Lecture 2: Cellular and Molecular Physiology II

• Claude Bernard stated that vital mechanisms aim to preserve life's conditions in the internal environment, finding causation of life's external phenomena in the inner environment's physico-chemical conditions.

Homeostasis

• It refers to "constant stand".

Internal vs. External Environment

• Intracellular refers to inside cells
• Extracellular refers to outside cells

Phospholipid Bilayer

• It is composed of choline, phosphate group, glycerol, and fatty acids.

Membrane Structure and Functions

• Plasma membrane forms the outer boundary of every cell and is an extremely thin layer.
• The plasma membrane encloses the intracellular contents.
• It acts as a mechanical and electrical barrier or insulator.
• Plasma membrane determines cell composition and controls the entry/exit of nutrients, molecules, and waste products via selective permeability.

Overview of Membrane Transport

• Lipid-soluble and small water-soluble substances permeate the plasma membrane unassisted, which is a passive process.
• Active forces require energy to move particles across the membrane.
• Passive forces do not require energy expenditure.

Accumulation/Exclusion of Substances by Cells

• Different concentrations across the membrane indicate something more than passive diffusion is occurring.

Solute Movement Across Cell Membrane

• Channel proteins and carrier proteins facilitate the movement of solutes.

Origins of Bioelectricity

• Electricity is present and used throughout the human body.
• Cells are specialized to conduct electrical currents.
• The nervous system requires electricity to send signals throughout the body and brain, enabling movement, thought, and feeling.

Molecular Basis of Cell Excitability

• Potassium concentration is 4-5 mM outside the cell and 140 mM inside the cell.

Equilibrium vs. Steady-State

• Equilibrium is a passive state.
• Steady-state requires active processes to maintain.

Polarized Cell

• In a polarized cell at steady-state, Potassium concentration is typically 4-5 mM outside the cell and 140 mM inside the cell.

Measuring Voltage Across the Plasma Membrane

• Voltage is the potential difference and potential energy, viewed as a "battery."
• Current is the flow of charge and kinetic energy.

Introduction to Excitable Cells

• Nerve and muscle cells are excitable tissues, along with some endocrine cells.
• Excitable tissues produce electrical signals when excited.
• Neurons use electrical signals to receive, process, initiate, and transmit messages.
• Electrical signals are critical to the function of the nervous system and all muscles.

Electrochemical Equilibrium

• Electrochemical equilibrium refers to the state where the chemical "push" equals the electrical "pull."
• There is no NET flux at electrochemical equilibrium.
• EK is the voltage at which Potassium is at electrochemical equilibrium, where NET movement of Potassium is zero.

Nernst Equation

• Natural processes move to the state of lowest free energy or -ΔG.
• At 37°C, Ex = (61/z) * log(co/ci).
• Potassium equilibrium potential (EK) = 61 log (5/140) = -90mV.
• Sodium equilibrium potential (ENa) = 61 log (140/10) = +70mV.
• Calcium equilibrium potential (ECa) = 61/2 log (10^-3/10^-7) = +120mV.

Electrochemical Potentials

• Ions are selfish, and permeability is power.
• Potassium (K) is the only biological ion with Ex below RMP (resting membrane potential).

Working with E-C Potentials

• If Vm = +90 mV and sodium channels (Na) open, Na will efflux to lower Vm.
• High voltages can move ions against their concentration gradient.
• Sodium (Na) will always move to bring Vm to ENa.
• When Vm = +60mV, there is no NET flux of Na.
• At equilibrium, electrical and chemical forces are equal.

Action Potentials and Ion Channels

• Novocaine and Lidocaine are anesthetics.
• Ion channels create action potentials.

Understand These Terms

• Depolarization means a decrease in potential, making the membrane less negative.
• Repolarization is the return to resting potential after depolarization.
• Hyperpolarization refers to an increase in potential, making the membrane more negative.

Threshold Potential

• The threshold potential is approximately +25 mV from the resting membrane potential (RMP).
• Opening sodium channels occurs at the threshold potential.
• Action potentials exhibit an "All or None" phenomenon.
• The threshold potential filters out small, random depolarizations.

Sodium-Potassium Pump and Resting Membrane Potential

• The Sodium-Potassium pump actively transports Sodium out of the cell and Potassium into the cell.
• Potassium tends to drive membrane potential to its equilibrium potential of -90 mV.
• Sodium tends to drive membrane potential to its equilibrium potential of +60 mV.
• Potassium exerts the dominant effect on resting membrane potential because the membrane is more permeable to Potassium.
• Resting potential (-70 mV) is much closer to Potassium's equilibrium potential than Sodium's.
• Relatively large net diffusion of Potassium outward during resting potential doesn't produce -90 mV because the membrane is slightly permeable to Sodium.
• Relatively small net diffusion of Sodium inward neutralizes some potential created by Potassium alone, bringing resting potential to -70 mV, slightly less than Potassium's.
• Negatively charged intracellular proteins that cannot cross the membrane remain unbalanced inside the cell, making the inside more negative than the outside.