copyright © 2012 pearson education, inc. chapter 6 the muscular system betty mcguire cornell...
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Chapter 6
The Muscular System
Betty McGuireCornell University
Lecture Presentation
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The Muscular System
Function and characteristics of muscles Skeletal muscles working in pairs Contraction of muscles Voluntary movement Energy for muscle contraction Slow-twitch and fast-twitch muscle cells Building muscle
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Function and Characteristics of Muscles
Three types of muscle Skeletal Cardiac Smooth
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Function and Characteristics of Muscles
All muscles are Excitable (they respond to stimuli) Contractile (they can shorten) Extensible (they can stretch) Elastic (they can return to their original
length after being shortened or stretched)
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Function and Characteristics of Muscles
Skeletal muscles are voluntary muscles responsible for Moving our body Maintaining posture Supporting internal organs Pushing against veins and lymphatic
vessels to move blood and lymph along Generating heat
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Skeletal Muscles Working in Pairs
The body has more than 600 skeletal muscles
Synergistic muscles Muscles that must contract at the same
time to cause movement Antagonistic muscles
Movement is produced when one muscle of the pair contracts and the other relaxes
Example: the biceps muscle and triceps muscle of the upper arm
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Skeletal Muscles Working in Pairs
Tendon Band of connective tissue that attaches
a muscle to a bone Origin of a muscle
The end attached to the bone that remains relatively stationary during movement
Insertion of a muscle The end attached to the bone that
moves
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Skeletal Muscles Working in Pairs
Tendinitis Condition of having an inflamed tendon Caused by overuse, misuse, or age Healing is slow because tendons have a
poor blood supply Most effective treatment is rest
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Skeletal Muscles Working in Pairs
Muscle pull Also called a muscle strain or tear Caused by overstretching that damages
the muscle or tendon Treatment includes ice to reduce
swelling and keeping the muscle stretched
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Contraction of Muscles
Fascicle A bundle of muscle cells
A skeletal muscle has many fascicles Each fascicle is surrounded by its own
connective tissue sheath The connective tissue sheaths of fascicles
merge at the ends of muscles to form tendons that attach the muscle to bone
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Contraction of Muscles
A muscle cell = a muscle fiber When skeletal muscle cells are viewed
under a microscope, they have distinct bands called striations The striations are formed by the
arrangement of myofibrils within the cell Myofibrils are specialized bundles of
proteins
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Contraction of Muscles
Each myofibril contains two types of myofilaments Myosin (thick) filaments Actin (thin) filaments
Actin filaments are more numerous
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Contraction of Muscles
Each myofibril has tens of thousands of contractile units, called sarcomeres The ends of each sarcomere are marked by
dark protein bands called Z lines
Within each sarcomere the actin and myosin filaments are specifically arranged
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Contraction of Muscles
One end of each actin filament is attached to a Z line
Myosin filaments lie in the middle of the sarcomere, and their ends partially overlap with surrounding actin filaments
The degree of overlap increases when the muscle contracts
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Contraction of Muscles
Muscle contraction occurs at the molecular level
According to the sliding filament model, a muscle contracts when actin filaments slide past myosin filaments, shortening the sarcomere
Myosin molecules are shaped like two-headed golf clubs The club-shaped myosin heads are key
to moving actin filaments
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Contraction of Muscles
The myosin head, also known as a cross-bridge, attaches to a nearby actin filament Then the head bends and swivels,
pulling the actin filament toward the midline of the sarcomere
The myosin head disengages from the actin filament
The movements of myosin require ATP The cycle begins again
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Contraction of Muscles
Muscle contraction is controlled by the availability of calcium ions
Muscle cells contain the proteins troponin and tropomyosin
The troponin-tropomyosin complex and calcium ions regulate muscle contraction at the actin-myosin binding sites
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Contraction of Muscles
When a muscle is relaxed, the troponin-tropomyosin complex covers the actin-myosin binding sites
Muscle contraction occurs when calcium ions bind to troponin, causing it to change shape
This change in shape moves tropomyosin, exposing the actin-myosin binding sites
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Contraction of Muscles
Sarcoplasmic reticulum Form of smooth endoplasmic reticulum
found in muscle cells Stores calcium ions
Transverse tubules (T tubules) Pockets in the plasma membrane of a
muscle cell Carry signals from motor neurons deep
into the muscle cell to every sarcomere
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Contraction of Muscles
Rigor mortis Muscle contraction will occur as long
ATP is present Without ATP, cross-bridges cannot
be broken Within 3 to 4 hours after death, the
muscles become stiff = rigor mortis Actin and myosin gradually break
down and muscles relax again after 2 to 3 days
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Contraction of Muscles
Role of nerves in muscle contraction Neuromuscular junction
Junction between the tip of a motor neuron and a skeletal muscle cell
A nerve impulse travels down a motor neuron to the neuromuscular junction, where it causes the release of acetylcholine (a neurotransmitter) from the motor neuron
Acetylcholine diffuses across a small gap and binds to receptors on the plasma membrane of the muscle cell
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Contraction of Muscles
Role of nerves in muscle contraction (cont.) The acetylcholine causes changes in
the permeability of the muscle cell, resulting in an electrochemical message similar to a nerve impulse
The message travels along the plasma membrane into the T tubules and then to the sarcoplasmic reticulum, releasing calcium ions for muscle contraction
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Contraction of Muscles
Web Activity: Muscle Structure and Function
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Contraction of Muscles
Muscular dystrophy (MD) If too many calcium ions enter a muscle cell, then
proteins may be destroyed, eventually causing the cell to die; on a large scale, muscles weaken
MD = a group of inherited conditions in which muscles weaken
Duchenne muscular dystrophy One of the most common forms The gene for production of the protein
dystrophin is defective Lack of dystrophin allows excess calcium ions to
enter muscle cells, eventually killing the cells
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Voluntary Movement
Motor unit A motor neuron and all the muscle cells
it stimulates All the muscle cells in a given motor unit
contract together
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Voluntary Movement
The number of muscle cells in a motor unit is highly variable Muscles responsible for precise movements
have fewer muscle cells in each motor unit than do muscles responsible for less precise movements
On average, there are 150 muscle cells in a motor unit
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Voluntary Movement
Motor units and recruitment The strength of muscle contraction can
be increased by increasing the number of motor units that are stimulatedThis process, performed by the
nervous system, is called recruitment
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Voluntary Movement
Muscle twitch Contraction of a muscle in response to a
single stimulus Twitches are very brief and typically not part
of normal movements
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Voluntary Movement
If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first, due to summation
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Voluntary Movement
Tetanus A sustained, powerful contraction
caused by very frequent stimuli Fatigue sets in when a muscle is unable to
contract even when stimulated Changing the frequency of stimulation is
another way to vary the contraction of muscles
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Energy for Muscle Contraction
Muscle contraction requires an enormous amount of energy
ATP for muscle contraction comes from many sources, typically used in sequence ATP stored in muscle cells Creatine phosphate stored in muscle cells Anaerobic metabolic pathways Aerobic respiration
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Slow-Twitch and Fast-Twitch Muscle Cells
Slow-twitch muscle cells Contract slowly, with great endurance Abundant mitochondria Packed with myoglobin (oxygen-binding
pigment) Dark, reddish appearance
Myoglobin Rich blood supply
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Slow-Twitch and Fast-Twitch Muscle Cells
Fast-twitch muscle cells Contract rapidly and powerfully but with
much less endurance Can make and break cross-bridge
attachments more rapidly Have more actin and myosin Rely on anaerobic metabolic pathways to
generate ATP and therefore tire quickly
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Building Muscle
Aerobic exercise Enough oxygen is delivered to the muscles
to keep them going for long periods Increases endurance and coordination Promotes development of new blood vessels Increases the number of mitochondria Typically does not increase size of muscles Examples: walking, jogging, swimming
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Building Muscle
Resistance exercise Builds strength Muscles increase in size when they are
repeatedly made to exert more than 75% of their maximum force
Increases in muscle size reflect increases in the diameter of existing muscle cells
Example: weight lifting
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Building Muscle
PLAY | Mitchell Report
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Building Muscle
PLAY | Steroids