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Muscle slides 2019

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Muscle
1. Striated muscles move skeletal parts by
contracting
2. Interactions between myosin and actin generate
force during muscle contractions
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Smooth muscle
- Contracts involuntarily, eg. movement of
food through the digestive tract
• Cardiac muscle
- Contraction is involuntary
• Skeletal muscle
-
Under voluntary control
-
Essential for locomotion
Structure and Function of Vertebrate Skeletal
Muscle:
• The sarcomere is the functional unit of muscle
contraction.
• Thin filaments consist of two strands of actin and
one tropomyosin coiled about each other.
• Thick filaments consist of myosin molecules.
Reticulum = little net
Terminal cisterna are storage sacs
Sarcoplasmic reticulum is adapted for Ca2+ storage and release
Sarcolemma - plasma membrane surrounding a muscle cell
2. Interactions between myosin and actin
generate force during muscle contractions
The sliding-filament model of muscle contraction
3. Calcium ions and regulatory proteins
control muscle contraction
4. Diverse body movements require
variation in muscle activity
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Motor nerve fiber
Neuromuscular
junction
Cross section
of spinal cord
Neuromuscular
junctions
Part of muscle fiber
Spinal nerve
Muscle
Motor nerve fiber
10 µm
Excitation of muscle fiber
EPP = endplate potential
MAP= muscle action potential
3. Control of muscle contraction
• At rest tropomyosin blocks the myosin
binding sites on actin.
• Calcium binds to the troponin complex
to expose myosin binding sites
• When an action potential meets the muscle
cell’s sarcoplasmic reticulum (SR) stored
Ca2+ is released.
The events controlling contraction of a
skeletal muscle
1. EPP
MAP
2. signal conducted along T-tubules
3. Signal spreads from T-tubules to sarcoplasmic
reticulum
4. Release of Ca 2+
5. Calcium binds to troponin and a conformational
change takes place
6. Change in position of the tropomyosin
molecule
7. Cross bridges attach to the actin filaments
8. Active sliding of actin filaments into the A-band
9. ADP released, ATP binds – myosin head
detaches from the actin filament.
During a single contraction, each cross bridge
attaches, pulls and detaches many times.
10. Active calcium uptake by SR.
Tropomyosin inhibits cross bridges.
Muscle relaxes.
4. Variation in muscle activity
• An individual muscle cell (fibre) either contracts completely
or not all.
• Individual muscles, composed of many individual muscle
fibers, can contract to varying degrees.
The three phases of a muscle twitch are:
1. Latent period – first few milliseconds after stimulation
when excitation-contraction coupling is taking place
2. Period of contraction – cross bridges actively form
and the muscle shortens
3. Period of relaxation – Ca2+ is reabsorbed into the
SR, and muscle tension goes to zero
Graded Muscle Responses
• Variations in the degree of muscle contraction
• Required for proper control of skeletal movement
• Responses are graded by changing:
• frequency of stimulation
• strength of the stimulus
Muscle Response:
• Threshold stimulus – the stimulus strength at
which the first observable muscle contraction
occurs
• Beyond threshold, muscle contracts more
vigorously as stimulus strength is increased
Muscle tension depends on
1. Frequency of stimulation
2. Number of motor units involved
• Graded muscle contraction also controlled by regulating
number of motor units involved in the contraction.
• Each motor unit controls between 4 and hundreds of fibres
• Force of contraction is controlled by multiple motor unit
summation, called recruitment.
• Recruitment of motor neurons increases the number of
muscle cells involved in a contraction.
• Some muscles are always at least partially contracted (eg.
posture).
• Fatigue is avoided by rotating among motor units.
Two types of muscle contractions:
• Isometric contraction – increasing muscle tension
(muscle does not shorten during contraction)
• Isotonic contraction – decreasing muscle length
(muscle shortens during contraction)
Fast muscle fibers are adapted for rapid,
powerful contractions.
• Fatigue relatively quickly.
Slow muscle fibers are adapted for sustained
contraction.
• Relative to fast fibers, slow fibers have:
• Less SR  Ca2+ remains in the cytosol
longer.
• More mitochondria, a better blood supply,
and myoglobin.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Antagonistic action of skeletal muscles
• Agonist muscle contracts
• Antagonist muscle relaxes
• Groups of muscles work together
• Separate, closely timed stimuli produce smooth,
sustained contraction
Adaptations for strength and speed
Long muscles shorten faster
- linear function of the number of sarcomeres
Levels of functional organisation in a skeletal muscle
Muscle consists of bundles of muscle fibers (fascicles)
Each fascicle has individual muscle fibers.
Each muscle fiber contains hundreds of myofibrils and is
innervated by a motor neuron
Myofibrils are bundles of myofilaments (actin and myosin)
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