Cellular Organization in the Human Body

Cells, Tissues, Organs, and Organ Systems

The human body is organized into hierarchical levels, each with distinct functions and structures. Understanding these levels is fundamental to anatomy and physiology.

• Cell: The basic unit of life, capable of performing all life processes.

• Primary Tissue: Groups of similar cells with a common function. The four major types are epithelial, connective, muscle, and nervous tissue.

• Organ: A structure composed of two or more tissue types working together for a specific function.

• Organ System: A group of organs that work together to perform major body functions.

Primary Tissue Types and Their Functions

Each tissue type has specialized roles in the body:

• Epithelial Tissue: Covers surfaces, lines cavities, forms glands; functions in protection, absorption, and secretion.

• Connective Tissue: Supports, binds, and protects organs; includes bone, blood, fat.

• Muscle Tissue: Contracts to produce movement; includes skeletal, cardiac, and smooth muscle.

• Nervous Tissue: Transmits electrical signals; found in the brain, spinal cord, and nerves.

Homeostasis and Feedback Mechanisms

Definition of Homeostasis

Homeostasis is the maintenance of a stable internal environment despite changes outside the body.

Components of Negative Feedback

• Sensor: Detects change.

• Control Center: Processes information.

• Effector: Responds to restore balance.

Negative Feedback Mechanisms

Negative feedback mechanisms counteract changes, with effectors acting to reverse the change and restore balance.

Membrane Transport Mechanisms

Passive vs. Active Transport

• Passive Transport: No energy required; moves substances down their concentration gradient.

• Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

Types of Passive Transport

• Simple Diffusion: Molecules move directly through the membrane (e.g., O2, CO2).

• Facilitated Diffusion: Uses carrier or channel proteins for larger or charged molecules.

• Osmosis: Diffusion of water through a selectively permeable membrane.

Leak Channels vs. Gated Channels

• Leak Channels: Always open, allow ions to move freely.

• Gated Channels: Open/close in response to stimuli:

  • Voltage-gated: Open with changes in membrane potential.

  • Ligand-gated: Open when a chemical binds to the channel.

  • Mechanically-gated: Open in response to physical deformation.

Facilitated Diffusion via Carrier Proteins

Carrier proteins move molecules across the membrane down their concentration gradient by changing shape upon binding the molecule.

Active Transport Mechanisms

• Primary Active Transport: Direct use of ATP (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy from another gradient (e.g., Na+-glucose symporter).

Gap Junctions

Specialized connections between cells allowing direct communication and passage of ions/small molecules.

Chemical Messengers and Signal Transduction

Modes of Chemical Messenger Action

• Autocrine: Acts on the same cell that secreted it.

• Paracrine: Acts on nearby cells.

• Endocrine: Travels through the bloodstream to distant cells.

Signal Transduction Mechanisms

• Intracellular Receptors: Lipid-soluble messengers cross the membrane, bind to receptors inside the cell, and alter gene expression.

• Membrane Enzymes (e.g., Tyrosine Kinase): Messenger binds, enzyme is activated, and phosphorylation of proteins triggers cellular responses.

Fast vs. Slow Gated Ion Channels

• Fast: Open/close rapidly (e.g., voltage-gated Na+ channels).

• Slow: Open/close more slowly (e.g., some ligand-gated channels).

G Protein-Linked Channel and Enzyme Activation

• G Protein-Linked Ion Channel Activation:

  1. Messenger binds receptor.

  2. G protein activated.

  3. G protein opens/closes ion channel.

• G Protein-Linked Enzyme Activation:

  1. Messenger binds receptor.

  2. G protein activated.

  3. G protein activates enzyme (e.g., adenylate cyclase).

  4. Second messengers produced (e.g., cAMP).

Termination of Cell Signaling

• Removal of messenger, breakdown of second messengers, or deactivation of receptors.

Neural Signaling and Action Potentials

Action Potential

An action potential is a rapid, temporary change in membrane potential that travels along the axon.

Phases of Action Potential

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Returns to resting potential.

• Hyperpolarization: Becomes more negative than resting potential.

Gated Ion Channels in Action Potentials

• Voltage-gated Na+ and K+ channels are involved.

Sequence of Channel Opening/Closing

• Na+ channels open (depolarization), then close; K+ channels open (repolarization), then close (hyperpolarization).

Absolute Refractory Period

Ensures one-way propagation and limits frequency of action potentials.

Graded Potentials vs. Action Potentials

• Graded: Vary in size, decay with distance, can sum.

• Action: All-or-none, do not decay, propagate along axon.

Conduction of Action Potentials

• Non-myelinated Axons: Continuous conduction (slower).

• Myelinated Axons: Saltatory conduction (jumps between nodes, faster).

Function and Formation of Myelin

• Function: Insulates axon, increases speed of conduction.

• Peripheral Nervous System (PNS): Schwann cells wrap around axons.

• Central Nervous System (CNS): Oligodendrocytes wrap around multiple axons.

Components of Neurons

• Cell Body (Soma): Contains nucleus and most organelles; have ligand-gated channels; produce graded potentials.

• Dendrites: Reception of incoming information.

• Axon: Transmits electrical impulses called action potentials; voltage-gated sodium and potassium channels.

• Axon Hillock: Where axon originates and action potentials initiated; high concentration of voltage-gated channels.

• Axon Terminal: High concentration of voltage-gated Ca2+ channels; releases neurotransmitter.

Neuronal Synapses and Neurotransmitters

Neuronal Synapses

Synapses are junctions where neurons communicate. Arrival of action potential at the axon terminal results in Ca2+ influx, neurotransmitter release, and binding to postsynaptic receptors.

Neurotransmitters

• Chemical messengers stored in vesicles; classified as amino acids, peptides, monoamines, etc.

Anatomy of a Typical Synapse

• Presynaptic terminal

• Synaptic cleft

• Postsynaptic membrane

Transmission Across the Synapse

• Action potential → Ca2+ influx → neurotransmitter release → postsynaptic effect

Afferent vs. Efferent Neurons

• Afferent: Carry sensory info to CNS.

• Efferent: Carry motor commands from CNS to effectors.

Summary Table: Types of Membrane Transport