooth endoplasmic reticulum surrounding each myofibril
Most run longitudinally
Pairs of terminal cisterns form perpendicular cross channels
Functions in regulation of intracellular Ca2+ levels
Stores and releases Ca2+

T Tubules

Continuations of sarcolemma
Lumen continuous with extracellular space
Increase muscle fiber's surface area
Penetrate cell's interior at each A band–I band junction
Associate with paired terminal cisterns to form triads that encircle each sarcomere

Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle.

Part of a skeletal

muscle fiber (cell)

Myofibril

Sarcolemma

I band

A band

I band

Z disc

H zone

Z disc

line

Sarcolemma

Triad:

• T tubule

• Terminal

cisterns of

the SR (2)

Tubules of

the SR

Myofibrils

Mitochondria

Triad Relationships

T tubules conduct impulses deep into muscle fiber; every sarcomere
Integral proteins protrude into intermembrane space from T tubule and SR cistern membranes and connect with each other
T tubule integral proteins act as voltage sensors and change shape in response to voltage changes
SR integral proteins are channels that release Ca2+ from SR cisterns when voltage sensors change shape

Sliding Filament Model of Contraction

Generation of force
Does not necessarily cause shortening of fiber
Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening

Sliding Filament Model of Contraction

In relaxed state, thin and thick filaments overlap only at ends of A band
Sliding filament model of contraction
During contraction, thin filaments slide past thick filaments  actin and myosin overlap more
Occurs when myosin heads bind to actin  cross bridges

Sliding Filament Model of Contraction

Myosin heads bind to actin; sliding begins
Cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere
Causes shortening of muscle fiber
Pulls Z discs toward M line
I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same)

Figure 9.6 Sliding filament model of contraction.

Slide 2

Fully relaxed sarcomere of a muscle fiber

Figure 9.6 Sliding filament model of contraction.

Slide 3

Fully contracted sarcomere of a muscle fiber

The Nerve Stimulus and Events at the Neuromuscular Junction

Skeletal muscles stimulated by somatic motor neurons
Axons of motor neurons travel from central nervous system via nerves to skeletal muscle
Each axon forms several branches as it enters muscle
Each axon ending forms neuromuscular junction with single muscle fiber
Usually only one per muscle fiber

Events of Excitation-Contraction (E-C) Coupling

AP propagated along sarcomere to
T tubules
Voltage-sensitive proteins stimulate Ca2+ release from SR
Ca2+ necessary for contraction

Role of Calcium (Ca2+) in Contraction

At low intracellular Ca2+ concentration
Tropomyosin blocks active sites on actin
Myosin heads cannot attach to actin
Muscle fiber relaxed

Role of Calcium (Ca2+) in Contraction

At higher intracellular Ca2+ concentrations
Ca2+ binds to troponin
Troponin changes shape and moves tropomyosin away from myosin-binding sites
Myosin heads bind to actin, causing sarcomere shortening and muscle contraction
When nervous stimulation ceases, Ca2+ pumped back into SR and contraction ends

Cross Bridge Cycle

Continues as long as Ca2+ signal and adequate ATP present
Cross bridge formation—high-energy myosin head attaches to thin filament
Working (power) stroke—myosin head pivots and pulls thin filament toward M line

Cross Bridge Cycle

Cross bridge detachment—ATP attaches to myosin head and cross bridge detaches
"Cocking" of myosin head—energy from hydrolysis of ATP cocks myosin head into high-energy state

Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.

Slide 1

Actin

Ca2+

Thin filament

Myosin

cross bridge

Thick

filament

Myosin

ATP

hydrolysis

In the absence of ATP, myosin heads will not detach, causing rigor mortis.

*This cycle will continue as long

as ATP is available and Ca2+ is

bound to troponin.

Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming

a cross bridge.

Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. *

Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”).

The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent

low-energy state. As a result it pulls the actin filament toward the M line.

Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.

Slide 2

Actin

Thin filament

Myosin

cross bridge

Thick

filament

Myosin

Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge.

Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.

Slide 3

The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result it pulls the actin filament toward the M line.

Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.

Slide 4

Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”).

Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.

Slide 5

Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. *

ATP

hydrolysis

*This cycle will continue as long

as ATP is available and Ca2+ is

bound to troponin.

Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.

Slide 6

A&P Flix™: The Cross Bridge Cycle

PLAY

Actin

Ca2+

Thin filament

Myosin

cross bridge

Thick

filament

Myosin

ATP

hydrolysis

In the absence of ATP, myosin heads will not detach, causing rigor mortis.

*This cycle will continue as long

as ATP is available and Ca2+ is

bound to troponin.

Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming

a cross bridge.

Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. *

Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”).

The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent

low-energy state. As a result it pulls the actin filament toward the M line.

Homeostatic Imbalance

Rigor mortis
Cross bridge detachment requires ATP
3–4 hours after death muscles begin to stiffen with weak rigidity at 12 hours post mortem
Dying cells take in calcium  cross bridge formation
No ATP generated to break cross bridges