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Body movement
Maintenance of posture
Respiration
Production of body heat
Communication
Constriction of organs and vessels
Heart beat
• Contractility: ability of a muscle to shorten with force
• Excitability: ability of muscle to receive & respond to
stimuli
• Extensibility: muscle can be stretched beyond its
normal resting length and is still able to contract
• Elasticity: ability of muscle to recoil to original resting
length after stretched
• Muscles are responsible for all types of
body movement
• Three basic muscle types are found in the
body
– Skeletal muscle
– Cardiac muscle
– Smooth muscle
• These types differ in structure, location,
function, and means of activation
• Skeletal and smooth muscle cells are
elongated and are called muscle fibers
• Muscle contraction depends on two kinds
of myofilaments – actin and myosin
• Muscle terminology is similar
– Prefixes – myo, mys, and sarco all refer to
muscle
– Sarcolemma – muscle plasma membrane
– Sarcoplasm – cytoplasm of a muscle cell
• Skeletal muscles that attach to and
cover the bony skeleton
• Has obvious stripes called striations
• Is controlled voluntarily (i.e., by
conscious control)
• Contracts rapidly but tires easily
• Responsible for locomotion, facial
expressions, posture, respiratory
movements, other types of body
movement
• Is extremely adaptable and can exert
forces ranging from a fraction of an
ounce to over 70 pounds
• Found in the walls of hollow
visceral organs, such as the
stomach, urinary bladder, and
respiratory passages
• Some functions: propel urine, mix
food in digestive tract,
dilating/constricting pupils,
regulating blood flow
• Not striated
• Controlled involuntarily by
endocrine and ANS
• Occurs only in the heart
• Striated like skeletal muscle
• Involuntary
• Heart: major source of
movement of blood
• Autorhythmic
• Controlled involuntarily by
endocrine and ANS
• Composed of muscle
cells (fibers), connective
tissue, blood vessels,
nerves
• Fibers are long,
cylindrical,
multinucleated
• Striated appearance due
to light and dark banding
• The three connective tissue
sheaths are:
– Endomysium – fine sheath of
connective tissue composed of
reticular fibers surrounding
each muscle fiber
– Perimysium – fibrous
connective tissue that
surrounds groups of muscle
fibers called fascicles
– Epimysium – Dense regular
connective tissue that surrounds the
entire muscle (many fascicles)
• Each muscle is served by one
nerve, an artery, and one or more
veins
• Each skeletal muscle fiber is
supplied with a nerve ending that
controls contraction
• Contracting fibers require
continuous delivery of oxygen
and nutrients via arteries
• Wastes must be removed via
veins
• Most skeletal muscles span joints
and are attached to bone in at least
two places
• When muscles contract the movable
bone, the muscle’s insertion moves
toward the immovable bone, the
muscle’s origin
•
Muscles attach:
– Directly – epimysium of the
muscle is fused to the periosteum
of a bone
– Indirectly – connective tissue
wrappings extend beyond the
muscle as a tendon or
aponeurosis
• Outer boundary of the cell is made of
plasma membrane – sarcolemma
• Multiple nuclei present inside sarcolemma
• Cytoplasm of muscle cell - sarcoplasm
• Endoplasmic reticulum – Sarcoplasmic
reticulum
• Most of muscle fiber is packed with
myofibrils
• Other organelles, such as mitochondria,
glycogen granules, sarcoplasmic
reticulum, and T tubules are found
between myofibrils
•
Myofibrils: are thread like
organelles
–
Composed of protein threads
called myofilaments:
–
–
thin (actin)
thick (myosin)
–
Sarcomeres: repeating units of
myofilaments
•
Interaction between actin and
myosin filaments leads muscle to
shorten or contract
•
The tropomyosin/troponin complex
regulates the interaction between
actin and myosin
• Consists of two strands of fibrous (F)
actin, troponin and tropomyosin
molecules
• Strands of F actin coiled to form double
helix
• Each F actin strand is composed of G
actin monomers each of which has an
active site
• Each F actin strand consists of 200 G
actin monomers
• Active site binds with myosin during
muscle contraction
• Tropomyosin: an elongated
protein winds along the groove of
the F actin double helix
• Troponin is composed of three
subunits: one that binds to actin, a
second that binds to tropomyosin,
and a third that binds to calcium
ions
• The tropomyosin/troponin
complex regulates the interaction
between active sites on G actin
and myosin
•
Thick filaments are composed of
the protein myosin
•
Shaped like golf clubs
•
Each myosin molecule has a rodlike tail and two globular heads
– Tails – two interwoven, heavy
polypeptide chains
– Heads – Each head contains
two smaller, light polypeptide
chains
•
Each myosin filaments consists
of about 300 myosin molecules
•
Properties of Myosin heads
1. Can bind to active sites on
the actin molecules to form
cross-bridges
2. Attached to the rod portion
by a hinge region, bend and
straighten during contraction
3. Heads have ATPase activity,
activity that breaks down
ATP, releasing energy
•
The smallest contractile
unit of a muscle
•
The region of a myofibril
between two Z discs
•
The point where actin
originates is called Z
disk
•
Each sarcomere has
alternating actin and
myosin filaments
•
The arrangement of myosin
(dark in color and is called
anisotropic band or A band)
•
And actin (light in color and
is called isotropic or I band)
alternatively
•
Gives the muscle a striated
appearance
•
H zone (bare zone) - lacks
actin filament
•
M line: middle of H zone;
delicate filaments holding
myosin in place
• Upon stimulation, myosin
heads bind to actin and sliding
begins
• Actin myofilaments slide over
myosin to shorten sarcomeres
– Actin and myosin do not
change length
– Shortening sarcomeres
responsible for skeletal
muscle contraction
• Nervous system controls muscle contractions
through action potentials
• Electrical signals, called action potentials
travel from brain or SC via axons of the nerve
to muscle fibers and cause them to contract
• Skeletal muscles are stimulated by motor
neurons
• Axons of neurons branch (axon terminals) as
they enter muscles
• Each axonal branch forms a neuromuscular
junction with a single muscle fiber
•
Neuromuscular junctions – association site of
nerve and muscle
• The neuromuscular junction is
formed from:
– Axonal endings, which have
small membranous sacs
(synaptic vesicles) that
contain the neurotransmitter
acetylcholine (ACh)
– Sarcolemma of the muscle
fiber is highly folded that
contains ACh receptors and
helps form the
neuromuscular junction
• Axonal ends and muscle fibers are
always separated by a space
called the synaptic cleft
• When a nerve impulse reaches the
end of an axon at the
neuromuscular junction:
– Voltage-regulated calcium
channels open and allow Ca2+
to enter the axon
– Ca2+ inside the axon terminal
causes synaptic vesicles to fuse
with the axonal membrane
– This fusion releases ACh
into the synaptic cleft via
exocytosis
– ACh diffuses across the
synaptic cleft to ACh
receptors on the
sarcolemma
– Binding of ACh to its
receptors initiates an
action potential in the
muscle
• ACh bound to ACh receptors is
quickly destroyed by the enzyme
acetylcholinesterase
• This destruction prevents continued
muscle fiber contraction in the
absence of additional stimuli
• At rest, membranes are polarized
• There is voltage difference across
membranes
• Difference between charge inside and
outside cell membrane = RESTING
MEMBRANE POTENTIAL (RMP)
• Inside of cell membrane is more
negative charge than outside
•
Due to the presence of more positive
ions (Na+) outside the cell
• Axonal terminal of a motor
neuron releases ACh and binds
to the receptors of sarcolemma
• Causes the opening of Sodium
channels
•
Sodium ions enter rapidly
– RMP becomes more positive
• If the stimulus is strong enough,
reaches threshold
• Depolarization occurs
• An action potential is initiated
• Thus, the action potential travels
rapidly along the sarcolemma
• Once initiated, the action
potential is unstoppable, and
ultimately results in the
contraction of a muscle
• Immediately after the depolarization wave
passes, the sarcolemma permeability
changes
• Na+ channels close and K+ channels open
• K+ diffuses from the cell
• Repolarization takes place
• The ionic concentration of the resting state
is restored by the Na+-K+ pump
•
All-or-none principle: like camera flash
system
• AP produced in sarcolemma of muscle
lead to muscle fiber contraction by
Excitation-Contraction Coupling
• Involves
– Sarcolemma
– Transverse (T) tubules: invaginations
of sarcolemma
– Sarcoplasmic reticulum: smooth ER
– Terminal cisternae: Enlarged SR
– Triad: T tubule, two adjacent terminal
cisternae
– Ca2+
– Troponin
• AP produced at neuromuscular
junction:
– Is propagated along the
sarcolemma
– Travels down the T tubules
– Triggers Ca2+ release from
terminal cisternae
• Ca2+ released from SR binds to
troponin and causes:
– The blocking action of
tropomyosin to cease
• Active binding sites of actin are
exposed
• Myosin heads attach with actin and
forms cross bridges
• Thin filaments move toward the
center of the sarcomere
• Hydrolysis of ATP powers this
cycling process
• Ca2+ is removed into the SR,
tropomyosin blockage is restored,
and the muscle fiber relaxes
•
A muscle twitch is contraction of muscle in response to
a stimulus that causes action potential in one or more
muscle fibers
• There are three phases to a muscle twitch
– Lag (latent) phase
– Contraction phase
– Relaxation phase
• Lag phase – first few msec
after stimulus; EC coupling
taking place
• Contraction phase – cross
bridges form; muscle
shortens
• Relaxation phase –
Reentry of Ca2+ in SR
• In response to each action potential muscle fiber
produces contraction of equal force
• Follow All-or-none law - muscle fibers
• Sub-threshold stimulus: no action potential;
no contraction
• Threshold stimulus: action potential;
contraction
• Stronger than threshold : action potential;
contraction equal to threshold stimulus
• Muscle fiber contraction is “all or none”
• But in whole muscle not all fibers may be stimulated
during the same interval
• Different combinations of muscle fiber contractions may
give differing responses – Graded Response
• Motor units (Nerve-Muscle Functional Unit) : a single
motor neuron and all muscle fibers innervated by it
• Strength of contraction is
graded: ranges from weak to
strong depending on stimulus
strength
• Multiple motor unit summation:
strength of contraction depends
upon no. of motor units
• A muscle has many motor units
– Submaximal stimuli
– Maximal stimulus
– Supramaximal stimuli
• As the frequency of action
potentials increase, the
frequency of contraction
increases
– Incomplete tetanus: muscle
fibers partially relax between
contraction
– Complete tetanus: no
relaxation between
contractions
– Multiple-wave summation:
muscle tension increases as
contraction frequencies
increase
• Graded response
• Occurs in muscle rested for
prolonged period
• Each subsequent contraction
is stronger than previous until
all equal after few stimuli
• Possible explanation: more
and more Ca2+ remains in
sarcoplasm and is not all
taken up into the
sarcoplasmic reticulum
• Isometric Contraction: no change in muscle length but tension
increases during contraction
– Eg. Postural muscles of body, muscles that hold the spine
erect while a person is sitting or standing
• Isotonic Contraction: change in length but tension constant
during contraction, eg. Movement of upper limbs, fingers such
as waving, using a computer
– Concentric: Tension in muscle overcomes opposing
resistance and muscle shortens, eg. Lifting a loaded
backpack from the floor to table
– Eccentric: tension maintained, enough opposing resistance
to cause the muscle to increase in length, eg. Person slowly
lowers the heavy weight
• Decreased capacity to work and reduced efficiency of
performance
• Types
– Psychological: depends on emotional state of
individual
– Muscular: results from ATP depletion
– Synaptic: occurs in Neuromuscular junction due to
lack of acetylcholine
– Physiological contracture: state of fatigue where
due to lack of ATP neither contraction nor
relaxation can occur
– Rigor mortis: development of rigid muscles
several hours after death. Ca2+ leaks into
sarcoplasm and attaches to myosin heads and
cross bridges form. Rigor ends as tissues start to
deteriorate
• ATP provides immediate energy for muscle contractions
• Produced from three sources
– The interaction of ADP with creatine phosphate (CP)
– Anaerobic respiration
– Aerobic respiration
• Direct phosphorylation of
ADP by creatine phosphate
– Muscle cells contain creatine
phosphate (CP)
• CP is a high-energy molecule
– ADP is left, after ATP is
depleted,
– CP transfers energy to ADP, to
regenerate ATP
– CP supplies are exhausted in
about 15 seconds
• Aerobic Respiration
– Series of metabolic pathways occur
in the mitochondria, require oxygen
– Known as oxidative phosphorylation
– Glucose is broken down to carbon
dioxide and water, & release energy
in the form of ATP
– This is a slower reaction that
requires continuous oxygen and
nutrient fuel
– 36 ATP/ glucose
• Anaerobic glycolysis
– Reaction that breaks
down glucose without
oxygen
– Glucose is broken
down to pyruvic acid to
produce some ATP
– Pyruvic acid is
converted to lactic acid
• Anaerobic glycolysis
• This reaction is not as
efficient, but is fast
• 2ATP/glucose
• Huge amounts of
glucose are needed
• Lactic acid produces
muscle fatigue
• Vigorous exercise causes dramatic changes in muscle
chemistry
• For a muscle to return to a resting state:
–
–
–
–
Oxygen reserves must be replenished
Lactic acid must be converted to pyruvic acid
Glycogen stores must be replaced
ATP and CP reserves must be resynthesized
• Slow-twitch oxidative
– Contract more slowly, smaller in diameter, better blood
supply, more mitochondria, more fatigue-resistant than
fast-twitch, large amount of myoglobin.
– Postural muscles, more in lower than upper limbs. Dark
meat of chicken.
• Fast-twitch
– Respond rapidly to nervous stimulation, contain myosin
ATPase that can break down ATP more rapidly than that
in Type I, less blood supply, fewer and smaller
mitochondria than slow-twitch
– Lower limbs in sprinter, upper limbs of most people.
White meat in chicken.
• Not striated, fibers smaller than those in
skeletal muscle
• Spindle-shaped; single, central nucleus
• More actin than myosin
• Caveolae: indentations in sarcolemma;
may act like T tubules
• Ca2+ required to initiate contractions;
binds to calmodulin which regulates
myosin kinase. Cross-bridging occurs
• Relaxation: caused by enzyme myosin
phosphatase
Smooth Muscles
• Arranged in two layers:
• circular layer
• longitudinal layer
• These two layers alternately
contract and relax
• And move food through digestive
tract, emptying the bowels & bladder
• Maintain housekeeping activities
• Slow and steady
• Visceral or unitary: cells in sheets; function as a unit
– eg. Digestive, reproductive, urinary tracts
– Numerous gap junctions
– Allow Action potential to pass from cell to cell
– Often autorhythmic
• Multiunit: cells or groups of cells act as independent
units
– Sheets (blood vessels); bundles (arrector pili and
iris); single cells (capsule of spleen)
– Fewer gap junctions
– Contracts when stimulated by nerves or
hormones
• Slow waves of depolarization
and repolarization transferred
from cell to cell
• Depolarization caused by
spontaneous diffusion of Na+
and Ca2+ into cell
• Does not follow all-or-none law
• Contraction regulated by
nervous system and by
hormones
• Some visceral muscle exhibits autorhythmic contractions
• Tends to contract in response to sudden stretch but not
to slow increase in length
• Exhibits relatively constant tension: smooth muscle tone
• Amplitude of contraction remains constant although
muscle length varies
• Innervated by autonomic nervous system
• Neurotransmitters are acetylcholine and
norepinephrine
• Hormones important as epinephrine and
oxytocin
• Receptors present on plasma membrane to
which neurotransmitters or hormones bind
determines the response of smooth muscle
• Found only in heart
• Striated
• Each cell usually has one
nucleus
• Has intercalated disks and gap
junctions
• Autorhythmic cells
• Action potentials of longer
duration and longer refractory
period
• Ca2+ regulates contraction
• Reduced muscle mass
• Increased time for muscle to contract in
response to nervous stimuli
• Reduced stamina
• Increased recovery time
• Loss of muscle fibers
• Decreased density of capillaries in muscle