Lecture Outline: The Skeletal System
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- Functions of the Skeletal System
- Support: Provides the framework for the body
- Protection: Encloses important organs, like the brain (cranium) and heart/lungs (rib cage)
- Movement: Provides stiff bones for muscles to pull on, enabling body movement
- Breathing: Muscles pull on ribs to change lung volume, facilitating air intake and exhalation
- Mineral Storage: Serves as a large reservoir of calcium and phosphate in the form of hydroxyapatite (calcium phosphate)
- Regulates blood calcium levels as part of homeostasis
- Calcium ions are crucial for heart beating and skeletal muscle movement
- Phosphate is essential for many reasons, including being a component of plasma membranes
- Hemopoiesis: Production of blood cells (red and white) occurs in the marrow within bones
- Bone Tissue and Structure
- Bone Composition
- Mainly hydroxyapatite (calcium phosphate), which makes bones hard
- Hardest part of the body, second only to teeth enamel
- Types of Bone Tissue
- Compact Bone
- Much less space, therefore denser and stronger
- Forms the superficial layer on any bone
- Composed of cylindrical structures called osteons
- Spongy Bone
- Located deep to compact bone
- Has many pores/holes, giving it a sponge-like appearance, but it is still hard and strong
- Makes bones lighter while retaining strength
- Spaces contain marrow (red and yellow)
- Composed of tiny beam-like structures called trabeculae
- Bone Cells
- Osteoblasts: Cells that create the extracellular matrix material
- Osteocytes: Mature bone cells that maintain the extracellular matrix; locked in lacunae (little lagoons)
- Osteoclasts: Cells that degrade the extracellular matrix, breaking down hydroxyapatite to release calcium when needed
- Osteons (in Compact Bone)
- Central Canal: Passageway for blood vessels and nerves
- Lamellae: Several concentric layers of hard material surrounding the central canal
- Bone Development and Dynamics
- Bone Shapes
- Long bones: Longer than they are wide (e.g., humerus, femur)
- Flat bones: Thin and flattened (e.g., sternum)
- Short bones: Cube-shaped, not particularly long in any dimension (e.g., ankle/tarsal bones, wrist/carpal bones)
- Irregular bones: Complex, irregular shapes (e.g., vertebrae)
- Long Bone Anatomy
- Diaphysis: The long, central shaft of the bone
- Epiphysis: The two tips at either end of the long bone
- Medullary Cavity: An open, deep internal area within thicker bones, which contains marrow
- Ossification (Bone Hardening)
- Initial skeleton (pre-birth) is primarily made of pliable cartilage (mostly hyaline cartilage) or connective tissue membranes (skull)
- Epiphyseal Plate: A cartilage plate between the diaphysis and epiphysis in growing bones, where new material is added to make the bone longer
- Epiphyseal Line: Formed when the epiphyseal plate ossifies (turns into bone) around age 20, indicating that longitudinal growth has stopped
- Bone Remodeling
- Bones are dynamic and continuously being partially destroyed and rebuilt throughout life
- Involves both osteoclasts (tearing down old material) and osteoblasts (building new material)
- Bones adapt to stress; for example, trabeculae can be added and rearranged to withstand greater force
- Fracture Healing (Four Stages)
- Hematoma Formation: A large blood clot (hematoma) forms at the fracture site due to severed blood vessels, providing immediate but weak support
- Fibrocartilage Formation: The hematoma is gradually replaced by fibrocartilage, which is stronger than a blood clot
- Ossification of Fibrocartilage: The fibrocartilage turns into bony material (a bony callus), making the bone strong enough for use
- Remodeling: The bony callus is gradually broken down and reshaped to restore the bone to its original form, often resulting in a site stronger than it was originally
- Fetal Skeleton Development
- In a fetus, dark areas on X-rays indicate bony material, while lighter areas are still cartilage
- Fontanelles: Soft spots in a baby's skull, consisting of dense connective tissue membranes, which eventually ossify to form skull sutures
- Divisions of the Skeleton
- Overall Bone Count in Adulthood: 206 bones
- Humans are born with more bones that fuse over time (e.g., mandible, frontal bone)
- Over half (106) of the bones are in the feet and hands
- 56 bones (over a quarter) are in the fingers and toes (phalanges), allowing for dexterity
- Axial Skeleton: Forms the central long axis of the body
- Skull
- Total: 22 bones
- Cranium (Brain Case): 8 bones that are in direct contact with the brain, forming the protective brain case
- Facial Bones: 14 bones that attach to the front of the cranium
- Examples of skull bones to know:
- Parietal bone (forms side walls of cranium, "parietal" means wall)
- Temporal bone (orange, near temples, often first place for graying hair, indicating time)
- Occipital bone (unpaired, at the back, may have a larger protuberance in males)
- Frontal bone (unpaired, forms forehead, starts as two halves in fetus)
- Zygomatic bone (blue, cheekbone, forms part of the zygomatic arch, which is like a yoke)
- Maxillae (2, upper jawbones, stay separate)
- Mandible (lower jawbone, starts as two halves, fuses into one)
- Vertebral Column (Spine)
- Encloses and protects the spinal cord (part of the central nervous system)
- Made of many individual bones (vertebrae) whose slightly movable joints allow overall flexibility
- Categories of Vertebrae:
- Cervical Vertebrae (C1-C7): 7 bones in the neck ("cervix" means neck/narrowing)
- Thoracic Vertebrae (T1-T12): 12 bones in the chest ("thorax" means chest), which articulate with the ribs
- Lumbar Vertebrae (L1-L5): 5 bones in the lower back, typically have the largest bodies due to weight-bearing
- Sacrum: A single bone fused from 5 sacral vertebrae, part of the pelvic girdle
- Coccyx: A single bone fused from 2-4 coccygeal bones, also known as the tailbone
- Normal Curvatures: The vertebral column is not straight but has natural curves in the cervical, thoracic, and lumbar regions
- Curvature Disorders:
- Scoliosis: Abnormal sideways curvature
- Kyphosis: Exaggerated thoracic curvature (hunchback)
- Lordosis: Exaggerated lumbar curvature
- Vertebra Structure:
- Vertebral Foramen: The central hole in each vertebra, which collectively forms a tunnel for the spinal cord; the hole gets smaller inferiorly
- Body: The main weight-bearing part of the vertebra; gets larger inferiorly
- Intervertebral Discs: Tough pads of fibrocartilage located between vertebral bodies, allowing slight movement
- Herniated Disc: Occurs when part of an intervertebral disc bulges out and presses on spinal nerves, causing pain (discs do not "slip")
- Special Cervical Vertebrae:
- C1 (Atlas): The superior-most vertebra, named after Atlas from mythology as it holds up the skull; has no body, allows nodding ("yes" movement)
- C2 (Axis): Has a pivot point (dens) that articulates with the atlas, allowing for twisting/rotation of the head ("no" movement)
- Distinguishing Vertebrae Types:
- Cervical vertebrae are the only ones with three holes (one vertebral foramen and two additional holes for blood vessels)
- Thoracic vertebrae (side view) resemble giraffe heads
- Lumbar vertebrae (side view) resemble moose heads, and have very large bodies
- Ribs: 12 pairs (24 total) associated with thoracic vertebrae
- True Ribs (Pairs 1-7): Connect directly to the sternum via their own band of hyaline cartilage
- False Ribs (Pairs 8-12): Do not connect directly to the sternum
- Pairs 8-10: Have cartilage that connects to the cartilage of other ribs
- Pairs 11-12 (Floating Ribs): Connect only to the vertebrae and have no anterior connection
- Sternum: Breastbone, central bone in the chest to which true ribs and some false ribs indirectly attach
- Appendicular Skeleton: Consists of limbs and the structures that attach them to the axial skeleton
- Pectoral Girdle: Forms a ring (though not complete) around the neck/shoulders
- Clavicles (2): Collar bones
- Scapulae (2): Shoulder blades
- Upper Limbs
- Humerus: The single bone of the arm (brachium), from shoulder to elbow
- Antibrachium (Forearm) Bones (2):
- Radius: Runs parallel to the ulna in anatomical position; rotates for pronation (palm down) and supination (palm up)
- Ulna: Forms the hinge joint at the elbow with the humerus
- Manus (Hand) Bones:
- Carpus (Wrist): The proximal part of the hand, made of 8 carpal bones arranged in two rows
- Metacarpals: The 5 bones of the palm, beyond the carpals
- Phalanges (Fingers/Digits): The 14 bones of the fingers (2 in the thumb, 3 in each of the other four fingers)
- Pelvic Girdle: Forms a complete ring, attaching the lower limbs to the axial skeleton
- Composed of two Ossa Coxae (pelvic bones) and the sacrum (part of the axial skeleton)
- Ossa Coxae: Each pelvic bone fuses from three parts in adulthood:
- Ilium: The superior and largest part
- Ischium: The posterior, rounded part that one sits on ("sitting bones")
- Pubis: The anterior part, where the two pubes meet in the front
- Sexing a Skeleton using the Pelvic Girdle:
- Female pelvis has a more U-shaped angle where the pubes meet, creating a wider birth canal
- Male pelvis has a more V-shaped angle where the pubes meet
- Lower Limbs
- Femur: The single bone of the thigh (proximal part of lower limb), and the largest bone in the skeleton
- Leg Bones (from knee to foot):
- Tibia: The larger of the two leg bones, bears all the weight from above and connects to the femur
- Fibula: The smaller bone, situated lateral to the tibia; does not bear weight but supports when shifting to the side, connecting only to the tibia and tarsal bones
- Pedis (Foot) Bones:
- Tarsus (Ankle): The proximal part of the foot, made of 7 tarsal bones (one fewer than carpal bones)
- Metatarsals: The 5 bones that make up the arch of the foot, beyond the tarsals
- Phalanges (Toes/Digits): The 14 bones of the toes (2 in the great/big toe, 3 in each of the other four toes)