Download Muscular System

The Muscular System
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Types of Muscle:
1. Smooth/Visceral
a. Characteristics
– Spindle-shaped cells (fusiform)
– Single nucleus
– No striations
– Involuntary
– Capable of mitosis
b. Function
Line arteries/veins, GI tract, ureters, urethra.
Types of Muscle:
2. Cardiac
a. Characteristics
– Striated
– Branched
– 1-2 nuclei
– Intercalated discs- allow
heart to contract as a unit.
– Involuntary
– Incapable of mitosis
b. Function
Cause heart to beat
Types of Muscle:
3. Skeletal
a.
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Characteristics
Striated
Multi-nucleated
Voluntary
b. Function
Movement
Posture
Heat production
Communication/facial expression
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Terms related to skeletal muscle
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Tendons- anchor muscle to bone.
Origin - attachment to stationary bone.
Insertion - attachment to moveable bone.
Bursae - sacs between tendons and bones.
Synovial membrane/fluid- lines bursae.
Makes it easier for tendon to slide over
bone.
• Tendon sheath- encloses some tendons.
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Overall Structure of a Skeletal Muscle
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Microscopic Structure of a single muscle fiber:
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Microscopic Structure of a
Skeletal Muscle
• Each muscle is made up of 1000s of muscle cells
or muscle fibers.
• Each muscle fiber contains myofibrils, which are
large bundles (myofilaments) of proteins:
– Thick filaments are made of myosin.
– Thin filaments are made of actin.
• Sarcomere - contractile unit of a muscle fiber.
– Repeating sarcomeres are separated from each other by
dark bands called Z lines.
• Striations in skeletal muscle are caused by:
– A bands (dark); overlapping actin and myosin
– I bands (light); actin only.
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The Sliding Filament Model:
Overview
• During contraction, actin and myosin attach
to each other forming “bridges” that pull
myofilaments past each other. This requires
ATP.
– The connecting bridges will only form properly
is calcium is present.
• During the relaxed state, calcium is the ER(SR).
• Calcium is released when the muscle is stimulated
by a nerve.
Types of proteins:
• Contractile proteins
– Actin
– Myosin
• Regulatory proteins (on/off switch):
– Troponin
– Tropomyosin
• Structural proteins (keep thin and thick filaments
aligned):
– Dystrophin: when missing causes Muscular Dystrophy
– Titin
Sliding Filament Model: Contraction
1) Motor neuron releases acetylcholine at
neuromuscular junction.
2) Ca2+ is released out of SR into muscle cell body.
3) Ca2+ binds to troponin/tropomyosin, uncovering the
active sites on actin.
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4) Myosin heads form bridges on/with active
sites on actin.
5) Actin is pulled over myosin as heads move
in like paddles.
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Sliding Filament Model:
Relaxation
1) Ca2+ is pumped back into SR.
2) Troponin and tropomyosin cover active
sites on actin.
*Refractory period: 5 milliseconds
-time during which muscle cannot receive
another stimulus
Physiology of a Smooth Muscle
• The sliding filament model applies to all muscle
tissue.
• Unique to smooth muscle:
– Myofilaments are not arranged in a definite striped
pattern = no striations.
– Contain same amount of actin but less myosin.
– Contains no troponin (other proteins instead).
– Dense bodies are analogous to Z lines in striated
muscle.
Anatomy of a Smooth Muscle
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Rigor Mortis
• Upon death, muscle cells are unable to
prevent calcium entry from the SR into the
cytosol.
• This allows myosin to bind with actin.
• Since there is no ATP made the myosin
cannot unbind and the body remains in a
state of rigidity for the next couple of days.
Functions of Skeletal Muscle
1) Movement
– Muscles can only pull, never push
– only insertion bone moves.
– Prime mover: the one muscle primarily
responsible for producing a particular
movement.
– Synergists: helpers muscles.
• Ex. Bending the elbow:
– Prime mover = biceps brachii
– Synergist= brachialis
– When synergists immobilize a bone they
are called fixators.
• Ex. Rhomboids and levator scapulae keep scapula
from moving during actions such as lifting with the
arms.
– Antagonists: muscles that relax as prime mover
and synergists contract.
• Ex. Biceps brachii and triceps brachii
2) Posture
– Maintained by tonic contraction.
• Relatively few of a muscle’s fibers shorten at
one time so the muscle as a whole does not
shorten and no movement occurs.
• Counteracts with gravity.
3) Heat Production
– The contraction of muscle fibers produces most
of the heat required to maintain body
temperature.
– Some energy lost as heat during hydrolysis of
ATP.
Muscle Stimulus
• Threshold: the minimal level of stimulation
required to cause a muscle fiber to contract.
• Contraction is all or none.
• The power of the contraction depends on
how many muscle fibers are activated.
Contraction of a Skeletal Muscle
• Muscle tension: the force exerted by a
contracting muscle on an object.
• Load: the opposing force exerted on the
muscle by the weight of an object to be
moved.
• The Motor Unit: a motor neuron and all the
muscle fibers it innervates.
– The number of muscle fibers per motor unit varies
greatly depending on their location in the body.
• Ex. Muscles of the eye: precise control is needed. A
motor unit is 4-6 muscle fibers.
• Leg muscle - a single motor neuron may control
1000-2000 muscle fibers.
Type of Skeletal Muscle
Contractions
1) Isotonic
– If the muscle tension developed overcomes the
load and the muscle shortens.
Ex. Lifting free weights, walking, running, breathing.
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2) Isometric:
Is muscle tension develops but the muscle
does not shorten and the load is not moved.
Allows muscles to grow larger and stronger.
Ex. Yoga poses. Holding but not lifting free
weights.
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3) Twitch: a quick, jerky response to a stimulus.
4) Tetanic: a more sustained and steady response.
– Produced by a series of stimuli bombarding the
muscle in rapid succession.
– Contractions melt together to produced a sustained
contraction (tetanus).
• Not to be confused with the bacterial disease
tetanus that causes severe involuntary
contractions.
Two types of skeletal muscle
fibers:
1) Slow Twitch Fibers (“red” muscle)
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Contract for long periods but with little force.
Muscle is dense with capillaries (red color)
and mitochondria.
It can carry more O2 and sustain more aerobic
activity
Best suited for endurance activities
2) Fast Twitch fibers (“white” muscle)
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Contract quickly and powerfully but fatigue
easily
Best suited for short term, power activities.
Is anaerobic, less dense in mitochondria
• Why do chickens have white breast meat
and dark leg meat? What does this say
about the activities of associated muscles?
• Why do ducks have dark breast meat?
Muscle Fatigue
– If repeated stimulation of a muscle occurs the
strength of the contraction decreases and the
muscle looses its ability to contract.
– When there is not enough oxygen the muscle
cells switch to anaerobic respiration which
produces lactic acid as a bi-product = burning
sensation.
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Effects of Exercise on Skeletal
Muscles
• Disuse atrophy: shrinking of muscle in
mass due to prolonged inactivity
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• Hypertrophy: an increase in muscle size by
increasing the number of myofilaments
within a muscle cell.
– Strength training: isometric
exercises
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• Endurance/aerobic training: increases a muscle’s ability to
sustain moderate exercise of a long period.
– Isotonic movements: increase number of blood vessels
to a muscle
• Increased blood flow=more efficient delivery of
oxygen to muscle fibers.
– Increases the number of mitochondria. Allows
greater production of ATP as a rapid energy
source.
Movements produced by skeletal
muscle contractions
• Flexion: Flex
• Extension: Extend
• Abduction: moving a part away from the
midline of body.
• Adduction: moving a part toward the
midline.
• Rotation: movement along a longitudinal axis.
(head roll).
• Circumduction: rotation that moves a part so that
its distal end moves in a circle. (throwing a pitch)
• Supination: rotation of forearm so that hand faces
up.
• Pronation: rotation of forearm so that hand faces
down.
• Dorsiflexion: foot flexes so toes point up.
• Plantar flexion: foot flexes so toes point
down.
Muscular Injury
• Fibromyostis (charley horse): tendon
inflammation
• Myalgia: muscle pain
• Cramp: prolonged muscle spasm - tetanic
contraction.
• Strain (pulled muscle): excessive stretching and
sometimes tearing of muscle.
• Contusion: muscle bruise
Myopathies (diseases of the
muscle)
• Muscle infections:
– Poliomyelitis: viral infection of nerves that control
skeletal muscle movement.
• Muscular Dystrophy:
– Genetic disease that causes muscle atrophy
– Protein Dystrophin, which keep thick and thin filaments
aligned is missing
• Myasthenia Gravis:
– Autoimmune disease in which immune system attacks
muscle cells at neuromuscular junction.
– Nerve impulses cannot fully stimulate muscle.