L8 Graded Potential & Action Potential (PHYSIOLOGY) PDF

‫بسم الله الرحمن‬ ‫الرحيم‬ PHYSIOLOGY OF FOUNDATION BLOCK L8: GRADED POTENTIAL & ACTION POTENTIAL  Objectives Step-by-step, explain how an action potential is produced Define graded potential and action potential. Describe charac...

‫بسم الله الرحمن‬ ‫الرحيم‬ PHYSIOLOGY OF FOUNDATION BLOCK L8: GRADED POTENTIAL & ACTION POTENTIAL  Objectives Step-by-step, explain how an action potential is produced Define graded potential and action potential. Describe characteristics of graded potential Describe the characteristics of action potential.  When the cell membranes are stimulated!!! When the cell membranes are stimulated, there is a change in the permeability of the membrane to sodium ions (Na+).The membrane becomes more permeable to Na+ and K+ sodium ions diffuse into the cell down a concentration gradient. The entry of Na+ disturbs the resting potential and causes the inside of the cell to become more positive relative to the outside. A rapid transient changes in the membrane potential of the excitable cells when excited. which is known as action potential or nerve impulses  Action Potential ( Stages ) THRESHOLD: The threshold potential refers to the crucial membrane potential level that must be depolarized in order to generate an action potential. Depolarization Membrane becomes less polarized(less negative) than at resting potential IONIC CAUSE Na(INFLUX ) Repolarization Membrane returns to resting potential after having been depolarized IONIC CAUSE K (EFFLUX ) Hyperpolarization Membrane becomes more polarized(membrane becomes more negative) than at resting potential IONIC CAUSES ???  Potential Changes Two kinds of potential change( electrical signals) Graded potentials Serve as short-distance signals Action potentials Serve as long-distance signals  GRADED POTENTIAL Graded Potential is local change in the membrane potential. E.g. RMP changes from -70 mv to -60 mv (a 10mv change). Graded Potential is due to Na+ entry at the small specialized region of plasma membrane. Graded Potential can be summated by giving stronger stimulus. The Stronger a triggering event is, The larger the resultant graded Potential Graded potentials die over short distances Occurs in small, specialized region of excitable cell membranes  Action Potential Brief, rapid large (100mV) changes in membrane potential during which potential actually reverses Do not decrease in strength as they travel from their site of initiation throughout remainder of cell membrane  Action Potential Action Potential follows All or None Law. It means excitable membrane either responds to a stimulus with a maximal action potential or it does not respond with an action potential at all.  Action Potential Refractory period is that period ,during which no new action potential can be initiated. Refractory Period – Two Types: 1– Absolute Refractory period: It is that period of action potential during which no new action potential can be initiated even by strong stimulus. 2– Relative Refractory : It is that period during which second action potential can be produced by very strong stimulus Comparison of Graded Potentials and Action Potentials Graded Potential Action Potential 1. Stimulus does not reach 1. Stimulus reaches threshold threshold level. level therefore causes AP. 2. Stimulus causes local change 2. Stimulus causes depolarization in membrane potential e.g. -70 to threshold level. to -60mv 3. It dies down over short 3. It is propagated. distance. 4. Can not be summated. 4. Can be summated. 5. Obeys all or none law. 5. Does not obey all or none law.  References Human physiology by Lauralee Sherwood, 9th edition Text Book Of Physiology by Guyton & Hall, 12th edition Review of Medical Physiology by Ganong, 24th edition