Chapter 6 The Muscular System
Because flexing muscles look like mice scurrying beneath the skin, some scientist
long ago dubbed them muscles, from the Latin word mus meaning "little mouse."
Indeed, the rippling muscles of professional boxers or weight lifters is often the first
thing that comes to mind when one hears the word muscle. But muscle is also the
dominant tissue in the heart and in the walls of other hollow organs of the body. In
all its forms, it makes up nearly half the body's mass.
The essential function of muscle is contraction, or shortening-a unique characteristic
that sets it apart from any other body tissue. As a result of this ability, muscles are
responsible for essentially all body movement and can be viewed as the "machines"
of the body.
Overview of Muscle Tissues
 Describe similarities and differences in the structure and function of the
three types of muscle tissue and indicate where they are found in the body.
o Skeletal – single, very long, cylindrical, multinucleated cells with very obvious striations,
voluntary via nervous system controls, attach to bones for movement, slow to fast
contractions
o Cardiac – branching chains of cells, uninucleated, striated, involuntary, found only in the
heart, slow rhythmic contractions
o Smooth – single, fusiform, uninucleated, no striations, involuntary, can stretch, found
mostly in walls of hollow visceral organs but not the heart, very slow contraction
 Define muscular system.
o The function of muscle tissue is to contract or shorten to cause movement, maintain
posture, stabilize joints, and generate heat
o The term muscular system applies specifically to skeletal muscle
 Define and explain the role of the following: endomysium, perimysium, epimysium, tendon, and
aponeurosis.
o Endomysium – delicate connective tissue sheath enclosing muscle fibers
o Perimysium – coarser fibrous membrane that wraps several sheathed muscle fibers to
form a bundle of fibers called a fascicle
o Epimysium – even tougher overcoat of connective tissue that covers many fascicles bound
together to form the muscle
o Tendon – strong, cords of dense fibrous tissue attaching a muscle indirectly to bones
cartilages, or connective tissue coverings of each other
o Aponeurosis – sheet-like bundles of fibrous or membranous sheet connecting a muscle
and the part it moves
Microscopic Anatomy of Skeletal Muscle
 Describe the microscopic structure of skeletal muscle and explain the role of actin- and myosincontaining myofilaments.
o See diagram on page 167
o Sarcolemma – the many oval nuclei of a skeletal muscle cell
o Myofibrils – the long ribbon-like fibers that fill the cytoplasm – actually chains of tiny
contractile units called sarcomeres – the alternating light and dark bands along the length of
the aligned myofibrils give the muscle cells its striped appearance
Light band has a midline interruptions, a darker area called the Z disk, and the
dark A band has a lighter central area called the H zone
Sarcomeres – functional unit of the muscle cell
Myofilaments – found within the sarcomeres that actually produce the banding pattern –
two types of threadlike proteins
 Thick filaments – myosin fibers – made of bundles molecules of the protein
myosin and contain ATPase enzymes – extend the entire length of the dark A band
– midparts of the thick filaments are smooth but ends are studded with small
projections called myosin heads also called cross bridges because they link the
thick and thin filaments together during contraction
 Thin filaments – composed of the contractile protein actin and some regulatory
proteins that play a role in allowing (or preventing) myosin head binding to actin –
also called actin filaments, are anchored to the Z disk, a disc-like membrane)
 Light I band includes parts of two adjacent sarcomeres and contains only the
thin filaments, and although the thin filaments overlap the ends of the thick
filaments, they do not extend into the middle of a relaxed sarcomere, and this
the central region (the H zone, which lacks actin filaments and looks a bit
lighter) is sometimes called the bare zone
 When contraction occurs, and the actin-containing filaments slide toward each
other into the center of the sarcomeres, these light zones disappear because
the actin and myosin filaments are completely overlapped
Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum with interconnecting
tubules and sacs – the SR surround each and every myofibril – function is to sore calcium
and to release it on demand
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Skeletal Muscle Activity
 Describe how an action potential is initiated in a muscle cell.
o One motor neuron may stimulate a few muscle cells or hundreds of them, depending on the
particular muscle and the work it does
o One neuron and all the skeletal muscle cells it stimulates are a motor unit and when a long
threadlike extension of the neuron, called the nerve fiber or axon, reaches the muscle, it
branches into a number of axonal terminals, each of which forms junctions with the
sarcolemma of a different muscle cell – these junctions are called neuromuscular
junctions
o Even though the nerve endings and the muscle cells’ membranes are very close, they never
touch leaving a gap between them called the synaptic cleft, which is filled with tissue fluid,
interstitial fluid
o When the nerve impulse reaches the axonal terminals, a chemical called neurotransmitter
(acetylcholine) is released and diffuses across the synaptic cleft and attaches to
membrane protein receptors attached to the sarcolemma – if enough acetylcholine is
released, the sarcolemma becomes temporarily permeable to sodium ions (Na+), which
rushes into the muscle cell giving the interior of the cell a positive charge generating the
electrical current called an action potential – once begun, this action potential is
unstoppable (all or none response) and it travels over the entire surface of the sarcolemma,
conducting the electrical impulse from one end of the cell to the other resulting in the
contraction of the muscle cell
o The events that return the cell to its resting state include diffusion of potassium ions (K+)
out of the cell and the operation of the sodium-potassium pump, the active transport
mechanism that moves the sodium and potassium ions back to their initial positions
 Describe the events of muscle cell contraction.
o When muscle fibers are activated by the nervous system, the cross bridges attach to
myosin binding sites on the thin filaments, and sliding begins
o Energized by ATP, each cross bridge attaches and detaches several times during
contraction, pulling the thin filaments toward the center of the sarcomere – this event
occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens
o The attachment of myosin cross bridges to actin requires calcium ions, and the action
potential leading to contraction causes the sarcoplasmic reticulum to release stored
calcium ions into the sarcoplasm
o When the action potential ends, calcium ions are reabsorbed into the SR storage areas, and
the muscle cell relaxes and settles back to its original length
o Acetylcholine is being broken down by enzymes on the sarcolemma making sure there is
only one contraction preventing continued contraction without a nerve impulse
 Define graded response, tetanus, isotonic and isometric contractions, and muscle tone as these
terms apply to a skeletal muscle.
o Graded response – the all-or-none law of muscle contraction applies to the muscle cell not
the whole muscle – whole muscles will react to stimuli with different degrees of shortening
 Grades response is done in two ways
 By changing the frequency of muscle stimulation
 By changing the number of muscle cells being stimulated
 Muscle twitches – single, brief, jerky contractions that occur as a result of nervous
system problems
o Tetanus – two types – not to be confused with the bacterium toxin that can lead to death
 Fused or complete tetanus – when the muscle is stimulated so rapidly that no
evidence of relaxation is seen and the contractions are completely smooth and
sustained
 Unfused or incomplete tetanus – stage muscles are in until complete tetanus
state is achieved
o Isotonic contractions – same tone or tension contractions – they myofilaments are
successful in sliding movements, the muscle shortens, and movement occurs like when you
lift a book
o Isometric contractions – same measurement contractions – contractions when the
muscle do no shorten – the myosin myofilaments are trying to slide and the tension in the
muscle keeps increasing because the muscle is pitted against some immoveable object like
when you push against the wall
o Muscle tone – state of continuous partial contraction – the result of different motor units
scattered throughout the muscle being stimulated by the nervous system in a systematic
way – not consciously controlled
 Describe three ways in which ATP is regenerated during muscle activity.
o Direct phosphorylation of ADP by creatine phosphate – high energy molecule creatine
phosphate (CP) transfers a phosphate to ADP to make 1 ATP/CP – CP supplies exhausted
soon but no oxygen is required
o Aerobic respiration – occurs in mitochondria involving the oxidative phosphorylation
pathways starting with glucose releasing carbon dioxide and water as well as 36
ATP/glucose molecule – slow and requires continuous supply of oxygen and nutrients
o Anaerobic glycolysis and lactic acid formation – no oxygen required in the first steps of
glucose breakdown which occurs in the cytosol to produce pyruvic acid and 2 ATP/glucose –
occurs during intense muscle activity when oxygen is absent resulting in pyruvic acid being
converted to lactic acid instead of going through the rest of aerobic respiration
 Define oxygen debt and muscle fatigue and list possible causes of muscle fatigue.
o Oxygen debt – occurs during prolonged muscle activity when there is not enough oxygen
being taken in fast enough to keep cell respiration making ATP
o Muscle fatigue – occurs when muscles strenuously for a long time – a muscle is fatigued
when it is unable to contract even though it is still being stimulated – will begin to contract
more weakly until it finally ceases reacting and stops contracting
 Describe the effects of aerobic and resistance exercise on skeletal muscles and other body organs.
o Aerobic or endurance exercise – result in stronger, more flexible muscles with greater
resistance to fatigue due in part to increased blood supply to muscles and the individual
muscle cells produce more mitochondria and store more oxygen – does not cause the
muscle to increase much in size – benefits metabolism, digestion, neuromuscular
coordination and skeleton as well as the heart, reduced fat deposits in vessels, and lungs
o Resistance or isometric exercise – results in the bulging muscles seen in body builders –
occurs when muscles are pitted against some immoveable object such as a wall – key is to
force the muscle to contract with as much force as possible – increased the size and
strength is due to enlargement of individual muscles cells as they make more contractile
filaments – the amount of connective tissue that reinforces the muscle also increases
Muscle Movements, Types, and Names
 Define origin, insertion, prime mover, antagonist, synergist, and fixator as they relate to muscles.
o Origin – attached to the immovable or less moveable bone
o Insertion – attached to the movable bone and when the muscle contracts, the insertion
moves toward the origin
o Prime mover – the muscle that has the major responsibility for causing a particular
movement
o Antagonist – muscles that oppose or reverse a movement – when a prime mover is active,
its antagonist is stretched and relaxed – antagonists can be prime movers
o Synergist – help prime movers by producing the same movement or by reducing
undesirable movements such as when a muscle crosses two or more joints
o Fixator – specialized synergists that hold a bone still or stabilize the origin of a prime
mover so all the tension can be used to move the insertion bone
o The actions of antagonistic and synergistic muscles are important for smooth, coordinated,
precise movements
 Demonstrate or identify the different types of body movements.
o Flexion – movement that decreases the angle of the joint and brings two bones closer
together – hinge joints and ball-and-socket joints
o Extension – movement that increases the angle between two bones – straightening the
knee or elbow
o Rotation – movement of a bone around its longitudinal axis – ball-and-socket joints,
movement of the atlas around the dens of the axis (shaking your head no)
o Abduction – moving a limb away from the midline, or median plane, of the body – also the
fanning movement of fingers and toes
o Adduction – movement of limbs toward the body midline
o Circumduction – combination of flexion, extension, abduction, and adduction – seen in
ball-and-socket joints such as shoulder where the proximal end of the limb is stationary and
the distal end moves in a circle
o Dorsiflexion and plantar flexion – up and down movement of the foot at the ankle –
lifting the foot = dorsiflexion while depressing the foot = plantar flexion
o Inversion and eversion – inversion of the foot turns it medially (inward toward the other
foot) – eversion of the foot turns it laterally (outward and away from the other foot)
Supination and pronation – refer to movements of the radius around the ulna –
supination occurs when the forearm rotates laterally so that the palm faces anteriorly and
the radius and ulna are parallel while pronation occurs when the forearm rotated medially so
that the palm faces posteriorly and the radius crosses the ulna so the two bones form an X
o Opposition – the action by which you move your thumb to touch the tips of the other
fingers on the same hand
 List some criteria used in naming muscles.
o Direction of the muscle fibers – rectus = straight – oblique = slanted
o Relative size of the muscle – maximus vs. minimus and longus
o Number of origins – bi, tri, quad
o Location of the muscle’s origin and insertion
o Shape of the muscle
o Action of the muscle – flexor, adductor
o
Gross Anatomy of Skeletal Muscles
 Name and locate the major muscles of the human body (on a torso model, muscle chart, or
diagram) and state the action of each.
o See diagrams on pages 182 – 192
Developmental Aspects of the Muscular System
 Explain the importance of a nerve supply and exercise in keeping muscles healthy.
o If the nerve supply to a muscle is destroyed and the muscle is no longer stimulated, it loses
tone and becomes paralyzed and soon after, it becomes flaccid, or soft and flabby, and
begins to atrophy
 Describe the changes that occur in aging muscles.
o Increasing muscular control reflects the maturation of the nervous system – muscle control
is achieved in a cephalic/caudal and proximal/distal direction
o As we age, the amount of connective tissue in the muscle increases and the amount of
muscle mass decreases and the muscles become more stringier (sinewy) but exercise helps
retain muscle mass and strength – lose of muscle mass leads to weight lose – loss in muscle
mass also leads to a decrease in muscle strength (strength decreases by about 50% by the
age 80)