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