The Skeletal System and Muscular System Lecture Notes

Overview of the Skeletal and Muscular Systems

• The skeletal and muscular systems are the two systems primarily responsible for the movement of the human body.
• The skeleton provides a structural framework that allows muscles to attach themselves.
• Movement is the direct result of the combined action of muscles pulling against this skeletal framework.

The Skeletal System: Composition and Structure

• Classification: Humans are classified as vertebrates, meaning they possess an internal skeleton known as an endoskeleton.
• Composition: The human skeleton consists of a total of $206$ bones.
• Biological Nature: Despite its hard exterior, bone is a living tissue. It is comprised of:     * Blood vessels.     * Nerves.     * Living bone cells.
• Mineralization: Bone hardness is attributed to its high mineral content, specifically the minerals calcium ($Ca$) and phosphorus ($P$).
• Bone Layers:     * Compact Bone: This forms the hard, dense outer layer of the bone.     * Spongy Bone: This composes the inner layer of the bone. It is within the spongy bone that bone marrow is formed.

Anatomical Components of the Human Skeleton

• The Cranium and Face:     * Skull.     * Cranium.     * Mandible (jawbone).
• The Upper Body and Torso:     * Clavicle (collarbone).     * Scapula (shoulder blade).     * Sternum (breastbone).     * Ribs (including the Twelfth rib).     * Humerus (upper arm bone).     * Radius (forearm bone, thumb side).     * Ulna (forearm bone, pinky side).
• The Spine and Pelvis:     * Vertebrae (individual bones of the spine).     * Pelvis.     * Sacrum.     * Coccyx (tailbone).
• The Hands:     * Carpal bones (wrist).     * Metacarpal bones (palm).     * Phalanges (fingers).
• The Lower Body:     * Femur (thigh bone).     * Patella (kneecap).     * Tibia (shin bone).     * Fibula (calf bone).
• The Feet:     * Tarsal bones (ankle).     * Metatarsal bones (foot).     * Phalanges (toes).

The Vertebral Column

• Structure: The vertebral column is an S-shaped spring-like structure. It extends from the base of the skull through the entire length of the trunk of the body.
• Function: It serves as the central supporting structure of the skeleton and protects the spinal cord housed within it.
• Composition: In adults, the column consists of $26$ separate bones known as vertebrae. While distinct in early development, the vertebrae of the sacrum and coccyx become fused in adulthood.
• Vertebral Distribution:     * Cervical (neck) Vertebrae: $7$ bones (including the atlas and axis vertebrae).     * Thoracic (chest) Vertebrae: $12$ bones.     * Lumbar (back) Vertebrae: $5$ bones.     * Sacral (fused): $1$ bone (consisting of $5$ fused vertebrae).     * Coccyx (fused): $1$ bone (consisting of $3$ to $5$ fused vertebrae).     * Total Count: $26$ bones.
• Intervertebral Discs:     * Individual vertebrae are separated by fibrous cartilage called intervertebral discs.     * Purpose: These discs allow for slight movement within the vertebral column.     * Benefit: They serve as cushions for the backbone, acting as shock absorbers against impact.

Key Functions of the Skeleton

• Body Support: The skeleton provides the fundamental shape and form of the human body.
• Movement:     * Bones acting as levers for muscles.     * Skeletal muscles are attached at both ends to bones via tendons. Tendons are tough, inelastic tissues.     * Muscle contraction produces force that pulls the bone, resulting in movement.     * Joints provide the necessary points of articulation for movement to occur.
• Protection of Internal Organs:     * Skull: Encases and protects the brain.     * Vertebrae: Shield the spinal cord.     * Pelvis: Shields the reproductive organs.     * Rib Cage: Protects the heart and lungs.
• Production of Blood Cells: Red bone marrow, found in specific bones, is the site for the production of red and white blood cells.
• Storage of Mineral Ions:     * Bones store calcium ($Ca$) and phosphorus ($P$) ions.     * Iron ($Fe$) Recycling: The liver breaks down old red blood cells and extracts iron. This iron is sent to the bones to be incorporated into hemoglobin for the creation of new red blood cells.

Classification and Anatomy of Joints

• Definition: A joint is the point where two adjacent bones meet.
• Main Categories:     1. Fixed or Immovable Joints: Also known as sutures. The bones are held together so firmly that no movement is possible. Examples include the bones of the skull and the junction between the sacrum and pelvic bones.     2. Movable (Synovial) Joints: These joints allow for easy movement.

Anatomy of Synovial Joints

• Ligaments: Tough bands of fibrous tissue that hold bones together. They are strong but flexible, forming a capsule around the joint.
• Synovial Membrane: Lines the inside of the capsule.
• Synovial Fluid: An oily fluid produced by the synovial membrane. It lubricates the joint to reduce friction, ensuring smooth movement and preventing bone damage.
• Cartilage: The ends of bones at a joint are covered in smooth cartilage. This further reduces friction, prevents bones from rubbing against each other, and acts as a shock absorber.

Types of Movable Joints

• Hinge Joint: Allows movement in only one direction. Examples: Elbow, knee, jaw, toes, and fingers.
• Gliding Joint: Occurs when bones slide across one another. Examples: Found between adjacent vertebrae and in the bones of the wrist (carpals) and ankle (tarsals).
• Pivot Joint: Specifically found between the first two vertebrae (the atlas and axis). It allows for up-and-down and side-to-side (rotational) movement.
• Ball and Socket Joint: Consists of a rounded bone head fitting into a cup-like cavity of another bone. This allows for movement in all directions (a high degree of mobility). Examples: Shoulder and hip joints.

The Muscular System and Tissue Types

• Total Count: The human body contains approximately $600$ muscles.
• Skeletal Muscle:     * Attached to the skeleton to enable movement.     * Voluntary: We have conscious control over its actions.     * Appearance: Under a microscope, it appears striped or "striated" due to alternating light and dark fiber regions.
• Cardiac Muscle:     * Located exclusively in the heart.     * Involuntary: Not under conscious control.     * Endurance: Works continuously throughout life without experiencing fatigue.
• Smooth Muscle:     * Involuntary: Not under conscious control.     * Location: Found in the linings of various organs such as blood vessels, the bladder, the uterus, and digestive organs.     * Peristalsis: The rhythmic contraction and relaxation of smooth muscle. This process moves substances through the digestive tract, including the esophagus, stomach, and small and large intestines.

Mechanics of Muscle Movement

• Binary Action: Muscles can only perform two actions: they contract (shorten) or relax (return to original length).
• Unidirectional Force: Muscles can pull on bones when they contract, but they cannot push.
• Antagonistic Pairs: Because muscles only pull, they must work in pairs where one muscle works in the opposite direction of the other.     * Flexor Muscle: Bends the joint.     * Extensor Muscle: Straightens the joint.
• Example: Forearm Movement:     * Flexing the arm: The biceps (flexor) contracts, pulling on the radius to bend the elbow, while the triceps (extensor) relaxes.     * Straightening the arm: The triceps contracts to pull the arm straight, while the biceps relaxes.

Musculoskeletal Health

• Exercise: Required to keep muscles flexible and toned. It also serves to increase muscle strength and bone density.
• Diet: Must include adequate supplies of Calcium ($Ca$) and Vitamin D.

Injuries of the Skeletal and Muscular System

• Fracture: A break in a bone. Treatment requires immobilizing the bone using a cast.
• Dislocation: Occurs when a sudden force pulls a bone out of its joint, potentially damaging ligaments and the synovial membrane. This is common in the shoulder's ball and socket joint. A doctor must manually put the bone back into place.
• Sprain: The tearing of ligaments caused by forcing a joint beyond its normal range of movement. Characterized by pain and swelling. Common in the wrist and ankle.
• Arthritis: Inflammation in the joints.     * Osteoarthritis: The most common form, often affecting the elderly. It occurs when cartilage wears away, allowing bones to rub together. This leads to pain, stiffness, and eventual loss of movement. Treatment may involve artificial joint replacement.