Bones, The Skeletal System, & Articulations
Chapter 6: Functions and Components of the Skeletal System
Functions of the Skeletal System
Support: The skeletal system provides a stable framework for the body, supporting not only the body structure but also cradling soft organs such as the heart and lungs, allowing for upright posture.
Protection: The skeleton acts as a shield for vital organs; for example, the skull encloses and protects the brain from injury, while the ribcage protects the heart and lungs from trauma.
Movement: Bones serve as levers for muscle action, providing attachment points for muscles. Joints formed between bones allow for flexible movement in various planes, which is vital for activities such as walking, running, and grasping objects.
Storage: Bones serve as a reservoir for essential minerals such as calcium and phosphorus, releasing them into the bloodstream when needed. Additionally, bones store adipose tissue within the yellow marrow for fat storage, which can be utilized as an energy source.
Hematopoiesis: The process of blood cell formation occurs in the red bone marrow, which is located primarily within the cavities of certain bones, such as the sternum and pelvis, producing red blood cells, white blood cells, and platelets.
Acid-base Homeostasis: Bones play a key role in regulating the body’s pH balance. By absorbing or releasing minerals such as calcium and phosphate, bones help maintain a stable internal environment critical for cellular functions.
Types of Bones
Long Bones: Characterized by their elongated shape, these bones are longer than they are wide and consist of a diaphysis (shaft) and epiphyses (ends). Examples include the femur, ulna, and humerus, which are crucial for limb movement.
Short Bones: Nearly equal in length and width, short bones provide stability and support with limited movement, such as the carpals and tarsals in the wrist and foot respectively.
Flat Bones: These thin and often curved bones provide extensive surfaces for muscle attachment and protection for underlying structures. Examples include the sternum, cranial bones of the skull, and ribs which all play critical roles in body armor and muscle leverage.
Irregular Bones: With complex shapes that don’t fit into other categories, irregular bones serve multiple functions. Examples include the vertebrae, which protect the spinal cord and support the body, and some facial bones that provide structural integrity and support for facial features.
Macroscopic Parts of a Long Bone
Diaphysis: The central shaft of long bones composed primarily of compact bone, it provides strength and support, enabling the bone to withstand tensions and compressive forces.
Epiphysis: The ends of long bones, which are often covered with articular cartilage to reduce friction at joint surfaces and facilitate smooth movement. The epiphysis contains spongy bone filled with red marrow, playing a crucial role in blood cell production.
Epiphyseal Line and Plate: The epiphyseal plate is crucial during growth, consisting of hyaline cartilage that allows for lengthwise growth, while the line indicates where growth has ceased, marking the transition from cartilage to bone tissue.
Articular Cartilage: A smooth, white tissue that covers the ends of bones in synovial joints, reducing friction and absorbing shock during movement.
Periosteum: A dense, fibrous membrane that covers the outer surface of bones, containing blood vessels, nerves, and lymphatic vessels; it is essential for bone health and repair and serves as an attachment point for muscles and tendons.
Medullary Cavity: The hollow space within the diaphysis filled with yellow marrow in adults, which serves as a fat storage depot and plays a less active role in hematopoiesis than red marrow during periods of high energy demand.
Bone Marrow: There are two types, red marrow responsible for hematopoiesis and yellow marrow that stores fat. The composition and function of marrow can change based on the body’s physiological needs.
Components of Osseous Tissue
Inorganic Matrix: This includes hydroxyapatite crystals, which provide rigidity and the compressive strength essential for resisting mechanical stress. This mineral composition is why bones can withstand substantial weight and force without breaking.
Organic Matrix: Composed mainly of collagen fibers and ground substance, which provide flexibility and tensile strength, allowing bones to absorb shock and resist bending forces without fracturing.
Bone Cells: - Osteocytes: These mature bone cells reside in lacunae and function in the maintenance of bone matrix and the regulation of mineral content.
Osteoblasts: Immature bone-forming cells vital for ossification, these cells produce the bone matrix and are crucial during the growth and healing of bones.
Osteoclasts: Large multinucleated cells that resorb bone tissue, critical in bone remodeling and maintaining calcium homeostasis in the body.
Microscopic Structure of Bone
Compact Bone: This dense tissue forms the outer layer of bones, providing strength and protection. Within this layer is the osteon (Haversian system), the fundamental unit of structure featuring:
Lamellae: Concentric rings of bone matrix surrounding the central canal, providing support and strength to withstand forces.
Central (Haversian) Canal: Contains blood vessels and nerves essential for bone nutrition and sensation.
Lacunae: Small cavities that house osteocytes, responsible for monitoring and maintaining bone health.
Canaliculi: Tiny channels connecting lacunae, facilitating nutrient and waste exchange between osteocytes and the central canal.
Spongy Bone: Lighter than compact bone, it consists of an trabecular network and is primarily found in the epiphyses of long bones, providing structural integrity while minimizing weight and serving as a site for hematopoiesis.
Ossification Processes
Intramembranous Ossification: This process involves the direct formation of bone from mesenchymal tissue, primarily for flat bones such as the skull, where osteoblasts differentiate directly from mesenchyme and begin bone formation.
Endochondral Ossification: Involves the replacement of hyaline cartilage with bone, essential for the development of long bones and responsible for longitudinal growth during infancy and childhood.
Bone Growth
Length: Achieved at the epiphyseal plate, where chondrocytes proliferate and later are replaced by osteoblasts, leading to the growth of long bones as the individual matures.
Width: This process involves bone formation by osteoblasts on the outer surface while osteoclasts break down bone on the inner surface of the medullary cavity, allowing bones to grow in diameter while maintaining strength.
Hormonal Influence: Hormones such as growth hormone from the pituitary gland, sex hormones (estrogen and testosterone), and thyroid hormones influence the regularity of bone growth, impacting when growth plates close and affecting overall bone density.
Bone Remodeling and Repair
Bone Deposition: New bone formation through the action of osteoblasts, essential for growth, healing, and adaptation to stress. It is stimulated by mechanical stress on bones.
Bone Resorption: The process by which osteoclasts break down bone tissue, crucial for calcium regulation, and reshaping bones in response to physical demands placed on the skeletal system.
Factors Influencing Remodeling:
Specific Sex Hormones: Estrogens and androgens play critical roles in promoting bone formation, as well as regulating bone density throughout life.
Age: Bone remodeling efficiency typically decreases with age, leading to increased susceptibility to fractures and conditions like osteoporosis.
Dietary Factors: Adequate intake of essential nutrients like calcium, vitamin D, and protein is vital for bone health, whereas deficiencies can lead to weakened bone structure.
Calcium Ion Homeostasis: The regulation of calcium levels in the bloodstream is crucial for various physiological functions, including muscle contraction and nerve transmission, and is primarily controlled by the actions of parathyroid hormone (PTH) and calcitonin.
Chapter 7: The Skeletal System Structure and Function
Axial vs. Appendicular Skeleton
Axial Skeletal System (80 bones):
Skull: Comprises cranial bones that protect the brain and facial bones that form the features of the face, allowing for sensory functions and articulation.
Ribs & Sternum: Protect the thoracic cavity, facilitate respiration, and support the pectoral girdle.
Vertebrae: 23 individual vertebrae (7 cervical, 12 thoracic, 5 lumbar) along with the sacrum and coccyx protect the spinal cord and support the body's weight, allowing for flexibility and movement.
Appendicular Skeletal System (126 bones):
Girdles: The pectoral girdle (clavicles and scapulae) and pelvic girdle (hip bones) connect the limbs to the axial skeleton, providing attachment points for muscles and support for locomotion and posture.
Limbs: Comprises the bones of the upper limbs (humerus, radius, ulna, carpal, metacarpal, and phalanges) and lower limbs (femur, tibia, fibula, tarsals, metatarsals, and phalanges), which facilitate a wide range of movements from fine motor skills to weight-bearing activities.
Bone Markings
Identification: Proper naming and spelling are crucial for understanding anatomy, as different surface features (e.g., processes, fossae, foramina) serve specific functions, including the attachment of muscles, articulation with other bones, and passageways for nerves and blood vessels (reference Skeletal System checklist).
Structural Features of Bones
Skull: Includes cranial bones (e.g., frontal, parietal) that encase the brain and facial bones (e.g., maxilla, mandible) providing structure and function for facial expression and mastication.
Vertebral Column: Composed of cervical, thoracic, lumbar vertebrae, sacrum, and coccyx; it houses and protects the spinal cord, provides support for the head and trunk, and facilitates various movements of the spine.
Thoracic Cage: Formed by the ribs and sternum; protects vital organs like the heart and lungs, accommodates for respiration by expanding and contracting with the diaphragm and intercostal muscles.
Pectoral Girdle & Upper Limb: Composed of the scapula and clavicle connecting the arms to the torso, comprising the humerus, radius, ulna, and hand bones facilitating a wide range of movements required for manipulation and mobility.
Pelvic Girdle & Lower Limb: Includes coxal bones (hip bones) that connect the lower limbs to the axial skeleton, supporting weight during bipedal locomotion and housing organs; comprised of the femur, tibia, fibula, and foot bones suited for both stability and locomotion.
Anatomical Features of Specific Cavities
Orbit: Bony cavity that houses the eye and is composed of several bones (e.g., frontal, zygomatic, maxillary) forming a protective socket for the eye, facilitating vision and movement.
Nasal Cavity: The interior space bordered by the nasal and maxillary bones crucial for respiration, filtration of air, and olfaction, lined with mucous membranes to trap debris and moisten inhaled air.
Paranasal Sinuses: Air-filled spaces that lighten the skull, enhance voice resonance, and play a role in the respiratory system; includes frontal, maxillary, ethmoid, and sphenoid sinuses, which may influence pressure and drainage issues that affect sinus health.
Comparison of Skull Development
Adult vs. Infant: Infants are born with fontanels (soft spots) that allow for brain growth and skull deformation during childbirth. These fontanels gradually ossify and close as the child develops, resulting in a solid bony structure by adulthood.
Classes of Vertebrae
Cervical: Seven vertebrae (C1-C7) support the neck; the atlas (C1) allows head nodding, while the axis (C2) enables head rotation.
Thoracic: Twelve vertebrae (T1-T12) articulate with ribs, supporting the upper back and maintaining thoracic cavity integrity for lung and heart protection.
Lumbar: Five vertebrae (L1-L5) are the largest, supporting most of the body’s weight, allowing for flexion, extension, and minimal rotational movement.
Sacrum: Composed of five fused vertebrae, it forms the posterior aspect of the pelvis, connecting the spine to the pelvis and distributing weight to the lower limbs.
Coccyx: Known as the tailbone, it is a vestigial structure formed by the fusion of four vertebrae that provides attachment for ligaments and muscles of the pelvic floor.
Intervertebral Discs and Ligaments
Intervertebral Discs: Composed of an outer annulus fibrosus and an inner nucleus pulposus; these discs cushion the vertebrae, allowing for flexibility, absorbing shock during movement, and helping to maintain spinal alignment.
Vertebral Ligaments: These strong connective tissues, such as the anterior longitudinal and posterior longitudinal ligaments, strengthen and stabilize the vertebral column, providing support against excessive motion and injury.
Chapter 8: Joints, Classification and Movement
Functional Classification of Joints
Synarthroses: Completely immovable joints, such as cranial sutures and the gomphosis joint of teeth in sockets; these joints provide stability and protection.
Amphiarthroses: Slightly movable joints, like those in the pubic symphysis, allowing limited flexibility to accommodate movement and absorb shock.
Diarthroses: Freely movable joints; examples include synovial joints like the knee and shoulder, allowing for a wide range of motions due to their anatomical structure.
Structural Classification of Joints
Fibrous Joints: Characterized by dense connective tissue with no joint cavity, these joints resist movement; examples include syndesmoses (between long bones) and sutures of the skull.
Cartilaginous Joints: Bones connected by hyaline cartilage or fibrocartilage, allowing for limited movement; examples include synchondroses (costal cartilage) and symphyses (pubic symphysis).
Synovial Joints: Features a fluid-filled joint cavity allowing for extensive mobility; components include articular cartilage for smooth movement, joint capsules, and synovial fluid for lubrication and nourishment of cartilage.
Synovial Joint Types **
Types of Synovial Joints: Includes ball-and-socket (shoulder, hip), hinge (elbow, knee), pivot (atlantoaxial joint), condyloid (wrist), saddle (thumb), and gliding joints (carpals), each permitting different ranges of motion critical for diverse activities.
Movements at Synovial Joints
Flexion: Decreases the angle between body parts, such as bending the elbow.
Extension: Increases the angle, like straightening the knee.
Abduction: Moving a limb away from the body’s midline, such as raising arms sideways.
Adduction: Moving a limb toward the midline of the body, like bringing arms down to the sides.
Plantar Flexion: Extending the foot downwards (pointing toes) enabling standing on tiptoes.
Dorsiflexion: Flexing the foot upwards, bringing toes toward the shin.
Rotation: Turning a bone on its own axis, such as twisting the head side to side.
Circumduction: Moving a limb in a circular motion, as seen with the arm rotating at the shoulder.
Pronation: Rotating the forearm so that the palm faces downwards (turning a doorknob).
Supination: Rotating the forearm so the palm faces upwards (holding soup).
Inversion: Turning the sole of the foot inward, assisting in maintaining balance.
Eversion: Turning the sole of the foot outward, which can be important in various movements during walking or running.
Comparison of Classes and Subclasses of Joints
Collectively, understanding the differences and similarities among these classifications in terms of mobility, structural features, and biomechanical relationships is critical for comprehending overall joint function and health.