A&P Chapter 6 The Muscular System
1. List the functions of muscles.
---essential function of muscle is contraction (or shortening)
---separates it from any other body tissue
---responsible for essentially all body movement
---can be viewed as “machines” of the body
---in all its forms, makes up nearly half the body’s mass
2. Name the types of muscle in the human body.
---skeletal, cardiac, and smooth
---share certain similarities; all muscle cells are elongated
and are called muscle fibers
---ability to shorten depends on two types of myofilaments
--muscle cell equivalents of microfilaments of cytoskeleton
---“myo,” “mys,” and “sarco” refer to muscle
--“sarcoplasm” = cytoplasm in muscle cells
3. Describe skeletal muscle cells.
---skeletal muscle fibers packaged into organs called skeletal
muscles that attach to body’s skeleton
---known as striated muscle because fibers appear striped
---is only muscle subject to conscious control
---keys words are skeletal, striated, and voluntary
---each muscle fiber enclosed in connective tissue sheath called
endomysium
---several sheathed muscle fibers wrapped by coarser fibrous
membrane called perimysium to form bundle of fibers called
a fascicle
---many fascicles bound together by overcoat of connective
tissue called epimysium which covers the entire muscle
---epimysia blend into strong, cordlike tendons (sheetlike
aponeuroses) which attach muscles indirectly to bones,
cartilages, or connective tissue coverings of each other
---observe and study overhead 6.1
4. Relate sarcomere to the myofibril.
---muscle cell plasma membrane called sarcolemma
---long ribbonlike organelles called myofibrils nearly fill
cytoplasm
---alternating light (I) and dark (A) bands along length
of myofibrils give striped appearance
---light I band has midline interruption, a darker area called
the Z line
---dark A band has lighter central area called the H zone
---myofibrils are chains of tiny contractile units called
sarcomeres which are aligned end to end like boxcars in train
---is arrangement of smaller myofilaments within sarcomeres that
actually produces banding pattern
5. Describe myofilament arrangement.
---two types of threadlike protein myofilaments within each
sarcomere
---larger, thick filaments (myosin filaments) extend entire
length of dark A band
--midparts of thick filaments are smooth but ends are studded
with small projections (myosin heads or cross bridges)
---thin filaments (actin filaments) are anchored to Z line
which is actually disclike membrane
---light I band is area that includes parts of two adjacent
sarcomeres and contains ONLY thin filaments
---thin filaments overlap ends of thick filaments, they do NOT
extend into middle of relaxed sarcomere, thus central region
(H zone) looks bit lighter
---when contraction occurs, actin containing filaments slide
toward each other into center of sarcomere and light zones
disappear because actin and myosin filaments completely
overlap
6. List the events of sarcomere contraction.
---when muscle fibers activated by nervous system, cross
bridges on myosin attach to myosin binding sites on the
thin filaments (actin)
---each cross bridge attaches and detaches several times
during contraction and pulls thin filaments toward the
center of sarcomere
---as this event occurs simultaneously in sarcomeres throughout
cell, muscle cell shortens
---Z lines move closer together
---H zone disappears
---A bands move closer together but do NOT change in length
---contraction of millions of sarcomeres in millions of fibers
results in contraction of entire skeletal muscle
7. Describe the connective tissue wrappings of skeletal muscle.
---endomysium wraps each individual muscle fiber (cell)
---perimysium wraps bundles of fibers into a fascicle
---epimysium covers the entire muscle
---the epimysium is continuous with tendons or aponeuroses
8. Briefly describe the smooth muscles.
---smooth muscle has NO striations and is involuntary
---found in walls of hollow visceral organs like stomach,
urinary bladder, digestive tract, bronchi, uterus, blood
vessels
---key terms are visceral, nonstriated, and involuntary
---are spindle-shaped, have single nucleus, and arranged
in sheets or layers
---contractions are slow and sustained
--does not tire easily
---movement of food through digestive tract, emptying bowels
and bladder, maintenance of blood pressure
9. Briefly describe cardiac muscle.
---keys terms are cardiac, striated, and involuntary
---branching cells joined by special junctions called
intercalated disks
---arranged in spiral shape
---allows contractions to closely coordinated
10. Summarize the nerve aspects of skeletal muscle contraction.
---each muscle fiber must be stimulated separately by nerve
impulses to contract
---one motor neuron (nerve cell) and all skeletal muscle cells
it stimulates are a motor unit
---threadlike extensions of neuron (nerve fiber/axon) branch
into number of axonal terminals at muscle
--each axonal terminal forms junctions with sarcolemma of
different muscle cell
---these junctions are called neuromuscular junctions
--nerve endings and muscle cells’ membranes NEVER touch
--gap between them called synaptic cleft and is filled with
interstital fluid
---when nerve impulse reaches axonal terminals, neurotransmitter
is released
--specific neurotransmitter that stimulates muscle cells
is acetylcholine (Ach)
---acetylcholine diffuses across synaptic cleft
---attaches to receptors on sarcolemma
--if enough acetylcholine released, sarcolemma becomes
temporarily permeable to sodium ions (Na+) which rush
into muscle cell
---generates electrical current called action potential
---action potential travels over entire surface of sarcolemma
conducting impulse from one end of cell to the other
---result is contraction of the cell
11. List the steps in the sliding-filament theory of muscle
contraction.
---nerve impulse reaches neuromuscular junction
---acetylcholine released
---acetylcholine causes action potential in sarcolemma
---action potential in sarcolemma causes sarcoplasmic
reticulum to release stored calcium ions into sarcoplasm
---calcium ions cause cross-bridges to form
---thin myofilaments (actin) pulled over thick (myosin)
myofilaments
--energy provided by ATP
---sarcomere contracts
---action potential ends, calcium ions reabsorbed
---cross-bridges turn loose
---sarcomere relaxes
---neurotransmitter acetylcholine broken down by enzymes
in synaptic cleft
---prevents continuous stimulation of muscle fiber
---acetylcholinesterase (care for some Raid, anyone?)
12. Distinguish between twitch, incomplete tetanus, and complete
tetanus.
---a muscle fiber contracts in all-or-none fashion
---whole muscles DO NOT CONTRACT THAT WAY
---skeletal muscles are organs composed of 1000s of muscle
cells and they react to stimuli with graded responses (or
different degrees of shortening
---graded muscle contractions are produced in two ways
--changing the speed of muscle stimulation
--changing the number of muscle cells being stimulated
---muscle twitch is a single, brief, jerky contraction that
occurs as result of certain nervous system problems
--NOT the way muscle normally operates
--single stimulus-contraction-relaxation sequence in
muscle fiber
--twitches in skeletal muscle do not accomplish anything
useful
---incomplete tetanus results when nerve impulses are delivered
to muscle at very high rate
--so rapidly that cells do not get chance to relax completely
between stimuli
--stimulation continues and muscle never allowed to relax
completely will cause tension to peak
--muscle producing peak tension during rapid cycles of
contraction and relaxation is said to be in
incomplete tetanus
---complete tetanus results when muscle is stimulated so
rapidly that no evidence of relaxation is seen and
contractions are completely smooth and sustained
13. Relate the number of fibers stimulated to load moved.
---complete tetanus major role is to produce smooth and
prolonged muscle contractions
---how forceful a muscle contracts depends in large part on
how many of its cells are stimulated
---few cells stimulated = contraction of muscle as whole slight
---all muscle cells stimulated = muscle contraction as strong
as it can be
---muscle contractions can be slight or vigorous, depending on
what work has to be done
14. Discuss muscle fatigue and oxygen debt.
---if a muscle is subject to continual contraction for a long
time, muscle fatigue occurs
---muscle is fatigued when it is unable to contract even though
it is still be stimulated
---without rest, active/working muscle begins to tire and
contracts more weakly until it finally ceases reacting and
stops contracting
---muscle fatigue is believed to result from oxygen debt that
occurs during prolonged muscle activity
---person not able to take in oxygen fast enough to supply
muscles with oxygen they need
--work muscle can do and how long it can work without
becoming fatigued depend on how good its blood supply is
---if muscle runs out of oxygen, it must depend on glycolysis
for ATP and converts pyruvic acid to lactic acid
---lack of adequate ATP and increasing acidity cause muscle
to contract less and less effectively and finally to stop
contracting all together
15. Distinguish between isotonic and isometric contractions.
---isotonic (same tone/tension) contractions more familiar
---myofilaments are successful in their sliding movements,
muscle shortens, and movement occurs
---bending knee, rotating arms, and smiling are examples of
isotonic contractions
---isometric (same measurement/length) contractions are
contractions in which muscles do NOT shorten
---myosin myofilaments are skidding their wheels and tension
in muscle keeps increasing
--trying to slide but muscle pitted against some more or
less immovable objects
--trying to lift 400 lb. dresser alone or pushing on
immovable wall
16. Explain muscle tone.
---when muscle is voluntarily relaxed, some of its fibers are
contracting; first one group then another
---as result, muscle remains firm, healthy, and ready for action
---this state of continuous partial contractions is called
muscle tone
--is result of different motor units which are scattered
through muscle being stimulated by nervous system in
systematic way
---nerve supply to muscle is destroyed, muscle no longer
stimulated in this manner, it loses tones and becomes
paralyzed
---soon after, becomes flaccid (soft/flabby) and begins to
atrophy (waste away)