Tissues and Histology

Four Main Tissue Categories

The human body is composed of four primary tissue types, each with distinct structures and functions:

• Epithelial Tissue: Covers body surfaces, lines cavities, and forms glands. Functions include protection, absorption, secretion, and sensation.

• Connective Tissue: Supports, binds, and protects organs. Includes bone, blood, cartilage, adipose, and more.

• Muscle Tissue: Specialized for contraction and movement. Types include skeletal, cardiac, and smooth muscle.

• Nervous Tissue: Conducts electrical impulses and processes information. Found in the brain, spinal cord, and nerves.

Cell Junctions: Gap vs. Tight Junctions

• Gap Junctions: Allow direct communication between adjacent cells via connexons, permitting ions and small molecules to pass. Important in cardiac and smooth muscle for coordinated contraction.

• Tight Junctions: Create a seal between cells, preventing passage of substances between them. Found in epithelial linings such as the intestines and blood-brain barrier.

Types of Cell Layering in Epithelia

• Simple Epithelium: Single layer of cells; functions in absorption, secretion, and filtration.

• Stratified Epithelium: Multiple layers; provides protection against abrasion and stress.

Transitional Epithelium

• Location: Found lining the urinary bladder, ureters, and part of the urethra.

• Function: Allows stretching and recoiling as organs fill and empty.

Protein Fibers in Connective Tissue

• Collagen Fibers: Strong, flexible, and resist stretching; most abundant.

• Elastic Fibers: Stretch and recoil; provide elasticity to tissues like skin and lungs.

• Reticular Fibers: Thin, branching fibers forming supportive networks in soft organs (e.g., spleen, lymph nodes).

Protein in Dense Connective Tissue

• Collagen: The primary protein, providing tensile strength to tendons and ligaments.

Dense Regular vs. Dense Irregular Connective Tissue

• Dense Regular CT: Collagen fibers aligned in parallel; found in tendons and ligaments; resists tension in one direction.

• Dense Irregular CT: Collagen fibers arranged randomly; found in dermis of skin; resists tension in multiple directions.

Cartilage Types and Locations

• Hyaline Cartilage: Most common; found in nose, trachea, larynx, costal cartilages, and articular surfaces of bones.

Membranes in the Body

• Mucous Membranes (Mucosa): Line body cavities open to the exterior (e.g., digestive, respiratory, urinary tracts).

• Serous Membranes (Serosa): Cover organs and line closed cavities; consist of two layers:

  • Parietal Layer: Lines cavity wall.

  • Visceral Layer: Covers organs.

• Categories: Pleura (lungs), Pericardium (heart), Peritoneum (abdominal organs).

Muscle Tissue

Types of Muscle Tissue

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

• Cardiac Muscle: Involuntary, striated, branched; found in heart; intercalated discs present.

• Smooth Muscle: Involuntary, non-striated; found in walls of hollow organs (e.g., intestines, blood vessels).

Muscle Coverings

• Epimysium: Surrounds entire muscle.

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

• Endomysium: Surrounds individual muscle fibers.

Muscle Structure Terms

• Sarcomere: The functional contractile unit of muscle fiber, defined as the region between two Z lines.

• Transverse (T) Tubules: Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

Sliding Filament Theory

• Describes how muscle contraction occurs as thin (actin) and thick (myosin) filaments slide past each other, shortening the sarcomere.

Neuromuscular Junction Components

• Acetylcholine (ACh): Neurotransmitter that stimulates muscle contraction.

• Acetylcholinesterase: Enzyme that breaks down ACh, terminating the signal.

Regulatory Proteins

• Tropomyosin: Blocks active sites on actin when muscle is relaxed.

• Active Sites: Regions on actin where myosin heads bind during contraction.

Muscle Fiber Types

• Fast Fibers (Type II): Contract quickly, fatigue rapidly; adapted for short bursts of power.

• Slow Fibers (Type I): Contract slowly, resist fatigue; adapted for endurance.

Muscle Contraction Types

• Isotonic Contraction: Muscle changes length; includes:

  • Concentric: Muscle shortens (e.g., lifting a weight).

  • Eccentric: Muscle lengthens (e.g., lowering a weight).

• Isometric Contraction: Muscle length does not change; tension increases (e.g., holding a weight steady).

Tetanus

• Tetanus: Sustained muscle contraction due to rapid stimulation.

• Incomplete (Unfused) Tetanus: Partial relaxation between stimuli.

• Complete (Fused) Tetanus: No relaxation; maximal tension.

Motor Unit

• A motor neuron and all the muscle fibers it innervates; fundamental unit of muscle contraction.

Myoblasts

• Embryonic cells that fuse to form muscle fibers.

Aponeurosis

• A broad, flat tendon that connects muscles to bones or other muscles.

Nervous Tissue and Nervous System

Types of Nerve Cells

• Neurons: Conduct electrical impulses; main functional cells of the nervous system.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons; do not conduct impulses.

Neuron Structure

• Dendrite: Receives signals from other neurons.

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

• Axolemma: Plasma membrane of the axon.

• Axon Hillock: Region where action potentials are initiated.

• Telodendria: Terminal branches of the axon, ending in synaptic terminals.

Neuron Functional Types

• Sensory (Afferent) Neurons: Transmit impulses from receptors to CNS.

• Motor (Efferent) Neurons: Transmit impulses from CNS to effectors (muscles/glands).

• Interneurons: Connect sensory and motor neurons within CNS; process information.

Receptor Types

• Exteroreceptors: Detect external stimuli (e.g., touch, temperature).

• Interoreceptors: Detect internal stimuli (e.g., from organs).

• Proprioceptors: Detect body position and movement (e.g., in muscles, tendons).

Wallerian Degeneration

• Process where the distal portion of a damaged axon degenerates after injury.

Neuroglia Types and Functions

• Astrocytes: Maintain blood-brain barrier, regulate environment, support neurons (CNS).

• Oligodendrocytes: Form myelin sheaths in CNS.

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

• Microglia: Phagocytic cells; remove debris and pathogens (CNS).

• Satellite Cells: Support neuron cell bodies in PNS ganglia.

• Schwann Cells: Form myelin sheaths in PNS.

• Nodes of Ranvier: Gaps in myelin sheath; facilitate saltatory conduction.

White Matter vs. Gray Matter

• White Matter: Regions with myelinated axons; responsible for rapid signal transmission.

• Gray Matter: Regions with neuron cell bodies, dendrites, and unmyelinated axons; site of processing and integration.

Sodium-Potassium Exchange Pump

• Maintains resting membrane potential by moving 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

Ion Channels

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Chemically-Gated (Ligand-Gated) Channels: Open when specific molecules bind (e.g., neurotransmitters).

• Mechanically-Gated Channels: Open in response to physical deformation (e.g., touch receptors).

Action Potential Properties

• All-or-None Principle: Once threshold is reached, an action potential occurs fully.

• Threshold Stimulus: Minimum stimulus required to trigger an action potential.

• Refractory Period: Time during which a neuron cannot fire another action potential.

• Saltatory Propagation: Action potentials jump from node to node in myelinated axons, increasing speed.

Neurotransmitters

• Cholinergic Neurotransmitters: Use acetylcholine (ACh); found at neuromuscular junctions and in parasympathetic nervous system.

• Adrenergic Neurotransmitters: Use norepinephrine (noradrenaline); found in sympathetic nervous system.

• Dopamine: Involved in reward, motivation, and motor control.

• Serotonin: Regulates mood, appetite, and sleep.

• Nitric Oxide (NO): Gaseous neurotransmitter; involved in vasodilation and neural signaling.

Polarization Terms

• Polarization: Resting state; inside of neuron is negative relative to outside.

• Depolarization: Membrane potential becomes less negative (more positive); Na+ enters cell.

• Repolarization: Return to resting potential; K+ exits cell.

Additional info: Where content was brief, academic context and definitions were expanded for clarity and completeness.