Cellular Physiology

• Extracellular Fluid (ECF) is the fluid which bathes the cell and its composition is maintained by organ systems.
• Approximately 14/15 L of ECF is composed of major cations such as Na+ and Ca2+, and major anions like Cl- and HCO3-.
• Intracellular Fluid (ICF) is the fluid within the cell and its composition is maintained by the cell membrane.
• Approximately 28 L of ICF is composed of major cations like K+ and Mg2+, and major anions like Proteins and organic phosphate (AMP, ADP, ATP).
• Total Body Water (TBW) is the total amount of water in the body, which is 42/43 L in an adult human.
• Solute concentration increases until the carriers are saturated, a process known as Transport Maximum (Tm).
• At low solute concentration, facilitated diffusion typically proceeds faster than simple diffusion because of the carrier.
• Simple diffusion proceeds as long as there is a concentration gradient for the solute and is more rapid than facilitated diffusion.
• Counter-transport, exchange, or anti-port occurs when solutes move in opposite directions.
• However, at higher solute concentration, the carriers become saturated and facilitated diffusion will level off.
• Structurally related solutes compete for transport sites on carrier molecules, for example, Galactose is a competitive inhibitor of glucose transport in the small intestines.
• Transport of two or more solutes is coupled in secondary active transport, requires an indirect input of metabolic energy, and the metabolic energy is provided indirectly by the Na+-K+ pump.
• Facilitated diffusion is a process where a carrier facilitates diffusion, occurs down an electrical gradient, and does not require metabolic energy.
• Co-transport or symport occurs when the solutes move in the same direction across the cell membrane.
• Extracellular Fluid (ECF) constitutes 1/3 of body weight and is also known as internal environment or milieu intérieur because all cells live within it.
• Passive tension is the tension developed by simply stretching a muscle to different lengths, with the ability to generate greatest tension at its resting length.
• Anaerobic metabolism provides more tension for a greater time.
• Active tension represents the active force developed during cross-bridge cycling, and is proportional to the number of cross-bridges that cycle.
• Isotonic contraction is a shortening of muscle at a constant afterload, with the tension generated remaining the same for a set length.
• Isometric contraction is a contraction in which a muscle exerts force (tension) without changing length, allowing the muscle to develop tension at a preset length but not allowing it to shorten.
• Total tension is the sum of passive tension during stretch and active tension during cross bridge formation.
• More time is required for cross-bridge formation against increased resistance, and with further afterload increases, shortening and velocity may decrease to zero.
• The velocity of shortening reflects the speed of cross-bridge cycling, and decreases with an increase in afterload.
• Intracellular Fluid (ICF) constitutes 2/3 of body weight and is also known as cytosol.
• Trans-Cellular fluid is a specialized form of ECF found in spaces such as synovial, peritoneal, pericardial, and intraocular spaces, and CSF.
• The length of dendrites and the flux of Cl contribute to the decrement of electrotonic conduction.
• Serotonin is produced from tryptophan in serotonergic neurons in the midbrain and in the GIT.
• Neurotransmitters include Choline Esters, Biogenic Amines, Amino Acids, Neuropeptides, Acetylcholine (ACh), Dopamine, Epinephrine, Norepinephrine, Histamine, serotonin, γ-Aminobutyric acid (GABA), Glutamate, glycine, Adrenocorticotropin (ACTH), Cholecystokinin, Dynorphin, Endorphins, Enkephalins, Glucose-dependent insulinotropic peptide (GIP), Glucagon, Neurotensin, Oxytocin, Secretin, Substance P, TRH, VIP.
• Decrement of Electrotonic Conduction refers to the decrease in membrane potential as it spreads electrotonically along dendrites toward the soma due to leaks in dendrite allowing for leakage of electrical charge.
• Serotonin has a high concentration.
• Acetylcholine is the neurotransmitter of preganglionic neurons.
• Acetylcholine is the only neurotransmitter that is utilized at the neuromuscular junction.
• Dopamine is prominent in the substantia nigra and is usually inhibitory, inhibiting prolactin.
• Norepinephrine is synthesized in the adrenal medulla and is the primary neurotransmitter released from postganglionic sympathetic neurons.
• Epinephrine is synthesized in the adrenal medulla.
• Approximate Water Distribution in Adult Humans (70Kg person) is as follows: TBW constitutes 60% of body weight in men and 50% in women, ICF constitutes 40% of body weight in both men and women, ECF constitutes 20% of body weight in both sexes, and interstitial fluid constitutes 75% of ECF in both sexes.
• Relaxation involves: Ca 2+ is re-accumulated in the SR by the Ca 2+ ATPase of the SR membrane (SERCA); Ca 2+ decreases to less than 10 -7 M, insufficient Ca 2+ for binding to troponin C; tropomyosin returns to its resting position, preventing myosin interaction with actin.
• Glycolysis is the process that binds P to ADP and restores phosphocreatine.
• Steps of Cross-bridge formation include: before contraction begins, the heads of the cross-bridges bind with ATP; ATPase cleaves ATP to ADP & P, with P bound to the myosin head; myosin is still extended perpendicularly toward the actin filament with no contact to actin; Troponin + Ca 2+ -tropomyosin complex is cleared, exposing binding site; myosin & actin bridges; conformational change due to bridging allows for myosin to bend towards arm of myosin, which is the power stroke; walk-along/ratchet theory; previously cleaved ATP provided energy for power stroke; ADP & P are released to form ATP
• Energy for contraction is stored in ATP, which lasts for 1-2 seconds; phosphocreatine extends tension to 5-8 seconds; ATP regeneration is dependent on oxidative metabolism, specifically glycolysis, which lasts for approximately