Muscles and Skin Overview

Muscles and Skin

• Skin is the largest organ by weight and surface area.
• Skin thickness varies; thinnest in eyelids, thickest in heels.
• Skin functions include:
  • Protection: Barrier against pathogens, chemicals, radiation, and physical injury.
  • Regulation: Regulates body temperature via sweat production and blood flow.
  • Sensation: Contains nerve endings sensitive to touch, pain, temperature, and pressure.
  • Metabolism: Involved in vitamin D synthesis when exposed to sunlight.
  • Immune Defense: Plays role in the body's immune response.

Structure of the Skin

• Two major layers:
  • Epidermis: Outer, thinner layer; composed of epithelial tissue, avascular.
  • Dermis: Inner, thicker vascular layer.

Epidermis

• Outermost layer; composed of keratinized stratified squamous epithelium.
• Provides protective barrier.
• Contains cells like:
  • Keratinocytes: Most abundant; filled with keratin, acting as a barrier.
  • Melanocytes: Produce melanin (skin pigment).
  • Langerhans cells: Involved in immune defense against pathogens.
  • Tactile epithelial cells (Merkel cells): Fewer in number; responsible for touch sensation.

Dermis

• Deeper skin layer.
• Composed of dense irregular connective tissue with collagen and elastic fibers.
• Provides tensile strength and elasticity.
• Contains fibroblasts, macrophages, nerve cells, and adipocytes.
• Houses blood vessels, nerves, glands, and hair follicles.

Subcutaneous Layer (Hypodermis)

• Located beneath the dermis; not part of the skin.
• Composed of loose connective tissue and adipose (fat) tissue.
• Contains collagen and elastin fibers; blood vessels and nerves.
• Attaches skin to underlying tissue/organs.
• Plays roles in heat insulation, energy storage, anchoring, and cushioning the underlying structures.

Appendages of the Skin

• Nails: Keratinized plates on dorsal surfaces of fingers and toes.
• Hair follicles: Invaginations of epidermis into dermis; where hair grows.
  • Hair shaft: Visible part.
  • Hair follicle: Structure beneath the skin producing hair.
    • Sebaceous glands: Associated with hair follicle; produce sebum (oils hair and scalp).
    • Arrector pili muscles: Small muscles attached to hair follicles; cause goosebumps.
    • Hair bulb: Base of hair follicle; where hair growth begins.
• Sweat glands: Tubular glands distributed over most of the body; produce sweat for temperature regulation & pathogen defense.

The Nails

• Keratinized plates on the dorsal surfaces of fingers and toes.
• Proximal edge is the root of the nail.
• Nail folds surround the nail.
• Nail bed is the surface of skin the nail covers.

Hair and Hair Follicles

• Distributed over the body (except lips, palms, and soles).
• Follicles are invaginations of epidermis into dermis; hair grows out from them.

Sebaceous Glands

• Located beneath the epidermis, near hair follicles.
• Secrete sebum (oil) onto hair shafts.
• Helps maintain hair flexibility and oils the skin around the follicle opening.

Sweat Glands

• Long, tubular glands distributed over most of the body (except red margins).
• Secrete sweat to regulate body temperature and offer some pathogen defense.

Types of Muscles

• Skeletal muscle:
  • Striated, voluntary, multi-nucleated fibers.
  • Attached to skeleton.
• Smooth muscle:
  • Non-striated, involuntary, spindle-shaped fibers.
  • Usually in the walls of internal organs.
• Cardiac muscle:
  • Striated, involuntary, branched fibers.
  • Only in the heart walls.

Skeletal Muscle Terminology

• Muscle: Special connective tissue with contractility.
• Agonist: The prime mover producing a specific action.
• Antagonist: Muscle opposing the agonist's action.
• Synergist: Muscles assisting the agonist, stabilizing the joint, and improving efficiency.
• Origin: Muscle attachment moving the least (usually proximal).
• Insertion: Muscle attachment moving the most (usually distal).
• Belly: Fleshy part of the muscle.
• Aponeurosis: Sheet-like fibrous tissue connecting muscles or muscles to other structures, not bones.
• Tendons: Connect muscles to bones.

Thorax

• Region between neck and abdomen.
• Separated from the neck and abdomen by the diaphragm.
• Protects lungs and heart.
• Provides attachment points for thorax, upper extremity, abdomen, and back muscles.

Thoracic Cavity

• Divided into:
  • Mediastinum: Contains heart, major blood vessels, thymus, and lymph nodes.
  • Two pleural cavities: Each contain a lung.

Thoracic Wall

• Formed by:
  • Thoracic portion of vertebral column.
  • Sternum and costal cartilages.
  • Ribs and intercostal spaces.
  • Suprapleural membrane.
  • Diaphragm.

Chest Wall

• Skin, superficial fascia (subcutaneous tissue), and respiratory accessory muscles (e.g., serratus anterior).
  • Superficial fascia: Loose connective tissue layer containing blood vessels, fat, and nerves; supports and protects underlying tissue.

Intercostal Spaces

• Spaces between ribs containing muscles for respiration:
  • External intercostals.
  • Internal intercostals.
  • Innermost intercostals.
  • Intercostal nerves and blood vessels (arranged in VAN order).
• Endothoracic fascia: A loose connective tissue layer separating the innermost intercostal muscles from the pleura.

Pleura

• Sac composed of two mesothelial layers (visceral and parietal).
• Visceral pleura: Covers the lungs following their contours.
• Parietal pleura: Lines the inner chest wall, diaphragm, and superior mediastinum.
• Pleural cavity: Space between visceral and parietal pleura containing fluid reducing friction during respiration.
• Functions: Lubrication, pressure regulation, protection.

Cardiovascular System

• Composed of heart, blood vessels, and blood.
• Heart acts as a pump pushing blood to organs.
• Blood carries oxygen and nutrients to cells.
• Blood carries carbon dioxide and waste products back to the heart.

Heart

• Muscular organ (250-400 grams).
• Pyramidal shape, within the pericardium, behind the sternum.
• Composed of four chambers forming two separate pumps.
  • Chambers: Right atrium, Right ventricle, Left atrium, Left ventricle
  • Valves: Tricuspid, Pulmonary, Mitral (Bicuspid), Aortic

Heart Valves

• Control blood flow within the heart:
  • Tricuspid: Right atrium to right ventricle.
  • Pulmonary: Right ventricle to pulmonary artery.
  • Mitral (Bicuspid): Left atrium to left ventricle.
  • Aortic: Left ventricle to aorta.

Arterial Supply of the Heart

• Supplied by the right and left coronary arteries.
• Arise from the ascending aorta.
• Coronary arteries and branches distribute over the surface of the heart.

Conducting System of the Heart

• Generates rhythmic contractions (70-90 beats/minute).
• Impulse originates spontaneously in the conducting system (Sinoatrial node).
• Atria contract first; followed by ventricles.
• Delay in impulse transmission allows atria to empty before ventricles contract.

Major Blood Vessels

• Aorta: Major artery branching off to many branches.
• Pulmonary arteries: Carry deoxygenated blood to lungs.
• Pulmonary veins: Return oxygenated blood to the heart.
• Superior vena cava: Major vein returning blood from upper body.
• Inferior vena cava: Major vein returning blood from lower body.

Blood Circulation

• Oxygen-rich blood from lungs to body.
• Oxygen-poor blood from body to lungs.

Lungs

• Two organs in thoracic cavity for gas exchange.
• Lobe-segment structure.
• Fissures divide different lobes.

Bronchial Tree

• Trachea divides at carina into right and left main bronchi.
• Bronchi branch into secondary bronchi (lobar) for each lobe.
• Further branching into segmental bronchi and bronchioles to alveolar ducts and alveoli for gas exchange

Lung Lobes and Fissures

• Lungs are divided into lobes.
• Fissures separate lobes.

Blood Supply

• Pulmonary arteries: Carry deoxygenated blood to lungs.
• Pulmonary veins: Return oxygenated blood to the heart.
• Bronchial arteries: Supply oxygenated blood to lung tissue.

Gas Exchange

• Occurs in alveoli.
• Gas diffuses between air in alveoli and blood in capillaries.
• Oxygen enters blood, carbon dioxide leaves.

This quiz covers the fundamental concepts of muscles and skin, including the structure and functions of skin as the largest organ. You'll explore the layers of skin, from the epidermis to the dermis, and learn about the roles they play in protection, regulation, and sensation.