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

Organization of the Body

Levels of Organization

The human body is organized into hierarchical levels, each with specific roles in maintaining life and function.

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

• Tissues: Groups of similar cells that perform a common function.

• Organs: Structures composed of two or more types of tissues working together to perform specific functions (e.g., esophagus).

• Organ Systems: Groups of organs that work together to accomplish a particular task (e.g., 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, generating force and movement.

• Nerve: Initiates and transmits electrical impulses for communication.

• Connective: Connects, anchors, and supports body structures (e.g., adipose (fat) tissue).

Homeostasis and Feedback Mechanisms

Definition of Homeostasis

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

Components of Negative Feedback

• Set point: Desired level of a regulated variable.

• Sensors: Detect changes in the variable and send input to an integration center.

• Integration center: Compares actual value to set point and sends output to effectors.

• Effectors: Act to return the variable toward the set point.

Negative feedback mechanisms maintain homeostasis by counteracting deviations from the set point, restoring normal conditions.

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) and transport proteins; moves substances from areas of low to high concentration.

Types of Passive Transport

1. Simple diffusion

2. Diffusion through ion channels

3. Facilitated diffusion

Mechanisms of Passive Transport

• Simple diffusion: Direct movement through the membrane (lipid-soluble substances).

• Diffusion through ion channels: Movement through protein channels (mediated transport).

• Facilitated diffusion: Transport via carrier proteins in the membrane (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 and Active Transport

• Facilitated diffusion: Carrier proteins bind specific molecules, change shape, and allow passage across the membrane without energy input.

• Active transport: Moves molecules against a concentration gradient using ATP (primary) or energy from another gradient (secondary).

Example: The Na+/K+ pump uses ATP to move sodium and potassium ions against their gradients.

Cell Communication and Signal Transduction

Types of Cell Signaling

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

• Paracrine: Acts on nearby cells.

• Endocrine: Travels through the 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.

• Some receptors have enzyme activity (e.g., tyrosine kinases) and autophosphorylate upon activation, triggering intracellular signaling pathways.

Gated Ion Channels and G Proteins

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

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

G Proteins

• 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 (e.g., cAMP).

5. Cellular response triggered.

Termination of cell signaling: Messenger is removed or degraded, receptor is inactivated or internalized, and second messengers are broken down.

Neural Signaling and Action Potentials

Action Potentials

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

• 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.

• Channels involved: Voltage-gated Na+ and K+ channels.

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

• 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 the axon.

Myelination and Conduction

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

• Non-myelinated axons: Slower, continuous conduction.

• Function of myelin: Insulates axon, speeds conduction.

• Formation: Schwann cells (PNS) and oligodendrocytes (CNS) wrap around axons to form myelin sheaths.

Neurons and Synapses

Components of Neurons

• Cell body (soma)

• Dendrites

• Axon

• Axon terminals

Synapses and Neurotransmission

• Synapse: Junction between neurons.

• Action potential arrives at axon terminal, causing Ca2+ channels to open and Ca2+ to enter.

• Neurotransmitters are released into the synaptic cleft and bind to receptors on the postsynaptic cell.

Neurotransmitters

• Released into the synaptic cleft.

• Bind to receptors on postsynaptic cells.

• Classified by function (excitatory/inhibitory) or structure (e.g., amino acids, peptides).

Afferent vs. Efferent Neurons

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

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

Gap Junctions

• Specialized connections between cells.

• Allow direct passage of ions and small molecules.

• Enable rapid communication and coordination (e.g., heart muscle cells).

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2 across membrane
Facilitated Diffusion	No	High to Low	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump

Key Equations

• Nernst Equation (for equilibrium potential of an ion):

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

• Ohm's Law (for membrane current):

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

Additional info: Equations are provided for context; students should understand their application in membrane physiology.