biophysics 2

• Membrane potential


  Voltage difference between the inside of the cell and the outside of the cell. it is about - 60mV
• Membrane potential


  • Fundamental feature of living cells
  • Used as a source of potential energy to perform work in the form of membrane transport for a special group of cells called excitable cells
  • Can be changed for the purpose of signaling
• Distribution of charges


  Physical features of a membrane potential in a cell: lipid bilayer (plasma membrane) separating intracellular and extracellular compartment, on either sides of the plasma membrane there are dissolved ions, the concentrations of the ions (specifically the amount of charge dissolved in each compartment) are equal to each other, equal number of positive and negative charges for both the outside and the inside fluids
• Ionic membrane potential


  • 2 requirements to produce a membrane potential in cells: the sodium potassium pump brings in 2 potassium ions and extrudes 3 sodium ions per cycle, using ATP, creating concentration gradient, Potassium channel allows potassium to flow from inside to outside the cell down its electrochemical gradient
• Leaky potassium channels


  1. most cells have an ubiquitous expression of these leaked potassium channels, 2- Anions are unable to flow across the membrane because they lack ion channels, gaining positive charge on the outside and accumulation of unbalanced negative charge on the inside because anion's flow is restricted
• Resting membrane potential


  The excess positive charge on the outside develops an electrostatic interaction across the lipid bilayer with the excess negative charge on the inside, the presence of these charges along the lipid bilayer develops an electrical field that is the membrane potential, a situation where we have accumulated sufficient amount of charges to produce a membrane potential which is strong enough to oppose potassium tendency to move down its concentration gradient, concentration gradient and voltage gradient in the form of our membrane potential are equal and opposite, at this point potassium movement is at equilibrium
• Membrane potential genesis


  For Most mammalian cells we have a high concentration of potassium inside and low concentration of potassium outside, positive and negative charges electrostatically interact across the membrane producing our membrane potential
• Nernst equation


  Equation that describes the relationship between concentration and membrane potential
• Cardiac muscle cells


  • Striated (contains actin and myosin) and branching, adjacent cells are connected by intercalated discs, which contain: Desmosomes - mechanically hold cells together as heart contracts, Gap junction - electrically connect cells
• Functional syncytium


  Because regions of the heart are connected electrically by gap junctions, they form a "functional syncytium", when one cell undergoes an action potential, the AP spreads to all connecting cells, they all contract together, Atria form one group/syncytium, Ventricles form another group/syncytium, Heart muscle contraction is ALL-OR-NONE (no graded contractions)
• Intrinsic conduction system


  • The heart's electrical activity is independent of the Nervous System, Action Potentials in the heart are generated by the Intrinsic Conduction System, a set of electrical pacemaker cells that coordinates and synchronizes heart activity, Sequence of Excitation: SA Node -> AV Node -> Bundle of His -> Right and left bundle branches -> Purkinje fibers (sub endocardial conducting network)
• SA Node


  Located in the upper right atrium, fastest, sets the pace of the heart 70 – 80 action potentials per minute at rest, Activity spreads to BOTH atria and to the AV Node, it is the heart's pacemaker, and its characteristic rhythm, called sinus rhythm, determines heart rate
• AV Node


  Located in the lower right atrial wall, near the inter ventricular septum, 2nd fastest, only sets the pace if there is damage to the SA Node, 40-60 action potentials per minute, Activity pauses first, then spreads to bundle of His, AV Nodal Delay = 100 ms, due to fibrous tissue, slow conduction, small diameter of Fibers
• Bundle of His and Purkinje fibers


  The bundle of His is located within the interventricular septum, and has a right and a left branches, the purkinje fibers travel up the outer walls of the ventricles, Slowest, not life sustaining, 20-40 action potentials per minute, Activity spreads to ventricles, Although the atria and ventricles are adjacent to each other, they are not connected by gap junctions, The AV bundle is the only electrical connection between A and V, The fibrous cardiac skeleton is non conducting and insulates the rest of the AV junction
• Ectopic focus


  Premature ventricular contraction, Defects in the intrinsic conduction system can cause irregular heart rhythms, or arrhythmias, they may also cause uncoordinated atrial and ventricular contractions, or even fibrillation, a condition of rapid and irregular contractions in which control of heart rhythm by the SA node is disrupted by rapid activity in other heart regions, Rhythmicity is high in S-A node > A-V node > His bundle & Purkinje fibers
• Intrinsic Conduction of Heart Contractions

  P wave - SA node to atria, atrial depolarization, QRS complex - AV node to purkinje fibers, ventricular depolarization, T-wave - ventricular repolarization
• Electrocardiogram (ECG/EKG)


  Measures heart depolarization and repolarization
• AV bundle


  The only electrical connection between A and V
• Fibrous cardiac skeleton


  • Non conducting
  • Insulates the rest of the AV junction
• Ectopic focus


  Premature ventricular contraction
• Defects in the intrinsic conduction system


  Can cause irregular heart rhythms, or arrhythmias
• Defects in the intrinsic conduction system

  Can cause uncoordinated atrial and ventricular contractions, or even fibrillation
• Fibrillation


  Rapid and irregular contractions in which control of heart rhythm by the SA node is disrupted by rapid activity in other heart regions
• Rhythmicity


  • High in S-A node
  • High in A-V node
  • High in His bundle & Purkinje fibers
• Intrinsic Conduction of Heart Contractions

  • P
  • QRS
  • AV delay 100 ms
• Heart electrical activity


  Could be traced by ECG waveform
• Electrocardiogram (ECG)


  Measures heart depolarization and repolarization
• ECG waveform


  • P-wave (SA node to atria, atrial depolarization)
  • QRS complex (AV node to purkinje fibers, ventricular depolarization)
  • T-wave (ventricular repolarization)
• ECG


  A graphic record of heart activity
• ECG leads


  • 3 bipolar leads
  • 9 unipolar leads
• The 12 leads provide a comprehensive picture of the heart's electrical activity
• An ECG tracing is a composite of all the action potentials generated by nodal and contractile cells at a given time, not a tracing of a single action potential
• Repolarization


  Slower than depolarization
• The wave representing atrial repolarization is normally obscured by the large QRS complex
• Pacemaker cells

  Generate action potentials in the intrinsic conduction system, they are self-generating, self-excitatory
• Pacemaker cells

  Have a lowest membrane potential of -60 mV, but do not stay at -60 mV, they have no rest
• Pacemaker action potential

  1. Membrane potential drifts back up to -40 mV

  2. Threshold is reached spontaneously at -40 mV

  3. Peak depolarization at +10 mV

  4. Repolarization back to -60 mV, and the cycle restarts
• Funny Na+ channels (If)

  Open at hyperpolarization (-60 mV), Na+ enters cell, depolarizes
• Transient-type (T-type) Ca2+ channels

  Open briefly at low depolarization (-50 mV), Ca2+ enters cell
• Long-lasting (L-type) Ca2+ channels

  Open at threshold (-40 mV), Ca2+ enters cell, leading to full depolarization