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Introduction to Muscles:
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Muscle tissue is found in every organ
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Muscles participate in every activity that requires movement
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Large proportion of body weight is muscle
-40% of body weight in males
-32% of body weight in females
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Skeletal muscle: attaches to skeleton and provides strength and mobility
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Cardiac muscle: exclusively in the heart
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Smooth muscle: walls of digestive tract, blood vessels, uterus, ureters
Muscles Produce Movement or Generate Tension:
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Muscles may produce movement
-voluntary: conscious control over movement (picking up a pen)
-involuntary: unconscious control over movement (beating of heart)
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Many muscles resist movement
-maintenance of posture
-maintenance of blood pressure
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Muscles generate heat
The Fundamental Activity of Muscle is Contraction:
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Excitable: contract in response to electrical or chemical stimuli
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All muscle cells have one mechanism of action
-they contract (shorten), then relax (lengthen)
Skeletal Muscles Cause Bones to Move:
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600 skeletal muscles
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Synergistic muscles: muscles that work together to create the same movement
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Antagonistic muscles: muscles that oppose each other
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Many muscles attach to bones via tendons
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Origin: end of muscle that attaches to relatively stationary bone
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Insertion: end of muscle attached to another bone across a joint; action pulls insertion
toward origin
A Muscle is Composed of Many Muscle Cells
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Muscles
-group of muscle cells with same origin, insertion and function
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Fasicles
-bundles of muscle fibers (cells) wrapped in connective tissue (fascia)
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Muscles fibers (muscle cells)
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-long, tube shaped
-vary in length from few mm to 30cm
-multinucleate
-packed with myofibrils, which are long cylindrical structures that contain proteins actin
and myosin
The Muscle Contractile Unit is the Sarcomere:
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Sarcomere: contractile unit
-myosin: forms thick filament
-actin: forms thin filaments
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Z Lines: attachment points for sarcomeres
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A sarcomere is a segment of myofibril extending from one Z line to the next
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Arrangement of filaments gives rise to striated appearance of skeletal muscle
Individual Muscle Cells Contract and Relax:
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Muscle contraction: each sarcomere shortens a little
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Basic process of contraction
1. Skeletal muscle must be activated by a nerve
2. Nerve activation increases the concentration of calcium ions in the vicinity of the
contractile proteins
3. Presence of calcium permits contractions
4. When nerve stimulation stops, contraction stops
Nerves Activate Skeletal Muscles:
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Motor neurons stimulate muscle contraction
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Acetylcholine is released from motor neuron at neuromuscular junction
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Electrical impulse is transmitted along T tubules
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Calcium (Ca++) is released from sarcoplasmic reticulum (modified sooth endoplasmic
reticulum)
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Ca++ initiates chain of events that cause contraction when it contacts the myofibrils
Activation Releases Calcium:
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Thick filaments: myosin
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Thin filaments: actin
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Contraction: formation of cross-bridges between thick and thin filaments
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Ca++ must be present for cross-bridges to form
- In absence of Ca++, protein complex of troponin-tropomyosin covers myosin
binding sites on actin molecules
- in presence of Ca++, binding sites are available
Calcium Initiates the Sliding Filament Mechanism:
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1. Calcium is released from sarcoplasmic reticulum
2. Calcium binds to troponin
3. Troponin- tropomyosin complex shifts position
4. Myosin binding site is exposed
5. Myosin heads form cross-bridged with actin
6. Actin filaments are pulled toward center of sarcomere
7. Sarcomere shortens
When Nerve Activation Ends, Contraction Ends:
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Nerve activation ends, contraction ends
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Ca++ pumped back into sarcoplasmic reticulum (requires ATP)
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Ca++ is no longer bound to troponin
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Myosin binding site is covered
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ATP must bind to myosin before myosin heads can detach from actin
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No calcium= no cross-bridges
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Muscle relaxes
Muscles Require Energy to Contract and to Relax:
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Principle source of energy: ATP
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ATP required for contraction
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ATP required for relaxation
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ATP is replenished by a variety of means
-creatine phosphate
-stored glycogen
-aerobic metabolism of glucose, fatty acids, and other high-energy molecules
The Activity of Muscles Can Vary:
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Isotonic contractions: muscle shortens, while maintaining a constant forc, movement
occurs
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Isometric contractions: force is generated, muscle doesn’t shorten, no movement
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Degree of Nerve Activation Influences Force
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Terms to know:
-motor unit
-muscle tension
-all-or-none principle
-muscle tone
-recruitment
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Motor unit
-motor neuron and all the muscle cells it controls
-smallest functional unit of muscle contraction
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Muscle tension
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-mechanical force that muscles generate when they contract
-determined by:
-motor unit size
-number of active motor units
-frequency of stimulation of motor units
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All-or-none principle
-individual muscle cells are completely contracting or are relaxed
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Muscle Tone:
-whole muscles- maintain intermediate level of force known as muscle tone
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Recruitment
-activation of additional motor units increase muscle tone
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Complete cycle of contraction-relaxation in response to stimulus
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Can be observed using a myogram (laboratory recording of muscle activity)
-latent period
-contraction
-relaxation
-summation
-tetanic contraction
Exercise Training Improves Muscle Mass, Strength and Endurance:
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Strength training
-resistance training
-short, intense
-builds more myofibrils, particularly in fast-twitch fibers
Aerobic training:
-builds endurance
-increases blood supply to muscle cells
-increase in mitochondria and myoglobin
-reach target heart rate for at least 20 minutes, three times a week
Cardiac and Smooth Muscles Have Special Features:
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Involuntary
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Able to contract entirely on their own in absence of nerve stimulation
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Cardiac muscle cells are joined by intercalated discs
-have gap junctions allowing cells to electrically stimulate the next one
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Smooth muscle cells joined by gap junctions, allowing cells to activate each other
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Cardiac and smooth muscle cells respond to stimulation from autonomic nervous
system, which can modify the degree of contraction
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Arrangement of Myosin and Actin Filaments:
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Cardiac muscle
-sarcomere arrangement of thick and thin filaments
-striated appearance
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Smooth muscle
-filaments arranged in criss-crossed bundles, not sarcomeres
-no striations
Speed and Sustainability of Contraction:
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Skeletal muscle: fastest
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Cardiac muscle: moderate
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Smooth muscle
-very slow
-partially contracted all of the time
-almost never fatigues
Diseases and Disorders of the Muscular System:
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Muscular dystrophy
-genetic disease: Duchenne muscular dystrophy
-modified dystophin protein enables leakage of Ca++ into cells
-extra Ca++ activates enzymes that destroy proteins
-muscle weakening and wasting
-muscle mass is replaced with fibrous connective tissue
-life expectancy: approx. 30 years
Tetanus:
-infection of deep wound by bacteria, Clostridium tetani
-Bacteria produce tetanus toxin, which causes muscles to contract forefully
-death due to respiratory failure
-preventable by tetanus vaccine
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Muscle cramps: often caused by dehydration and ion imbalances
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Pulled muscles: result from overstretching of a muscle, fibers tear apart
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Fasciitis: inflammation of fascia
-plantar fasciitis: sole of foot
