Homeostasis and Feedback Mechanisms

Homeostasis

• Homeostasis is a steady state that involves energy consumption
• Equilibrium is a state that does not use energy
• A vital parameter (e.g., blood glucose) is well-regulated when in a steady state
• The body carefully manages actions that can raise or lower the parameter value to maintain a constant value

Negative Feedback

• Negative feedback is the most common type of feedback mechanism
• It reverses any deviation from a stable point
• If a factor becomes too high or too low, a series of changes return the factor towards its mean value
• This maintains homeostasis

Positive Feedback

• Positive feedback loops maintain or reinforce the stimulus
• The cycle exacerbates deviations from a stable point
• Repetition of this cycle can lead to instability or death

Electrolyte Content of Body Fluids

• Data presented includes the plasma mEq/L, interstitial fluid mEq/L, and intracellular fluid mEq/L for various electrolytes
  • Cations (Na+, K+, Ca2+, Mg2+) and Anions (Cl-, HCO3-, SO42-, PO43-, protein) are listed
  • Specific measurements are provided

Osmotic Pressure and Reflection Coefficient

• The relation between osmotic pressure and reflection coefficient are reviewed
• Isotonic solution is always iso-osmotic, but an iso-osmotic solution may not be isotonic
• Urea is freely permeable and equilibrates across compartments
• Impermeable solute concentration determines osmotic pressure

Tonicity of Solutions

• Isotonic solutions cause no change in cell volume
• Hypotonic solutions cause cell swelling
• Hypertonic solutions cause cell shrinking
• Tonicity depends on the concentration of impermeable solutes

Transport Function of the Plasma Membrane

• Simple Diffusion: Follows electrochemical gradient; through phospholipid layer
• Facilitated Diffusion: Follows electrochemical gradient; requires a protein carrier
• Primary Active Transport: Against electrochemical gradient; Uses ATP directly
• Secondary Active Transport: Against electrochemical gradient; Uses the electrochemical gradient produced by primary active transport
• Bulk Transport: Against electrochemical gradient; Endocytosis and exocytosis
• Examples of substances transported using each method are given*

Fick's Law of Diffusion

• Relates diffusive flux to concentration gradient across a membrane
• Factors affecting the rate of diffusion are:
  • Permeability coefficient
  • Partition coefficient
  • Diffusion coefficient
  • Thickness of the membrane
  • Cross-sectional area
  • Concentration difference

Saturation in Carrier-Mediated Transport

• Carrier proteins have a limited number of binding sites for solutes.
• At low concentrations, transport rate increases steeply
• At high concentrations, transport rate levels off (transport maximum) at the point where all binding sites are occupied

Stereospecificity in Carrier-Mediated Transport

• Binding sites for solutes on transport proteins are stereospecific.
• Glucose transporter recognizes and transports only D-glucose and not L-glucose
• The difference between D and L is displayed*

Competition in Carrier-Mediated Transport

• Binding sites are specific, but related solutes can be transported
• D-galactose can inhibit the transport of D-glucose

Ion Channel Characteristics

• Conductance depends on the probability that the channel is open, the higher the probability, the higher the conductance/permeability.
• Gates on ion channels controlled by different types of sensors:
  • Voltage-gated channels
  • Ligand-gated channels
  • Second-messenger-gated channels
  • Mechanically-gated channels

Mechanisms Responsible for the Resting Membrane Potential

• Chemical gradients generated by active transport pumps
• Selective membrane permeability (more permeable to potassium ions)
• Electrical gradients (create negative intracellular charge)
• Electrochemical equilibrium

Equilibrium Potential

• The equilibrium potential for an ion is the membrane potential at which the diffusive force is exactly balanced by the electrical force
• The overall current flow of an ion directly relates to the net force and conductance

Driving Force for Diffusion

• How driving forces affect ion diffusion across the cell membrane
• Driving force is calculated by taking the resting membrane potential minus the cell's equilibrium potential for a given ion
• When the result is negative, ions flow into the cell, if the result is positive, ions flow out of the cell*
• Examples of ions (Na+, Cl+, K+, Ca 2+) driving force are displayed graphically

Characteristics of Action Potentials

• Stereotypical size and shape
• Non-decremental propagation
• All-or-none response
• Examples of action potentials are given to demonstrate this concept*

Refractory Periods

• Absolute refractory period: no new action potential is possible, due to inactivation gates in Na+ channels.
• Relative refractory period: a new AP can be generated, but only with a larger stimulus due to the hyperpolarizing after-potential

Propagation of Action Potentials

• Action potentials are propagated down a nerve or muscle fiber by local currents
• The action potentials start at the initial segment/hillock of the axon, and propagate down the axon by spread of local currents

Factors Affecting Conduction Velocity in Nerves

• Myelination, axon diameter, temperature affect the speed of conduction
• A table comparing faster and slower conduction factors is shown*

Different Receptors, Signaling Pathways, and Messengers

• Various receptors and signaling pathways described and categorized
• Different receptor types and signaling pathways are mentioned with examples of their ligands.*
• For each, Second Messengers and what effect they have are given*

Good Study Habits

• Consistent study schedule
• Note-taking
• Study breaks
• Organized workspace
• Prioritized tasks
• Distraction-free environment
• Peer study groups
• Periodic review sessions