skeletal system - unit 7

GNUR 157: Anatomy & Physiology I - Chapter 07: The Skeletal System

Structure of the Skeleton and Skeletal Cartilages

  • The Skeleton

    • Consists of approximately 206 bones and associated skeletal cartilages located at joints or articulations.

    • Skull

      • The most complex structure of the skeleton, comprising 22 bones.

      • 8 cranial bones encase the brain.

      • 14 facial bones form the framework for the face.

    • Vertebral Column

      • Includes 33 vertebrae.

      • The top 24 vertebrae encase the spinal cord.

      • The 2 inferior bones, the sacrum and coccyx, are made of fused vertebrae.

    • Thoracic (Rib) Cage

      • Comprises 12 pairs of ribs, the sternum, and part of the vertebral column.

      • Primary function is to protect the structures within the thoracic cavity.

    • Pectoral Girdle

      • Includes the clavicle and scapula.

      • Supports the upper limb and anchors it to the trunk.

    • Upper Limb

      • Arm: Includes the humerus.

      • Forearm: Includes the radius and ulna.

      • Wrist: Includes the carpals.

      • Hand and Fingers: Include the metacarpals and phalanges.

    • Pelvic Girdle

      • Composed of 2 pelvic bones and the sacrum.

      • Each pelvic bone consists of an ilium, ischium, and pubis.

      • Supports the lower limb and anchors it to the trunk.

    • Lower Limb

      • Thigh: Includes the femur.

      • Leg: Includes the tibia and fibula.

      • Ankle: Includes the tarsals.

      • Foot: Includes the metatarsals and phalanges.

  • Structural Divisions of the Skeleton

    • Axial Skeleton

      • Forms the longitudinal axis of the body.

      • Structured primarily for protection.

      • Components: Skull, Vertebral column, Thoracic cage.

    • Appendicular Skeleton

      • Structured primarily for motion.

      • Components: Bones of the girdles (pectoral and pelvic) and the upper and lower limbs.

Bone Markings

  • Definition: Surface features found on bones.

  • Types of Bone Markings

    • Depressions

      • Description: Clefts of varying depth in a bone; located where a bone meets another structure (e.g., another bone or a blood vessel).

      • Function: Allow blood vessels and nerves to travel along a bone, or provide an articulation point for two bones.

      • Facet: Shallow convex or concave surface where two bones articulate. Example: Rib: Articular facet for articulation with a transverse process.

      • Fossa (plural, fossae): Indentation in a bone into which another structure fits. Example: Humerus: Distal portion with olecranon fossa.

      • Fovea: Shallow pit. Example: Femur: Fovea capitis.

      • Groove (or sulcus): Long indentation along which a narrow structure travels. Example: Rib: Costal groove.

    • Openings

      • Description: Holes that allow blood vessels and nerves to travel through a bone; permit access to the middle and inner ear; encase delicate structures and protect them from trauma.

      • Function: Enclose delicate structures and allow them to pass through a bone.

      • Canal (or meatus): Tunnel through a bone. Example: Temporal bone: External acoustic meatus.

      • Fissure: Narrow slit in a bone or between adjacent parts of bones. Example: Sphenoid bone: Superior orbital fissure.

      • Foramen (plural, foramina): Hole in a bone. Example: Frontal bone: Supraorbital foramen.

    • Projections

      • Description: Bony extensions of varying shapes and sizes; some provide locations for attachment of muscles, tendons, and ligaments; some fit into depressions of other bones to stabilize joints.

      • Function: Provide sites for ligament/tendon attachment or where bones articulate.

      • Condyle: Rounded end of a bone that articulates with another bone. Example: Mandible: Mandibular condyle.

      • Crest: Ridge or projection. Example: Ilium: Iliac crest.

      • Head: Round projection from a bone's epiphysis. Example: Humerus: Head.

      • Tubercle and Tuberosity: Small, rounded bony projection; a tuberosity is a large tubercle. Example: Humerus: Deltoid tuberosity.

      • Epicondyle: Small projection usually proximal to a condyle. Example: Humerus: Medial epicondyle.

      • Process: Prominent bony projection. Example: Scapula: Coracoid process.

      • Spine: Sharp process. Example: Scapula: Spine.

      • Protuberance: Outgrowth from a bone. Example: Occipital bone: External occipital protuberance.

      • Trochanter: Large projection found only on the femur. Example: Femur: Greater trochanter.

      • Line: Long, narrow ridge. Example: Femur: Linea aspera.

Overview of Skull Structure

  • Skull Bones

    • All adult skull bones are united by immoveable joints called Sutures, with the sole exception of the mandible (lower jaw bone).

    • Cranial Bones (Cranium)

      • Single Bones: Frontal, Occipital, Ethmoid, Sphenoid.

      • Paired Bones: Temporal, Parietal.

    • Facial Bones

      • Single Bones: Mandible, Vomer.

      • Paired Bones: Maxilla, Zygomatic, Nasal, Lacrimal, Palatine, Inferior Nasal Concha.

  • Cavities of the Skull

    • Cranial Cavity: Surrounds the brain. Divided into:

      • Superior Cranial Vault (Calvaria)

      • Inferior Cranial Base: Contains the Anterior, Middle, and Posterior Cranial Fossae (indentations) that support the brain.

    • Orbits: Contain the eyeballs.

    • Nasal Cavity: Houses the sensory receptors for smell.

    • Oral Cavity: Surrounds the teeth and tongue.

    • Paranasal Sinuses: Air-filled, membrane-lined spaces.

    • Other small cavities house the sense organs for hearing and balance.

  • Paranasal Sinuses

    • Located within the frontal, sphenoid, ethmoid, and maxillary bones.

    • Lined with mucous membranes and connect to the nasal cavity.

    • Functions: Lighten the skull and enhance voice resonance.

    • Infections of these sinuses can result in sinusitis.

Fetal Skull

  • Characteristics

    • Includes membranous areas where ossification is not complete, known as Fontanels (or soft spots).

    • These fontanels remain until a child is between 18–24 months old.

    • They provide crucial flexibility during the birth process.

  • Specific Fontanels

    • Anterior Fontanel: Located between the developing frontal and parietal bones, at the meeting point of the coronal and sagittal sutures.

    • Posterior Fontanel: Situated between the developing parietal and occipital bones, at the apex of the lambdoid suture.

    • Sphenoid Fontanels: Found in each temple on the right and left sides, where the sphenoid bone meets other cranial bones.

    • Mastoid Fontanels: Located at the junction of the lambdoid and squamous sutures, where the developing parietal, temporal, and occipital bones meet.

Forensic Skull Anatomy

  • Determining a Person's Sex from Skull Bones

    • Male: Forehead is typically sloped, supraorbital ridge is more prominent, mandibular angle is approximately 90 degrees, and the mastoid process is larger.

    • Female: Forehead is generally straight, supraorbital ridge is less prominent, mandibular angle is greater than 90 degrees, and the mastoid process is smaller.

  • Other Identifying Factors

    • Approximating a Person's Age: The size of the skull, the appearance of the sutures, and the condition of the teeth can be used.

    • Clues to an Individual's Ethnic Heritage: Other specific skull features can provide indications.

Hyoid Bone

  • Description: A small, C-shaped bone suspended in the superior neck by muscles and ligaments.

  • Attachments: Attaches to the styloid processes of the temporal bone and to the larynx.

  • Function: Serves as an important attachment point for muscles involved in swallowing and speech.

Overview of the Vertebral Column

  • Vertebral Column (Spine)

    • Comprises an average of 33 Vertebrae.

    • Cervical Vertebrae: 7 vertebrae located in the neck.

    • Thoracic Vertebrae: 12 vertebrae that articulate with the ribs.

    • Lumbar Vertebrae: 5 vertebrae located in the lower back.

    • Sacral (Sacrum): 5 fused vertebrae that articulate with the pelvic bones.

    • Coccygeal (Coccyx): 3–5 fused vertebrae located at the most inferior end of the vertebral column.

  • Spinal Curvatures

    • Primary Curvatures: The Thoracic and Sacral curvatures are present during the fetal period and are convex posteriorly.

    • Secondary Curvatures: The Cervical and Lumbar curvatures develop after the fetal period and are concave posteriorly.

    • Specific Curvature Posture:

      • Cervical Curvature: Concave.

      • Thoracic Curvature: Convex.

      • Lumbar Curvature: Concave.

      • Sacral Curvature: Convex.

      • Lumbosacral junction-Coccyx.

  • Abnormal Spinal Curvatures

    • Scoliosis:

      • Description: Lateral curvatures of the spine that appear C or S shaped.

      • Causes: May be congenital (deformities present at birth), neuromuscular (resulting from trauma to nerves and muscles), or idiopathic (unknown cause).

      • Clinical Impact & Treatment: Mild cases may be asymptomatic and require no treatment. Severe cases may necessitate back braces, physical therapy, or surgery.

    • Lordosis (known as “swayback”)

      • Description: Exaggerated cervical and lumbar curvatures.

      • Occurrence: Some degree is normal in children. Abnormal lordosis (hyperlordosis) is observed in adults with extra abdominal weight, including pregnant women.

      • Clinical Impact & Treatment: May lead to lower back pain. Treatment typically involves weight loss (if needed) and physical therapy.

    • Kyphosis (gives a “humpback” appearance)

      • Description: Exaggerated thoracic curvature.

      • Causes: Can be caused by arthritis, osteoporosis, vertebral fractures, and developmental abnormalities.

      • Clinical Impact & Treatment: Mild cases generally require no treatment. Severe cases can be debilitating, causing heart and lung dysfunction, nerve compression, and significant pain, often requiring surgery.

Structure of the Vertebrae

  • Cervical Vertebrae

    • Smallest vertebrae.

    • Unique feature: Possess Transverse Foramina within their transverse processes, which allow for the passage of vertebral arteries and veins.

    • C1 Atlas: Named for the Greek Titan that supported the world, it supports the skull. It lacks a vertebral body and a spinous process.

    • C2 Axis: Features the Dens (Odontoid Process), which projects superiorly from its body and fits into the vertebral foramen of the atlas, forming the atlantoaxial joint. This joint is held in place by the transverse ligament.

    • C3-C7: Have oval bodies, large triangular vertebral foramina, and short, forked spinous processes.

    • C7 (Vertebra Prominens): Characterized by a unique, longer spinous process that is not forked. This process is palpable and serves as a significant clinical landmark.

  • Thoracic Vertebrae

    • Generally larger than cervical vertebrae, with size increasing progressively down the vertebral column.

    • Features: Superior Costal Facets and Inferior Costal Facets are present on the lateral sides of their bodies, providing articulation points for the ribs.

      • T10-T12: Typically have only one costal facet on each side.

      • T1-T10: Also possess Transverse Costal Facets which articulate with another portion of the ribs.

  • Lumbar Vertebrae

    • Largest and heaviest of the mobile vertebrae, designed to bear the significant weight of the torso.

    • Characteristics: Kidney-shaped bodies; thick spinous processes that point posteriorly; vertebral foramina shaped like a flattened triangle; lumbar pedicles and laminae are thicker and shorter compared to other vertebrae.

  • Comparison of Cervical, Thoracic, and Lumbar Vertebrae Summary

    Characteristic

    Cervical Vertebrae

    Thoracic Vertebrae

    Lumbar Vertebrae


    Body shape and size

    Small and oval; C1 lacks a body; C2 has the dens

    Larger and heart-shaped; contain costal facets

    Largest and kidney-shaped


    Vertebral foramen

    Triangular

    Circular

    Flattened triangular


    Transverse processes

    Contain transverse foramina

    Long; contain articular facets for ribs

    Short with no facets or foramina


    Spinous processes

    Most are fork-shaped; C1 lacks a spinous process; C7 is long and not forked

    Long; point inferiorly

    Thick; point posteriorly

    • Sacrum (S1-S5)

    • Forms the posterior wall of the pelvic cavity.

    • The 5 sacral vertebrae typically fuse by ages 20–25.

  • Coccyx

    • Generally composed of 4 vertebrae.

    • These coccygeal vertebrae typically fuse around age 25.

Vertebral Compression Fractures

  • Definition: A fracture of the vertebral body that results in a reduction of the vertebra's height by more than 20%.

  • Causes: Typically result from severe trauma or diseases that degrade bone quality, such as osteoporosis and bone tumors.

  • Types:

    • Wedge Fractures: Involve primarily the anterior portion of the vertebra.

      • Commonly observed in elderly women with kyphosis and osteoporosis.

      • Can cause radiating pain.

    • Burst Fractures: Involve the entire vertebral body.

      • Result from severe trauma.

      • Bone fragments produced by these fractures may damage the spinal cord and nearby nerves.

  • Treatments: Include bed rest, specialized bracing, physical therapy, and surgery if the injury is severe.

Intervertebral Discs

  • Definition: Pads located between adjacent vertebrae.

  • Quantity: There are 23 intervertebral discs throughout the vertebral column.

  • Structure:

    • Nucleus Pulposus: The inner, jelly-like core that acts as a shock absorber for the spine.

    • Anulus Fibrosus: The tough, outer ring composed of fibrocartilage that encircles the nucleus pulposus. Its role is to hold the nucleus pulposus in place and bind adjacent vertebrae together.

  • Additional Support: The vertebral column is further supported by numerous ligaments and muscles.

Herniated Disc

  • Definition: Also known as a