Chapter 5 – The Skeletal System

Subdivisions of the Skeleton

The human skeleton is divided into two main subdivisions: the axial and appendicular skeletons. Each subdivision has distinct anatomical components and functions.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage. It supports and protects the brain, spinal cord, and organs in the thorax.

• Appendicular Skeleton: Comprises the limbs and girdles (pectoral and pelvic). It facilitates movement and interaction with the environment.

Functions of the Skeletal System

The skeletal system performs several vital functions essential for human health and activity.

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., brain, heart, lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores calcium and phosphorus.

• Blood Cell Production: Houses bone marrow for hematopoiesis.

Classifications of Bones

Bones are classified based on their shapes and functions.

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).

Anatomical Areas of the Long Bone

Long bones have distinct anatomical regions important for growth and function.

• Diaphysis: Shaft of the bone, composed of compact bone.

• Epiphyses: Ends of the bone, mostly spongy bone.

• Metaphysis: Region between diaphysis and epiphysis, includes growth plate.

• Medullary Cavity: Central cavity containing bone marrow.

• Periosteum: Outer fibrous covering.

Microscopic Structure of Compact Bone

Compact bone is organized into structural units called osteons (Haversian systems).

• Osteon: Cylindrical structure containing concentric lamellae.

• Central Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient exchange.

Bone Formation and Remodeling

Bones form and remodel through two main processes: endochondral and intramembranous ossification.

• Endochondral Ossification: Bone develops from hyaline cartilage (e.g., long bones).

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of skull).

• Bone Remodeling: Continuous process involving osteoblasts (bone formation) and osteoclasts (bone resorption).

Structural Categories of Joints

Joints are classified based on their structure and the amount of movement they allow.

Types of Synovial Joints and Movements

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow.

Osteoporosis: Definition and Risk Factors

Osteoporosis is a condition characterized by decreased bone density and increased fracture risk.

• Modifiable Risk Factors: Diet low in calcium/vitamin D, sedentary lifestyle, smoking, excessive alcohol.

• Non-Modifiable Risk Factors: Age, gender (female), genetics, menopause.

Chapter 6 – The Muscular System

Types of Muscle Tissue

Muscle tissue is classified into three types, each with unique structure and function.

Muscle Coverings

Connective tissue sheaths surround muscle fibers and groups.

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds bundles (fascicles) of muscle fibers.

• Epimysium: Surrounds the entire muscle.

Functions of the Muscular System

• Movement: Locomotion and manipulation.

• Posture: Maintains body position.

• Stabilization: Stabilizes joints.

• Heat Production: Generates heat during contraction.

Microscopic Structure of Skeletal Muscle

Skeletal muscle fibers are multinucleated and contain myofibrils composed of sarcomeres.

• Sarcomere: Functional unit of contraction, composed of actin and myosin filaments.

• Myofibril: Rod-like unit within muscle fiber.

• Sarcoplasmic Reticulum: Stores calcium ions.

• T-tubules: Invaginations of the sarcolemma for signal transmission.

Initiation of Action Potential in Muscle Cell

An action potential in a muscle cell is initiated at the neuromuscular junction.

• Acetylcholine (ACh): Neurotransmitter released from motor neuron.

• Depolarization: Sodium ions enter muscle fiber, generating action potential.

Events of Muscle Cell Contraction (Sliding Filament Theory)

Muscle contraction occurs as actin and myosin filaments slide past each other.

• Cross-Bridge Formation: Myosin heads bind to actin.

• Power Stroke: Myosin pulls actin filament inward.

• ATP Binding: Myosin releases actin and resets.

Equation:

Isometric vs. Isotonic Contractions

• Isometric: Muscle tension increases, length remains constant (e.g., holding a weight steady).

• Isotonic: Muscle changes length, tension remains constant (e.g., lifting a weight).

Five Golden Rules of Skeletal Muscle Activity

1. All muscles cross at least one joint.

2. Typically, the bulk of the muscle lies proximal to the joint crossed.

3. All muscles have at least two attachments: origin and insertion.

4. Muscles can only pull; they never push.

5. During contraction, the muscle insertion moves toward the origin.

Common Types of Body Movements

• Flexion/Extension: Decreasing/increasing angle between bones.

• Abduction/Adduction: Moving limb away/toward midline.

• Rotation: Movement around longitudinal axis.

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

Criteria for Naming Skeletal Muscles

• Direction of muscle fibers (e.g., rectus, transverse)

• Relative size (e.g., maximus, minimus)

• Location (e.g., temporalis)

• Number of origins (e.g., biceps, triceps)

• Location of origin/insertion (e.g., sternocleidomastoid)

• Shape (e.g., deltoid)

• Action (e.g., flexor, extensor)

Arrangements of Muscle Fascicles

• Parallel: Fascicles run parallel to long axis (e.g., sartorius).

• Pennate: Fascicles attach obliquely to tendon (e.g., rectus femoris).

• Convergent: Fascicles converge toward a single tendon (e.g., pectoralis major).

• Circular: Fascicles arranged in rings (e.g., orbicularis oris).

Chapter 7 – The Nervous System

General Functions of the Nervous System

The nervous system is responsible for rapid communication and control throughout the body.

• Sensory Input: Detects changes inside and outside the body.

• Integration: Processes and interprets sensory input.

• Motor Output: Initiates responses by activating muscles or glands.

Structural and Functional Classifications of the Nervous System

Structures and Functions of Neurons and Neuroglia

Neurons are the functional units of the nervous system, specialized for transmitting electrical signals.

• Cell Body (Soma): Contains nucleus and organelles.

• Dendrites: Receive signals from other neurons.

• Axon: Transmits impulses away from cell body.

Neuroglia (glial cells) support, protect, and nourish neurons. Six types exist:

• Astrocytes: Support and regulate environment (CNS).

• Microglia: Immune defense (CNS).

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid (CNS).

• Oligodendrocytes: Myelinate axons in CNS.

• Satellite Cells: Support neurons in PNS.

• Schwann Cells: Myelinate axons in PNS.

General Structure of a Neuron and Important Regions

• Axon Hillock: Initiates action potential.

• Nodes of Ranvier: Gaps in myelin sheath for rapid signal transmission.

• Synaptic Terminals: Release neurotransmitters to communicate with other cells.

Myelin: Function and Cells Responsible

Myelin is a fatty substance that insulates axons, increasing the speed of nerve impulse transmission.

• Function: Enables saltatory conduction, allowing impulses to jump between nodes of Ranvier.

• Cells Responsible: Oligodendrocytes (CNS) and Schwann cells (PNS).