The Skeletal System I: Joints, Skull, and Vertebral Column

The Skeletal System I

Overview

The skeletal system provides the structural framework for the human body, protects vital organs, and enables movement through its joints and articulations. This section focuses on the classification of joints, the structure of the skull, and the vertebral column.

Arthrology: The Study of Joints

Definition and Importance

• Joint/Arthrosis: A site where two skeletal elements come together, also known as an articulation.

• Joints are essential for movement and stability in the skeletal system.

Classification of Joints

• Solid Joints: Bones are joined by connective tissue or cartilage, allowing little or no movement.

• Synovial Joints: Bones are separated by a synovial cavity filled with synovial fluid, allowing free movement.

Types of Solid Joints

• Fibrous Solid Joints:

  • Syndesmosis: Bones connected by a ligament (e.g., interosseous membrane between radius and ulna).

  • Suture: Immovable joints found between skull bones.

  • Gomphosis: Joint between teeth and their sockets.

• Cartilaginous Solid Joints:

  • Synchondrosis: Bones joined by hyaline cartilage (e.g., epiphyseal plates in growing bones).

  • Symphysis: Bones joined by fibrocartilage (e.g., pubic symphysis, intervertebral discs).

Types of Synovial Joints

• Characterized by a synovial cavity, articular cartilage, and a joint capsule.

• Allow a wide range of movements.

Components of Synovial Joints

• Essential Components (found in all synovial joints):

  • Articular Cartilage: Covers the ends of bones, reducing friction and absorbing shock.

  • Synovial Cavity: Space filled with synovial fluid for lubrication.

  • Joint Capsule: Encloses the joint, consisting of an outer fibrous membrane and an inner synovial membrane.

• Common Associated Structures (not found in all synovial joints):

  • Reinforcing Ligaments: Strengthen and stabilize the joint.

  • Tendinous Synovial Sheaths: Protect tendons as they pass over joints.

  • Articular Discs (Menisci): Pads of fibrocartilage that improve fit and absorb shock.

  • Bursae: Fluid-filled sacs that reduce friction between moving structures.

Classification of Synovial Joints by Movement

• Uniaxial: Movement in one plane (e.g., hinge, pivot joints).

• Biaxial: Movement in two planes (e.g., condyloid, saddle joints).

• Multiaxial: Movement in multiple planes (e.g., ball and socket joints).

• Gliding: Limited movement (e.g., intercarpal joints).

Examples: The elbow is a hinge joint (uniaxial), while the shoulder is a ball and socket joint (multiaxial).

Axial Skeleton: Skull and Vertebral Column

Skull: Neurocranium vs. Viscerocranium

• The skull consists of 22 bones, divided into two main parts:

• Neurocranium: The cranial vault that protects the brain (8 bones).

• Viscerocranium: The facial skeleton (14 bones) that forms the face and supports sensory structures.

Neurocranium Bones

• Frontal bone (1)

• Parietal bones (2)

• Temporal bones (2)

• Occipital bone (1)

• Sphenoid bone (1)

• Ethmoid bone (1)

Viscerocranium Bones

• Nasal bones (2)

• Lacrimal bones (2)

• Zygomatic bones (2)

• Maxillae (2)

• Palatine bones (2)

• Inferior nasal conchae (2)

• Vomer (1)

• Mandible (1)

Key Features of the Skull

• Sutures: Immovable fibrous joints connecting skull bones (e.g., coronal, sagittal, lambdoid sutures).

• Zygomatic Arch: Formed by the zygomatic process of the temporal bone and the zygomatic bone.

• Mandible: The lower jawbone, the only movable bone of the skull.

Temporomandibular Joint (TMJ)

• Classification: Modified synovial hinge joint.

• Structure: Formed by the mandibular condyle and the mandibular fossa of the temporal bone, with an articular disc for smooth movement.

• Function: Allows opening, closing, and side-to-side movement of the jaw.

Vertebral Column

Structure and Regions

• The vertebral column consists of approximately 33 vertebrae, divided into regions:

Region	Number of Vertebrae
Cervical	7 (C1–C7)
Thoracic	12 (T1–T12)
Lumbar	5 (L1–L5)
Sacrum	5 (fused)
Coccyx	3–5 (fused)

Curvatures of the Spine

• Primary Curvatures (Kyphotic): Concave anteriorly; present at birth (thoracic and sacral regions).

• Secondary Curvatures (Lordotic): Concave posteriorly; develop after birth (cervical and lumbar regions).

Example: The cervical curvature develops as infants begin to lift their heads.

General Features of Vertebrae

• Body: Weight-bearing portion.

• Vertebral Arch: Encloses the vertebral foramen.

• Processes: Spinous, transverse, and articular processes for muscle attachment and articulation.

• Intervertebral Foramina: Openings for spinal nerves.

Specialized Vertebrae

• Atlas (C1): Supports the skull, allows nodding motion.

• Axis (C2): Has the dens (odontoid process) for rotation of the head.

Joints of the Vertebral Column

• Atlanto-occipital Joint: Between the atlas and occipital bone; allows nodding.

• Atlantoaxial Joint: Between atlas and axis; allows rotation.

• Intervertebral Joints: Between vertebral bodies, containing intervertebral discs (symphysis type joint).

Intervertebral Discs

• Annulus Fibrosus: Outer fibrocartilage ring.

• Nucleus Pulposus: Gel-like center for shock absorption.

Movements of the Vertebral Column

• Flexion and Extension: Bending forward and backward.

• Lateral Flexion: Bending to the side.

• Rotation: Twisting motion, especially in the cervical and thoracic regions.

Ligaments of the Vertebral Column

• Anterior Longitudinal Ligament: Runs along the anterior surface of vertebral bodies; prevents hyperextension.

• Posterior Longitudinal Ligament: Runs along the posterior surface of vertebral bodies; prevents hyperflexion.

Summary Table: Types of Joints

Joint Type	Subtypes	Movement	Examples
Fibrous (Solid)	Suture, Syndesmosis, Gomphosis	Immovable or slight	Skull sutures, interosseous membrane, tooth socket
Cartilaginous (Solid)	Synchondrosis, Symphysis	Slight	Epiphyseal plate, pubic symphysis, intervertebral disc
Synovial	Hinge, Pivot, Ball and Socket, etc.	Freely movable	Elbow, shoulder, knee

Additional info: This summary is based on anatomical and physiological principles relevant to psychology students studying the biological basis of behavior, especially in areas such as neuropsychology and sensorimotor function.