THE MUSCULAR SYSTEMChapter 6

MUSCULAR SYSTEM Responsible for all

types of body movement

Three basic muscle types in the body Skeletal Cardiac Smooth

Know these characteristics!

CHARACTERISTICS OF SKELETAL MUSCLES

Cells are elongated (cigar-shaped) Muscle cell= Muscle fiber

Most are attached to bones by tendons Cells are multinucleate Have striations (bands, stripes) Voluntary (we consciously control them)

They are also activated by reflexes Cells are surrounded and bundled by

connective tissue

Key words: skeletal, striated, voluntary

CONNECTIVE TISSUE WRAPPINGS OF SKELETAL MUSCLE

Endomysium Covering around

single fiber/cell Perimysium

Around a bundle of fibers Bundle of fibers

known as Fascicle Course, fibrous

membrane around a bundle of muscle cells

CONNECTIVE TISSUE WRAPPINGS OF SKELETAL MUSCLE

Epimysium Covers the entire

skeletal muscle Outer covering

Fascia On the outside of

the epimysium Like “skin” on

muscle Think ‘skin’ like

membrane on skinless raw chicken

SKELETAL MUSCLE ATTACHMENTS

Epimysium blends into connective tissue attachment Tendon

Cord-like structure Tough; Small; pass over body projections

Aponeuroses Sheet-like structure

Both attaches muscle to bones, cartilage, connective tissue, or each other

SMOOTH MUSCLE CHARACTERISTICS No striations Spindle-shaped cells Single nucleus Involuntary-we can’t

control them Found mainly in walls

of hollow organs Stomach Intestines Bladder Respiratory passages

Key words: Visceral, Involuntary, Non-striated

CARDIAC MUSCLE CHARACTERISTICS Has striations Usually has one

nucleus Joined to other

muscle cells at intercalated disc Junction between

cardiac muscle cells Involuntary Found in heart Key words: cardiac,

striated, involuntary

MUSCLE FUNCTION

Produce movement Maintain posture Stabilize joints

Tendons help reinforce joints Generate heat

Heat is a byproduct of muscle activity

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

Cells are multinucleate Nuclei are beneath sarcolemma

Fancy word for cell membrane of muscle Literally “muscle husk”

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

Myofibril Bundles of

myofilaments Prefix “myo” means

muscle Aligned to give

distinct bands (striations) I band = light band A band = dark band

Sarcomere Contractile unit of

muscle fiber

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

Organization of the sarcomere Thick filaments = Myosin

filaments (RED) LARGE Made of the protein

Myosin Length of A band Contains enzymes that

help with muscle contraction

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

Organization of the sarcomere Thin filaments=

Actin filaments (BLUE) Made of protein Actin Anchored to Z-disc Light I band is made

only of thin filaments

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

Myosin filaments have heads (extentions, or cross bridges) Look like balloons in

the picture Myosin and actin

overlap At rest, there is a

bare zone that lacks actin Known as H zone

SKELETAL MUSCLE ACTIVITY

Muscles need to have: Irratibility

Ability to receive and respond to a stimulus Contractility

Ability to shorten when an adequate stimulus is received

NERVE STIMULUS TO MUSCLES

Muscles must be stimulated by a nerve to contract

Parts of a Motor Unit One Neuron (nerve

cell) All the muscle cells

stimulated by that neuron

NERVE STIMULUS TO MUSCLES

Neuromuscular Junction Association site of

nerve and muscle Where nerve/muscle

come together

Synaptic cleft Gap between nerve

and muscle Nerve and muscle

don’t make contact This area is filled with

fluid

TRANSMISSION OF NERVE IMPULSE TO MUSCLE Neurotransmitter (NT)

Chemical released by nerve when impulse arrives Acetylcholine is the NT for skeletal muscle (uh-seet-ill-co-lean is pronunciation)

NT attaches to receptors on sarcolemma (muscle cell membrane)

The sarcolemma is then temporarily permeable to sodium (Na+)

Sodium rushes into the cell This changes electrical conditions in the cell (because

of extra + ions) Generates an action potential (electrical current) Causes contraction…Impulse goes from one end of

cell to the other Once contraction starts, you can’t stop it.

MUSCLE CONTRACTION, CONTINUED

Like lighting a match under a dry twig… Twig is charred, becomes hot enough to burst

into flame Flame then consumes twig Flame = Action Potential that sweeps over

the cell

SLIDING FILAMENT THEORY OF CONTRACTION Activation by nerve

causes myosin heads to attach to actin

They then bind to the next site of the actin (thin filament)

This continues, causing the myosin to ‘slide’ along the actin

Result = shortened muscle (contracted muscle)

SLIDING FILAMENT THEORY OF CONTRACTION

CONTRACTION OF A SKELETAL MUSCLE

Muscle fiber contraction is all or none Muscles can’t partially contract

Not all muscle fibers may be stimulated at the same time

Different combos of fiber contractions produce different responses Graded responses

Different degrees of skeletal muscle shortening

TYPES OF GRADED RESPONSES Twitch

Single, brief contraction NOT a normal function

Tetanus (summing of contractions) Once contraction is

immediately followed by another

Muscle does not completely return to resting state

Effects get added Unfused (incomplete)

tetanus Some relaxation occurs

between contractions Results are summed

Fused (complete) tetanus No relaxation between

contractions Sustained muscle contraction

MUSCLE RESPONSE TO STRONG STIMULI

Muscle force depends on how many fibers are stimulated

More fibers contracting means greater muscle tension

Muscles can continue to contract unless they run out of energy

ENERGY FOR MUSCLE CONTRACTION

Muscles use stored ATP for energy Bonds of ATP are broken, energy is released There is only 4-6 seconds worth of ATP stored in

muscles What happens after this is used?

See next slide

ENERGY FOR MUSCLE CONTRACTION

Direct Phosphorylation Muscles contain

creatine phosphate (CP) High energy molecule CP transfers energy

to ADP (what’s left over after ATP is used) to make ATP again

Creatine Phosphate supplies are depleted after about 20 seconds

ENERGY FOR MUSCLE CONTRACTION

Aerobic Respiration Series of pathways

that occur in mitochondria

Glucose (sugar) is broken down to release energy Produces carbon

dioxide and water Slow reaction that

requires continuous oxygen

ENERGY FOR MUSCLE CONTRACTION Anaerobic Glycolysis

Breaks down glucose without oxygen

Glucose is converted to pyruvic acid to make ATP

Pyruvic acid is converted to lactic acid

NOT efficient, but is FAST

Need huge amounts of glucose

Lactic acid produces muscle fatigue

HOW IT WORKS

Running a race We tend to breathe shallowly when running Not getting enough O2 to muscles, so body

can’t use Aerobic respiration to make ATP to power muscles to keep going

Uses Anaerobic respiration to make ATP instead

This produces lactic acid, which builds up in muscles and causes cramping

Proper breathing technique can reduce or eliminate cramps related to this

MUSCLE FATIGUE AND OXYGEN DEBT

Muscle fatigue= unable to contract muscle Common cause = Oxygen debt

Oxygen debt must be “repaid” to tissue to remove it

Oxygen is required to get rid of accumulated lactic acid in muscle cells This is why athletes with cramps commonly have the

cramping muscle massaged-it increases blood flow to the area

Blood carries oxygen, when then helps remove lactic acid from muscle

Increasing acidity (from lactic acid) and lack of ATP cause muscle to contract less This is why you get tired after running for long periods of

time (or doing some other physical activity)

TYPES OF MUSCLE CONTRACTIONS

Isotonic Contractions Myofilaments slide past each other Muscle shortens Ex: Lifting weights

Isometric Contractions Increases tension in the muscle Muscle does not shorten Ex: Pushing against a brick wall

MUSCLE TONE

In relaxed muscle, some fibers are still contracted

Different fibers contract at different times to provide muscle tone

This is involuntary

MUSCLES AND MOVEMENTS

Movement is due to a muscle moving an attached bone

Muscles are attached to at least two points Origin

Attachment to IMMOVABLE bone

Insertion Attachment to

MOVEABLE bone

WHAT ABOUT EXERCISE?

Results of Increased Muscle Use Increase in muscle size Increase in muscle strength Increase in muscle efficiency Muscle becomes more fatigue resistant

Decreased muscle use has the opposite affect Muscle atrophy (wasting away of muscle)

TYPES OF MUSCLES Prime Mover

Has major responsibility for a certain movement

Antagonist Muscle that opposes or

reverses a prime mover Returns limb to normal

position) Synergist

Helps a prime mover in a movement, prevents rotation

Smaller muscles often help large muscle neighbors

Fixator Stabilizes the origin of a

prime mover Stabilize joints so motion is

smooth

Prime Mover/ Antagonist Pairs Biceps/Triceps Forearm Flexors/ Extensors Pectorals/Latissimus Dorsi Trapezius/Deltoid Quadriceps/Hamstrings

NAMING SKELETAL MUSCLES 7 Criteria:

Direction of muscle fibers Ex: rectus (straight)

Relative size of muscle Ex: gluteus maximus (largest)

Location of muscle Many are named for bones Ex: Temporalis

Number of origins Ex: Triceps (3 heads); Biceps (2 heads)

Location of muscle’s origin and insertion Ex: Sternocleidomastoid (sterno for sternum)

Shape of the muscle Ex: Deltoid (triangular, like Greek letter delta)

Action of the muscle Flexor or extensor (flexes or extends a bone)