Fundamental Concepts of Human Anatomy and Physiology

Posted on Oct 21, 2025 in Biology

Unit 1: Foundations of Anatomy and Physiology

Core Principles in A&P

Complementarity of Structure and Function

The principle that structure is always best suited for its function, often summarized as form follows function, applies to each level of organization.

• Example: Tissues in the lungs are thin, allowing gases to cross rapidly.
• Example: The size of a red blood cell is relative to the size of veins and arteries.
• Example: The arrangement of fibers in dense regular and irregular connective tissues reflects their functional roles.

Characteristics of Living Organisms

• Cellular composition: Organisms are made of cells.
• Metabolism: The sum of all chemical reactions in the body.
• Growth: Increase in size or number of cells.
• Excretion: Removal of waste products.
• Responsiveness: Ability to sense and react to stimuli.
• Movement: Motion of the organism or its parts.
• Reproduction: Production of offspring.

Levels of Organization

Chemical Level (atom, molecule, macromolecule) → Cellular Level (organelle, cell) → Tissue Level (two or more cell types, cells, and ECM) → Organ Level (two or more tissues) → Organ System Level (two or more organs) → Organism

Anatomical Position

A common frame of reference for the body:

1. Body is erect.
2. Feet are together.
3. Palms face forward.
4. Thumbs point away from the body.

Homeostasis

Homeostasis is the maintenance of a stable internal environment. Disturbances in homeostasis can lead to disease or death if uncorrected.

Negative Feedback Loop

This mechanism opposes the initial change in a regulated variable to reduce output and return to a set point.

• Stimulus: A regulated variable moves outside its normal range.
• Receptor: The change is detected by a receptor.
• Control Center/Effector: The control center processes the information and signals an effector.
• Response: The effector’s action opposes the initial stimulus.
• End Point Reached: The variable returns to its normal range, and the feedback loop is turned off.

Positive Feedback Loop

In this mechanism, the effector’s activity increases and reinforces the initial stimulus, moving the variable further from the set point.

• Example: Blood Clotting
  • Stimulus: An injury occurs to a blood vessel.
  • Receptor: Damage is detected by receptors on platelets.
  • Control Center/Effector: Activated platelets release chemicals that attract and activate more platelets.
  • Response: Platelets form a plug to seal the blood vessel.
  • End Point Reached: Once the vessel is sealed, platelet activity decreases.
• Example: Childbirth
  • Stimulus: The baby’s head stretches the cervix.
  • Receptor: Data is sent to the brain.
  • Control Center: The brain signals the uterus.
  • Effector: The pituitary gland produces oxytocin.
  • Response: Oxytocin stimulates uterine contractions, pushing the baby’s head further.

Organic Compounds

Organic compounds are those that contain carbon-hydrogen (C-H) bonds.

Carbohydrates

Serve as fuel and have structural roles. Glycogen is the storage polymer of glucose in animals.

Lipids

Hydrophobic molecules including fats and oils.

• Fatty acids: Lipid monomers consisting of carbon chains.
  • Saturated: Solid at room temperature (e.g., animal fat, palm/coconut oil); associated with increased cardiovascular risk.
  • Unsaturated: Liquid at room temperature.
  • Omega-3 fats: Found in flaxseed oil and fish oil; have positive effects on the cardiovascular system.
  • Trans fats: Partially hydrogenated oils; associated with increased cardiovascular risk.
• Triglycerides: Three fatty acids linked to a glycerol molecule. Used primarily for energy and are the most common lipid in the body.
• Phospholipids: Composed of a glycerol backbone, two fatty acid tails (hydrophobic), and one phosphate head (hydrophilic). They are a major component of the cell membrane, forming the phospholipid bilayer (fluid mosaic model).
• Steroids: Characterized by a four-ring hydrocarbon structure. Cholesterol is a component of the cell membrane and is used to synthesize sex hormones.

Proteins

Macromolecules with diverse functions: enzymes, structural roles, movement, body defenses, fuel, hormones, receptors, and antibodies. Amino acids are the building blocks of proteins; there are 20 different types.

Nucleotides and Nucleic Acids

Nucleic acids are built from monomers of nucleotides and make up genetic material.

• Nucleotide Structure:
  • A nitrogenous base with a hydrocarbon ring structure.
  • A five-carbon sugar (ribose or deoxyribose).
  • A phosphate group.
• DNA (Deoxyribonucleic Acid): Found in the nuclei of cells, it forms a double helix. It contains genes, which are the codes for protein synthesis.
  • Structure: Deoxyribose sugar alternating with a phosphate group.
  • Bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
  • Complementary Base Pairing: A pairs with T, and C pairs with G.
• RNA (Ribonucleic Acid): A single strand of nucleotides that moves between the nucleus and cytosol. It is critical for making proteins.
  • Bases: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
  • Complementary Pairing: A pairs with U, and C pairs with G.

ATP (Adenosine Triphosphate)

The main source of chemical energy in the body. It is formed from ADP + P. The production of large quantities of ATP requires oxygen, which is why we breathe air.

Cell Structure

• Plasma Membrane: Provides structural support, communication, and cell identification. It separates the intracellular fluid (ICF)/cytosol from the extracellular fluid (ECF).
• Cytoplasm: Contains the ICF/cytosol, organelles, and cytoskeleton.
• Nucleus: Contains most of the cell’s DNA and is the primary location for making most RNA. DNA and RNA control organelle functions by coding for and synthesizing proteins.

The Phospholipid Bilayer Membrane

Structure

• A phosphate group (hydrophilic head).
• Two fatty acids (hydrophobic tails).

Membrane Proteins

• Transport substances: Act as protein channels or carrier proteins.
• Receptors: Bind to chemical messengers (ligands) to trigger events within the cell.
• Enzymes: Speed up chemical reactions, vital for homeostasis.
• Structural support: Give cells shape and maintain integrity.
• Link adjacent cells: Anchor cells within a tissue and allow for cell-to-cell communication.

Other Membrane Components

• Cholesterol: A lipid molecule that stabilizes the plasma membrane’s fluid structure during temperature changes.
• Glycolipids and Glycoproteins: Serve to identify the cell as part of the body.

Drugs and Membrane Receptors

Many drugs are designed to resemble ligands that bind to membrane receptors:

• Agonists: Mimic a ligand’s actions by stimulating the receptor (e.g., morphine mimics the actions of endorphins).
• Antagonists: Inhibit a ligand’s actions by blocking the receptor (e.g., antihistamines block receptors for histamine).

Membrane Transport

Passive Transport

Uses kinetic energy and follows a concentration gradient until equilibrium is reached.

• Diffusion: The movement of solute molecules from a high to a low concentration.
  • Simple Diffusion: Mostly nonpolar solutes (oxygen, carbon dioxide, lipids) pass straight through the phospholipid bilayer.
  • Facilitated Diffusion: Involves charged or polar solutes (ions, glucose) that cross the bilayer with the help of a carrier or channel protein.
• Osmosis: The movement of water across a selectively permeable membrane down its concentration gradient. Water moves from an area with a low solute concentration to an area with a high solute concentration. Osmotic pressure is the driving force exerted by solute molecules.

Active Transport

Requires energy (ATP) to move solutes against their concentration gradients (from low to high concentration). Example: The Na+/K+ pump moves Na+ into the ECF and K+ into the ICF via ATP hydrolysis.

• Vesicular Transport: For large macromolecules that are too big for channels or carriers. Vesicles are small sacs enclosed in a phospholipid bilayer that can fuse with the plasma membrane. This process requires ATP.

Tonicity

A way to compare the osmotic pressure gradients between two solutions – the cytosol and the ECF.

• Isotonic ECF: The ECF concentration is equal to the ICF concentration.
• Hypertonic ECF: The solute concentration of the ECF is higher than inside the cell. Water is pulled out of the cell, causing it to shrink (crenate).
• Hypotonic ECF: The solute concentration of the ECF is lower than inside the cell. Water is pulled into the cell, causing it to swell and possibly rupture (lyse).

Cellular Organelles

• Mitochondria: The ‘powerhouse’ of the cell; membrane-bound organelles involved in chemical energy production (ATP).
• Peroxisomes: Use oxygen to carry out reactions that produce hydrogen peroxide (H2O2). They oxidize toxic chemicals, break down fatty acids, and synthesize certain phospholipids.
• Ribosomes: Sites of protein synthesis. They can be free in the cytosol (making proteins for use within the cell) or bound to other structures (making proteins for export).
• Rough Endoplasmic Reticulum (RER): Has ribosomes bound to its membrane. It packages secretory proteins into transport vesicles and produces membrane components.
• Smooth Endoplasmic Reticulum (SER): Not associated with ribosomes. It stores calcium ions, performs detoxification reactions, and is involved in lipid synthesis.
• Golgi Apparatus: The ‘FedEx’ of the cell. A group of flattened membranous sacs that further modify, sort, and package proteins and lipids from the ER for export.
• Lysosomes: Responsible for the digestion of worn-out cell components. They contain digestive enzymes called acid hydrolases.
• Centrosome/Centrioles: A microtubule-organizing center located close to the nucleus. The pair of centrioles is critical for cellular division.
• Cytoskeleton: A network of protein filaments.
  • Microvilli: Finger-like extensions of the plasma membrane that increase surface area for absorption.
  • Cilia: Hair-like projections that propel substances past the cells.
  • Flagella: Whip-like tails that propel the entire cell (found only on sperm cells in humans).
• Nucleus: The governing body that directs the activities of other cellular components.
  • Nuclear Envelope: A membrane surrounding the nucleoplasm, containing nuclear pores.
  • Chromatin: A loose arrangement of DNA and associated histone proteins in a non-dividing cell. During cell division, chromatin condenses into thick structures called chromosomes. Human cells contain 23 pairs of chromosomes (one maternal and one paternal pair). Each replicated chromosome consists of identical copies called sister chromatids.
  • Nucleoli: Sites for the synthesis of ribosomal RNA and the assembly of ribosomes.

Protein Synthesis

Gene expression is the production of a protein from a specific gene.

1. Transcription: DNA is copied, creating messenger RNA (mRNA).
2. Translation: mRNA binds with a ribosome, initiating the synthesis of a protein.

Flow of Information: DNA → Transcription → mRNA → Translation → Protein

Mutations are changes in DNA due to copying mistakes or mutagens (e.g., UV light, chemicals, viruses). DNA mutations are the basis for many diseases, including cancer.

The Cell Cycle

The cell cycle is required for growth, development, tissue repair, and renewal. It consists of interphase and the M phase (cell division).

Interphase

A period of growth and preparation for cell division.

• G1 phase (1st gap): The cell performs normal daily metabolic activities.
• S phase (synthesis): DNA synthesis (replication) occurs.
• G2 phase (2nd gap): Cellular growth continues, and proteins required for cell division are produced.

During interphase, the nuclear envelope is present, and individual chromosomes are not distinguishable.

M Phase (Cell Division)

Consists of two overlapping processes: mitosis and cytokinesis.

• Mitosis: The division of newly replicated genetic material between two daughter cells in somatic cells.
  • Prophase: Chromatin compacts into chromosomes. Centrioles migrate to opposite poles to organize spindle fibers, which attach to each sister chromatid. The nuclear envelope begins to break apart.
  • Metaphase: Spindle fibers pull sister chromatids to align along the middle of the cell.
  • Anaphase: Sister chromatids are pulled apart toward opposite poles. Cytokinesis may begin.
  • Telophase: The nuclear envelope reassembles, and chromosomes uncoil back into chromatin.
• Cytokinesis: The cell’s proteins, organelles, and cytosol are divided between the two daughter cells. A cleavage furrow forms, and the cells split apart.

Meiosis

A type of cell division that forms daughter cells with half the number of chromosomes (haploid, 1n). It proceeds through two successive divisions: Meiosis I and Meiosis II.

• Meiosis I: Separates homologous pairs to produce haploid cells. (Prophase I, Metaphase I, Anaphase I, Telophase I)
• Meiosis II: Separates the sister chromatids of each chromosome; the cells remain haploid. (Prophase II, Metaphase II, Anaphase II, Telophase II)

Unit 2: Tissues and the Integumentary System

Four Main Tissue Types

Epithelial Tissues (Epithelia)

• Tightly packed sheets of cells with no visible ECM.
• Cover and line all body surfaces and cavities.
• Form glands that manufacture secretions (sweat, saliva) or chemical messengers (hormones).
• Functions: Protection, immune defense, secretion, transport into other tissues, sensation.
• Avascular and classified by cell shape and number of layers.

Connective Tissues (CT)

• Connect tissues to one another; usually vascular.
• ECM is a prominent feature, with cells scattered throughout.
• Functions: Bind, support, protect, and transport substances.
• Cells:
  • Fibroblasts: Produce fibers.
  • Mast cells: Produce histamine, which causes inflammation.
  • Adipocytes: Fat cells.
  • Phagocytes: Includes macrophages that ingest foreign invaders.
• Connective Tissue Proper
  • Loose CT: Mostly ground substance, with fibers and fibroblasts. Located beneath the epithelium of the skin, in membranes lining body cavities, and within the walls of hollow organs.
  • Dense CT:
    • Irregular: Mostly disorganized collagen bundles. Located in the dermis, surrounding organs and joints.
    • Regular: Organized into parallel collagen bundles. Located in tendons and ligaments.
    • Elastic: Mostly parallel-oriented elastic fibers. Found in the walls of organs that need to stretch (e.g., large blood vessels).
  • Reticular CT: Composed mostly of reticular fibers that form fine networks supporting vessels. Found in lymph nodes, the spleen, the liver, and bone marrow. Forms part of the basement membrane.
  • Adipose CT: Composed of fat-storing adipocytes. Functions in fat storage (energy reserve), insulation, shock absorption, and protection.
• Specialized Connective Tissue
  • Cartilage: Found in joints, the ear, nose, and parts of the respiratory tract.
    • Hyaline Cartilage: Most abundant type. Found on the ends of long bones, in the trachea, nose, and most of the fetal skeleton.
    • Fibrocartilage: Has great tensile strength. Found in intervertebral discs, menisci of the knee, and the pubic symphysis.
    • Elastic Cartilage: Flexible. Found in the external ear, auditory tube, and epiglottis.
  • Bone Tissue (Osseous): Supports and protects, provides muscle attachments, stores calcium, and contains bone marrow (produces blood cells and stores fat). Contains osteoblasts, osteocytes (in lacunae), and osteoclasts.
  • Blood: Has a liquid ECM called plasma (mostly water, dissolved solutes, and proteins).
    • Erythrocytes (Red Blood Cells): Transport oxygen.
    • Leukocytes (White Blood Cells): Function in immunity.
    • Thrombocytes (Platelets): Cell fragments involved in blood clotting.

Muscle Tissues

• Contract using ATP as an energy source.
• Muscle cells (myocytes) are excitable (can respond to electrical or chemical stimulation).
• Three Types:
  • Skeletal Muscle: Attached to bone, striated, voluntary.
  • Cardiac Muscle: Found in the heart, striated, involuntary, contains intercalated discs.
  • Smooth Muscle: In the walls of hollow organs and blood vessels, non-striated, involuntary.

Nervous Tissues

• Consist of two cell types:
  • Neurons: Capable of generating, sending, and receiving messages.
  • Neuroglia: Support the activity of neurons.

Glands: Endocrine and Exocrine

Endocrine Glands

Ductless glands that secrete hormones directly into the bloodstream. Their products have widespread systemic effects on distant cells.

Exocrine Glands

Have ducts, and their secretions have only local effects. Can be unicellular (goblet cells secreting mucus) or multicellular (sweat glands, salivary glands).

• Secretion Types:
  • Merocrine: Fluid products released in vesicles (e.g., salivary and sweat glands).
  • Holocrine: The entire cell is released with the product (e.g., sebaceous gland).
  • Apocrine: A portion of the cell is pinched off with the product. Found in axillary and anal regions, functional at puberty, and associated with hair follicles.
• Modified Sweat Glands:
  • Ceruminous glands: Produce cerumen (earwax).
  • Mammary glands: Produce milk.

Membranes in the Body

• Serous Membranes: Line the pericardial, peritoneal, and pleural cavities (mesothelium).
• Synovial Membranes: Composed of connective tissue and line joints.
• Mucous Membranes: Line tubes and organs that connect to the outside of the body (e.g., nasal and oral cavities); secrete mucus.
• Cutaneous Membrane: The skin.

The Integumentary System

Consists of two main regions: the epidermis and the dermis.

Structure

• Accessory Structures: Sweat glands, sebaceous glands, hair, and nails.
• Sensory Receptors: Detect heat, cold, pain, and pressure.
• Arrector Pili Muscles: Small bands of smooth muscle associated with hair.
• Epidermis: Avascular (receives nutrients via diffusion).
• Dermis: Vascular.
• Hypodermis: Also known as superficial fascia or subcutaneous fat. It is deep to the dermis, not part of the skin, and anchors the skin to deeper structures. Composed of loose CT and adipose tissue; it is vascular.

Functions of the Integumentary System

1. Protection: Against mechanical trauma, pathogens, and the environment.
2. Sensation: Perceives changes in the body’s internal or external environment.
3. Thermoregulation: Relies on negative feedback loops to maintain a stable internal temperature.
   • If Body Temperature is ABOVE Normal: Thermoreceptors detect the increase. The hypothalamus stimulates sweating and vasodilation (VD) of vessels in the dermis to release heat.
   • If Body Temperature is BELOW Normal: Thermoreceptors detect the drop. The hypothalamus stimulates vasoconstriction (VC) of dermal blood vessels, decreased sweating, and shivering to conserve and produce heat.
4. Excretion: Elimination of waste products and toxins through sweat.
5. Synthesis: Production of Vitamin D. Calcitriol, formed from vitamin D, is necessary for the absorption of calcium (Ca++) by the small intestine. Ca++ is essential for nerve function, muscle contraction, and bone health.

Layers of the Epidermis

• Stratum Basale (Stratum Germinativum): The deepest layer; most metabolically and mitotically active.
• Stratum Spinosum: Still close to the blood supply; metabolically and mitotically active.
• Stratum Granulosum: Three to five layers of cells filled with keratin, providing water resistance.
• Stratum Lucidum: A narrow layer of clear, dead keratinocytes found only in thick skin.
• Stratum Corneum: The outermost layer, consisting of several layers of dead, flattened cells that are sloughed off (exfoliated).

Cells of the Epidermis

• Keratinocytes: Produce the protein keratin.
• Dendritic (Langerhans) Cells: Phagocytes of the immune system located in the stratum spinosum.
• Merkel Cells: Sensory receptors for light touch, located in the stratum basale.
• Melanocytes: Produce the skin pigment melanin, located in the stratum basale.

Layers of the Dermis

• Papillary Layer: Composed of loose CT. Contains dermal papillae with capillary loops and Tactile (Meissner) corpuscles for light touch.
• Reticular Layer: The deepest, thicker layer of the dermis. Composed mostly of dense irregular CT with collagen and elastic fibers. Rich in proteoglycans, which keep the skin firm and hydrated. Contains Lamellated (Pacinian) corpuscles for pressure and vibration. Blood vessels, glands, hair follicles, and adipose tissue are found here.

Unit 3: The Skeletal and Muscular Systems

The Skeletal System

Skeletal System Divisions

• Axial Skeleton: Skull, vertebral column, thoracic cage (ribs, sternum), ossicles, and hyoid bone.
• Appendicular Skeleton: Bones of the pectoral girdle (clavicle, scapula), upper limbs, pelvic girdle (coxal bones), and lower limbs.

Functions of the Skeletal System

• Protection: Guards vital organs.
• Mineral Storage and Acid-Base Homeostasis: Stores calcium and phosphate.
• Blood Cell Formation: Red bone marrow is involved in hematopoiesis.
• Fat Storage: Yellow bone marrow in the medullary cavity stores fat.
• Movement: Provides attachment sites for skeletal muscles.
• Support: Supports body weight and provides a structural framework.

Structure of a Long Bone

• Periosteum: Membrane surrounding the outer surface.
• Perforating Fibers (Sharpey’s Fibers): Anchor the periosteum firmly to the bone surface.
• Diaphysis: The shaft of a long bone.
• Epiphysis: The end of a long bone (proximal and distal).
• Articular Cartilage: Hyaline cartilage covering the ends of bones at joints.
• Marrow Cavity: Contains bone marrow (red or yellow).
• Endosteum: Thin membrane lining the marrow cavity.

Microscopic Bone Structure

• Compact Bone: Hard, dense outer region that allows bone to resist compression and twisting stresses.
• Spongy Bone (Cancellous Bone): Found inside cortical bone. A honeycomb-like framework of bony struts (trabeculae) that resists forces from many directions.
• Epiphyseal Lines: Remnants of the epiphyseal plates that separate the epiphyses from the diaphysis in adult bones.
• Epiphyseal Plates (Growth Plates): Hyaline cartilage found in the developing bones of children, allowing for growth in length.

Composition of Bone Tissue (ECM)

• Inorganic Matrix (65%): Consists of minerals, primarily hydroxyapatite salts of calcium (Ca) and phosphorus (P).
• Organic Matrix (35%): Known as osteoid, it consists of collagen fibers and the usual ECM components.

Bone Cells

• Osteogenic Cells: Differentiate into osteoblasts.
• Osteoblasts: Perform bone deposition (build bone).
• Osteocytes: Mature bone cells found in lacunae.
• Osteoclasts: Perform bone resorption (break down bone) by secreting acid and enzymes.

Histology of Bone

• Structure of Compact Bone: The Osteon (Haversian System)
  • Lamellae: Concentric rings of thin layers of bone.
  • Central Canal: Contains blood vessels and nerves.
  • Lacunae: Spaces that house osteocytes.
  • Canaliculi: Little canals connecting lacunae.
  • Perforating Canals (Volkmann’s Canals): Perpendicular to central canals, connecting them.
• Structure of Spongy Bone
  • Not weight-bearing in the same way as compact bone.
  • Not organized into osteons.
  • Contains trabeculae (bony struts).

Ossification (Bone Formation)

• Endochondral Ossification: Bone develops from a cartilage template. This process forms all bones below the head except the clavicles.
• Intramembranous Ossification: Bone develops from fibrous connective tissue. This process forms the bones of the skull and the clavicles.

Hormonal Regulation of Bone

• PTH (Parathyroid Hormone): Secreted by the parathyroid gland, it stimulates effects that increase blood Ca+2 levels by increasing osteoclast activity, increasing calcium absorption from the gut, and inhibiting calcium loss in urine.
• Calcitonin: Secreted by the thyroid gland, it causes a decrease in blood Ca+2 levels by inhibiting osteoclasts and increasing calcium loss in urine.

The Muscular System

Organization of Skeletal Muscle

Muscle Fiber → Endomysium → Fascicle → Perimysium → Epimysium → Tendon

• Myocytes (Muscle Cells): Contain sarcoplasm (cytoplasm), sarcolemma (plasma membrane), and sarcoplasmic reticulum (smooth ER).
• Myofibrils: Made up of many myofilaments.
  • Thick Filaments: Composed of myosin.
  • Thin Filaments: Composed of actin, tropomyosin, and troponin.
  • Elastic Filaments: Composed of titin.

Functions of the Muscular System

• Movement of bones at joints.
• Contraction to generate heat.
• Contraction of the diaphragm for respiration.
• Maintaining posture.
• Facial expressions and swallowing.
• Sphincters for conscious control over body openings.
• Support of soft tissues (e.g., abdominal walls, pelvic floor).

Properties of Muscle Cells

• Contractility: The ability to contract, where proteins in the cell draw closer together.
• Excitability: The ability to respond to a stimulus.
• Conductivity: The ability to conduct electrical changes across the plasma membrane.
• Extensibility: The ability to be stretched without being ruptured.
• Elasticity: The ability to return to its original length after being stretched.

Muscle Types

All muscle types generate a force called muscle tension.

• Skeletal Muscle: Attached to bone, striated, voluntary.
• Cardiac Muscle: Found in the heart, striated, involuntary, contains intercalated discs.
• Smooth Muscle: In the walls of hollow organs and blood vessels, non-striated, involuntary.

Sliding Filament Mechanism

This mechanism explains how tension is generated during muscle contraction within the sarcomere, the functional unit of contraction.

• Striations:
  • Light Bands (I bands): Contain only thin filaments.
  • Dark Bands (A bands): Contain both thin and thick filaments.
• Bands/Zones of the Sarcomere:
  • I Band: Light band.
  • A Band: Dark band.
  • H Zone: Area in the A band with thick filaments only.
  • Z Disc (Line): The boundary at the beginning and end of a sarcomere.
  • M Line: The middle line of the sarcomere, with thick filaments only.
• Steps of Contraction: The I band and H zone narrow, while the A band remains unchanged. The Z-discs move closer together, shortening the sarcomere. When sarcomeres contract simultaneously, the whole muscle fiber shortens.

The Neuromuscular Junction (NMJ)

The synapse (junction) between a motor neuron and a muscle cell.

• The axon terminal of the neuron contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).
• The synaptic cleft is the space between the axon terminal and the muscle fiber.
• The motor end plate is a specialized region of the muscle plasma membrane with ACh receptors.

Phases of Contraction and Relaxation

1. Excitation: An action potential signals the release of ACh into the synaptic cleft. ACh binds to receptors on the motor end plate, opening channels that allow Na+ ions to enter the muscle fiber, generating a muscle potential.
2. Excitation-Contraction Coupling: The muscle potential signals the sarcoplasmic reticulum (SR) to release Ca++ into the cytosol. Calcium ions bind to troponin, causing tropomyosin to move and expose the active sites of actin.
3. Contraction: The myosin head, cocked by ATP, binds to the active site of actin, forming a crossbridge. A power stroke occurs when ADP + Pi are released. Myosin binds to another ATP, which breaks the link with actin. The cycle repeats as long as the stimulus and ATP are available.
4. Relaxation: Acetylcholinesterase (AChE) degrades ACh. ATP breaks the crossbridges. Calcium ions are actively pumped back into the SR. Troponin and tropomyosin block the active sites of actin again.

Rigor Mortis

The progressive stiffening of skeletal muscles begins about 3–4 hours after death. Without ATP, calcium pumps fail, and Ca++ remains in the cytosol, initiating contraction. Myosin heads cannot detach from actin without ATP, so muscles remain contracted until the myofilaments begin to degenerate, about 48–72 hours after death.

Unit 4: The Nervous System

Organization of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS): Brain and spinal cord. Performs integrative functions like decision-making, interpreting sensory information, planning movement, maintaining homeostasis, and higher mental functions.
• Peripheral Nervous System (PNS): All nerves outside the skull and vertebral column. Carries out motor and sensory functions.

Functional Divisions

• Sensory (Afferent) Division: Gathers information and carries it toward the CNS.
  • Somatic Sensory Division: Signals from skeletal muscles, bones, joints, skin, and special senses (vision, hearing, etc.).
  • Visceral Sensory Division: Signals from viscera (organs).
• Integrative Functions (CNS): Analyze and interpret incoming sensory information and determine a response.
• Motor (Efferent) Division: Carries information away from the CNS to perform actions.
  • Somatic Nervous System: Transmits information to skeletal muscle.
  • Autonomic Nervous System (ANS): Transmits information to smooth muscle, cardiac muscle, and glands.

Neurons

Excitable cells responsible for sending and receiving signals as action potentials (AP).

Structure

1. Cell Body (Soma): Contains the nucleus, organelles, and Nissl bodies (RER, gives gray color).
2. Dendrites: Receive information from other neurons and conduct impulses toward the soma.
3. Axon (Nerve Fiber): Conducts impulses away from the soma. Includes the axon hillock and axon terminals (synaptic knobs).

Components

• In the CNS:
  • Nuclei: Clusters of neuron cell bodies.
  • Tracts: Bundles of axons.
• In the PNS:
  • Ganglia: Clusters of neuron cell bodies.
  • Nerves: Bundles of axons.

Structural Classification

• Multipolar Neurons: Single axon and multiple dendrites (>99% of all neurons, typically motor neurons).
• Bipolar Neurons: One axon, one dendrite, with the cell body between them (found in the eye and olfactory epithelium, typically sensory).
• Pseudounipolar Neurons: Have one fused axon that extends from the cell body and divides into two processes (typically sensory).

Functional Classification

• Sensory (Afferent) Neurons: Carry information toward the CNS (pseudounipolar or bipolar).
• Interneurons (Association Neurons): Transmit information within the CNS between sensory and motor neurons (most neurons in the body; multipolar).
• Motor (Efferent) Neurons: Carry information away from the CNS to muscles and glands (multipolar).

Neuroglia (Glial Cells)

Provide support and protection for neurons, maintain their environment, and can divide to fill space when a neuron dies.

CNS Neuroglia

• Astrocytes: Transport nutrients and gases between blood vessels and neurons; form the blood-brain barrier (BBB).
• Oligodendrocytes: Form myelin in the CNS.
• Microglia: Activated by injury into phagocytic cells.
• Ependymal Cells: Ciliated cells that manufacture and circulate cerebrospinal fluid (CSF).

PNS Neuroglia

• Schwann Cells: Produce myelin.
• Satellite Cells: Provide supportive functions.

The Myelin Sheath

• Myelin: Repeating layers of phospholipid plasma membrane that provide insulation.
• Nodes of Ranvier: Gaps between myelin sheaths.
• White Matter: Composed of myelinated axons.
• Gray Matter: Composed of neuron cell bodies and unmyelinated processes.

Electrophysiology of Neurons

• Local Potentials: Travel short distances.
• Action Potentials: Travel the entire length of an axon, beginning at the trigger zone.
• Ion Channels: Ions rely on specific protein channels for diffusion.
• Resting Membrane Potential (RMP): -70mV, due to a difference in ion distribution. The cell is polarized (positive on the outside, negative on the inside). This is maintained by the Na+/K+ pump and the higher permeability to K+ through leak channels.
• Depolarization: Na+ channels open, and Na+ flows into the cell, making the membrane potential more positive.
• Repolarization: K+ channels open, and K+ flows out of the cell, making the cell more negative and returning it to RMP.
• Hyperpolarization: The cell becomes more negative than the normal RMP due to the efflux of K+ and influx of Cl-.

Events in an Action Potential

1. A local potential must depolarize the axon to reach the threshold (usually -55 mV).
2. Depolarization: Voltage-gated sodium channels open, and Na+ rushes in (an example of positive feedback).
3. Repolarization: Voltage-gated potassium channels open, and K+ rushes out.
4. Hyperpolarization: May occur as K+ channels are slow to close.

Propagation of Action Potentials

• Saltatory Conduction: In myelinated axons, the AP ‘jumps’ between Nodes of Ranvier, increasing the speed of conduction.
• Continuous Conduction: In unmyelinated axons, every section of the axolemma must propagate the AP, resulting in slower conduction.

The Chemical Synapse

Events at a Chemical Synapse

1. An AP in the presynaptic neuron triggers calcium ion channels in the axon terminal to open.
2. The influx of calcium ions causes synaptic vesicles to release a neurotransmitter into the synaptic cleft.
3. The neurotransmitter binds to receptors on the postsynaptic neuron.
4. Ion channels open, leading to a local potential (and possibly an AP if the threshold is reached).

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): The membrane potential moves closer to the threshold.
• Inhibitory Postsynaptic Potential (IPSP): The membrane potential moves farther away from the threshold.

Neurotransmitters

• Acetylcholine (ACh): Excitatory [cholinergic].
• Biogenic Amines: Excitatory, including catecholamines (NE, Epi, dopamine) [adrenergic] and serotonin.
• Amino Acids: Glutamate (excitatory) and GABA (inhibitory).
• Neuropeptides: Can be excitatory or inhibitory (e.g., endorphins).

The Brain

A soft, whitish-gray organ in the cranial cavity, continuous with the spinal cord. It is composed mostly of nervous tissue and requires ~20% of cardiac output. It has ventricles filled with cerebrospinal fluid (CSF).

Four Divisions of the Brain

1. Cerebrum: Consists of left and right hemispheres responsible for higher mental functions, sensory perception, and motor control. Each hemisphere has five lobes: Frontal, Parietal, Temporal, Occipital, and Insula. The Cerebral Cortex is the gray matter covering the hemispheres. The Limbic System (including the hippocampus and amygdala) is involved in memory, learning, emotion, and behavior.
2. Diencephalon: Deep to the hemispheres; processes, integrates, and relays information; involved in homeostasis and biological rhythms.
   • Thalamus: Gateway for sensory information (except smell) to the cerebral cortex.
   • Hypothalamus: Regulates the ANS, sleep/wake cycle, thirst, hunger, and body temperature. Secretes hormones.
   • Epithalamus: Includes the pineal gland, which secretes melatonin.
   • Subthalamus: Works with the basal nuclei to control movement.
3. Cerebellum: Inferior to the occipital lobe; coordinates voluntary motor activities. Features the arbor vitae (white matter).
4. Brainstem: Consists of the midbrain, pons, and medulla oblongata. Manages reflexes, homeostasis, and relays information.
   • Midbrain: Surrounds the cerebral aqueduct. Contains the superior and inferior colliculi (visual and auditory reflexes) and the substantia nigra (controls movement, produces dopamine).
   • Pons: Regulates movement, breathing, reflexes, sleep, and arousal.
   • Medulla Oblongata: The most inferior structure; regulates breathing and other vital activities.

The Spinal Cord

Located in the vertebral cavity, extending from the foramen magnum to L1-L2. It is responsible for relaying and processing information (reflexes). It has a central canal filled with CSF.

• Spinal Meninges: Similar to cranial meninges.
• Epidural Space: Between the dura mater and vertebral foramina; filled with veins and adipose tissue.
• Subarachnoid Space: Between the arachnoid and pia mater; filled with CSF. A common site for a lumbar puncture (LP).

Brain and Spinal Cord Structure

• White Matter: Myelinated axons. Bundles are called tracts in the CNS.
• Gray Matter: Cell bodies, dendrites, and unmyelinated axons. Forms the cerebral cortex and the central H-shape of the spinal cord. Clusters of cell bodies are called nuclei.

Protection of the Central Nervous System

1. Cranial Meninges: Three layers of membranes surrounding the brain.
   • Dura Mater: Outermost layer.
   • Arachnoid Mater: Middle layer, with the CSF-filled subarachnoid space beneath it.
   • Pia Mater: Innermost layer, in direct contact with brain tissue.
2. Cerebrospinal Fluid (CSF): A fluid that bathes the brain and fills cavities (ventricles). It is produced by the choroid plexuses and reabsorbed by arachnoid granulations. It cushions the brain, maintains temperature, removes wastes, and provides buoyancy.
3. Blood-Brain Barrier (BBB): Prevents many substances in the blood from entering the brain and its cells.

Spinal Nerves

Part of the PNS, they carry sensory and motor impulses to and from the spinal cord.

• Posterior (Dorsal) Nerve Root: Carries sensory information. The dorsal root ganglion contains the cell bodies of sensory neurons.
• Anterior (Ventral) Nerve Root: Carries motor information.
• Connective Tissue Coverings:
  • Epineurium: Outermost layer, holds motor and sensory axons together.
  • Perineurium: Surrounds fascicles (bundles of axons).
  • Endoneurium: Surrounds an individual axon.

Nervous System Role in Homeostasis

• Autonomic Nervous System (ANS): Maintains vital functions (heart rate, blood pressure, digestion), mainly controlled by the hypothalamus.
• Respiration: Regulated by the pons and medulla.
• Body Temperature: Regulated by the hypothalamus. A fever is caused by pyrogens that increase the hypothalamic set point. Antipyretics (e.g., aspirin) work by blocking pyrogen formation.

The Autonomic Nervous System (ANS)

Also known as the visceral motor division, it maintains homeostasis.

Sympathetic Division (SNS)

• Origin: Thoracolumbar division.
• Function: ‘Fight or flight’ division; prepares the body for emergency situations.
• Effects: Increases heart rate and force of contraction, dilates bronchioles, dilates blood vessels to skeletal and cardiac muscle, constricts blood vessels to digestive/urinary/integumentary systems, dilates pupils, increases sweating.

Parasympathetic Division (PSNS)

• Origin: Craniosacral division.
• Function: ‘Rest and digest’ division; maintains homeostasis at rest.
• Effects: Decreases heart rate and blood pressure, constricts bronchioles, increases digestive tract motility, promotes urination and defecation, increases salivation and lacrimation.

Nervous System Disorders

• Infectious Meningitis: A potentially life-threatening infection of the meninges. Diagnosis involves examining CSF. Viral meningitis is generally mild, while bacterial meningitis can be fatal and requires aggressive antibiotic treatment.
• Parkinson’s Disease: A common movement disorder characterized by difficulty initiating and terminating movement (hypokinetic), minimal facial expression, shuffling gait, and a resting tremor. Caused by the degeneration of dopamine-secreting neurons of the substantia nigra.
• Dementia: A progressive loss of recent memory, degeneration of cognitive functions, and changes in personality. Common forms include Alzheimer’s disease (AD), vascular dementia, and Lewy body dementia. There is no cure.
• Amyotrophic Lateral Sclerosis (ALS) (Lou Gehrig’s Disease): Degeneration of motor neuron cell bodies in the spinal cord and cerebral cortex. It leads to progressive muscle weakness and death, usually within 5 years of onset. There is no cure.