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The Muscular System

Three types of muscles…Smooth Cardiac Skeletal

This chapter focuses on skeletal muscle

Smooth muscle…walls of the

viscera (organs), blood vessels, bronchioles

INVOLUNTARY muscle

NONSTRIATED (no stripes)

The contraction of smooth muscle enables the viscera to perform their functions

Smooth muscle

Cardiac muscleFound only in the HEART Striated involuntary Cardiac muscle cells are long branching cells that fit together tightly at junctions called INTERCALATED DISCS

Intercalated discs promote rapid conduction of electrical signals throughout the heart

Cardiac muscle

Skeletal muscleGenerally attached to bone VOLUNTARY muscle Cells are long cylinders

Composed of proteins arranged to make the muscle appear striped, or STRIATED

Skeletal muscles: –produce movement –maintain body posture – stabilize joints –produce HEAT to maintain body

temperature

Skeletal MusclePrecursor cell: Myoblast

Fuses together with other myoblasts to form one large cell with many nuclei called a muscle fiber(myotube)

Skeletal muscleMany muscle fibers make a muscle Have the ability to shorten or contract

Skeletal Muscle CellsSarcolemma: plasma membrane in

muscles Myofibrils: long ribbon-like

organelles Sarcomeres: chains of contractile

units

Sarcomere: functional unit of muscle

Sarcomere

Sliding Filament Model of Muscle Contraction

Thin and thick filaments slide past each other to shorten sarcomere. Actin: thin filaments Myosin: thick filaments

How muscles contract…When muscles contract, they shorten

Muscles shorten because the sarcomeres shorten

The sarcomeres shorten because the actin and myosin filaments slide past each other

How does the sarcomere shorten?When stimulated, the myosin heads

make contact with the actin, forming temporary connections called CROSSBRIDGES

The crossbridges pull the actin to the center of the sarcomere

The actin slides past the myosin

Crossbridges in action…

Remember….The sarcomeres shorten not because the actin and myosin proteins shrink or shrivel up, but because the proteins slide past one another

The role of calcium and ATP ATP and CALCIUM play important

roles in the contraction and relaxation of muscle

ATP helps the myosin heads form and break the crossbridges with the actin

However, ATP can only perform its role if calcium is present

Calcium is stored in the sarcoplasmic reticulum (ER of muscle cells)

When the muscle is stimulated, calcium is released from the sarcoplasmic reticulum and causes the actin, myosin, and ATP to interact

Muscle Contraction controlled by the Nervous system

Skeletal muscle contraction can take place only if the muscle is first stimulated by a nerve

This type of nerve is called a MOTOR or SOMATIC NERVE

The area where the motor nerve meets the muscle is called the NEUROMUSCULAR JUNCTION (NMJ)

The NMJ

What happens at the NMJ?The stimulated nerve causes an electrical

signal, or nerve impulse, to move along the nerve toward the nerve ending

Stored in pouches at the nerve ending is a NEUROTRANSMITTER called ACETYLCHOLINE (ACh)

The ACh is released into the NMJ space The ACh diffuses across the space and

binds to receptor sites on the muscle membrane The ACh stimulates the receptors and

causes an electrical signal to develop along the muscle membrane

The electrical signal stimulates the sarcoplasmic reticulum to release calcium The calcium allows for the interaction

of actin, myosin, and ATP – the muscle contracts

Energy SourcesCreatine phosphate phosphorylates

ADP ADP ! ATP (no oxygen used)

Aerobic respiration: constant oxygen and glucose supply

Anaerobic glycolysis and lactic acid fermentation No oxygen used Not much energy produced this way but much faster than aerobic respiration

Lactic acid buildup: muscle fatigue and soreness

Muscle Fatigue and Oxygen DebtFatigue: muscle unable to contract

even though it is being stimulated.

pH denatures ATPase and excess H+ ions interfere with Ca2+ ions

Muscle Fatigue and Oxygen DebtOxygen debt occurs during

prolonged muscle activity Amount of work and length of

workout depends on a muscle’s blood supply

Muscle Fatigue and Oxygen DebtOxygen debt from vigorous activity

must be paid back Rapid deep breathing

Responses of a whole muscle..The sliding filament hypothesis explains

the contraction and relaxation of a single muscle fiber

A whole skeletal muscle, however, is composed of thousands of muscle fibers

The contraction of a whole muscle differs from that of a single muscle fiber

A single muscle fiber contracts in an ALL-OR-NOTHING response – the fiber contracts as strongly as possible

It never partially contracts

A whole muscle, however, is capable of contracting partially

It can contract weakly or very strongly

Lifting a pencil may require the contraction of several hundred muscle fibers, in an all-or-nothing manner

But only a few fibers are contracting

Lifting a 100 lb weight, however, requires contraction of thousands of fibers, all contracting in an all-or-nothing manner

The greater muscle force is achieved by using, or recruiting, additional fibers

This process is called RECRUITMENT

The strength of skeletal muscle contraction can be varied by RECRUITMENT of additional muscle fibers

Classification of MusclesSlow twitch –More mitochondria, more blood supply

(dark meat) –Slower contraction for endurance

Classification of MusclesFast twitch –Specialized for rapid, forceful

contractions –Short term contractions – Less blood supply (white meat)

Most muscles are a mixture of both Genetically determined Rigorous training can alter the ratio

Types of Muscle ContractionIsotonic: actin and myosin filaments slide past each other and the muscle shortens

Types of Muscle ContractionIsometric: filaments try to slide but

muscle is pitted against an immovable object

Muscle tone: continuous partial contraction

If nerve supply to muscle is destroyed, muscle is no longer stimulated, loses tone & becomes paralyzed, flaccid and atrophied.

Muscle attachment…Muscles form attachments to other

structures:

a tendon attaches muscle to bone

a flat, sheet-like fascia, called APONEUROSIS connects muscle to muscle, or muscle to bone

Tendon Ligament

aponeurosis

Origin and insertion….These terms refer to the sites of

muscle attachment:

ORIGIN of the muscle attaches to the stationary (immovable) bone

INSERTION attaches to the moveable bone

Although most movement is accomplished through the cooperation of groups of muscles, a single muscle is generally responsible for most of the movement

The “chief muscle” is called the PRIME MOVER

Assisting the prime mover are “helper muscles” called SYNERGISTS

Synergists cooperate with other muscles

In contrast, ANTAGONISTS are muscles that oppose the action of another muscle

Movement occurs from the contraction and relaxation of alternating muscles

When one contracts (prime mover), the other must relax (antagonist)

Concentric movement: contraction Eccentric movement: relaxation – Muscle exert

force even as they lengthen

Naming skeletal muscles…There are 656 muscles in the human body!!

The names of various muscles are generally based on one or more of the following characteristics:

–Size –Shape –Direction of fibers – Location –Number of origins –Origin and insertion –Muscle action

SizeVastus – huge Maximus – large Longus – long Minimus – small Brevis - short

ShapeDeltoid – triangular Latissimus – wide Trapezius – trapezoid Rhomboideus – rhomboid Teres – round

Fiber direction

Fibers are lined up in several directions:

Rectus – straight Oblique – diagonal Transverse – across Circularis - circular

locationThe names of muscles often reflect

their location:

Pectoralis – chest Gluteus – buttock Brachii – arm Supra – above Infra – below Sub – underneath Lateralis - lateral

Number of originsMuscle may be named according to

the number of sites to which it is anchored:

Biceps – 2 sites Triceps – 3 sites Quadriceps – 4 sites

Muscle actionAbductor muscle – moves

the limb away from the midline of the body

Adductor muscle – moves the limb toward the midline

Common Body MovementsFlexion: movement in (generally)

sagittal plane that decreases the angle of the joint and brings bones closer together Extension: opposite of flexion;

creates distance between bones or parts of body –Hyperextension: greater than 180

degrees

Common Body MovementsRotation: movement of a bone around

its longitudinal axis

Common Body MovementsAbduction: moving a limb away

(usually on the frontal plane) from the midline Fanning of fingers or toes also Adduction: opposite of abduction

Common Body MovementsCircumduction: combination of

flexion, extension, abduction, and adduction (circular movement)

Special Movements (occur only at certain joints

Dorsiflexion: lifting the foot so that superior surface approaches the shin (standing on heels)

Special Movements (occur only at certain joints

Plantar flexion: pointing the toes

Special Movements (occur only at certain joints

Inversion: turn sole medially Eversion: turn sole laterally

Special Movements (occur only at certain joints

Supination: forearm rotates laterally so that palms face forward Pronation: forearm rotates medially

so palms face posteriorly

Special Movements (occur only at certain joints

Opposition: move thumb to touch the tips of the other fingers on the same hand

Five Golden Rules on Muscle Activity

All muscles cross at least one joint. The bulk of the muscle lies proximal

to the joint crossed. All muscles have at least two

attachments: origin and insertion. Muscles can only pull, they never

push. During contraction, the muscle

insertion moves toward the origin.

Exercise Inactivity: muscle weakness and

wasting. Use it or lose it!

ExerciseAerobic or endurance exercise –Stronger, flexible muscles with greater

resistance to fatigue – Increased blood supply – Increased number of mitochondria – Improved digestion and elimination,

neuromuscular coordination, stronger skeleton, increased heart stroke volume, increased lung capacity

–No increase in muscle size

ExerciseResistance (isometric) – Increased muscle size and strength

What is wrong with “bulking up”?

Nothing, as long as it is done through weight lifting and exercise Increase in muscle size is due to

increase in actomyosin within the cell Bulking up with the use of

steroids is dangerous!

Steroids are believed to cause liver cancer, atrophy (shrinking) of the testicles in males, hypertension, and severe psychotic mood swings

American College of Sports Medicine advises resistance training with non-maximal

weights for adolescents

Optimal bone mass Reduce obesity Decrease sports

injuries

Homeostatic ImbalancesSprain: Ligament stretches or tears

Homeostatic ImbalancesStrain: injury to muscle or tendon

RICERest Ice Compression elevation

Homeostatic ImbalancesMuscular dystrophy: muscles enlarge due to fat and connective tissue deposit, but muscle fibers degenerate and atrophy Sex-linked, more common in males

Homeostatic ImbalancesAmyotrophic lateral sclerosis (ALS) Lou Gehrig’s disease Loss of control of voluntary muscles

due to destruction of nerves that innervate them.