The Skeletal System: The Axial Skeleton

The Skeletal System: The Axial Skeleton (Chapter 7)

The Axial Skeleton and Homeostasis

• The bones of the axial skeleton play a crucial role in maintaining homeostasis, primarily by:

  • Protecting organs: The cranium encases the brain; the vertebrae surround the spinal cord; the ribs protect the heart and lungs.

  • Calcium storage and release: Bones serve as a reservoir for calcium, an essential ion, and regulate its release into the bloodstream.

Divisions of the Skeletal System

• The human skeleton comprises $206$ named bones, categorized into two main divisions:

  • Axial Skeleton ($80$ bones):

    • Location: Forms the longitudinal axis of the body.

    • Components: Skull bones, auditory ossicles ($6$), hyoid bone ($1$), ribs ($24$), sternum ($1$), vertebral column ($26$), and sacrum.

  • Appendicular Skeleton ($126$ bones):

    • Location: Bones of the upper and lower extremities, and the girdles (pectoral and pelvic) that attach the limbs to the axial skeleton.

Table of Adult Skeletal System Bones

Types of Bones

Based on Shape

• Almost all bones (except sutural bones) are classified into five main types:

  • Long bones: Greater length than width (e.g., femur, humerus).

  • Short bones: Cube-shaped; nearly equal in length and width (e.g., carpals, tarsals).

  • Flat bones: Thin layers of parallel plates; provide extensive surfaces for muscle attachment and protection (e.g., cranial bones, sternum).

  • Irregular bones: Complex and varied shapes that do not fit into other categories (e.g., vertebrae, certain facial bones).

  • Sesamoid bones: Small, independent bone or nodule of cartilage developed in a tendon where it passes over an angular structure, typically functioning to protect the tendon from stress and wear (e.g., patella, some bones in hands and feet). Named for their resemblance to a sesame seed.

Based on Location

• Sutural bones (Wormian bones):

  • Small, extra bone plates located within the sutures of cranial bones.

  • Sutures: These are the immovable, jointed areas where flat bones of the skull come together.

Bone Surface Markings

• Bones possess unique surface markings which are structural features adapted for specific functions.

• There are two major categories of surface markings:

  • Depressions and Openings:

    • Functions: Allow for the passage of soft tissues (nerves, blood vessels, ligaments, tendons) or form joints.

    • Examples:

      • Fissure: A narrow slit between adjacent parts of bones through which blood vessels or nerves pass (e.g., superior orbital fissure of sphenoid bone).

      • Foramen: An opening (hole) through which blood vessels, nerves, or ligaments pass (e.g., optic canal of sphenoid bone).

      • Fossa: A shallow depression (e.g., coronoid fossa of humerus, mandibular fossa).

      • Sulcus: A furrow (groove) along a bone surface that accommodates a blood vessel, nerve, or tendon (e.g., intertubercular sulcus of humerus).

      • Meatus: A tubelike opening or passageway (e.g., external acoustic meatus of temporal bone).

  • Processes:

    • Functions: Projections or outgrowths that either form joints or serve as attachment points for ligaments and tendons.

    • Processes that form joints:

      • Condyle: A large, rounded protuberance with a smooth articular surface at the end of a bone (e.g., lateral condyle of femur).

      • Facet: A smooth, flat, slightly concave or convex articular surface (e.g., superior articular facet of vertebra).

      • Head: A usually rounded articular projection supported on the neck (constricted portion) of a bone (e.g., head of femur).

    • Processes that form attachment points for connective tissue:

      • Crest: A prominent ridge or elongated projection (e.g., iliac crest of hip bone).

      • Epicondyle: A typically roughened projection above a condyle (e.g., medial epicondyle of femur).

      • Line (linea): A long, narrow ridge or border (less prominent than a crest) (e.g., linea aspera of femur).

      • Spinous process: A sharp, slender projection (e.g., spinous process of vertebra).

      • Trochanter: A very large projection (found only on the femur) (e.g., greater trochanter of femur).

      • Tubercle: A variably-sized rounded projection (e.g., greater tubercle of humerus).

      • Tuberosity: A variably-sized projection that has a rough, bumpy surface (e.g., ischial tuberosity of hip bone).

Skull: An Overview

• The adult skull contains $22$ bones, not including the $6$ auditory ossicles (three in each ear: malleus, incus, stapes).

• These bones feature numerous processes, ridges, lines, depressions, and foramina.

Cranial Bones

• These bones (total of $8$) enclose and protect the brain.

  • Frontal Bone: Forms the forehead, roof of orbits.

    • Features: Supraorbital foramen/notch, frontal squama.

  • Parietal Bones ($2$): Form the superior and lateral aspects of the cranium.

  • Temporal Bones ($2$): Form the inferolateral aspects of the skull, house the middle and inner ear structures.

    • Features: Temporal squama, zygomatic process, mastoid portion/process, styloid process, external acoustic meatus, mandibular fossa, articular tubercle.

  • Occipital Bone: Forms the posterior base of the cranium.

    • Features: External occipital protuberance, occipital condyle, foramen magnum, superior and inferior nuchal lines.

  • Sphenoid Bone: Often called the "keystone bone" as it articulates with all other cranial bones. Bat-shaped, forms part of the cranial floor, orbit, and lateral skull.

    • Features: Greater and lesser wings, sella turcica (containing the hypophyseal fossa), dorsum sellae, tuberculum sellae, pterygoid processes, sphenoidal sinus. Touches almost all of the bones in the skull.

  • Ethmoid Bone: Lies between the sphenoid and nasal bones, forming part of the nasal cavity, orbits, and anterior cranial floor.

    • Features: Crista galli, cribriform plate (with olfactory foramina), perpendicular plate, superior and middle nasal conchae, ethmoidal labyrinth (ethmoidal cells), orbital plate.

Facial Bones

• These bones (total of $14$) form the face, provide support for teeth, and muscle attachment for facial expression.

  • Vomer: Forms the inferior part of the nasal septum.

  • Maxillae ($2$): Large upper jaw bones, articulate with all facial bones except the mandible.

    • Features: Infraorbital foramen, alveolar process, palatine process.

  • Zygomatic Bones ($2$): The cheekbones.

    • Features: Zygomaticofacial foramen, temporal process.

  • Mandible ($1$): The lower jawbone.

    • Significance: It is the largest and strongest facial bone, and the only movable skull bone (excluding auditory ossicles).

    • Features: Mental foramen, mandibular foramen, alveolar process.

Unique Features of the Skull

• Sutures: Immovable joints that hold most skull bones together.

  • Coronal suture: Between frontal and parietal bones.

  • Sagittal suture: Between the two parietal bones.

  • Lambdoid suture: Between parietal and occipital bones.

  • Squamous suture: (X2)Between parietal and temporal bones.

• Paranasal Sinuses: Mucous membrane-lined air cavities within the frontal, maxillary, sphenoid, and ethmoid bones.

  • Functions: Act as resonating chambers to enhance the voice, increase the surface area of the nasal mucosa, and help moisten inhaled air.

• Fontanels: Soft spots in a fetal or infant skull.

  • Description: Areas of unossified mesenchyme that develop into dense connective tissue.

  • Closure: They gradually close through intramembranous ossification, typically by $2$ years of age. Posterior close by 1 year.

Principal Foramina of the Skull

Hyoid Bone

• Unique Feature: Unlike most bones, the hyoid bone does not articulate directly with any other bone.

• Function: It is suspended by ligaments and muscles from the styloid processes of the temporal bones.

• Role: Supports the tongue and provides an attachment site for several muscles of the neck and pharynx, important for swallowing and speech.

The Vertebral Column

• Also known as the spinal column, backbone, or spine.

• Composition: Composed of $26$ vertebrae divided into $5$ distinct regions.

• Primary Function: Protects the delicate spinal cord.

Curves of the Vertebral Column

• Normal Curves: In adults, the vertebral column has four normal curves that increase its strength and maintain balance.

  • Primary Curves: Thoracic and Sacral curves, which are present at birth (fetal curve).

  • Secondary Curves: Cervical and Lumbar curves, which develop after birth as the child learns to hold its head up (cervical) and stand/walk (lumbar).

  • Cervical curve (formed by $7$ cervical vertebrae)

  • Thoracic curve (formed by $12$ thoracic vertebrae)

  • Lumbar curve (formed by $5$ lumbar vertebrae)

  • Sacral curve (formed by $5$ fused sacral vertebrae)

Intervertebral Discs

• Location: Found between the bodies of adjacent vertebrae, from the second cervical vertebra (C2) down to the sacrum.

• Composition: Each disc consists of:

  • An outer ring made of fibrocartilage, called the annulus fibrosus.

  • An inner, soft, gel-like nucleus, called the nucleus pulposus.

  • A layer of hyaline cartilage on the superior and inferior surfaces of each disc, connecting it to the vertebral bodies.

• Functions: These discs are crucial for:

  • Shock absorption: Cushioning the vertebrae during movement and impact.

  • Separation: Keeping the vertebrae apart, allowing room for spinal nerves to exit.

Vertebrae of Each Region

• While all vertebrae share common structures, each region (cervical, thoracic, lumbar) also possesses unique identifying characteristics.

Cervical Vertebrae ($C1-C7$)

• Atlas (C1): The first cervical vertebra.

  • Unique Features: Lacks a body and spinous process. Ring-like structure with large superior articular facets that articulate with the occipital condyles, permitting nodding movement.

• Axis (C2): The second cervical vertebra.

  • Unique Features: Characterized by the presence of the dens (odontoid process) that projects superiorly from its body, forming a pivot point for the atlas, allowing head rotation.

• Typical Cervical Vertebrae (C3-C7):

  • Features: Small bodies, a bifid (forked) spinous process (C2-C6), and transverse foramina in their transverse processes, through which the vertebral arteries pass.

Thoracic Vertebrae ($T1-T12$)

• Primary Function: Support the ribs.

• Unique Features: Possess special structures called costal facets (or demifacets) on their bodies and transverse processes (for T1-T10) for articulation with the heads and tubercles of the ribs, respectively.

• Spinous Process: Generally long and projects inferiorly.

Lumbar Vertebrae ($L1-L5$)

• Unique Features: They are the largest and strongest vertebrae in the spinal column, designed to bear the most weight. Lacking unique structures like transverse foramina or costal facets found in cervical and thoracic vertebrae.

Comparison of Vertebral Characteristics

Sacrum and Coccyx

• Sacrum:

  • Description: A large, triangular bone formed by the fusion of $5$ sacral vertebrae (S1-S5).

  • Location: Forms the posterior wall of the pelvic girdle.

  • Features: Superior articular processes, sacral promontory, anterior and posterior sacral foramina, median sacral crest, sacral canal, sacral hiatus, auricular surface.

• Coccyx:

  • Description: A small, triangular bone, commonly known as the tailbone, formed by the fusion of $4$ coccygeal vertebrae (Co1-Co4).

  • Location: Articulates superiorly with the sacrum.

  • Features: Coccygeal cornua.

The Thorax

• Definition: The entire chest region.

• Components: The bony framework, known as the thoracic cage, is composed of the sternum, ribs, and costal cartilages.

Sternum (Breastbone)

• Composition: A flat, elongated bone located in the center of the chest, composed of three segments:

  • The upper manubrium.

  • The middle body.

  • The lower xiphoid process.

• Articulations: The sternum articulates with the clavicles (collarbones) and the costal cartilages of the ribs.

Ribs

• Number: There are $12$ pairs of ribs.

• Function: Provide structural support to the thoracic cavity and protect the thoracic organs.

• Classification based on sternal attachment:

  • True (Vertebrosternal) Ribs (Pairs 1-7): Their costal cartilages directly connect to the sternum.

  • False (Vertebrochondral) Ribs (Pairs 8-10): Their costal cartilages indirectly connect to the sternum, usually by joining the cartilage of the rib above.

  • Floating (Vertebral) Ribs (Pairs 11-12): These ribs do not connect to the sternum at all; their anterior ends terminate in the abdominal musculature.

• General Rib Features: Head (articulates with vertebral body), neck, tubercle (articulates with transverse process of vertebra), body (shaft), costal angle, and costal groove.

Clinical Connections

Temporomandibular Joint (TMJ) Dysfunction

• Symptoms: Dull pain around the ear, tender jaw muscles, and a clicking noise when opening or closing the mouth.

• Causes: Can be attributed to improperly aligned teeth, grinding of teeth (bruxism), trauma to the head, or arthritis.

• Treatment: Often includes conservative measures such as moist heat or ice application, consumption of soft foods, and pain relievers.

Deviated Nasal Septum

• Description: A condition where the nasal septum, the wall dividing the nasal cavity into halves, is significantly displaced from the midline.

• Causes: Frequently caused by trauma to the nose or a developmental abnormality.

• Symptoms: May lead to complications such as infection, inflammation, chronic nasal congestion, headaches, and nosebleeds.

• Treatment: In severe cases, surgery may be required to correct the deviation and improve breathing.

Disorders Affecting the Skeletal System

Herniated Disc

• Description: Occurs when the nucleus pulposus of an intervertebral disc protrudes through the annulus fibrosus, often compressing nearby spinal nerves or the spinal cord.

• Causes: Can result from trauma or may simply be associated with the aging process (degenerative changes).

Abnormal Curves of the Vertebral Column

• The normal curves of the spinal column can become exaggerated, leading to various pathologies:

  • Scoliosis: An increased lateral (sideways) curvature of the vertebral column, often appearing as an S- or C-shape.

  • Kyphosis: An increased thoracic curve, resulting in a

  • Lordosis: An increased lumbar curve