MSK 2

Bone Tissue

• Osteocytes are the primary cell type found in bone tissue.
• Osteoid refers to the matrix of the bone that contains proteoglycans and glycoproteins, providing flexibility and strength.
• Collagen fibers are primarily responsible for the tensile strength of bone, contributing to its flexibility and ability to dissipate energy.
• The organic components of bone, like collagen fibers, proteoglycans, and glycoproteins, provide a buffer against mechanical forces, reducing the risk of fractures.
• Calcium phosphates are the primary inorganic component of bone, contributing to its hardness.
• Approximately 50% of bone's volume and 70% of its weight is made up of inorganic components.
• Mineral salts crystalize around collagen fibers, increasing the hardness of the bone.
• Calcium phosphate plays a crucial role in contributing to the hardness and strength of bone.
• Hydroxyapatite is the main mineral component of bone, contributing to its hardness.
• Bone marrow is the soft, spongy tissue found inside bones, responsible for blood cell production and fat storage.
• Osteoblasts are responsible for building new bone tissue.
• Osteoclasts are responsible for breaking down bone tissue.

Bone Tissue: Cells and Composition

• Osteocytes are the primary cell type found in bone tissue
• Osteoid refers to the organic matrix of bone, which contains proteoglycans and glycoproteins
• Collagen fibers provide tensile strength and flexibility to bone, contributing to its toughness and ability to dissipate energy
• Proteoglycans and glycoproteins contribute to the strength and flexibility of bone, acting as a buffer against mechanical forces and reducing the risk of fractures
• Calcium phosphates are the primary inorganic component of bone, contributing to its hardness
• Mineral salts make up approximately 50% of bone's volume and 70% of its weight, crystalizing around collagen fibers to increase hardness
• Osteogenic cells are the precursor cells that give rise to osteoblasts and most other bone cells
• Osteoblasts form new bone by secreting bone matrix, a process known as osteogenesis
• Osteocytes are formed when osteoblasts become embedded in the bone matrix and communicate mechanical stress signals to remodeling cells
• Osteoclasts are responsible for absorbing bone tissue during growth and healing
• Bone lining cells maintain the bone matrix on surfaces not undergoing remodeling
• Osteocytes are not directly involved in the formation or absorption of bone tissue.

Bone Structure

• Osteocytes are the primary cell type in bone tissue. They are responsible for maintaining the bone matrix and communicating mechanical stress signals to remodeling cells.
• Osteoid is the unmineralized portion of bone, containing proteoglycans and glycoproteins.
• Collagen fibers provide the tensile strength of bone.
• Hydroxyapatite (calcium phosphate) provides compressive strength to bone.
• Bone's organic components (collagen and osteoid) buffer against mechanical forces and reduce the risk of fractures.
• Around 50% of bone's volume is made up of mineral salts, accounting for 70% of bone's weight.

Bone Cells

• Osteoblasts form new bone by secreting bone matrix during the process called osteogenesis.
• Osteoclasts absorb bone tissue during growth and healing.
• Osteogenic cells are the precursors to both osteoblasts and other bone cells.
• Bone lining cells maintain the matrix on surfaces not undergoing remodeling.

Bone Remodeling

• Bone remodeling involves coordinated activity of osteoblasts and osteoclasts.
• Osteoblasts lay down new bone matrix to form new bone.
• Osteoclasts resorb bone tissue and break down bone.
• The continuous process of bone remodeling is essential for maintaining bone strength and responding to changes in mechanical stress.

Osteon

• The osteon is also known as the Haversian system.
• It is a structural unit of compact bone.
• The primary feature is a hollow tube of bone matrix.
• Collagen fibers in adjacent lamellae (concentric rings of bone matrix) run in different directions.
• This arrangement of collagen fibers provides resistance to strain and torsion.
• Bone salts align with and alternate the direction of collagen fibers, further strengthening the osteon.
• The osteon's structure is an example of how structure relates to function.
• Elongated cylinders of an osteon are oriented parallel to the long axis of the bone.

Osteon Structure and Function

• The osteon, also known as the Haversian system, is a primary structural unit of compact bone.
• The osteon is characterized by a hollow tube of bone matrix, which provides strength and resistance to strain and torsion.
• Collagen fibers within adjacent lamellae of an osteon run in different directions, contributing to the bone’s strength and flexibility.
• Tiny weight-bearing pillars within osteons provide resistance to strain and torsion, enhancing the bone’s structural integrity.
• Crystals of bone salts, such as hydroxyapatite, align with and alternate the direction of collagen fibers within the lamellae, contributing to the bone’s hardness and rigidity.
• The structure of the osteon is a clear example of how structure relates to function, as its intricate design maximizes strength and resilience.
• The elongated cylinders of an osteon are oriented parallel to the long axis of the bone, providing support and stability.
• Lamellae, concentric rings of bone matrix, form the structural framework of the osteon, encasing the central canal which contains blood vessels and nerves.

Osteon Structure

• The central canal or Haversian canal, is the main channel within the osteon, containing blood vessels and nerves that supply the bone tissue.
• Osteocytes, mature bone cells, reside in small spaces called lacunae, which are embedded within the lamellae, concentric rings of bone matrix.
• Canaliculi are tiny channels that connect the lacunae to the central canal, allowing for communication and transport of nutrients and waste products between osteocytes.

Osteocyte Communication and Function

• Osteocytes rely on the canaliculi network to communicate with each other and with the central canal.
• This communication system enables the passage of nutrients, oxygen, and waste products, ensuring the survival and function of osteocytes.
• The canaliculi are essential for maintaining the health and integrity of the bone tissue.

Cells of the Osteon

• Osteocytes are the mature bone cells responsible for maintaining the bone tissue, and they reside in the lacunae within the osteon.
• Osteoblasts are responsible for forming new bone matrix, while osteoclasts break down bone tissue, contributing to bone remodeling.
• The Haversian canal serves as the main route for the delivery of nutrients and the removal of waste products from osteocytes.

Bone Vascularization

• Bones have a rich blood supply which is essential for their growth, healing, and overall health.
• Haversian canals and Volkmann's canals contain blood vessels that supply nutrients to the bone.
• Haversian canals run longitudinally through the bone, while Volkmann's canals connect them to each other and the periosteum.
• Blood vessels within the canals deliver nutrients and oxygen to osteocytes, removing waste products as well.

Blood Supply and Bone Health

• A compromised blood supply can lead to weakened bones, impaired healing, and stunted growth.
• Nutrient delivery and waste removal via blood vessels are crucial for maintaining bone health.
• Medullary cavity is a large central space within the bone also supplies nutrients and blood cells.

Further Key Points

• Osteocytes, the mature bone cells, reside in lacunae within the bone matrix.
• Canaliculi are small channels that connect lacunae to each other and to the Haversian canals, facilitating communication between osteocytes.

Spongy Bone vs. Compact Bone

• Spongy bone is lighter and less dense than compact bone.

Spongy Bone Structure

• Trabeculae and bars of bone make up the framework of spongy bone.
• Red bone marrow is found within the small, irregular cavities adjacent to trabeculae.

Canaliculi Function

• Canaliculi in spongy bone connect to adjacent cavities instead of a central Haversian canal.

Trabeculae Advantages

• Trabeculae provide maximum strength.
• Trabeculae can realign to follow lines of stress, which allows for bone adaptation.

Blood Cell Production

• Spongy bone is primarily involved in blood cell production due to the presence of red bone marrow.

Spongy Bone Design

• Spongy bone design allows it to absorb shock and reduce weight while maintaining strength.

Negative Feedback Loops

• Maintain homeostasis by reversing deviations from a set point.
• Act to counter changes in the body's internal environment.

Calcium in the Body

• Essential for:
  • Muscle contraction
  • Nerve impulse transmission
  • Bone structure

Calcium Homeostasis

• High blood calcium levels: Trigger the release of calcitonin, which lowers calcium levels.
• Low blood calcium levels: Trigger the release of parathyroid hormone (PTH), which increases calcium levels.
• PTH:
  • Stimulates calcium reabsorption in the kidneys.
  • Increases calcium absorption from the gut.
  • Promotes bone resorption (breakdown of bone tissue).

Bone Remodeling

• Mechanical forces:
  • Stimulate bone formation in areas subjected to stress.
  • Influence the alignment and strength of bone trabeculae.
• Prolonged inactivity:
  • Decreases bone density due to lack of mechanical stress.

Calcium in the Blood

• The normal blood calcium ion (Ca²⁺) concentration ranges between 9 and 11 mg/dl.

Calcium in the Body

• The human body typically contains between 1200 and 1400 grams of calcium.
• 99% of the body's calcium is stored in bones.
• 1.5 grams of calcium are typically present in the blood.

Calcium Homeostasis

• The hormonal control loop is the primary mechanism responsible for maintaining stable blood calcium levels.
• Calcitonin is a hormone that decreases blood calcium levels when they are elevated.
• Parathyroid hormone (PTH) increases blood calcium levels. PTH stimulates bone resorption, calcium reabsorption in the kidneys, and calcium absorption in the intestines.

Importance of Calcium

• Calcium is crucial for various bodily functions, including:
  • Muscle contraction
  • Nerve function
  • Blood clotting

Osteoclasts and Bone Resorption

• Osteoclasts release proteolytic enzymes and H⁺ ions to digest the bone matrix.
• The primary function of osteoclasts is to release calcium and phosphorus into the blood circulation.
• Osteoclasts are responsible for the breakdown of bone tissue during bone remodeling.

Osteoblasts and Bone Formation

• Osteoblasts secrete osteoid, an unmineralized matrix that contributes to bone formation.
• After secretion, osteoid undergoes mineralization, a process that is not fully understood.
• Approximately 5% of adult bone is remodeled each week.

Bone Remodeling

• Bone remodeling is a continuous process that allows for the renewal and maintenance of bone tissue. This involves both bone formation by osteoblasts and bone resorption by osteoclasts.

Bone Remodeling and Deposition

• Bone remodeling and deposition are most vigorous up to age 25.
• Between ages 25 and 35, the rate of bone remodeling slowly declines.
• After age 40, bone reabsorption exceeds bone formation.
• About 10% of bone mass is lost each decade after age 40.
• This decline in bone formation after age 35 leads to a higher risk of fractures and osteoporosis.

Factors Influencing Bone Health

• Physical activity levels and diet significantly influence the rate of bone remodeling and deposition as one ages.
• Estrogen and testosterone, which decrease with age, are key hormones that influence bone remodeling.

Maintaining Bone Health After Age 40

• It is crucial to maintain bone health after age 40 to reduce the risk of osteoporosis and fractures.

Cartilage Types and Development

• Hyaline cartilage is the primary cartilage in the skeleton before 8 weeks of gestation.

Primary Ossification

• Primary ossification is the process of bone formation from hyaline cartilage.
• The first step is the secretion of osteoid by periosteal osteoblasts.
• A bone collar forms around the diaphysis during primary ossification.

Endochondral Ossification

• Endochondral ossification is the process by which cartilage is replaced by bone.
• Calcification of cartilage occurs during this process.

Periosteal Bud and Ossification

• The periosteal bud, containing blood vessels, nerves, and red marrow, infiltrates the diaphysis during ossification.

Bone Formation and Elongation

• As the diaphysis elongates and the medullary cavity appears, spongy bone is formed.

Epiphyseal Ossification

• The epiphyses ossify, but hyaline cartilage persists at the epiphyseal plates and articular surfaces.
• Hyaline cartilage remains only at the epiphyseal plates and articular cartilages after the epiphyses ossify.

Fracture Types

• Nondisplaced fracture: Bone remains in its normal position, no misalignment.
• Displaced fracture: Bone ends are out of alignment, requiring medical intervention.
• Complete fracture: Break extends completely through the bone.
• Incomplete fracture: Break does not go all the way through the bone, it's partial.
• Open (compound) fracture: Bone protrudes through the skin, increasing risk of infection.
• Closed (simple) fracture: Skin remains intact, no bone protrusion.

Red Marrow Location and Function

• Red marrow is found in the medullary cavity of the diaphysis and all areas of spongy bone in infants.
• The primary function of red marrow is haematopoiesis (production of blood cells).
• In adults, red marrow is mostly found in the diploe of flat bones like the pelvis, sternum, and skull.
• In adults, red marrow is specifically found in the head of long bones.

Yellow Marrow

• Yellow marrow primarily consists of fat.
• In adults, yellow marrow extends into the epiphyses of long bones.

Bone Marrow Biopsy

• Bone marrow is most commonly taken from the hip bone (pelvis) for clinical bone marrow biopsies.
• The flat bones, such as the pelvis, are typically the best source of red marrow for clinical procedures in adults.

Osteocalcin and Insulin

• Osteocalcin is a hormone secreted by osteoblasts, primarily influencing insulin production and uptake.
• Osteocalcin targets pancreatic beta cells to divide and produce insulin.
• Osteocalcin levels are reduced in patients with type 2 diabetes.
• Insulin activates inactive osteocalcin in bone.

Osteocalcin and Adiponectin

• Osteocalcin stimulates the release of adiponectin from adipocytes.
• Adiponectin restricts fat storage.

FGF23

• Fibroblast growth factor 23 (FGF23) regulates phosphate reabsorption in the kidneys.

Osteocalcin and Insulin: Synergistic Relationship

• Osteocalcin and insulin have a two-way synergistic mechanism.
• Osteocalcin promotes insulin production, and insulin activates osteocalcin.

Osteocalcin and Insulin Production

• Osteocalcin, secreted by osteoblasts, primarily influences insulin production and uptake.
• Osteocalcin directly targets pancreatic beta cells to promote their division and insulin production.
• In patients with type 2 diabetes, osteocalcin levels are reduced.
• Insulin activates inactive osteocalcin in bone.

Adiponectin and Fat Storage

• Adiponectin, a hormone released by adipocytes, is stimulated by osteocalcin.
• Adiponectin restricts fat storage and promotes energy expenditure.

FGF23 and Phosphate Regulation

• Fibroblast growth factor 23 (FGF23) regulates phosphate reabsorption in the kidneys.

The Relationship between Osteocalcin and Insulin

• There is a two-way synergistic mechanism between osteocalcin and insulin.
• This means that they both influence each other's function and activity.

Osteocalcin & Insulin

• Osteocalcin is secreted by osteoblasts and primarily influences insulin production and uptake.
• Osteocalcin targets pancreatic beta cells to divide and produce insulin.
• Osteocalcin levels are reduced in patients with type 2 diabetes.
• Insulin activates inactive osteocalcin in bone.

Osteocalcin & Adiponectin

• Osteocalcin stimulates the release of adiponectin from adipocytes.
• Adiponectin restricts fat storage.

Osteocalcin & FGF23

• FGF23 regulates phosphate reabsorption in the kidneys.

Relationship between Osteocalcin & Insulin

• There is a two-way synergistic mechanism between osteocalcin and insulin.

The Functional Contractile Unit of Skeletal Muscle

• The sarcomere is the functional contractile unit of a skeletal muscle.

Characteristics of Skeletal Muscle

• Excitability: The ability of skeletal muscle to respond to stimuli, such as nerve impulses.

Primary Function of Skeletal Muscle Contraction

• The primary function of skeletal muscle contraction is to move bones.

Muscle Contractility

• Contractility refers to the muscle's ability to shorten and produce movement.

The Sarcomere in a Skeletal Muscle

• The sarcomere is the area between two Z discs.

Neural Stimulation and Skeletal Muscle

• When skeletal muscle receives a neural stimulus, it becomes excitable and contracts.

The Role of Skeletal Muscle Contraction in Maintaining Posture

• Skeletal muscle contraction keeps the body upright and stable, maintaining posture.

Skeletal Muscle Contraction and Movement

• Skeletal muscles contract to produce movement.

Neuromuscular Junction

• The neuromuscular junction is a specialized synapse where a motor neuron communicates with a muscle fiber.
• A single motor neuron can innervate multiple muscle fibers.
• Acetylcholine is the neurotransmitter released at the neuromuscular junction.
• Acetylcholine binding to receptors on the muscle fiber membrane triggers an action potential in the muscle fiber.
• This action potential travels along the muscle fiber, leading to muscle contraction.

Muscle Contraction Initiation

• An action potential traveling down a motor neuron initiates muscle contraction.
• This action potential triggers the release of acetylcholine from the motor neuron's axon terminal.
• Acetylcholine diffuses across the synaptic space and binds to receptors on the muscle fiber membrane.
• This binding triggers an action potential in the muscle fiber, leading to the release of calcium ions from the sarcoplasmic reticulum.
• Calcium ions bind to troponin, initiating the sliding filament mechanism of muscle contraction.

The Neuromuscular Junction

• The neuromuscular junction is the specialized synapse between a motor neuron and a muscle fiber.

• It is responsible for transmitting signals from the nervous system to the muscles, initiating muscle contraction.

Muscle Contraction Initiation

• Muscle contraction is initiated by an action potential traveling down the motor neuron.

• This action potential triggers the release of acetylcholine from the motor neuron's axon terminal.

• Acetylcholine diffuses across the synapse and binds to receptors on the muscle fiber membrane.

• This binding triggers the muscle fiber to generate its own action potential.

Role of Calcium

• The action potential in the muscle fiber travels along the sarcolemma and into the T-tubules.

• This triggers the release of calcium ions from the sarcoplasmic reticulum.

• Calcium ions bind to troponin, a protein that regulates the interaction between actin and myosin filaments.

• This binding allows the actin and myosin filaments to slide past each other, shortening the muscle fiber and causing contraction.

Acetylcholine and Muscle Contraction

• Acetylcholine (ACh) binding to its receptors on the muscle fiber membrane opens ligand-gated ion channels
• These channels allow sodium (Na+) and potassium (K+) ions to pass through
• More Na+ diffuses into the muscle fiber than K+ diffuses out due to the electrochemical gradient favoring sodium influx
• This causes a transient change in membrane potential, known as local depolarization
• Local depolarization triggers the action potential to spread across the sarcolemma

Neuromuscular Junction: The Key to Muscle Activation

• The binding of acetylcholine (ACh) to its receptors at the neuromuscular junction opens ligand-gated ion channels
• These are not voltage-gated, mechanically-gated, or leak channels
• The opening of these channels allows for the influx of sodium and potassium ions, triggering the sequence of events that lead to muscle contraction

T-tubules in Muscle Cells

• T-tubules are continuous extensions of the sarcolemma (muscle cell membrane) that penetrate deep into the muscle fiber.
• They are essential for transmitting electrical impulses from the surface of the muscle fiber to the interior, ensuring a coordinated contraction of the entire cell.
• This transmission is crucial because it triggers the release of calcium ions from the sarcoplasmic reticulum (SR), which are necessary for muscle contraction.
• T-tubules are located at the A-band/I-band junction of the sarcomere, the functional unit of muscle contraction.
• They work closely with the SR to initiate muscle contraction by allowing the electrical impulse to reach the SR, stimulating calcium release.
• Without T-tubules, the electrical impulse would only affect the surface of the muscle fiber, resulting in a weak and uncoordinated contraction.
• The T-tubule system ensures the synchronized release of calcium ions along the entire length of the muscle fiber, resulting in a strong and coordinated contraction.

Sarcoplasmic Reticulum (SR) and Muscle Contraction

• Primary Role of SR: The SR's primary role is to regulate muscle contraction by storing and releasing calcium ions (Ca²⁺).

• Structure: The SR is an elaborate smooth endoplasmic reticulum that surrounds each myofibril within a muscle fiber.

• Calcium Release: When an action potential travels down a T-tubule, it triggers the release of Ca²⁺ from the SR.

• Contraction Initiation: The release of Ca²⁺ into the sarcoplasm allows myosin heads to bind to actin filaments, initiating muscle contraction.

• Calcium Reuptake: After contraction, Ca²⁺ is actively pumped back into the SR by the SR Ca²⁺ ATPase pump, leading to muscle relaxation.

• Relationship with Myofibrils: The SR is positioned around the myofibrils, ensuring that Ca²⁺ release is precisely targeted to the contractile proteins for efficient contraction.

Troponin

• A protein complex found in skeletal and cardiac muscle fibers
• Controls muscle contraction by regulating the interaction of actin and myosin
• Binds calcium ions (Ca²⁺) to trigger contraction
• Composed of three subunits:
  • Troponin C: Binds calcium.
  • Troponin I: Inhibits actin-myosin interactions.
  • Troponin T: Binds to tropomyosin.
• Elevated blood troponin levels indicate damage to cardiac muscle, often used as a biomarker for myocardial infarction (heart attack)

Tropomyosin

• A long, coiled protein found along the length of actin filaments
• Regulates muscle contraction by blocking myosin binding sites on actin in the absence of calcium.
• Composed of two identical polypeptide chains forming a coiled-coil structure
• Positions itself along actin filaments, stabilizing them.
• Allows contraction when calcium binds to troponin, triggering a shift in tropomyosin, exposing active sites on actin for myosin.

Interaction Between Troponin and Tropomyosin

• Both proteins work together to regulate muscle contraction.
• Calcium binding to troponin leads to a shift in tropomyosin, exposing active sites on actin.
• Essential for the contraction mechanism in both skeletal and cardiac muscle.

Muscle Contraction and Troponin

• Calcium ions (Ca2+) bind to troponin during muscle contraction.
• This binding causes troponin to change shape, pulling tropomyosin away from the myosin-binding sites on actin.
• The exposure of myosin-binding sites allows myosin heads to bind to actin, forming cross-bridges.
• Tropomyosin blocks myosin-binding sites on actin filaments when the muscle is at rest, preventing contraction.
• This blocking action is reversed when calcium binds to troponin.
• When calcium is removed from troponin, tropomyosin returns to its blocking position, preventing further cross-bridge formation and muscle contraction.

Excitation-Contraction Coupling

• The link between the action potential and muscle contraction.

Muscle Fiber Depolarization

• Triggered by the influx of sodium ions through ligand-gated channels.

T-tubules

• Transmit the action potential deep into the muscle fiber.

Calcium Release

• The action potential traveling down T-tubules triggers the release of calcium ions from the sarcoplasmic reticulum.

Calcium and Contraction

• Calcium binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin.
• This allows myosin heads to bind to actin filaments, initiating contraction.

Relaxation

• After the action potential is over, calcium is actively transported back into the sarcoplasmic reticulum.
• This allows tropomyosin to cover the myosin binding sites on actin again, ending the contraction.

Muscle Relaxation and Calcium

• When the action potential stops, calcium (Ca2+) is pumped back into the sarcoplasmic reticulum.
• This is achieved through active transport, which requires ATP.

Cross-Bridge Formation and Muscle Relaxation

• When calcium is removed from the cytoplasm, cross-bridge formation is inhibited.
• This is because calcium binds to troponin, which moves tropomyosin away from the myosin-binding sites on actin.
• Without calcium, tropomyosin blocks the myosin-binding sites, preventing cross-bridge formation.
• This allows actin and myosin filaments to slide past each other, leading to muscle relaxation.

Actin and Myosin Sliding

• During relaxation, actin and myosin filaments slide past each other due to contraction of another muscle or the muscle's own weight.
• This is not due to any active process within the muscle fiber itself.

The Sarcoplasmic Reticulum and Muscle Relaxation

• After muscle relaxation, calcium is stored in the cisternae of the sarcoplasmic reticulum, ensuring it's readily available for the next muscle contraction.
• This high concentration of calcium in the sarcoplasmic reticulum facilitates rapid diffusion into the cytoplasm when the next action potential arrives.

Preventing Cross-Bridge Formation

• The absence of calcium in the cytoplasm prevents cross-bridge formation during muscle relaxation.
• This is because calcium is essential for moving tropomyosin away from the myosin-binding sites on actin.

Rigor Mortis

• Rigor mortis refers to the stiffening of muscles after death.
• It starts about 4 hours after death, peaks at approximately 13 hours, and lasts about 50 hours.
• The lack of ATP after death causes the myosin heads to remain attached to the actin filaments, resulting in a sustained muscle contraction.
• As ATP is required to detach the myosin heads from the actin filaments and pump calcium back into the sarcoplasmic reticulum, the muscles stay contracted in rigor mortis.
• Calcium leakage from the sarcoplasmic reticulum into the cytoplasm further contributes to the formation of cross-bridges between actin and myosin.
• Without ATP, the muscle fibers cannot relax, leading to the stiffening associated with rigor mortis.

Rigor Mortis

• ATP Depletion

  • ATP is vital for muscle contraction & relaxation.
  • Muscle cells require ATP to detach myosin heads from the actin filaments after contraction.
  • ATP production ends after death due to lack of oxygen, preventing ATP regeneration.

• Cross-Bridge Formation

  • Without ATP, myosin heads remain attached to actin filaments, causing continuous & sustained contraction.
  • This inability of myosin to detach from actin results in muscle rigidity.

• Calcium Leakage

  • After death, calcium ions leak from the sarcoplasmic reticulum (SR) into the cytoplasm. - This leakage triggers calcium binding to troponin, moving tropomyosin away from actin's myosin-binding sites.
  • This promotes cross-bridge formation, but the lack of ATP prevents the cycle from completing, sustaining contraction.

• Lack of Relaxation

  • ATP is also needed to pump calcium back into the SR.
  • The persistent excess calcium in the cytoplasm maintains the contraction and rigidity.

•  Rigor Mortis onset: >4 hours after death

  • Rises to a peak at ~13 hours
  • Persists for ~50 hours

Muscle Tone

• Muscle Tone is a state of partial contraction that maintains muscle firmness without active movement.
• Spinal reflexes activate motor units, which are responsible for maintaining muscle tone.

Function of Muscle Tone

• The function of muscle tone is to maintain posture and stabilize joints.

Motor Units

• Small motor units control fine movements, while large motor units control large muscles.
• A small motor unit is a single neuron controlling a few muscle fibers, enabling precise movement.
• Large motor units are found in muscles that produce powerful, large-scale movements like those in the thighs or back.

Spinal Reflexes and Muscle Tone

• Spinal reflexes contribute to muscle tone by intermittently activating motor units, ensuring muscles are ready for action.

Importance of Muscle Tone

• Muscle tone, even in relaxed muscles, ensures muscles are primed for immediate response to stimuli.

Motor Units

• A motor unit is a single motor neuron and all the muscle fibers it innervates
• Each motor neuron can innervate multiple muscle fibers, but each muscle fiber is only innervated by one motor neuron

Muscle Contraction Phases

• Latent Phase: A brief delay between the action potential and the onset of contraction
• Contraction Phase: Muscle fibers generate tension and shorten
• Relaxation Phase: Calcium is pumped back into the sarcoplasmic reticulum, and tension decreases

Muscle Fiber Types and Contraction Speed and Duration

• There are three main types of muscle fibers:
  • Type I fibers (slow-twitch): Slow contraction speed, high resistance to fatigue, used for endurance activities
  • Type IIa fibers (fast-twitch, oxidative): Faster contraction speed than type-I, high resistance to fatigue, used for activities requiring both power and endurance
  • Type IIb fibers (fast-twitch, glycolytic): Fastest contraction speed, low resistance to fatigue, used for short bursts of power

Factors Influencing Contraction Speed and Duration

• Muscle fiber type: Type I fibers are slowest while Type IIb are fastest.
• Metabolic properties: Type I fibers are highly aerobic and use oxygen efficiently, while Type IIb fibers are glycolytic and utilize anaerobic metabolism.
• Number of motor units activated: Recruiting more motor units increases contraction strength and speed.
• Frequency of action potentials: More frequent action potentials lead to a stronger contraction.

Types of Muscle Contractions

• Twitch contraction: A rapid and brief response to a single action potential.
• Tetanic contraction: A sustained, powerful contraction resulting from a high frequency of action potentials.

Muscle Fiber Types

• Type I fibers are primarily associated with endurance activities.
• Type I fibers are slow-twitch and have a high myoglobin content, which allows for aerobic respiration.
• Type II fibers are fast-twitch and are best suited for activities like sprinting or weightlifting.
• Type II fibers primarily use anaerobic glycolysis for energy production.
• Type II fibers have a rapid and powerful contraction velocity.
• Type I fibers have a higher capillary density than Type II fibers to supply oxygen for aerobic metabolism.
• Type IIb fibers have a larger diameter than Type I fibers.
• Type I fibers can sustain prolonged activity due to increased myoglobin and capillary density.
• Type II fibers can be further divided into Type IIa and Type IIb.

### Muscle Fiber Types

• Type I fibers are primarily associated with endurance activities.
• Type I fibers have a high myoglobin content which allows for greater oxygen storage and aerobic metabolism.
• Type I fibers have a slower contraction velocity but can sustain prolonged activity.
• Type I fibers have a higher density of blood capillaries to deliver oxygen for sustained aerobic metabolism.
• Type I fibers have a smaller diameter compared to Type II fibers.

Fast-twitch Muscle Fibers

• Fast-twitch fibers (Type II) are best suited for high-intensity, short-duration activities such as sprinting or weightlifting.
• Type II fibers primarily rely on anaerobic glycolysis for energy production.
• Type II fibers have a rapid and powerful contraction velocity.
• Type II fibers have a larger diameter than Type I fibers.
• Type II fibers can be further divided into Type IIa and Type IIb.

### Type IIa and Type IIb Fibers

• Type IIa fibers are intermediate in their characteristics, possessing some qualities of both Type I and Type IIb fibers.
• Type IIb fibers are fastest-twitching and have the largest diameter. They are the most powerful but fatigue quickly

Muscle Fatigue and Fiber Types

• Fast-twitch (Type IIb) muscle fibers fatigue quickly due to lactic acid build-up from anaerobic metabolism.
• Slow-twitch (Type I) muscle fibers primarily use aerobic oxidative phosphorylation for energy production, allowing them to sustain prolonged activity.
• Slow-twitch fibers are advantageous for endurance activities due to their ability to constantly supply ATP.

Muscle Function and Fiber Type Adaptation

• Different activities and training can lead to adaptations in muscle fiber type distribution.
• Short bursts of high-intensity activity primarily utilize Type IIb fibers.

ATP Production and Prolonged Exercise

• Type I fibers maintain a constant ATP production through aerobic processes during prolonged exercise.

Endurance Training Adaptations

• Endurance training leads to an increased number of slow-twitch fibers and mitochondrial density.

Lactic Acid and Muscle Fatigue

• Lactic acid build-up during high-intensity exercise decreases muscle pH, contributing to fatigue.

Muscle Fatigue and Fiber Types

• Fast-twitch (Type IIb) muscle fibers fatigue quickly due to lactic acid build-up from anaerobic metabolism.
• Endurance muscles rely primarily on aerobic oxidative phosphorylation for energy production, fueled by slow-twitch (Type I) fibers.
• Slow-twitch fibers are suited for endurance activities because they can sustain prolonged activity with a constant supply of ATP.
• Muscle fiber type composition is adaptable based on activity and training.
• High-intensity, short-burst activities primarily utilize fast-twitch (Type IIb) fibers.
• Type I fibers maintain consistent ATP production through aerobic processes during prolonged exercise.
• Endurance training leads to increased slow-twitch fibers and mitochondrial density.
• Lactic acid contributes to muscle fatigue during high-intensity exercise by lowering pH in the muscle environment.

Muscle Adaptations to Training

• Endurance training leads to an increase in the number of mitochondria and capillaries in muscle fibers. This enhances the muscle's aerobic capacity.
• Strength training primarily leads to the fusion of satellite cells to increase nuclear content in muscle fibers. This increases the cross-sectional area of muscle fibers and thus muscle strength.
• Slow-twitch (Type I) muscle fibers are primarily recruited during endurance training, while strength training primarily impacts the cross-sectional area of muscle fibers.
• The number of myofibers in a muscle does not increase as a result of either endurance or strength training.
• Strength training increases the nuclear content of muscle fibers without increasing myofiber numbers, primarily due to an increase in the amount of actin and myosin.

Muscle Adaptations to Training

• Endurance training primarily leads to an increase in the number of mitochondria and capillaries within muscle fibers.
• Strength training results in an increase in the nuclear content of muscle fibers through the fusion of satellite cells. This does not increase the number of myofibers.
• The main goal of endurance training is to improve aerobic capacity and stamina.
• Strength training mainly leads to an increase in the cross-sectional area of muscle fibers.
• The increase in fiber diameter during strength training occurs because of an increase in the amount of actin and myosin.
• Endurance training mainly recruits slow-twitch (Type I) muscle fibers.