Download Chapter 6 The Muscular System - Tri

Chapter 6 The Muscular System
Biology 112
Tri-County Technical College
Pendleton, SC
Functions of Muscles
• Essential function of muscle is
CONTRACTION (shortening)
• Separates it from other body tissue
• Responsible for essentially all body
movement
• Can be viewed as “machines” of body
• Makes up nearly half of the body’s mass
Types of Muscle
• Skeletal, Cardiac, and Smooth
• Share certain characteristics
– Muscle cells are elongated and are called
MUSCLE FIBERS
– Ability to shorten depends on TWO types of
MYOFILAMENTS
• Muscle cells =“microfilaments of cytoskeleton”
• “Myo,” “Mys,” and “Sarco” refer to muscle
• Sarcoplasm-cytoplasm of muscle cells
Types of Muscles, visual
Skeletal Muscle
• Skeletal muscle fibers packed into organs called
skeletal muscles that attach to skeleton
• Known as striated muscle = fibers appear striped
• Only muscle subject to CONSCIOUS control
• Each fiber enclosed in CT sheath called
ENDOMYSIUM
• Several sheathed muscle fibers wrapped by
membrane called PERIMYSIUM to form bundle
of fibers called a FASCICLE
Skeletal Muscle, cont.
• Many fascicles bound by EPIMYSIUM which
covers entire muscle
• EPIMYSIS blend into strong, cordlike
TENDONS (sheetlike aponeuroses) which attach
muscles indirectly to bone, cartilages, or
connective tissue covering of each other
• Spend some time on 6.1; page 164
Skeletal Muscle, visual
Sarcomere & Myofibrils
• Plasma membrane of muscle cell called
SARCOLEMMA
• Long ribbonlike organelles called
MYOFIBRILS nearly fill cytoplasm
• Alternating light (I) and dark (A) bands along
length of myofibril = striped appearance
• Light I band has midline interruption; a darker
area called the Z DISC (line)
• Dark A band has lighter central area called the H
zone
Sarcomere, cont.
• Myofibrils are chains of tiny contractile units
called SARCOMERES which are aligned end to
end like boxcars in train
• IT is arrangement of smaller
MYOFILAMENTS within sarcomere that
actually produces banding pattern
• Spend some quality time on Figure 6.3; page
167
Myofilament Arrangement
• Each sarcomere contains two types of myofilaments
• Large thick filaments (MYOSIN) 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) 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
Myofilament Arrangement, cont.
• Thin filaments overlap ends of thick
filaments but DO NOT extend into middle
of relaxed sarcomere
• Thus central region (H zone) looks bit
lighter
• Contraction occurs, actin containing
filaments slide toward each other into center
of sarcomere and light zones disappear
– Actin and myosin filaments completely overlap
Sarcomere Contraction
• Fibers activated by NS, cross bridges on myosin attach
to myosin binding sites of actin filaments
• Attaches and detaches several times during contraction
and PULLS thin filaments toward center of sarcomere
• 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
• Millions of sarcomeres in millions of fibers =
contraction of entire skeletal muscle
Contraction Visual
Connective Tissue Wrappings
• Skeletal muscle wrapped by connective tissue
• ENDOMYSIUM wraps each individual muscle
fiber (cell)
• PERIMYSIUM wraps bundles of fibers into a
FASCICLE
• EPIMYSIUM covers the entire muscle
– Epimysium is continuous with tendons or
aponeuroses
Smooth Muscle
• Has NO striations and is involuntary
• Walls of hollow visceral organs like stomach, urinary
bladder, digestive tract, bronchi, uterus, and blood
vessels
• Visceral-nonstriated-involuntary
• Spindled-shaped fibers with single nucleus and
arranged in sheets/layers
• Contractions SLOW and SUSTAINED-does Not tire
easily
• Movement of food through digestive tract, emptying
bowels and bladder, & maintenance of blood pressure
Smooth Muscle, Visual
In a heartbeat…so to speak
• Cardiac muscle best described as
CARDIAC, STRIATED, AND
INVOUNTARY
• Branching cells joined by special junctions
called intercalated disks
• Cardiac muscle arranged in spiral shape
• Allows contractions to be closely
coordianted
Nerve Aspects
• Each muscle fiber must be stimulated separately
by nerve impulses to contract
• MOTOR UNIT-one motor neuron (nerve cell)
and all the skeletal muscle cells it stimulates
• Threadlike extensions of neuron (axon/nerve
fiber) branch into number of axonal terminals
at muscle
– Each axonal terminal forms junctions with
sarcolema of different muscle cell
• Junctions called neuromuscular junctions
Motor Unit, Visual
Neuromuscular Junction, Visual
Nerve Aspects, cont.
• Nerve endings and muscle cells’ membranes
NEVER TOUCH
– Gap between them called synaptic cleft and is
filled with interstitial fluid
• Nerve impulse reaches axonal terminals, a
neurotransmitter is released
– Specific neurotransmitter that simulates muscle
cells is ACETYLCHOLINE (Ach)
• Acetylcholine diffuses across synaptic cleft
Nerve Aspects, cont.
• Acetylcholine attaches to receptors on sarcolemma
– If enough released, sarcolemma becomes temporary
permeable to sodium ions (Na+) which rush into muscle
cell
• Generates electrical current called action
potential
• AP travels over entire surface of sarcolemma
conducting impulse from one end of cell to the
other
• Result is CONTRACTION of the cell
Action Potential, Visual
Sliding Filament Theory
• Nerve impulseneuromuscular junction 
acetylcholine releasedAP in sarcolemma
• AP in sarcolemma causes sarcoplasmic
reticulum to releases stored calcium ions
into sarcoplasm
• Calcium ions cause cross-bridges to from
• Thin myofilaments (actin) pulled over thick
(myosin) myofilaments
– Energy provided by ATP
SF Theory, cont.
•
•
•
•
Sacromere contracts
AP ends, calcium ions reabsorbed
Cross-bridges turn loose & sarcomere relaxes
Neurotransmitter acetylcholine degraded by
enzymes in synaptic cleft
• Prevents continuous stimulation of muscle
fiber
• Acetylcholinesterase (care for some Raid,
anyone?)
Contraction Mechanism, Visual
To twitch or not to twitch
• 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 which react to stimuli with
GRADED RESPONSES (different degrees of
shortening)
• Graded muscle contractions produced in 2
ways
– Changing speed of muscle stimulation
– Changing number of muscle cells being stimulated
Twitching time, cont.
• Muscle twitch is 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
– DOES NOT accomplish anything useful in
skeletal muscle
Twitch and more, cont.
• Incomplete tetanus results when nerve impulses
delivered to muscle at very high rate
• Delivered so rapidly 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/relaxation said to be in
incomplete tetanus
Enough on twitching already
• Complete tetanus results when muscle is stimulated
so rapidly that NO evidence of relaxation is seen
• Contractions are completely smooth and sustained
• Complete tetanus major role = smooth and prolonged
muscle contractions
• Force of muscle contraction depends on how many
of its cells are stimulated
• Few cells = contraction as whole is slight
• All cells = muscle contraction as strong as it can be
Fatigue and Debt
• Muscle subject to continual contraction for long
time = muscle fatigue occurs
• Muscle is fatigued when unable to contract even
though still be stimulated
• Without rest, active/working muscle begins to tire
and contracts more weakly until finally ceases
reacting and stops contracting
• MF believed to result from oxygen debt that
occurs during prolonged muscle activity
Fatigue & Debt, cont.
• Work muscle can do and how long it can
work w/o becoming fatigued depend on
how good its blood supply is
• If muscle runs out of O2, it must depend on
glycolysis for ATP and converts pyruvic
acid to lactic acid
• Lack of adequate ATP and >acidity cause
muscle to contract less effectively and
finally to stop contracting all together
Regeneration of ATP, Visual
Isotonic Contractions
• Isotonic (same tone/tension) contractions
most familiar
• Myofilaments are successful in sliding
movements, muscle shortens, and
movement occurs
• Bending knee, rotating arms, and smiling
are examples of isotonic contractions
Isometric Contractions
• Isometric (same measurement/length)
contractions are contractions in which muscles do
NOT shorten
• Mysoin myofilaments skidding their “wheels” and
tension in muscle keeps increasing
• Trying to slide but muscle pitted against some
more/less immovable object(s)
• Trying to lift 400 lb dresser along or pushing
against immovable wall
Muscle tone
• When muscle voluntarily relaxed, some of its
fibers are contracting (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 the result of different motor units scattered
through muscle being stimulated by nervous
system in systematic way
Muscle tone, cont.
• If nerve supply to muscle is destroyed,
muscle NO longer stimulated in this manner
• It loses TONE, and becomes paralyzed
• Soon after, becomes flaccid (soft/flabby)
and begins to atrophy (waste away)