Anatomy & Physiology Study Guide: Cells, Tissues, Membranes, and Neural Signaling

Organization of the Human Body

Cells, Tissues, Organs, and Organ Systems

The human body is organized into a hierarchy of structural levels, each with specific functions essential for life.

• Cell: The basic unit of life, capable of carrying out all life processes independently.

• Primary Tissue: A collection of cells of similar type and function. Four main types: epithelial, muscle, nerve, and connective.

• Organ: A structure composed of two or more types of tissues working together to perform specific functions. Example: Esophagus.

• Organ System: A group of organs that work together to accomplish a particular task. Example: Digestive system.

Primary Tissue Types and Their Functions

• Epithelial: Forms barriers between body and environment; involved in exchange, secretion, and protection.

• Muscle: Responsible for contraction, generation of force, and movement.

• Nerve: Initiates and transmits electrical impulses for communication.

• Connective: Connects, anchors, and supports other tissues. Example: Adipose (fat) tissue is a special connective tissue.

Homeostasis and Feedback Mechanisms

Definition of Homeostasis

Homeostasis is the process of maintaining a stable internal environment compatible with life.

• Set point: Desired level of a regulated variable.

• Sensors: Detect changes in the regulated variable and provide input to an integrating center.

• Integrating center: Compares actual value to set point and sends output to effectors to restore balance.

Negative Feedback Mechanisms

Negative feedback occurs when a change in a regulated variable triggers a response that counteracts the initial change, maintaining homeostasis.

• Example: Regulation of blood sodium levels.

Membrane Transport

Passive vs. Active Transport

• Passive Transport: No energy required; includes simple diffusion (no transporters) or mediated transport (via transport proteins), moving substances from areas of high to low concentration.

• Active Transport: Requires metabolic energy (ATP), uses pumps (membrane proteins/transporters), moves substances from areas of low to high concentration.

Types of Passive Transport

• Simple diffusion

• Diffusion through ion channels

• Facilitated diffusion

Mechanisms of Passive Transport

• Simple diffusion: Passive transport directly through the membrane (lipid soluble substances).

• Diffusion through ion channels: Passive transport via protein channels; selective for specific ions.

• Facilitated diffusion: Passive transport through carrier proteins in the membrane; a type of mediated transport.

Leak Channels vs. Gated Channels

• Leak channels: Always open, allowing constant passage of ions.

• Gated channels: Open or close in response to specific stimuli.

Types of Gated Channels

• Ligand (chemical) gated: Open when a specific chemical binds to the channel.

• Voltage-gated: Open or close in response to changes in membrane potential.

• Mechanical-gated: Open in response to mechanical force.

Facilitated Diffusion via Carrier Proteins

Carrier proteins bind specific molecules on one side of the membrane, change shape, and allow the molecule to pass through. This process does not require energy and moves substances down their concentration gradient.

Active Transport Mechanisms

• Primary active transport: Uses ATP directly (e.g., Na+/K+ pump).

• Secondary active transport: Uses energy from another gradient (e.g., glucose uptake via Na+ gradient).

Cell Communication and Signal Transduction

Types of Cell Signaling

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

• Paracrine: Acts on nearby cells.

• Endocrine: Travels through blood to distant cells.

Signal Transduction Mechanisms

• Messenger binds to receptor on cell surface or nucleus.

• Receptor-messenger complex acts as a transcription factor, altering gene expression.

• Receptor dimerizes and autophosphorylates tyrosine residues (for enzymes such as tyrosine kinases).

• Phosphorylated sites activate intracellular signaling pathways.

Gated Ion Channels: Fast vs. Slow

• Fast: Open/close quickly in response to stimuli (e.g., ligand-gated Na+ channels).

• Slow: Open/close more slowly, often via second messengers (e.g., G protein-coupled channels).

G Proteins and Their Functions

• Proteins that relay signals from receptors to effectors inside the cell.

Steps in G Protein-Linked Ion Channel Activation

1. Messenger binds receptor.

2. G protein activated (GDP → GTP).

3. G protein subunit opens ion channel.

4. Ions flow, changing membrane potential.

Steps in G Protein-Linked Enzyme Activation

1. Messenger binds receptor.

2. G proteins are activated.

3. G protein activates enzymes (e.g., adenylyl cyclase).

4. Second messenger produced (cAMP).

5. Cellular response triggered.

Termination of Cell Signaling

• Messenger removed or degraded.

• Receptor inactivated or internalized.

• Second messengers broken down.

Neural Signaling and Action Potentials

Action Potential Phases

• Depolarization: Na+ channels open, Na+ enters, membrane potential rises.

• Repolarization: K+ channels open, K+ exits, membrane potential falls.

• Hyperpolarization: K+ channels stay open briefly, membrane potential drops below resting.

Gated Channels in Action Potentials

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

Sequence of Channel Opening and Closing

• Na+ channels open first (depolarization), then K+ channels open (repolarization), then both close (return to resting).

Absolute Refractory Period

• Ensures one-way transmission and limits frequency of action potentials.

Graded Potentials vs. Action Potentials

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

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

Conduction of Action Potentials

• Non-myelinated axons: Slower conduction.

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

Function of Myelin

• Insulates axons, speeds conduction.

Formation of Myelin

• PNS: Schwann cells wrap around axon.

• CNS: Oligodendrocytes extend processes to multiple axons.

Neurons and Synapses

Components of Neurons

• Cell body (soma)

• Dendrites

• Axon

• Axon terminals

Junctions Between Neurons: Synapses

• Synapse: Junction between neurons.

• Action potential arrives at axon terminal, Ca2+ channels open, Ca2+ enters.

Neurotransmitters

• Released into synaptic cleft.

• Bind to receptors on postsynaptic cell.

• Stored in vesicles, classified as excitatory/inhibitory, or by structure (e.g., amino acids, peptides).

Afferent vs. Efferent Neurons

• Afferent: Carry signals to the CNS (sensory).

• Efferent: Carry signals from the CNS (motor).

Table: Comparison of Passive and Active Transport

Transport Type	Energy Required	Direction	Examples
Passive Transport	No	High to Low Concentration	Simple diffusion, facilitated diffusion, ion channels
Active Transport	Yes (ATP)	Low to High Concentration	Na+/K+ pump, secondary active transport

Key Equations

• Nernst Equation: Used to calculate equilibrium potential for an ion across a membrane.

$E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

• Ohm's Law (for membrane potential):

$I = g(V_m - E_{ion})$

Additional info: Equations added for academic completeness; not explicitly present in the original notes.