Human Skeletal System and Muscles

Learning Objectives for the Skeletal and Muscular Systems

• Functions of the Skeletal System: Describe the primary and secondary roles of the skeleton within the human body.

• Identification of Major Bones: Identify the main bones of the body and their locations on a skeletal diagram.

• Joint Classification: Describe the different types of joints found in the body and their range of motion.

• Muscle Mechanics: Describe how antagonistic muscle pairs work together to facilitate movement.

• Assessment Readiness: These objectives represent the critical knowledge required for successful completion of the course assessment.

Overview of the Human Skeletal System

• Bone Count: An adult human skeleton typically consists of $206$ bones.

• Living Tissue: Bone is a dynamic, living tissue that is continuously changing; new bone is made while old bone is broken down.

• Physiological Supply: Bones are richly supplied by blood vessels and nerves.

• Chemical Composition: Bone tissue is strengthened by calcium phosphate ($Ca_{3}(PO_{4})_{2}$) and calcium carbonate ($CaCO_{3}$).

Functions of the Skeletal System

• Framework: Provides a structural support system for the entire body.

• Movement: Facilitates movement by providing a system of levers that muscles pull against.

• Protection: Provides a physical barrier that protects internal organs (e.g., the skull protecting the brain, the ribs protecting the heart and lungs).

• Blood Cell Production: Produces blood cells within the bone marrow.

• Immune System Support: Contributes to the immune system through the production of white blood cells.

• Mineral Storage: Acts as a reservoir for essential minerals, specifically Calcium ($Ca$), including the storage and release of $Ca^{2+}$ and $PO$.

• Fat Storage: Stores and releases fat, which is kept within the bone structure.

Bone Structure and Anatomy

• Compact Bone: Also known as hard bone, this forms the dense outer shell of the bone.

• Spongy Bone: Located internally, it is lighter than compact bone and contains spaces filled with fat or bone marrow.

• Periosteum: A fibrous membrane that covers the surface of the bone.

• Peak Bone Mass: Humans reach their peak bone mass at approximately $30$ years of age. Following this peak, the body begins to lose slightly more bone mass than it generates.

Classification of Bone Types

• Long Bone: Characterized by a length greater than width (e.g., Femur).

• Short Bones: Often cube-shaped (e.g., Tarsals such as the Lateral cuneiform, Intermediate cuneiform, and Medial cuneiform).

• Flat Bone: Thin and often curved (e.g., Sternum).

• Irregular Bone: Complex shapes that do not fit other categories (e.g., Vertebra).

• Sesamoid Bone: Small, round bones embedded in tendons (e.g., Patella).

Cartilage

• Definition: A resilient and smooth elastic tissue that serves multiple structural and protective roles.

• Protective Role: It covers and protects the ends of long bones at the joints to reduce friction and absorb shock.

• Structural Role: It is a key component of the rib cage, the ear, the nose, the bronchial tubes, and the intervertebral discs.

• Physiological Limitation: Cartilage does not contain blood vessels or nerves.

The Axial Skeleton

The Axial Skeleton constitutes the central axis of the body.

• Skull:

  • Cranial Bones: Includes the Frontal and Temporal bones.

  • Facial Bones: Includes the Mandible, Maxilla, and Nasal bones.

  • Additional Features: Includes the Sutures (immovable joints) and the Styloid process.

• Vertebral Column: Divided into specific regions:

  • Cervical: $7$ vertebrae.

  • Thoracic: $12$ vertebrae.

  • Lumbar: $5$ vertebrae.

  • Sacrum: $5$ fused vertebrae.

  • Coccyx: $4$ fused vertebrae.

  • Associated Structures: Includes the Intervertebral foramen and Intervertebral discs.

• Thoracic Cage:

  • Sternum: The breastbone.

  • Ribs: $12$ pairs in total. $10$ pairs are attached to the sternum, while $2$ pairs are "floating ribs."

The Appendicular Skeleton

The Appendicular Skeleton "appends" to the axial skeleton and includes the limbs and their supporting girdles.

• Pectoral (Shoulder) Girdle: Consists of the Clavicle and Scapula.

• Upper Limb (Per Arm):

  • Humerus: Upper arm bone.

  • Radius and Ulna: Forearm bones.

  • Carpals: $8$ bones (wrist).

  • Metacarpals: $5$ bones (palm).

  • Phalanges: $14$ bones (fingers).

• Pelvic Girdle: Consists of the Coxal bone, which is composed of the Ilium, Ischium, and Pubic regions.

• Lower Limb (Per Leg):

  • Femur: Thigh bone.

  • Tibia and Fibula: Lower leg bones.

  • Tarsals: $7$ bones (ankle).

  • Metatarsals: $5$ bones (foot).

  • Phalanges: Bone segments for the toes.

Joints: Classification and Structure

Joints are categorized based on their degree of movement:

• Immovable or Fixed (Fibrous) Joints: These joints permit no movement; they are held together by fibrous connective tissue. Examples include the Sutures in the skull.

• Slightly Movable (Cartilaginous) Joints: These allow for limited movement. An example is the joint between the bodies of the vertebrae using the Intervertebral disk.

• Freely Movable (Synovial) Joints: These allow for a wide range of movement.

  • Anatomy of Synovial Joints:

    • Articular Cartilage: Covers and protects bone ends, lubricates movement, and absorbs shock.

    • Joint Capsule: A fibrous layer that surrounds the joint.

    • Synovial Layer: Produces synovial fluid to lubricate the joint and provide further shock absorption.

Types of Synovial Joints

• Hinge: Allows back-and-forth movement. Examples: Elbow, knee, fingers, and jaw.

• Ball & Socket: Provides the most extensive range of movement. Examples: Hip and shoulder.

• Gliding: Allows bones to slide over one another. Examples: Wrist and ankle.

• Saddle: Provides movement on two planes.

• Pivot: Allows for rotational movement.

Connective Tissues: Tendons and Ligaments

• Tendons: Cords of dense fibrous tissue that attach muscle to bone. They are described as "tying" muscles to bones.

• Ligaments: Bands of tough fibrous tissue that bind bone to bone. They are described as "linking" bones together.

The Muscular System

• Skeletal Muscles: Also known as striped or striated muscle.

• Contraction Mechanism: Muscles are made of fibres that contract when stimulated by a nerve.

• Muscle Pairs: Found in pairs to move bones (e.g., Biceps and Triceps).

• Performance: These muscles contract powerfully but tire easily.

• Voluntary Control: Muscles that move bones are voluntary because they are under conscious control.

Antagonistic Muscle Pairs

• Definition: Muscle pairs that work in opposite directions; when one muscle in the pair contracts, the other relaxes.

• Flexor Muscle: The muscle that contracts to bend a joint (e.g., Biceps).

• Extensor Muscle: The muscle that contracts to straighten a joint (e.g., Triceps).

• Leg Examples: The Quadriceps and Hamstrings are another common antagonistic pair.

• Common Muscle Group Nomenclature (Informal): Delts (Deltoids), Lats (Latissimus dorsi), Pecs (Pectorals), Quads (Quadriceps), Abs (Abdominals), Hammies (Hamstrings).

Skeletal System Disorders and Diseases

Multiple conditions can affect the integrity and function of the skeletal system:

• Bone Density Issues: Osteoporosis (characterized by reduced bone mass, often seen in aging), Osteopenia, Osteomalacia.

• Structural Deformities: Scoliosis (spinal curvature), Kyphosis, Spina Bifida.

• Inflammatory Conditions: Rheumatoid Arthritis, Bursitis, Gout.

• Bone Cancers: Osteosarcoma, Chondrosarcoma, Ewing's sarcoma.

• Developmental/Genetic Issues: Rickets (often associated with vitamin deficiencies), Osteogenesis imperfecta (brittle bone disease), Fibrous dysplasia.

• Other Diseases: Osteomyelitis (infection), Paget's Disease, Hypophosphatasia.

Classification of Bone Fractures

Fractures are categorized by the nature and direction of the break:

• Transverse: A break straight across the bone.

• Linear: A break running parallel to the bone's long axis.

• Oblique non-displaced: A slanted break where the bone remains in alignment.

• Oblique displaced: A slanted break where the bone ends are out of alignment.

• Spiral: A break caused by twisting forces.

• Greenstick: An incomplete break where the bone bends and cracks (common in children).

• Comminuted: A break where the bone shatters into several fragments.