Membrane Physiology Quiz

Membrane Physiology

• Extracellular Fluid Composition:
  • Na+: 142 mEq/L
  • K+: 4 mEq/L
  • Ca²⁺: 2.4 mEq/L
  • Mg²⁺: 1.2 mEq/L
  • HCO₃⁻: 103 mEq/L
  • Phosphates: 28 mEq/L
  • Glucose: 90 mg/dL
  • Amino acids: 30 mg/dL
  • Cholesterol: 0.5 g/dL
  • Phospholipids: 2-95 g/dl
  • Neutral fat: variable
  • PO₂: 46 mm Hg
  • PCO₂: 40 mm Hg
  • pH: 7.4
  • Proteins: 2 g/dL
• Intracellular Fluid Composition:
  • Na+: 10 mEq/L
  • K+: 140 mEq/L
  • Ca²⁺: 0.0001 mEq/L
  • Mg²⁺: 58 mEq/L
  • HCO₃⁻: 4 mEq/L
  • Phosphates: 10 mEq/L
  • Glucose: 0-20 mg/dL
  • Amino acids: variable
  • Others: variable
  • PO₂: 20 mm Hg
  • PCO₂: 50 mm Hg
  • pH: 7.0 -Proteins: 16 g/dL

Diffusion

• Molecules and ions in bodily fluids are constantly moving.
• This movement is called "heat."
• Motion never ceases except at absolute zero.

Transport Through Cell Membranes

• Transport through cell membranes occurs via diffusion and active transport.
• In diffusion, substances move across membranes either through intermolecular spaces or with a carrier protein.
  • The energy for diffusion comes from the kinetic motion of the substance itself.

Simple Diffusion

• Molecules or ions move through membrane openings or intermolecular spaces without interacting with carrier proteins.
• Rate depends on substance availability, kinetic motion velocity, and membrane opening number/size.

Facilitated Diffusion

• Carrier proteins aid molecule/ion passage by binding chemically with them.
• Diffusion rate approaches a maximum as the concentration of the diffusing substance increases.
• Glucose and amino acids are commonly transported via facilitated diffusion.

Diffusion Through Protein Pores and Channels

• Protein channels are selectively permeable to certain substances (e.g., Na+, K+).
• Channels open and close via gates.
• Voltage-gated channels open in response to electrical potential changes. Examples include sodium channels.
• Chemical (ligand)-gated channels open with chemical substance binding. Examples include acetylcholine-gated sodium channels.

Active Transport

• Movement of ions or substances across membranes against an energy gradient (uphill).
• This process utilizes carrier proteins.
• Energy required beside kinetic energy.

Types of Active Transport

• Primary Active Transport:
  • Energy derived directly from ATP breakdown. Often used to establish concentration gradients against a gradient.
• Sodium-Potassium Pump:
  • Pumps 3 Na+ ions out of the cell and 2 K+ ions in.
  • Essential for maintaining sodium/potassium concentration differences and voltage across membranes in nerve cells.
• Ca²⁺ Pumps:
  • Pumps calcium ions out of the cell or into intracellular vesicles.
• Hydrogen Pump:
  • Important in gastric glands and kidney tubules (active transport of hydrogen ions).

Co-transport

• Example: transport of glucose with Na⁺ or amino acids with Na⁺.

Counter-transport

• Example: Na⁺ counter-transport of Ca²⁺ and H⁺.

Membrane Potentials and Action Potentials

• Resting Membrane Potential:
  • The potential across the membrane of a neuron when not transmitting a signal. Typically -70 mV, inside being more negative.
• Action Potentials:
  • Rapid changes in membrane potential that propagate along nerve fibers.
  • Begin with a sudden change from negative to positive, then back towards the negative.

Stages of Action Potential

• Resting Stage: The membrane has a -70 mV potential.
• Depolarization Stage: The membrane becomes permeable to sodium ions, positively charged sodium rushes in, and the potential becomes more positive.
• Repolarization Stage: The sodium channels close and potassium channels open, allowing potassium ions to exit the cell, restoring the more negative potential.

Initiation of the Action Potential

• A stimulus that creates enough initial rise in transmembrane voltage opens voltage-gated sodium channels.
• The following positive feedback causes further rise in voltage to trigger action potential.

Threshold for Action Potential

• A sudden membrane voltage rise of 15 to 30 mV is usually necessary to initiate an action potential in a nerve fiber. This is often around -55 mV.

Propagation of the Action Potential

• An action potential elicited at one point on an excitable membrane often excites adjacent areas, causing propagation along the membrane.
• This propagation occurs in both directions.

Re-establishing Sodium and Potassium Ionic Gradients

• After an action potential, sodium that entered and potassium that exited must be transported back to create the original concentration gradients.

Plateau in Some Action Potentials

• Some membranes do not repolarize immediately after depolarization, instead entering a plateau phase.
• This prolonged depolarization duration is important for functions like heart muscle contraction.

Skeletal and Smooth Muscle Contraction

• Skeletal Muscle Fibers:

  • Comprised of numerous myofibrils that are responsible for skeletal muscle contraction.
  • Actin and myosin filaments interdigitate, resulting in striated appearance.
  • Contain the sarcolemma, a cell membrane; sarcoplasm, the cell's cytoplasm, and sarcoplasmic reticulum, an endoplasmic reticulum important for contraction.

• Smooth Muscle:

  • Composed of shorter and smaller fibers.
  • Multi-unit and single-unit smooth muscle are two types.
  • Multi-unit fibers contract independently of each other; often innervated by one nerve ending
  • Single-unit fibers contract together as one unit—are connected by gap junctions.
  • Have actin and myosin filaments but lack striated appearance
  • Intercellular proteins transmit contraction force between cells.

Sources of Energy for Muscle Contraction

• Glycolysis of Glycogen: Quick energy source, but limited.
• Phosphocreatine: Storage molecule providing energy for quick contraction.
• Oxidative Metabolism: The primary long-term energy source.

Excitation of Skeletal Muscle Fibers

• Neuromuscular Junction: Area where nerve and muscle tissue meet.
  • Acetylcholine (neurotransmitter) released from nerve to excite muscle cells.
  • Stimulation initiated when nerve terminal releases acetylcholine into the synaptic cleft, leading to contraction.
  • The signaling is halted as acetylcholine breaks down.

Excitation-Contraction Coupling

• Transfer of action potential from the sarcolemma into deeper regions of the muscle fiber through transverse tubules.
• T-tubule action potentials cause calcium release.

Smooth Muscle Contraction Regulation

• Different factors (nerve stimulation, hormones, stretch, and chemical environment) stimulate contraction.
• Smooth muscle fibers are often controlled by many signaling pathways unlike skeletal muscle.
• Calcium enters from extracellular space triggering smooth muscle contraction.

Smooth Muscle Contraction Factors

• Lack of oxygen (causes relaxation).
• Excess CO₂ (causes relaxation).
• Increased hydrogen ions (causes relaxation).