Muscular System

essential function of muscle is contraction or shortening

muscles are responsible for essentially all body movement

"machines" of the body

3 types (skeletal, cardiac, smooth) differ in cell structure, body location, & how they are stimulated to contract

all muscle cells are elongated and called muscle fibers

the ability of muscle to shorten or contract depends on two types of myofilaments ◦ (the muscle cell equivalents of the microfilaments

of the cytoskeleton)terminology is different

◦ (“myo” and “mys” mean muscle / “sarco” means flesh)

Skeletal Muscle organs that attach to the body's skeleton help form the smoother contours of the body fibers are cigar-shaped, multinucleate cells, and the

largest of the muscle fiber types known as striated muscle because its fibers appear

to be striped called voluntary muscle because it is the only

muscle type subject to conscious control often activated by reflexes contract rapidly and with

great force tires easily and must rest after short periods of

activity soft and surprisingly fragile can exert tremendous power

thousands of their fibers are bundled together by connective tissue◦ (provides strength and support to the muscle as a whole)

each muscle fiber is enclosed in a delicate connective tissue sheath called an endomysium

several sheathed muscle fibers are then wrapped by a coarser fibrous membrane called a perimysium◦ (forms a bundle of fibers called a fascicle)

many fascicles are bound together by an even tougher layer of connective tissue called an epimysium◦ (covers the entire muscle)

epimysia blend into the strong, cordlike tendons◦ (tendons anchor muscles, provide durability, and conserve

space around rough bony projections)

Smooth Muscleno striations, involuntary (not under conscious

control), slow and sustained contractionsare spindle-shaped with a single nucleus

arranged in sheets or layers◦ (most often there are two Iayers, one running

circularly and the other longitudinally)found in the walls of hollow visceral organs

such as the stomach, urinary bladder, respiratory passages

propels substances along a definite tract (pathway) within the body

Cardiac Musclestriated but involuntary heart musclecardiac fibers are cushioned by small amounts

of soft connective tissuearranged in spiral or figure 8-shaped bundleswhen the heart contracts, internal chambers

become smaller, forcing blood into large arteries leaving the heart

cardiac muscle fibers are branching cells joined by special junctions called intercalated disks

structure of muscle fibers allow heart activity to be closely coordinated.

Muscle Functionfour important roles in the body

(produces movement, maintains posture, stabilizes joints, generates heat)

Muscle FunctionsProducing Movement

◦skeletal muscles are responsible for all locomotion and manipulation

◦respond quickly to changes in the external environment

◦express our emotionsMaintaining Posture

◦skeletal muscles that maintain body posture

◦makes one tiny adjustment after another almost continuously

Producing Muscle FunctionsStabilizing Joints

◦muscle tendons are extremely important in reinforcing and stabilizing joints

Generating Heat◦by-product of muscle activity◦nearly ¾ of energy used to power

muscle contraction escapes as heat◦lost energy is important in

maintaining normal body temperature

Show Nerve Stimulation Video:http://highered.mcgraw-hill.com/s

ites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.html

The Nerve Stimulus and the Action Potential Part 1skeletal muscle cells must be stimulated by

nerve impulses to contractone motor neuron (nerve cell) may stimulate

hundreds of muscle cells◦ (depending on the particular muscle and the work

it does)one neuron and all the skeletal muscle cells it

stimulates are a motor unitnerve fiber (axon) forms neuromuscular

junctions with the sarcolemma of a different muscle cell ◦ they never touch and the synaptic cleft is filled with

tissue (interstitial) fluid

Part 2when nerve impulse reaches the axonal

terminals a neurotransmitter is released (acetylcholine - ACh)◦ACh diffuses across the synaptic cleft and

attaches to receptors in the sarcolemma◦ if enough acetylcholine is released the

sarcolemma becomes temporarily permeable to sodium ions (Na+)

◦Na+ rushes into the muscle cell◦cell interior has an excess of positive ions

which changes the electrical conditions of the sarcolemma

Part 3◦this generates an electrical current called an

action potential (unstoppable once it begins)

◦action potential travels over the entire surface of the sarcolemma

◦an electrical impulse is conducted from one end of the cell to the other

◦result is contraction of the muscle cell◦the cell returns to its resting state by

(1) diffusion of potassium ions (K+) out of the cell (2) activation of the sodium-potassium pump

(moves Na+ and K+ back to their initial positions)

Contraction of a Skeletal Muscle as a Whole

skeletal muscles are organs with thousands of muscle cells

muscles react to stimuli with graded responses (different degrees of shortening)

graded muscle contractions can be produced two ways◦(1) changing the speed of muscle

stimulation◦(2) by changing the number of muscle

cells being stimulated

muscle twitches single, brief, jerky contractions sometimes occur as a result of certain nervous system

problems nerve impulses are delivered to muscle at a very rapid

rate cells do not relax completely between stimuli contractions are "summed" and get stronger and

smoother when muscle is stimulated rapidly with no evidence of

relaxation, contractions are smooth/sustained, the muscle is said to be in tetanus

primary role of tetanus is to produce smooth and prolonged muscle contractions

depends mostly on how many muscle cells are stimulated

Providing Energy for Muscle Contraction

as a muscle contracts, ATP bonds are hydrolyzed to release energy

muscles only store 4 to 6 seconds' worth ATP

contracting muscles different pathways for ATP regeneration:

aerobic (cellular) respiration in the mitochondriaanaerobic glycolysis and lactic acid formation

◦ during intense muscle activity oxygen/glucose delivery is temporarily inadequate

◦ pyruvic acid generated during glycolysis is converted to lactic acid

◦ produces about 5% as much ATP as aerobic respiration

◦ works 2 ½ times faster and provides enough ATP for 30-40 seconds of activity

◦ uses huge amounts of glucose for a small ATP harvest◦ accumulated lactic acid promotes muscle fatigue and

muscle soreness

Muscle Fatigue and Oxygen Debta muscle is fatigued when it is unable to contract,

even if it is still being stimulatedwhen muscle begins to tire, it contracts more

weakly until it stops reacting /contractingbelieved to result from oxygen debt/lactic acid

accumulation occurring during prolonged muscle activity

muscle's ATP supply starts to run low increasing acidity / decreasing ATP cause muscle

to contract less effectively and finally stop individual breathes rapidly/deeply and muscles

receive sufficient oxygen◦ lactic acid broken down and ATP/creatine phosphate

made

Types of Muscle Contractions

Isotonic ◦ tension develops as actin/myosin

myofilaments attempt to slide past each other in muscle fibers

◦contractions occur when myofilaments slide, the muscle shortens, and movement occurs

Isometric ◦contractions in which the muscles do not

shorten because they are working against an immovable object

◦ tension in the muscle keeps increasing without movement

Muscle Toneeven a voluntarily relaxed muscle

has some contracting fibers contraction is not visible but

keeps muscle firm, healthy, ready

Effect of Exercise on Muscles

inactivity (loss of nerve supply, immobilization) leads to muscle weakness and wasting

regular exercise increases muscle size, strength, endurance

aerobic/endurance exercise (jogging, biking, or aerobics class)◦ stronger, more flexible muscles with greater

resistance to fatigue◦ cells form more mitochondria and store more

oxygen◦ overall body metabolism more efficient◦ improves digestion/elimination◦ enhances neuromuscular coordination◦ makes the skeleton stronger◦ heart enlarges (hypertrophies)

more blood is pumped with each beat, fat deposits are cleared, gas exchanges more efficient

resistances or isometric exercise ◦bulging muscles result mainly from

muscles being worked against some immovable object

◦require very little time and little or no special equipment

5 Golden Rules of Skeletal Muscle Activityall muscles cross at least one

jointbulk of the muscle lies proximal

to the joint crossedall muscles have at least two

attachments: the origin and the insertion

muscles puII not pushduring contraction the muscle

insertion moves toward the origin

Naming Skeletal Musclesmuscles come in many shapes/sizes

and named with several criteria◦direction of the muscle fibers - rectus

(straight), oblique (slanted)◦relative size of the muscle - maximus

(largest), minimus (smallest), longus (long)◦location of the muscle - named for the

bone with which they are associated◦number of origins - biceps, triceps, or

quadriceps have two, three, or four origins

◦location of the muscle's origin and insertion - named for their attachment sites sternocleidomastoid muscle – origin on

the sternum (sterno)/clavicle (cleido) and inserts on the mastoid process

◦shape of the muscle - distinctive shape that helps to identify them (deltoid means "triangular")

◦action of the muscle - flexor, extensor, abductor appear in names