BIOL 1344 Lecture 6 Outline of Last Lecture I. Layers of the HeartII. Valves in the Heart and Conditions with Malfunctioning ValvesIII. Coronary Arteries and Blood Pressure IV. Cardiac Muscle CellsOutline of Current Lecture V. Cell Polarization and DepolarizationVI. Generating Action PotentialA. Sodium/potassium PumpB. mV Levels in Cardiac Muscle CellsVII. Pacemaker Cell Cycle – Generating Action Potential Current LectureCells are either polarized or depolarized based on whether they are positive or negative on the inside. Cells contain proteins and phosphate which make them negative. However, they also contain potassium, or K+, which are positive but flow out of the cell through leak channels. If a cell is negative inside it is called polarized. If it is positive it is called depolarized. The sodium/potassium pump regulates these levels and restores ionic conditions in the cell. This pump uses ATP to continually pump out 3 Na+ and pump in 2 K+ in. This is why cells require food and oxygen for energy to maintain this system of homeostasis. Cardiac muscles generate action potential using this system of active transport in cells. There is a cycle that these cells go through which has four stages and lasts 250-350 milliseconds. These cells begin at about -90 mV then there is a rapid influx of Na+ through voltage-gated Na+ channels which shoots the cell up to +20 mV. This is the peak of the cycle and where the point of action potential is. This is the stimulus required to start the muscle contraction (systole). The next stage is slow Ca+2 entry through voltage-gated Ca+2 channels. The positive level of the cells drops a little bit during this stage. The third stage is fast K+ exit (or efflux) through voltage-gated K+ channels. The mV level of the cell drops almost back to -90 at this point. The last phase is the restoring of ionic conditions through the sodium/potassium pump so Na+ and Ca+ are pumped These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.out, K+ is pumped in. The mV level of the cell is restored to -90 and the muscle relaxation (diastole) is complete. Again, this entire cycle only lasts about 350 msec and there is no overlap between each cycle in cardiac muscle. In skeletal muscle there can be summation or stimuli building on each other to make an even greater muscle contraction but this cannot take place in cardiac muscle. If there ismore than one stimuli received during one cycle, nothing will happen, the cycle will continue as usual. Pacemaker cells have roughly this same cycle with a few differences. These cells begin at about -60 mV. The first stage is slow Na+entry through the voltage-gated channels which takes the cell to -40 mV. This is the pre-potential or pacemaker potential stage. Next, there is quick Ca+2 entry which brings the cell up to +10 mV where the action potential is reached. Then the cell levels quickly drop due to K+efflux. The duration of this cycle is about .8 seconds which results in about70 cycles per minute which is why 70 bpm is the average heart rate. These pacemaker cells is what keeps the heart
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