Dr. B Ch 09_lecture_presentation

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9The Muscular System: Skeletal Muscle Tissue and Organization

PowerPoint® Lecture Presentations prepared bySteven BassettSoutheast Community College Lincoln, Nebraska


Introduction

There are three types of muscle tissue: Skeletal muscle—Skeletal muscle tissue moves the body by pulling on

bones of the skeleton. Skeletal muscle fibers arise from embryonic cells called myoblasts. Work voluntarily. Striated microscopic pattern.

Cardiac muscle—Cardiac muscle tissue pushes blood through the arteries and veins of the circulatory system.

Work involuntarily. Striated microscopic pattern. Intercalated discs. Branchd fibers.

Smooth muscle—Smooth muscle tissues push fluids and solids along the digestive tract and hollow organs and perform varied functions in other systems.

Work involuntarily. Smooth microscopic pattern.


Introduction

Muscle tissues share four basic properties: Excitability: the ability to respond to stimulation

Skeletal muscles normally respond to stimulation by the nervous system.

Cardiac and smooth muscles respond to the nervous system and circulating hormones.

Contractility: the ability to shorten actively and exert a pull or tension that can be harnessed by connective tissues

Extensibility: the ability to continue to contract over a range of resting lengths

Elasticity: the ability of a muscle to rebound toward its original length after a contraction


Functions of Skeletal Muscles

Skeletal muscles are contractile organs directly or indirectly attached to bones of the skeleton.

They work voluntarily. Skeletal muscles perform the following functions:

Produce skeletal movement Maintain posture and body position Support soft tissues and stabilize the joints. Regulate entering and exiting of material Maintain body temperature


Skeletal muscle surrounding

• From inside out:• Sarcolema

• Plasma membrane of muscle fiber.• Endomysium

• Connective tissue surrounding the muscle fiber.• Perimysium

• Connective tissue surrounding the fascicle. Fascicle is a bundle of muscle fibers.

• Epimysium• Connective tissue surrounding the muscle.

• Deep fascia• Connective tissue surrounding the skeletal muscles together.


Anatomy of Skeletal Muscles

• Connective Tissue of Muscle• Tendons and Aponeuroses

• Epimysium, perimysium, and endomysium converge to form tendons

• Tendons connect a muscle to a bone• Aponeuroses connect a muscle to a muscle


Figure 9.1 Structural Organization of Skeletal Muscle

Epimysium

Muscle fascicle

Endomysium

Perimysium

Nerve

Muscle fibers

Blood vessels

SKELETAL MUSCLE(organ)

MUSCLE FASCICLE(bundle of cells)

Perimysium

Muscle fiber

Endomysium

Epimysium

Blood vesselsand nerves

Endomysium

Perimysium

Tendon

MUSCLE FIBER(cell)

Mitochondria

Sarcolemma

Myofibril

AxonSarcoplasm

Capillary

Endomysium

Myosatellitecell

Nucleus


Anatomy of skeletal muscle fiber

• Sarcolemma• The plasma membrane of the muscle fiber.

• Sarcoplasmic reticulum• the ER that stores calcium ions.

• Terminal cisterna• an expanded part of ER next to T tube.

• T tube• an extension of sarcolema into the muscle fiber.

• Triad• The combination of one T tube and two flaked Terminal Cisterna.


Figure 9.3ab The Formation and Structure of a Skeletal Muscle Fiber

Development of askeletal muscle fiber

External appearanceand histological view

Myoblasts

Muscle fibers developthrough the fusion ofmesodermal cellscalled myoblasts.

Myosatellite cell

Nuclei

Immaturemuscle fiber


Figure 9.3b–d The Formation and Structure of a Skeletal Muscle Fiber

External appearanceand histological view

The external organizationof a muscle fiber

Internal organization of a muscle fiber.Note the relationships among myofibrils,sarcoplasmic reticulum, mitochondria,triads, and thick and thin filaments.

Myofibril

Sarcolemma

Sarcoplasm

Nuclei

MUSCLE FIBER

Mitochondria

Sarcolemma

Myofibril

Thin filament

Thick filament

Triad T tubulesSarcoplasmicreticulum

Terminal cisterna

Sarcolemma

Sarcoplasm

Myofibrils


Figure 9.4b Sarcomere Structure

A corresponding view of a sarcomere in a myofibril inthe gastrocnemius muscle of the calf and a diagramshowing the various components of this sarcomere

Z line TitinH band

A bandI band

M lineZone of overlap Thinfilament

Thickfilament

Sarcomere

H band Z line

I band

Z line Zone of overlap M line

Sarcomere

TEM × 64,000

A band


Anatomy of Skeletal Muscles

• Levels of Organization• Skeletal muscles consist of muscle fascicles• Muscle fascicles consist of muscle fibers• Muscle fibers consist of myofibrils• Myofibrils consist of sarcomeres• Sarcomeres consist of myofilaments• Myofilaments are made of actin and myosin


Skeletal muscle

Sarcomere Organization Thick and thin filaments within a myofibril are organized in the

sarcomeres. All of the myofibrils are arranged parallel to the long axis of the

cell, with their sarcomeres lying side by side. A band: the dark area of sarcomere that contains thick filaments. I band: the light area between A bands. H band: the light area within the A band. M line: the dark line within the H band. Z line: the area of a myofibril, where actin filaments attach to one

another.


Skeletal muscle

Thin and Thick Filaments Each thin filament consists of a twisted strand of

several interacting proteins 5–6 nm in diameter and 1 μm in length. Troponin holds the tropomyosin strand in place.

Thick filaments are 10–12 nm in diameter and 1.6 μm in length, making them much larger than thin filaments.


Figure 9.5 Levels of Functional Organization in a Skeletal Muscle FiberSKELETAL MUSCLE

MUSCLE FASCICLE

MUSCLE FIBER

MYOFIBRIL

SARCOMERE

Surrounded by:Epimysium

Contains:Musclefascicles

Surrounded by:Perimysium

Contains:Muscle fibers

Surrounded by:Endomysium

Contains:Myofibrils

Surrounded by:Sarcoplasmicreticulum

Consists of:Sarcomeres(Z line to Z line)

Contains:Thick filaments

Thin filaments

I band A band

Z line M line

H band

Titin Z line


Figure 9.6ab Thin and Thick Filaments

The attachmentof thin filamentsto the Z line

The detailed structure of a thin filament showingthe organization of G actin, troponin, andtropomyosin

Myofibril

Z line M line

H band

Sarcomere

Actinin Z line Titin

Troponin Nebulin TropomyosinActive

siteG actin

molecules

F actinstrand


Figure 9.6cd Thin and Thick Filaments

Myofibril

Z line M line

H band

Sarcomere

A single myosin molecule detailing the structure andmovement of the myosin head after cross-bridgebinding occurs

The structure ofthick filaments

Titin

M line

Myosin tail

Myosin head

Hinge


Muscle Contraction

Contracting muscle fibers exert a pull, or tension, and shorten in length.

Sarcoplasmic reticulum stores calcium ions.

Caused by interactions between thick and thin filaments in each sarcomere

Triggered by presence of calcium ions Contraction itself requires the presence

of ATP.


Muscle Contraction

The Sliding Filament Theory Explains the following changes that occur

between thick and thin filaments during contraction: The H band and I band get smaller. The zone of overlap gets larger. The Z lines move closer together. The width of the A band remains constant

throughout the contraction.


Figure 9.7 Changes in the Appearance of a Sarcomere during Contraction of a Skeletal Muscle Fiber

A relaxed sarcomere showing locationof the A band, Z lines, and I band

During a contraction, the A band stays the samewidth, but the Z lines move closer together andthe I band gets smaller. When the ends of amyofibril are free to move, the sarcomeresshorten simultaneously and the ends of themyofibril are pulled toward its center.

I band A band

Z line Z lineH band


Muscle Contraction

The Start of a Contraction Triggered by calcium ions in the sarcoplasm Electrical events at the sarcolemmal surface

Trigger the release of calcium ions from the terminal cisternae

The calcium ions diffuse into the zone of overlap and bind to troponin.

Troponin changes shape, alters the position of the tropomyosin strand, and exposes the active sites on the actin molecules.


Figure 9.10a The Neuromuscular Synapse

A diagrammatic view of aneuromuscular synapse

Motorneuron

Axon

Muscle fiber

Path of actionpotential

Neuromuscularsynapse

Motorend plate

Myofibril


Figure 9.10ab The Neuromuscular Synapse

A diagrammatic view of aneuromuscular synapse

Motorneuron

Axon

Muscle fiber

Path of actionpotential

Neuromuscularsynapse

Motorend plate

Myofibril

One portion of aneuromuscular synapse

Myofibril

Mitochondrion

Sarcolemma

Glial cell

Synaptic terminal


Figure 9.10bc The Neuromuscular Synapse

One portion of aneuromuscular synapse

Myofibril

Mitochondrion

Sarcolemma

Glial cell

Synaptic terminal

Detailed view of a terminal,synaptic cleft, and motor endplate. See also Figure 9.2.

Synaptic vesicles

ACh

AChreceptorsiteAChE

molecules

Junctionalfold

Sarcolemma ofmotor end plate

Arriving actionpotential

Synaptic cleft


Skeletal muscle

The End of a Contraction When electrical stimulation ends:

The SR will recapture the Ca2+ ions. The troponin–tropomyosin complex will cover the

active sites. And, the contraction will end.


Figure 9.11 The Events in Muscle Contraction

ACh released, bindingto receptors

Sarcoplasmicreticulumreleases Ca2+

Active-siteexposure,cross-bridgeformation

ActionpotentialreachesT tubule

Ca2+

Actin

Myosin

SarcolemmaT tubule

Motorend

plateSynapticterminal

Contractionbegins

STEPS IN INITIATING MUSCLE CONTRACTION STEPS IN MUSCLE RELAXATION

ACh removed by AChE

Sarcoplasmicreticulumrecaptures Ca2+

Active sitescovered, nocross-bridgeinteraction

Contractionends

Relaxation occurs,passive return toresting length


Motor Units and Muscle Control

• A motor unit is all muscle fibers controlled by a single motor neurone.

• At the end of each motor neuron there are synaptic vesicles containing neurotransmitters. The neurotransmitter involved in the process of contraction is Acetylcholine.

• The enzyme in the synaptic cleft that destroys Ach and shuts down the contraction is Acetylcholineesterase (AchE)


Figure 9.12 The Arrangement of Motor Units in a Skeletal Muscle

Motor unit 1

Motor unit 2

Motor unit 3

KEY

SPINAL CORD

Muscle fibers

Axonsof motorneurons

Motornerve



Muscle Tone Some of the motor units of muscles are always

contracting, producing a resting tension in a skeletal muscle that is called muscle tone.

Resting muscle tone stabilizes the position of bones and joints.



Muscle Hypertrophy and Atrophy Exercise causes increases in

Number of mitochondria Concentration of glycolytic enzymes Glycogen reserves Myofibrils

Each myofibril contains a larger number of thick and thin filaments.

The net effect is an enlargement, or hypertrophy, of the stimulated muscle.

Disuse of a muscle results in the opposite, called atrophy.


Types of Skeletal Muscle Fibers

The features of fast fibers, or white fibers, are:

Large in diameter—due to many densely packed myofibrils

Large glycogen reserves Relatively few mitochondria

Their mitochondria are unable to meet the demand.

Fatigue easily Can contract in 0.01 seconds or less following

stimulation


Figure 9.13a Types of Skeletal Muscle Fibers

Note the difference in the size ofslow muscle fibers (above) andfast muscle fibers (below).

LM × 170

LM × 170

Slow fibersSmaller diameter,

darker color due tomyoglobin; fatigue

resistant

Fast fibersLarger diameter,

paler color; easily fatigued



Slow fibers, or red fibers, features are Only about half the diameter of fast fibers Take three times as long to contract after

stimulation Contain abundant mitochondria Use aerobic metabolism Have a more extensive network of capillaries

than do muscles dominated by fast muscle fibers.

Red color because they contain the red pigment myoglobin



Intermediate fibers have properties intermediate between those of fast fibers and

slow fibers. Intermediate fibers contract faster than slow

fibers but slower than fast fibers. Intermediate fibers are similar to fast fibers

except They have more mitochondria. They have a slightly increased capillary supply. They have a greater resistance to fatigue.


Figure 9.14ab Skeletal Muscle Fiber Organization

Parallel muscle(Biceps brachii muscle)

Parallel muscle with tendinous bands(Rectus abdominismuscle)

(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Fascicle

Cross section

Body (belly)


Figure 9.14e Skeletal Muscle Fiber Organization

(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Unipennate muscle(Extensor digitorum muscle)

Extendedtendon


Figure 9.14g Skeletal Muscle Fiber Organization

(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Multipennate muscle(Deltoid muscle)

Tendons

Cross section


Figure 9.14h Skeletal Muscle Fiber Organization

(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Circular muscle(Orbicularis oris muscle)

Contracted

Relaxed


Muscle Terminology

Origin: the end of the muscle that remains stationary Insertion: the end of the muscle that moves

Commonly the origin is proximal to the insertion.

Muscle Actions There are two methods of describing actions.

The first references the bone region affected. For example, the biceps brachii muscle is said to perform

“flexion of the forearm.”

The second method specifies the joint involved. For example, the action of the biceps brachii muscle is

described as “flexion of the elbow.”


Muscle Terminology

Muscles can be grouped according to their primary actions into three types:

Prime movers (agonists): are muscles chiefly responsible for producing a particular movement

Synergists: assist the prime mover in performing that action

Antagonists: are muscles whose actions oppose that of the agonist If the agonist produces flexion, the antagonist will produce

extension.

• Fixators• Agonist and antagonist muscles contracting at the same time to

stabilize a joint


Organization of Skeletal Muscle Fibers

• Most muscle names provide clues to theiridentification or location

Size of the muscle Magnus: large Brevis: short

Specific body regions Brachialis

Shape of the muscle Trapezius

Orientation of muscle fibers Rectus, transverse, oblique

Specific or unusual features Biceps (two origins)

Identification of origin and insertion Sternocleidomastoid

Primary functions Flexor carpi radialis

References to actions Buccinators


Table 9.2 Muscle Terminology


Levers and Pulleys: A Systems Design for Movement

First-class levers Second-class levers

Characteristics of second-class levers are: The force is magnified. The resistance moves more slowly and covers a shorter distance.

Third-class levers The characteristics of the third-class lever are:

Speed and distance traveled are increased. The force produced must be great.


Figure 9.15a The Three Classes of Levers

ResistanceFulcrum

Appliedforce

Movement completed

R

FAF

In a first-class lever, the applied force and the resistance are on oppositesides of the fulcrum. This lever can change the amount of force transmittedto the resistance and alter the direction and speed of movement.

R F

AF


Figure 9.15b The Three Classes of Levers

Movement completed

AF

F

F

F

AF

In a second-class lever, the resistance lies between the applied force andthe fulcrum. This arrangement magnifies force at the expense of distanceand speed; the direction of movement remains unchanged.

R


Figure 9.15c The Three Classes of Levers

In a third-class lever, the force is applied between the resistanceand the fulcrum. This arrangement increases speed and distancemoved but requires a larger applied force.

Movement completed

R

F

AF

AF

RF


Aging and the Muscular System

Skeletal muscle fibers become smaller in diameter.

Skeletal muscles become smaller in diameter and less elastic.

Tolerance for exercise decreases.

The ability to recover from muscular injuries decreases.

Dr. B Ch 09_lecture_presentation

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