Unit 5

MUSCULAR SYSTEM

IntroductionThe muscular system works with two

other systems to generate movement. The nervous system provides the electrical impulse that stimulates the movement and the skeletal system provides a framework for muscles to attach to and pull against. When all three systems work together, we gain the ability to move.

The essential function of muscle is contraction, or shortening.

Muscle TypesThree types:

SkeletalCardiacSmooth

Differ in cell structure, body location, and how they are stimulated to contract

All muscle cells are elongated all muscles are called muscle fibers

All muscle cells use microfilaments to contract

Skeletal MusclesSkeletal muscle fibers are cigar-shaped,

multinucleate cells and the largest of the muscle fiber types – some can be over 1 ft in length

It is striated muscle because its fibers appear to be striped.

It is voluntary muscle because it is the only muscle type subject to conscious control, although the muscles can also be activated by reflexes.

Individual muscle fibers are soft and fragile on their own, but strong and powerful when they are bundled together.

Skeletal Muscles

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 to form a bundle of fibers called a fascicle.

Multiple fascicles are bound together by an even tougher sheath called epimysium, which covers the entire muscle.

Skeletal Muscles

Epymysia blend into the tendons (connect muscle to bone) or into aponeuroses, which attach muscles indirectly to bones, cartilages, or connective tissue coverings of each other.

Tendons are mostly tough collagenic fibers and are smaller in size than muscles, which mean more can pass over a joint.

Smooth MusclesNo striationsInvoluntary controlFound mainly in the walls of hollow visceral organs

such as the stomach, urinary bladder, and respiratory passages, among others.

Propels substances along a definite tract (pathway) within the body.

Smooth muscle cells are spindle shaped with single nucleus, arranged into sheets or layers.

There are usually two layers of smooth muscle, one running circularly and one running longitudinally, that alternately contract and relax, changing the size and shape of the organ.

Cardiac MusclesStriatedInvoluntary controlFound in only one place in the body – the

heartCushioned by small amounts of soft

connective tissue and arranged in spiral or figure-8 shaped bundles

Cardiac muscle fibers are branching cells joined by intercalated discs

Muscle Functions1. Produces movement2. Maintains posture3. Stabilizes joints4. Generates heat

Producing MovementResult of muscle contractionMobility of body reflects the action of

skeletal musclesEnable us to respond to environmental

changes quicklyEmotional expression facial musclesSubstances are moved through the body

by smooth and cardiac muscles

Maintaining PostureMuscles function continuouslyMake small adjustments one after the

otherWe can maintain a standing or sitting

posture despite the downward pull of gravity

Stabilizing JointsHelp keep bones in placeEspecially important for poorly fitting

articulating surfaces, such as the shoulder joint

Generating HeatBy-product of muscle activityATP is used to power muscle contractions,

but nearly ¾ of its energy escapes as heatSkeletal muscle produces most heat

Microscopic Anatomy of Skeletal MuscleMultinucleateSarcolemma = plasma membrane (“muscle husk”)Myofibrils = ribbonlike organelles that push the

nuclei aside, have alternating light (I) and dark (A) bands

I band has a midline (darker area) called the Z discA band has a midline (lighter area) called the H

zoneBanding pattern reveals the working structure of

myofibrilsMyofibrils are actually chains of tiny contractile

units called sarcomeres lined up end to end like cars on a train along the length of the myofibrils

Microscopic Anatomy of Skeletal Muscle

Within the sarcomeres are myofilaments produce the banding pattern

Two types of myofilaments:Thick filaments = myosin filaments, split ATP to get

energy, extend entire length of dark A band, have small projections called myosin heads or cross bridges that link to thin filaments

Thin filaments = actin filaments, contractile protein, anchored to Z disc

H zone lacks actin looks lighter, “bare zone,” disappears during contractions

Sarcoplasmic reticulum (SR) = smooth endoplasmic reticulum, surrounds each myofibril, stores calcium to release “on demand”

Skeletal Muscle ActivityIrritability = ability to receive and

respond to a stimulusContractility = ability to shorten (forcibly)

when an adequate stimulus is receivedSkeletal muscles must be stimulated by

nerve impulses to contractOne motor neuron may stimulate a few

muscle cells or hundredsMotor unit = one neuron and all the

skeletal muscle cells it stimulates

Skeletal Muscle ActivityNerve fiber (axon) reaches the muscle and

branches out into several axonal terminals, each of which forms a junction with the sarcolemma of a different muscle cell neuromuscular junctions

The nerve endings and muscle cell membranes are very close, but they never touch

Synaptic cleft = space between nerve ending and muscle cell membrane, filled with interstitial fluid

Neurotransmitter = chemical released when a nerve impulse reaches the axonal terminals, for muscle cells this is acetylcholine (ACh)

Skeletal Muscle ActivityACh diffuses across the synaptic cleft and

attaches to receptors in sarcolemmaIf enough Ach is released, the sarcolemma

becomes temporarily permeable to sodium ions (Na+), which rush into the muscle cell

The interior of the cell now has an excess of positive ions, which generate an unstoppable electrical current called an action potential that travels over the entire surface of the sarcolemma

The result is the contraction of the muscle cell

Sliding Filament Theoryhttp://www.youtube.com/watch?v=gJ309LfHQ3M Cross bridges attach to myosin binding sites on the

thin filamentsEnergy from ATP allows cross bridges to attach

and detach several times during each contraction, pulling the thin filaments toward the center of the sarcomere

Occurs simultaneously in sarcomeres throughout the cell, causing cell to shorten

Requires calcium ions stored in SRAch is broken down a single nerve impulse = one

contraction, muscle relaxes unless more signals are sent

Contraction of a Skeletal Muscle as a Whole

The “all-or-nothing” nature of contraction only applies to the individual muscle cell, not to the muscle as a whole

Graded response = different degrees of contraction

Graded responses can be produced in two ways:1. Changing the frequency of muscle

stimulation2. Changing the number of muscle cells being

stimulated

Contraction of a Skeletal Muscle as a Whole

Muscle twitches = single, brief, jerky contractions, sometimes result from problems in the nervous system

Usually, nervous impulses are delivered so quickly that the cells do not get a chance to relax completely between stimuli.

Contractions get “added” together and become stronger and smoother muscle is in fused or complete tetanus (tetanic contraction)

Not to be confused with pathological conditionUntil muscle reaches this state, it is exhibiting

unfused or incomplete tetanus

Contraction of a Skeletal Muscle as a Whole

Primary role of tetanus is to produce smooth and prolonged muscle contractions.

How forcefully a muscle contracts depends largely on how many muscle cells are stimulated.

Only a few cells stimulated contraction will be slight

Strongest contractions = all muscle cells are stimulated

Providing Energy for Muscle Contraction

Muscles only store about 4-6 seconds worth of ATP needs to be regenerated for contraction to continue 3 main pathways1. Direct phosphorylation of ADP by creatine phosphate (CP)

CP transfers a high-energy phosphate group to ADP, making it ATP. This supply is exhausted in about 20 seconds.

2. Aerobic respiration During rest and light exercise, 95% of ATP comes from aerobic respiration, occurs in mitochondria, glucose is broken down into CO2 and water while the released energy is captured by ATP, fairly slow and requires constant oxygen

3. Anaerobic glycolysis and lactic acid formation No oxygen is used, glucose is broken down into pyruvic acid while small amounts of energy are captured by ATP. If previous two pathways cannot keep up with needs of muscles, pyruvic acid becomes lactic acid. This provides enough energy for 30-60 seconds of strenuous muscle activity.

Muscle Fatigue and Oxygen DebtMuscle fatigue = muscle is unable to contract

even though it is still being stimulatedIf a muscle does not have rest, it begins to

tire and contract more weakly until it finally stops reacting and contracting

Oxygen debt = occurs when a person is unable to take in enough oxygen to supply the energy needed for muscle contractions

Muscle activity is dependent on the blood supply and delivery of oxygen

Types of Muscle ContractionsIsotonic contractions = “same tone,”

myofilaments are successful in their sliding movements, movement occurs

Isometric contractions = “same measurement,” myofilaments are not successful in their sliding motions, movement is “blocked,” tension in muscle keeps increasing

Muscle ToneSometimes, even when a muscle is

voluntarily relaxed, some of its fibers are contracting

Contractions are not visible, but result in firm, healthy muscle

Muscle tone = state of continuous partial contractions

Effect of Exercise on MusclesMuscle inactivity, which can be due to any

number of things, such as loss of nerve supply, immobilization, etc., results in muscle weakness and wasting.

Regular exercise increases muscle size, strength, and endurance.

Aerobic or endurance exercises = result in stronger, more flexible muscles with greater resistance to fatigue, increased blood supply to muscle, more mitochondria per muscle cell, increases metabolism and coordinationExample: jogging, biking

Effect of Exercise on MuscleResistance exercises = based on isometric

contractions, muscles are pitted against an immovable (or difficult to move) object, results in increased muscle size due to enlargement of individual muscle cellsExample: lifting weights

5 Golden Rules of Skeletal Muscle Activity

1. All muscles cross at least one joint.2. Typically, the bulk of the muscle lies

proximal to the joint crossed.3. All muscles have at least two

attachments; the origin (attached to immovable or less movable bone) and the insertion (attached to moveable bone).

4. Muscles can only pull; they never push.5. During contraction, the muscle

insertion moves toward the origin.

Types of Body MovementsFlexion = movement that

decreases the angle of the joint and brings two bones closer together

Extension = movement that increases angle of the joint and moves two bones further apart

Hyperextension = extension greater than 180 degrees

Rotation = movement of a bone around its longitudinal axis

Types of Body MovementsAbduction = moving a limb

away from the midline of the body, also applies to the fanning of fingers and toes

Adduction = moving a limb toward the midline of the body

Circumduction = combination of flexion, extension, abduction, and adduction, commonly seen in ball-and-socket joints. The proximal end of the limb is stationary while the distal end moves in a circle

Types of Body MovementsDorsiflexion and

plantar flexion – up and down movements of the foot at the ankle

Inversion – turning the sole of the foot medially

Eversion – turning the sole of the foot laterally

Types of Body MovementsSupination – “turning

backward,” forearm rotates laterally so that the palm faces anteriorly, radius and ulna are parallel

Pronation – “turning forward,” forearm rotates medially so that palm faces posteriorly, radius comes across ulna, forming an X

Opposition – thumb moves to touch tips of the other fingers in the same hand

Types of MusclesPrime mover – muscle that has the major

responsibility for causing a particular movement

Antagonists – muscles that oppose or reverse a movement

Synergists – help prime movers by producing the same movement or by reducing undesirable movements

Fixators – specialized synergists, hold a bone still or stabilize the origin of a prime mover so all the tension goes to moving the insertion bone

Naming Skeletal MusclesDirection of the muscle fibers – reference to

imaginary line, such as midline of body or long axis of a limb bone. Rectus = parallel to the line, oblique = slanted to the line

Relative size of muscle – maximus = largest, minimus = smallest, longus = long

Location of muscle – some muscles are named for the bone with which they are associated

Number of origins – biceps = two origins, triceps = three origins, quadriceps = four origins

Naming Skeletal MusclesLocation of the muscle’s origin and

insertion – muscles are named for their attachment sites

Shape of the muscle – some muscles have a distinctive shape that helps identify them, deltoid = triangular

Action of the muscle – terms like flexor, extensor, and adductor are added to the name

GROSS ANATOMY OF SKELETAL MUSCLES

Head and Neck MusclesFrontalis – covers the frontal

bone as it runs from the cranial aponeruosis to the skin of the eyebrows, where it inserts. Allows you to raise your eyebrows and wrinkle your forehead. At the posterior end of the cranial aponeurosis is the small occipitalis muscle, which covers the posterior aspect of the skull and pulls the scalp posteriorly

Orbicularis oculi – fibers that run in circles around the eyes. Allows you to close your eyes, squint, blink, and wink.

Head and Neck MusclesOrbicularis oris – fibers that

run in circles around the lips, the “kissing” muscle, closes mouth, protrudes lips

Buccinator – fleshy muscle that runs horizontally across the cheek inserts into the orbicular oris. It flattens the cheek (as in blowing a trumpet) and is used to chew food.

Zygomaticus – extends from corner of mouth to the cheek, the “smiling” muscle

Chewing MusclesMasseter – covers the

angle of the lower jaw, runs from the zygomatic process of the temporal bone to the mandible, closes the jaw by raising the mandible

Temporalis – fan-shaped muscle overlying the temporal bone, inserts into the mandible, acts as a synergist to the masseter in closing the jaw

Neck MusclesPlatysma – single, sheetlike muscle

that covers anterolateral neck, originates from connective tissue covering of chest muscles and inserts into the area around the mouth, pulls down corners of the mouth

Sternocleidomastoid – paired muscles, one on each side of neck, two-headed, one head arises from the sternum, one from the clavicle. The heads fuse before inserting into the mastoid process of the temporal bone. Called the “prayer” muscle, helps flex the neck and bow the head.

Trunk Muscles – Anterior MusclesPectoralis major – large, fan-shaped

muscle covering the upper part of the chest, originates from the shoulder girdle and first six ribs, inserts into proximal end of humerus, forms the anterior wall of the axilla and adducts and flexes the arm

Trunk Muscles – Anterior MusclesIntercostal muscles – deep muscles found

between the ribs, external intercostals are important for breathing because they raise the ribcage (inhaling), internal intercostals depress the rib cage (exhaling)

Trunk Muscles – Anterior MusclesRectus abdominus – paired straplike

muscles, most superficial muscles of abdomen, run from pubis to rib cage, flex the vertebral column, compress the abdominal contents during defecation and childbirth

Trunk Muscles – Anterior MusclesExternal oblique – paired superficial

muscles that make up the lateral walls of the abdomen, fibers run downward and medially from the last eight ribs and insert into the ilium, flex vertebral column, rotate the trunk and bend it laterally

Trunk Muscles – Anterior MusclesInternal oblique – paired muscles deep to

the external obliques, fibers run at right angles to those of external obliques, originate from iliac crest and insert into the last three ribs, functions the same as external obliques

Trunk Muscles – Anterior MusclesTransverse abdominus – deepest muscle of

the abdominal wall, fibers run horizontally across the abdomen, arises from lower ribs and iliac crest and inserts into the pubis, compresses the abdominal contents

Trunk Muscles – Posterior Muscles

Trapezius – most superficial muscles of the posterior neck and upper trunk, form a diamond or kite-shaped muscle, broad origin, runs from occipital bone of skull to the end of the thoracic vertebrae, flares laterally to scapular spine and clavicle, extend the head, elevate, depress, adduct, and stabilize the scapula

Trunk Muscles – Posterior Muscles

Latissimus dorsi – large, flat muscle that covers the lower back, originates on the lower spine and ilium and then sweeps superiorly to insert into the proximal end of the humerus, extends and adducts the humerus

Trunk Muscles – Posterior Muscles

Erector spinae – prime mover of back extension, composite muscle consisting of three muscle colums (longissimus, iliocostalis, and spinalis), provides resistance that helps control the action of bending at the waist, will spasm after back injuries, common source of back pain

Trunk Muscles – Posterior Muscles

Deltoid – fleshy, triangle-shaped muscles that form the rounded shape of shoulders, bulkiness makes them a good injection site, origin at shoulder girdle from spine of scapula to clavicle, inserts into proximal humerus, prime movers of arm abduction

Muscles of the Upper LimbThree groups:

Muscles that arise from shoulder girdle and cross shoulder joint to insert into humerus (pectoralis major, latissimus dorsi, deltoid)

Muscles that cause movement at the elbow joint, enclose humerus and insert into forearm bones

Muscles of forearm that insert into hand bones

Muscles of the Humerus That Act on the Forearm

Biceps brachii – bulges when elbow is flexed, originates from two heads on the shoulder girdle, inserts into radial tuberosity, prime mover for flexion of the forearm and helps supinate the forearm

Brachialis – lies deep to biceps, helps flex elbow

Muscles of the Humerus That Act on the ForearmBrachioradialis – fairly weak

muscle, originates from humerus, inserts into distal forearm

Triceps brachii – only muscle fleshing out posterior humerus, three heads, originates from shoulder girdle and proximal humerus, inserts into olecranon process of ulna, antagonist of biceps brachii, the “boxer’s” muscle because it allows the arm to punch

Muscles of the Lower LimbCause movement at hip, knee, and foot

jointsLargest, strongest muscles in bodyMany span two joints and can cause

movement in both, therefore, the terms origin and insertion can be interchangeable

Muscles Causing Movement at the Hip Joint

Gluteus maximus – superficial muscle, forms flesh of buttock, powerful hip extensor, helps bring thigh in a straight line with pelvis, originates from sacrum and iliac bones and inserts on the gluteal tuberosity of the femur

Gluteus medius – lies under gluteus maximus, runs from ilium to femur, adducts hip, very fleshy, good injection site

Muscles Causing Movement at the Hip Joint

Iliopsoas – fused muscle composed of two muslces: the iliacus and the psoas major, runs from iliac bone and lower vertebrae deep inside the pelvis to the lesser trochanter of femur, prime mover of hip flexion, keeps upper body from falling backward

Adductor muscles – form muscle mass on medial side of thigh, press thighs together, become flabby very easily since gravity does most of their work for them, run from pelvis to proximal aspect of femur

Muscles Causing Movement at the Knee Joint

Hamstring group – form muscle mass of the posterior thigh, formed from three muscles: biceps femoris, semimembranosus, and semitendinosus, originate on ischial tuberosity, insert on both sides of proximal tibia, name comes from butchers using the tendons to hang hams

Muscles Causing Movement at the Knee JointSartorius – thin, straplike, most

superficial muscle of thigh, runs obliquely across thigh from anterior iliac crest to medial side of tibia, weak thigh flexor, the “tailor’s” muscle, synergist to cause cross-legged position

Quadriceps group – four muscles: rectus femorus and three vastus muscles, flesh out anterior thigh, originates from femur and pelvis, insert into tibial tuberosity, extends knee and flexes hip

Muscles Causing Movement at the Ankle and FootTibialis anterior – superficial

muscle on anterior leg, runs from upper tibia to tarsal bones, acts to dorsiflex and invert foot

Extensor digitorum longus – lateral to tibialis anterior, runs from lateral tibial condyle to phalanges, prime mover of toe extension and dorsiflexion of foot

Fibularis muscles – three muscles: longus, brevis, and tertius, found in lateral part of leg, run from fibula to metatarsals, plantar flex and evert foot

Muscles Causing Movement at the Ankle and FootGastrocnemius – forms

curved calf of posterior leg, two-headed, each head originating from each side of distal femur, inserts into heel of foot through Achilles tendon, prime mover for plantar flexion, “toe dancer’s” muscle

Soleus – deep to gastrocnemius, originates on tibia, inserts in heel, strong plantar flexor of foot

Muscular System DisordersMuscular dystrophy = muscle destroying

diseases, muscles enlarge due to fat and connective tissue deposits, muscle fibers degenerate and atrophy

http://www.youtube.com/watch?v=6wLnR7GJakY

Myasthenia gravis = disease characterized by drooping of upper eyelids, difficulty swallowing and talking, and general weakness and fatigue due to a shortage of ACh receptors at neuromuscular junctions, death usually due to respiratory failure