Chapter 5 – The Skeletal System

Subdivisions of the Skeleton

The human skeleton is divided into two main subdivisions: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage. It forms the central axis of the body.

• Appendicular Skeleton: Includes the limbs and girdles (pectoral and pelvic), responsible for movement and manipulation of objects.

Functions of the Skeletal System

The skeletal system performs several vital functions:

• 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:

• 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:

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

• Epiphyses: Ends of the bone, mostly spongy bone covered by articular cartilage.

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

• Medullary Cavity: Central cavity containing bone marrow.

• Periosteum: Outer fibrous covering of the bone.

Microscopic Structure of Compact Bone

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

• Osteon: Cylindrical structure containing concentric lamellae around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient exchange.

Bone Formation and Remodeling

Bone formation occurs via two processes:

• Endochondral Ossification: Bone develops from hyaline cartilage models (most bones).

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

• Bone Remodeling: Continuous process of bone resorption and formation, regulated by osteoclasts and osteoblasts.

Structural Categories of Joints

Joints are classified by their structure and movement:

• Fibrous Joints: Bones joined by dense connective tissue; immovable (e.g., sutures in the skull).

• Cartilaginous Joints: Bones connected by cartilage; slightly movable (e.g., intervertebral discs).

• Synovial Joints: Bones separated by a joint cavity filled with synovial fluid; freely movable.

Types of Synovial Joints and Movements

There are six types of synovial joints, each allowing specific movements:

• Plane (Gliding) Joint: Allows sliding movements (e.g., intercarpal joints).

• Hinge Joint: Permits flexion and extension (e.g., elbow, knee).

• Pivot Joint: Allows rotation (e.g., atlas and axis in the neck).

• Condyloid (Ellipsoid) Joint: Allows movement in two planes (e.g., wrist).

• Saddle Joint: Allows movement in two planes (e.g., thumb).

• Ball-and-Socket Joint: Allows movement in all axes (e.g., shoulder, hip).

Osteoporosis: Definition and Risk Factors

Osteoporosis is a condition characterized by decreased bone mass and increased fragility.

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

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

Chapter 6 – The Muscular System

Types of Muscle Tissue

Muscle tissue is classified into three types:

• Skeletal Muscle: Voluntary, striated, attached to bones for movement.

• Cardiac Muscle: Involuntary, striated, found only in the heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs.

Muscle Coverings

Connective tissue sheaths surround muscle fibers:

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds bundles of fibers (fascicles).

• Epimysium: Surrounds the entire muscle.

Functions of the Muscular System

• Movement: Locomotion and manipulation.

• Posture Maintenance: Stabilizes joints and maintains posture.

• Heat Production: Muscle contractions generate heat.

• Protection: Shields internal organs.

Microscopic Structure of Skeletal Muscle

Skeletal muscle fibers are multinucleated and striated, containing myofibrils composed of sarcomeres.

• Sarcomere: Functional unit of contraction, composed of actin (thin) and myosin (thick) filaments.

• Sarcoplasmic Reticulum: Stores calcium ions.

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

Initiation of Action Potential in Muscle Cell

An action potential is initiated when a motor neuron releases acetylcholine at the neuromuscular junction, depolarizing the muscle cell membrane.

• Depolarization: Sodium ions enter the cell, triggering an action potential.

• Propagation: The action potential travels along the sarcolemma and T-tubules.

Events of Muscle Cell Contraction (Sliding Filament Theory)

Muscle contraction occurs via the sliding filament mechanism:

• Calcium ions bind to troponin, exposing binding sites on actin.

• Myosin heads attach to actin, forming cross-bridges.

• Myosin heads pivot, pulling actin filaments inward (power stroke).

• ATP binds to myosin, causing detachment and re-cocking of the head.

Isometric vs. Isotonic Contractions

• Isometric Contraction: Muscle tension increases, but length remains the same (e.g., holding a weight steady).

• Isotonic Contraction: Muscle changes length to produce movement (e.g., lifting a weight).

Five Golden Rules of Skeletal Muscle Activity

• All muscles cross at least one joint.

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

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

• Muscles can only pull; they never push.

• During contraction, the insertion moves toward the origin.

Common Types of Body Movements

• Flexion: Decreases the angle between bones.

• Extension: Increases the angle between bones.

• Abduction: Moves limb away from midline.

• Adduction: Moves limb toward midline.

• Rotation: Movement around a longitudinal axis.

• Circumduction: Circular movement of a limb.

Criteria for Naming Skeletal Muscles

Muscles are named based on seven criteria:

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

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

• Location (e.g., temporalis)

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

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

• Shape (e.g., deltoid)

• Action (e.g., flexor, extensor)

Arrangements of Muscle Fascicles

Muscle fascicles are arranged in different patterns, affecting muscle function:

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

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

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

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

Chapter 7 – The Nervous System

General Functions of the Nervous System

The nervous system is responsible for:

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

• Integration: Processes and interprets sensory input.

• Motor Output: Responds by activating effector organs (muscles/glands).

Structural and Functional Classifications of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord; integration and command center.

• Peripheral Nervous System (PNS): Nerves outside the CNS; communication lines.

• Functional Divisions: Sensory (afferent) division and motor (efferent) division.

• Motor Division Subdivisions: Somatic (voluntary) and autonomic (involuntary) systems.

Structures and Functions of Neurons and Neuroglia

Neurons are the functional cells of the nervous system, transmitting electrical signals.

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

• Dendrites: Receive signals from other neurons.

• Axon: Transmits impulses away from the cell body.

Neuroglia (glial cells) support and protect neurons. Six types include:

• Astrocytes: Support and anchor neurons, regulate environment.

• Microglia: Phagocytic cells, remove debris.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• 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

• Cell Body: Metabolic center.

• Dendrites: Input region.

• Axon Hillock: Initiates action potential.

• Axon Terminals: Output region, releases neurotransmitters.

Myelin: Function and Cells Responsible for Myelination

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

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann Cells: Myelinate axons in the PNS.