Skeletal System and Bone Histology

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Questions and Answers

What are the main minerals stored in bones?

Magnesium and phosphorus

Calcium and phosphorus (correct)

Calcium and magnesium

Calcium and zinc

Which component of the bone imparts flexible strength to the bone matrix?

Elastic fibers

Proteoglycan

Collagen (correct)

Hydroxyapatite

Intramembranous ossification is the mode of development in these bones, except:

Mandible

Humerus bone (correct)

Frontal bones of the skull

Diaphysis of clavicle

Active bone resorption is a function of which of the following bone cells?

Osteoclast Signup and view all the answers

Blood calcium levels depend on the movement of this mineral in and out of the bone. Levels of calcium in the blood are controlled by?

Parathyroid hormone Signup and view all the answers

Which of the following is NOT a function of the skeletal system?

Digestion Signup and view all the answers

Which structure helps to hold bones together in the skeletal system?

Ligaments Signup and view all the answers

What role does cartilage play in the skeletal system?

Support and flexibility Signup and view all the answers

Which bone disorder is characterized by poorly mineralized bones with adequate osteoid production?

Osteomalacia Signup and view all the answers

Which condition describes an abnormal lateral curvature of the spine?

Scoliosis Signup and view all the answers

What blood test marker is indicative of enhanced osteoblastic activity?

Alkaline Phosphatase Signup and view all the answers

Which mineral is primarily stored in bones alongside calcium?

Phosphorus Signup and view all the answers

Which of the following bones does not undergo intramembranous ossification?

Humerus bone Signup and view all the answers

What is the primary function of osteoclasts?

To resorb bone Signup and view all the answers

Which type of bone is characterized by collagen fibers that are randomly oriented?

Woven bone Signup and view all the answers

What structure is known as the functional unit of compact bone?

Osteon Signup and view all the answers

How are osteocytes arranged in lamellar bone?

Sandwiched between lamellae Signup and view all the answers

Which feature distinguishes compact bone from spongy bone?

More bone matrix and less space Signup and view all the answers

What is the role of canaliculi in spongy bone?

To facilitate nutrient exchange Signup and view all the answers

Which bone type is organized into thin concentric sheets?

Lamellar bone Signup and view all the answers

What do perforating canals primarily connect?

Central canals of osteons Signup and view all the answers

What is the primary characteristic of trabeculae found in spongy bone?

Lamellae with osteocytes in lacunae Signup and view all the answers

What is the primary role of osteoblasts in bone histology?

To produce collagen and secrete matrix vesicles Signup and view all the answers

Which lamellae are found at the outer surfaces of compact bone?

Circumferential lamellae Signup and view all the answers

Which cells are responsible for the process of ossification?

Osteoblasts Signup and view all the answers

Which component of bone matrix is responsible for its organic characteristics?

Collagen and proteoglycans Signup and view all the answers

What is the function of canaliculi in bone histology?

To facilitate nutrient exchange among osteocytes Signup and view all the answers

What unique feature distinguishes osteoclasts from other bone cells?

They have a ruffled border Signup and view all the answers

What initiates the recruitment of osteoclasts to a bone remodeling site?

Osteoblasts recruiting osteoclasts Signup and view all the answers

What mainly comprises the inorganic component of bone matrix?

Hydroxyapatite Signup and view all the answers

Which statement accurately describes osteocytes?

They are capable of producing small amounts of matrix. Signup and view all the answers

What are osteochondral progenitor cells primarily derived from?

Mesenchymal cells Signup and view all the answers

Which of the following statements about osteoclasts is true?

They are multinucleated cells involved in bone resorption. Signup and view all the answers

What is the primary initiation site for intramembranous ossification?

Embryonic connective tissue membranes Signup and view all the answers

At what point in development does endochondral ossification begin?

Around the 8th week of embryonic development Signup and view all the answers

Which part of the skeletal system is formed primarily through intramembranous ossification?

Diaphyses of clavicles Signup and view all the answers

What is the main function of the epiphyseal plate in bone growth?

Cartilage growth and ossification Signup and view all the answers

How do hormones influence bone growth?

By stimulating interstitial and appositional bone growth Signup and view all the answers

What is the role of calcitonin in calcium homeostasis?

Lowers blood calcium levels Signup and view all the answers

Which vitamin is essential for collagen synthesis in bones?

Vitamin C Signup and view all the answers

What primarily regulates calcium levels in the blood?

Hormones such as parathyroid hormone and calcitriol Signup and view all the answers

What is the function of the basic multicellular unit (BMU) in bone remodeling?

Remodel bones and facilitate repair Signup and view all the answers

What happens to the epiphyseal plates as a bone reaches full growth during puberty?

They ossify and turn into the epiphyseal line Signup and view all the answers

What is the role of tropomyosin during muscle contraction?

It pulls away from actin to allow myosin binding. Signup and view all the answers

Which component invaginates into the interior of skeletal muscle fibers?

T tubule Signup and view all the answers

During an isometric contraction, which of the following occurs?

Muscle length remains unchanged while tension increases. Signup and view all the answers

Which sequence of events correctly describes the muscle contraction process?

Calcium ions combine with troponin, tropomyosin pulls away from actin, and myosin binds to actin. Signup and view all the answers

What is the primary function of ATPase associated with myosin?

To hydrolyze ATP, providing energy for muscle contraction. Signup and view all the answers

What is a primary cause of muscle cramps?

Dehydration Signup and view all the answers

Which condition is characterized by the chronic widespread pain in skeletal muscles?

Fibromyalgia Signup and view all the answers

What happens to muscle fibers as people age?

Atrophy of muscle fibers Signup and view all the answers

How is Duchenne muscular dystrophy primarily identified diagnostically?

Muscle biopsies Signup and view all the answers

What physiological mechanism is primarily associated with muscle fatigue?

Reduced ATP levels Signup and view all the answers

What is myasthenia gravis primarily classified as?

An autoimmune disease Signup and view all the answers

What common feature characterizes tendinitis?

Inflammation of a tendon Signup and view all the answers

Which of the following is associated with the aging process of the muscular system?

Decrease in the surface area of the NMJ Signup and view all the answers

What is the primary function of the neuromuscular junction in the muscular system?

It facilitates the stimulation of muscle cells by nerves. Signup and view all the answers

What is a common cause of muscle soreness after exercise?

Inflammatory chemical influx Signup and view all the answers

What does the presynaptic terminal do when an action potential arrives?

It opens voltage-gated calcium channels. Signup and view all the answers

Which of the following is an important diagnostic tool for identifying autoantibodies in myasthenia gravis?

Electromyography Signup and view all the answers

What role do acetylcholine receptors play at the neuromuscular junction?

They bind to acetylcholine to initiate muscle contraction. Signup and view all the answers

What is the space called that separates axon terminals and muscle fibers?

Synaptic cleft Signup and view all the answers

Which component of the neuromuscular junction contains synaptic vesicles?

Axonal endings Signup and view all the answers

Which statement accurately describes the motor end plate?

It has receptors for binding neurotransmitters. Signup and view all the answers

What is the primary trigger for muscle contraction at the neuromuscular junction?

Binding of acetylcholine to muscle fibers. Signup and view all the answers

What is the first step in the process of muscle contraction at the neuromuscular junction?

An action potential arrives at the presynaptic terminal. Signup and view all the answers

What is the role of troponin in muscle contraction?

Binds calcium to trigger muscle contraction Signup and view all the answers

During muscle relaxation, which process occurs first?

Cessation of action potential along the sarcolemma Signup and view all the answers

What characterizes isotonic contractions?

Tone remains constant while muscle shortens Signup and view all the answers

What triggers the rise in intracellular calcium levels necessary for muscle contraction?

Propagation of an action potential along the sarcolemma Signup and view all the answers

Which of the following components directly binds to active sites on actin molecules during contraction?

Myosin heads Signup and view all the answers

What is the primary function of myosin heads in muscle contraction?

Form cross-bridges with actin Signup and view all the answers

Which statement correctly describes the process of excitation-contraction coupling?

It directly links electrical signals to muscle contraction Signup and view all the answers

In which type of muscle contraction does the muscle shorten while maintaining constant tension?

Concentric contraction Signup and view all the answers

What happens to tropomyosin during muscle contraction?

It uncovers active sites on G actin Signup and view all the answers

What primarily occurs during isometric contractions?

Tension increases without change in muscle length Signup and view all the answers

What process involves the release of acetylcholine from synaptic vesicles?

Exocytosis Signup and view all the answers

What happens after acetylcholine binds to ligand-gated sodium channels on the motor end plate?

Depolarization occurs, generating an action potential Signup and view all the answers

What happens to acetylcholine after it unbinds from the ligand-gated channels?

It is broken down by acetylcholinesterase Signup and view all the answers

Which component of actin myofilament prevents active sites on G actin from being uncovered in relaxed muscle?

Tropomyosin Signup and view all the answers

What is the first step in the muscle contraction process at the neuromuscular junction?

Acetylcholine is released Signup and view all the answers

How is choline transported back into the presynaptic terminal?

By symport with sodium Signup and view all the answers

What structure does globular (G) actin form when it polymerizes?

Fibrous (F) actin Signup and view all the answers

What is the role of acetylcholinesterase in muscle contraction?

To break down acetylcholine in the synaptic cleft Signup and view all the answers

What initiates the generation of an action potential in a muscle fiber?

Sodium entering the muscle fiber after acetylcholine binds Signup and view all the answers

Which statement about tropomyosin is true?

It covers the active sites on G actin in relaxed muscle Signup and view all the answers

Study Notes

Skeletal System Functions

Support: Serves as the body's primary weight-bearing framework, providing rigid structure.
Protection: Safeguards vital organs.
Movement: Facilitates movement by providing attachment points for muscles.
Storage: Acts as a repository for essential minerals, primarily calcium and phosphorus.
Blood Cell Production: Red bone marrow within bones is responsible for hematopoiesis, the production of blood cells.

Bone Histology

Bone Matrix: Composed of organic and inorganic components.
- Organic (35%): Collagen and proteoglycans
- Inorganic (65%): Hydroxyapatite (calcium phosphate crystals)
Bone Cells:
- Osteochondral Progenitor Cells: Bone stem cells derived from mesenchymal cells.
- Osteoblasts: Responsible for bone formation, producing collagen and proteoglycans.
- Osteocytes: Mature bone cells responsible for maintaining bone matrix.
- Osteoclasts: Large, multinucleated cells responsible for bone resorption.

Bone Development

Intramembranous Ossification:
- Begins during the 8th week of embryonic development.
- Responsible for the development of skull bones, part of the mandible, and the diaphysis of clavicles.
- Characterized by the formation of bone directly within connective tissue membranes.
Endochondral Ossification:
- Also begins during the 8th week of embryonic development, but some processes may not begin until 18-20 years of age.
- Responsible for the development of the base of the skull, part of the mandible, the epiphyses of the clavicle, and most of the remaining skeletal system.
- Involves formation of bone within a cartilage model.

Bone Growth

Growth in Length:
- Occurs at the epiphyseal plate, a specialized cartilaginous region.
- Involves endochondral ossification.
- Epiphyseal plate closure marks the end of long bone growth.
Growth in Bone Width:
- Occurs at the periosteum, the outer covering of bones.
- Increases bone diameter.

Bone Remodeling

Continuous process of bone resorption and formation, replacing old bone with new bone.
Involves the basic multicellular unit (BMU), a group of osteoclasts and osteoblasts working together.
Essential for bone growth, shaping, adaptation to stress, repair, and calcium ion regulation.

Bone Repair

Involves a complex series of events:
- Hematoma Formation: A blood clot forms at the site of fracture.
- Soft Callus Formation: Fibrocartilaginous tissue forms a temporary bridge between the broken bone ends.
- Hard Callus Formation: The soft callus is gradually replaced by a hard callus of woven bone.
- Remodeling: Woven bone is remodeled into lamellar bone, restoring bone strength.

Abnormalities of Bone

Rickets:
- A disease of children characterized by inadequate mineralization of the epiphyseal plate, leading to soft and deformed bones.
Osteomalacia:
- A disease of adults characterized by poorly mineralized bone, leading to bone weakness and pain.
Osteomyelitis:
- Infection of bone tissue.
Osteoporosis:
- Characterized by decreased bone density, increasing the risk of fractures.

Diagnostic Tests

Blood Tests:
- Serum Calcium: Measures calcium levels in the blood.
- Alkaline Phosphatase: An enzyme that indicates increased osteoblastic activity.
- C-telopeptide: A marker for bone resorption.
- Procollagen Type 1 N-terminal Propeptide (P1NP): A marker for bone formation.
Serum 25 Hydroxyvitamin D Level: Measures vitamin D3 levels, important for calcium absorption.
Serum Vitamin C Level: Measures vitamin C levels, essential for collagen synthesis by osteoblasts.

Calcium Homeostasis

Parathyroid Hormone (PTH):
- Released in response to low blood calcium levels.
- Increases blood calcium by stimulating osteoclast activity, promoting calcium reabsorption in the kidneys, and indirectly enhancing calcium absorption in the intestines.
Calcitriol (Active Vitamin D):
- Increases blood calcium by promoting intestinal absorption of calcium.
Calcitonin:
- Lowers blood calcium by inhibiting osteoclast activity.

Neuromuscular Junction

The area where a muscle cell is stimulated by nerve
Consists of axonal endings, motor end plate, and the synaptic cleft
Axonal endings contain synaptic vesicles that store acetylcholine
Motor end plate is a specialized area of muscle plasma membrane containing acetylcholine receptors

Muscle Contraction Steps

Action potential arrives at presynaptic terminal
Voltage-gated calcium channels open in presynaptic membrane
Calcium triggers the release of acetylcholine from synaptic vesicles
Acetylcholine diffuses across the synaptic cleft
Acetylcholine binds to ligand-gated sodium channels on the motor end plate
Sodium enters the muscle fiber and depolarization occurs
If depolarization reaches threshold, an action potential is generated in muscle fiber
Acetylcholine unbinds from the ligand-gated channels, and sodium channels close
Acetylcholinesterase breaks down acetylcholine in the synaptic cleft
Choline is transported back into the presynaptic terminal with sodium
Acetylcholine is resynthesized within the presynaptic terminal

Actin and Myosin Myofilament Structure

Actin Myofilament: Consists of globular (G) actin, tropomyosin, and troponin.
- G actin subunits form fibrous (F) actin.
- Tropomyosin covers active sites on G actin in relaxed muscle.
- Troponin binds to actin and prevents tropomyosin from uncovering active sites.
- Troponin also binds calcium during muscle contraction.
Myosin Myofilament: Consists of two myosin heavy chains and two myosin heads.
- Myosin heads bind to active sites on actin during cross-bridge formation.
- Myosin heads have hinge regions that allow for movement.
- Myosin heads are ATPase enzymes that break down ATP to fuel muscle contraction.

Excitation-Contraction Coupling

Process that links electrical stimulation of muscle (action potential) to mechanical contraction
Occurs at the triad, which is a junction formed by a T tubule and two terminal cisternae of sarcoplasmic reticulum
Action potential travels along the sarcolemma and into the T tubule
This triggers the release of calcium from the sarcoplasmic reticulum
Calcium binds to troponin, causing a conformational change
This change in troponin pulls tropomyosin away from the active sites on actin
Myosin heads can now bind to actin and initiate the power stroke

Cross-Bridge Movement

Myosin head binds to active site on actin, forming a cross-bridge
Myosin head pivots, pulling the thin filament towards M line - This is the power stroke
ATP binds to the myosin head, causing it to detach from actin
ATP is hydrolyzed to ADP and phosphate, providing energy for the myosin head to return to its cocked position
Cycle repeats as long as calcium levels remain elevated and ATP is available

Muscle Relaxation

Acetylcholine release stops at the neuromuscular junction.
Action potentials cease, halting calcium release from the sarcoplasmic reticulum.
Calcium diffuses away from troponin molecules.
Tropomyosin moves back to cover the active sites on actin.
Cross-bridge formation is blocked, and muscle fibers relax.

Types of Muscle Contractions

Isometric contractions: Muscle does not shorten, but tension increases.
Isotonic contractions: Muscle shortens, and tone remains constant during contraction.
- Concentric contraction: Muscle shortens while tension increases.
- Eccentric contraction: Muscle lengthens while tension is maintained.

Disorders of the Muscular System

Aging: Muscle atrophy, loss of muscle fibers, reduced NMJ surface area, and fewer motor neurons.
Muscle fatigue: Temporary state of reduced work capacity due to factors like acidosis, ATP depletion, oxidative stress, and localized inflammation.
Muscle soreness: Painful condition after vigorous exercise, caused by inflammation.
Cramps: Painful, spastic contractions of muscles, often due to dehydration or electrolyte imbalance.
Tendonitis: Inflammation of a tendon or its attachment point, caused by overuse.
Fibromyalgia: Chronic, widespread muscle pain with no known cure.
Myasthenia gravis: Autoimmune disease where antibodies block acetylcholine receptors, causing muscle weakness.
- Diagnostics: Electromyography and antibody testing.
Duchenne muscular dystrophy: X-linked genetic disorder with progressive muscle weakness and degeneration.
- Diagnostics: Serum creatinine, muscle biopsy, and immunohistochemical studies.

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Description

Explore the essential functions of the skeletal system, including support, protection, movement, storage, and blood cell production. Additionally, delve into bone histology, examining the composition of the bone matrix and the various types of bone cells involved in its maintenance and formation.