Podcast
Questions and Answers
Which type of muscular tissue is responsible for moving blood through the heart?
Which type of muscular tissue is responsible for moving blood through the heart?
Muscular tissue has the ability to stretch without tearing.
Muscular tissue has the ability to stretch without tearing.
True
What is the scientific study of muscular tissue called?
What is the scientific study of muscular tissue called?
myology
The connective tissue layer that wraps around bundles of muscle fibers is called the _________.
The connective tissue layer that wraps around bundles of muscle fibers is called the _________.
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Match the types of muscular tissue with their primary functions:
Match the types of muscular tissue with their primary functions:
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Which of the following is true regarding muscle hypertrophy?
Which of the following is true regarding muscle hypertrophy?
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The thick filaments of a sarcomere are made of actin.
The thick filaments of a sarcomere are made of actin.
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What is the name of the structure that connects muscles to bones?
What is the name of the structure that connects muscles to bones?
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The plasma membrane of muscle fibers is called the __________.
The plasma membrane of muscle fibers is called the __________.
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Match the following muscle structures with their functions:
Match the following muscle structures with their functions:
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What is the primary role of dystrophin in muscle cells?
What is the primary role of dystrophin in muscle cells?
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The sarcomere lengthens during muscle contraction.
The sarcomere lengthens during muscle contraction.
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What is the term used for the cycle during which myosin binds and releases thin filaments?
What is the term used for the cycle during which myosin binds and releases thin filaments?
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Myosin releases thin filaments after binding a new ATP molecule.
Myosin releases thin filaments after binding a new ATP molecule.
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Which of the following ions is essential for muscle contraction?
Which of the following ions is essential for muscle contraction?
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Match the following terms with their descriptions:
Match the following terms with their descriptions:
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Voltage-gated sodium channels will open at any time regardless of membrane potential.
Voltage-gated sodium channels will open at any time regardless of membrane potential.
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What happens to the I band during muscle contraction?
What happens to the I band during muscle contraction?
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What is restored during repolarization of a muscle action potential?
What is restored during repolarization of a muscle action potential?
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Study Notes
Introduction
- Three types of muscular tissue: skeletal, cardiac, smooth
- All generate heat during contraction
- Myology is the study of muscular tissue
- Four properties of muscular tissue: electrically excitable, contractile, extensible, elastic
- Nerve tissues are also electrically excitable
Skeletal Muscle Structure
- Muscle fibers are the cells of skeletal muscle tissue
- Myofibrils are bundled protein filaments within muscle fibers
- Muscle: muscle fibers + connective tissue layers + nerve and blood supply
- Muscle is surrounded by fascia
- Fascia is connective tissue that physically groups muscles with similar functions and provides passage for nerves and vasculature
Fascia Layers
- Epimysium: superficial layer, dense irregular connective tissue, wraps muscle
- Perimysium: intermediate layer, dense irregular connective tissue, wraps fascicles (bundles of muscle fibers)
- Endomysium: deepest layer, mostly reticular fibers, wraps individual muscle fibers
Tendons
- Connect muscles to bones
- Aponeuroses are broad sheets of tendon, e.g., occipitofrontalis muscle connected by epicranial aponeurosis
Muscle Blood and Nerve Supply
- Muscular tissue requires a lot of oxygen for ATP production and is extensively vascularized
- Skeletal muscles are extensively innervated by somatic motor neurons
- Axons from spinal cord branch to muscles, typically 1 branch/muscle fiber
- Voluntary muscle contraction is regulated by somatic motor neurons
Muscle Fiber Structure
- Muscle fibers start as immature cells called myoblasts in the womb, fuse as they mature, resulting in large multinucleate cells
- Sarcolemma is the plasma membrane of myocytes, where electrical signals run along
- Sarcolemma folds inwards to form T-tubules
- Sarcoplasm is the cytoplasm of myocytes, densely packed with myofibrils, rich in glycogen
- Myoglobin is found only in muscle cells, binds oxygen at a heme group
- Myofibrils are long threads of contractile protein filaments, give striated appearance
- Sarcoplasmic reticulum (SR) is the smooth endoplasmic reticulum in muscle cells, extensively folded around each myofibril
- Membrane folds of the SR are called cisternae
- Triads form where two terminal cisternae meet a T-tubule
- Muscle fibers do not divide, but can increase in size (hypertrophy)
Hypertrophy
- Increased sarcoplasmic volume
- Increase in cellular contents, especially myofibrils, mitochondria, SR
- Response to increased mechanical stress, hormones, disease
Sarcomere Structure
- Myofilaments are thread-like structures within myofibrils
- Sarcomeres are contractile units within each myofilament
- Thick filaments extend from the M-line, made of myosin
- Thin filaments extend from the Z-discs, made of actin
Zones and Bands of the Sarcomere
- A band: region where thick and thin filaments overlap
- H zone: only thick filaments, located around the M-line
- I band: only thin filaments, located around the Z-discs
Muscle Contraction
- Muscle generate force by contraction
- Three types of proteins involved: contractile, regulatory, structural
Contractile Proteins:
- Myosin: motor protein, converts chemical energy in ATP to mechanical energy
- Each thick filament has ~300 myosin proteins
- Myosin heads extend radially from thick filaments and contact thin filaments, pulling them toward the M-line
- Each myosin head has ATP-binding and actin-binding sites
Contractile Proteins:
- Actin: cytoskeletal protein
- Long threads twisted together to form helical thin filaments
- Have myosin-binding sites
Regulatory Proteins:
- Troponin: binds Ca2+, moves tropomyosin
- Tropomyosin: blocks myosin-binding sites on thin filaments
Structural Proteins
- Stabilize and connect sarcomere and surrounding structures
- Titin: large elastic protein that spans the M-line to Z-discs, stabilizes thick filaments
- Dystrophin: connects thin filaments to integral membrane proteins in the sarcolemma, reinforces sarcomere structure, transmits tension of sarcomeres to tendons
Sliding Filament Model
- Sarcomere shortens as thin filaments slide over thick filaments
- Filaments do not change in length
Contraction Cycle
- Myosin binds thin filaments, pulls them towards the M-line, and releases them repetitively
- This is called the contraction cycle
Steps of the Contraction Cycle:
- Myosin binds and hydrolyzes ATP, energizes myosin, changes conformation
- Myosin binds thin filaments to form a cross-bridge
- Myosin pulls thin filaments towards the M-line (power stroke)
- Myosin releases thin filaments, requires binding of a new ATP molecule to myosin
Calcium and Muscle Contraction
- Myosin-binding sites blocked by tropomyosin until troponin binds Ca2+
- Ca2+ changes troponin conformation, moving tropomyosin
- Myosin can now form a cross-bridge
- Requires ATP and Ca2+ for contraction
Sarcomere Structure During Contraction
- Z-discs move closer together, sarcomere shortens
- H zone disappears
- I band narrows
Length-Tension Relationship
- Amount of filament overlap affects tension
- Completely overlapping filaments at rest = no tension
- Too little overlap = insufficient tension
- Optimal sarcomere length for maximal tension generation
Muscle Action Potentials
- Muscles store Ca2+ in the SR
- Somatic motor neurons release neurotransmitters (like acetylcholine) that bind to receptors on muscle cells, leading to an action potential
- Action potentials rapidly make a cell's membrane potential positive (depolarization)
- Repolarization is the restoration of a negative membrane potential after depolarization
Ion Channels and Membrane Potential Changes
- Voltage-gated ion channels open in response to changes in membrane potential
- Facilitate diffusion of ions down concentration gradients
- Voltage-gated sodium channels allow Na+ to enter the cell, causing depolarization
- Voltage-gated potassium channels allow K+ to flow out of the cell, causing repolarization
- Sodium channels close as membrane repolarizes
Excitation-Contraction Coupling
- Action potentials travel along sarcolemma to voltage-gated Ca2+ channels (VGCCs) at T-tubules
- VGCCs plug Ca2+ release channels in the SR membrane
- Action potentials open VGCCs, releasing and opening Ca2+ release channels in the SR
- Ca2+ floods the sarcoplasm and binds to troponin
- This initiates muscle contraction
Muscle Relaxation
- VGCCs close
- SR Ca2+ release channels close and reassociate with VGCCs at triads
- Ca2+ ATPases pump Ca2+ back into the SR and out of the cell
- This is called excitation-contraction coupling
Control of Muscle Tension
- One action potential usually equals one contraction
- More frequent action potentials = more tension
- Each somatic motor neuron axon can form multiple NMJs with muscle fibers
- Motor unit: 1 somatic motor neuron + all skeletal muscle fibers it synapses with
- Large muscles have many motor units distributed throughout
Muscle Twitch
- Contraction generated in all skeletal muscle fibers of one motor unit due to one action potential
- Three phases: latent period, contraction period, relaxation period
Refractory Period
- Muscle fiber cannot respond to new action potential while responding to a current one
- Temporarily unresponsive to new signals
Muscle Tone
- Some skeletal muscles maintain tension to stabilize positions
- Alternating involuntary contractions of motor units results in muscle stiffness
- This is called muscle tone
Types of Muscle Contractions
- Isotonic: constant tension in muscle as it changes length
- Concentric: muscle shortens to decrease the angle around a joint
- Eccentric: muscle lengthens while resisting a load
- Isometric: tension generated is not enough to overcome the resistance of the load, bones do not move
Muscle Metabolism
- Muscles require ATP for contraction cycle, active transport pumps
- ATP generated through three ways: creatine phosphate consumption, anaerobic glycolysis, aerobic respiration
Creatine Phosphate Consumption
- Creatine: small molecule made in the liver, kidneys, and pancreas
- Unused ATP dephosphorylated to create creatine phosphate at rest
- During work, creatine phosphate dephosphorylated to regenerate ATP
- Both phosphate transfers catalyzed by creatine kinase
Anaerobic Glycolysis
- Glucose broken down to pyruvate
- Does not require oxygen
- Yields 2 ATP molecules
- Produces lactic acid as a byproduct
Aerobic Respiration
- Requires oxygen
- Glucose broken down to CO2 and H2O
- Yields 38 ATP molecules
- Most efficient method of ATP production
Aerobic Respiration
- Glucose is broken down into two 3-carbon molecules called pyruvate in 10 chemical reactions.
- This process of splitting glucose is called glycolysis.
- Pyruvate is transported to the mitochondria if there is enough oxygen.
- Carbon from glucose is converted to CO2 and exhaled.
- Electrons from chemical bonds are transferred to the electron transport chain (ETC).
- The ETC releases energy, which is used to synthesize ATP.
- Oxygen accepts electrons at the end of the ETC.
Anaerobic Glycolysis
- If muscles have restricted access to oxygen, they cannot respire the products of glycolysis.
- Pyruvate will be fermented into lactic acid.
- Lactic acid fermentation converts pyruvate to lactic acid and regenerates NAD+, allowing glycolysis to continue to produce ATP.
Muscles Need Oxygen After Exercise
- Muscles need oxygen after exercise to replenish myoglobin, convert lactic acid back to glucose in the liver, and replenish creatine phosphate.
- This is called oxygen debt.
Types of Muscle Fibres
- There are 3 types of skeletal muscle fibers with different structures, functions and appearances.
Slow Oxidative Fibres
- Dark red in color due to high myoglobin content and capillaries.
- Have a slow contraction cycle (100-200 msec).
- Sometimes called "slow twitch".
- Do not fatigue easily, functioning during endurance activities and postural muscles.
- Rely primarily on aerobic respiration for energy.
Fast Oxidative-Glycolytic Fibres
- Dark red in color, with high myoglobin content and capillaries.
- Are the largest muscle fibers.
- Have a fast contraction cycle.
- Use a mixture of aerobic and anaerobic metabolism for energy.
Fast Glycolytic Fibres
- White in color with low myoglobin content and capillaries.
- Have a fast contraction cycle.
- Fatigue easily, primarily relying on anaerobic glycolysis for energy.
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Description
Test your knowledge on the types of muscular tissue, their properties, and the structural organization of skeletal muscle. This quiz covers key concepts related to myology and the layers of fascia that support muscle function.
