Electrical Physiology Excitable Cells Electrochemical Gradient Resting Membrane

Electrical Physiology Excitable Cells Electrochemical Gradient Resting Membrane Potential Action Potential

Excitable cells… § Cells that are able to produce and respond to electrical signals § Neurons § Muscles (skeletal muscle, heart and smooth muscle) § Secretory cells (such as pituitary, insulin producing cells of the pancreas) § Why electricity? § First you need a separation of charge electrochemical gradient potential energy

Electrochemical Gradient § Electrochemical gradient § The sum of all electrical and chemical forces acting across the cell membrane= potential energy § Due to concentrations of charged particles § Negative proteins and K+ inside § Na+ and Cl- outside

Electrochemical Gradient § Electrical Gradient § Develops when there are more positive or negative charges (ions) on one side of a membrane than on the other § Charges (ions) move toward the area of opposite charge § Positive toward negative and vice versa

Electrochemical Gradient § Chemical Gradient § Develops when there are more ions of a substance in one area than in another (i. e. , more Na+ extracellularly than intracellularly) § Ions tend to move from an area of high concentration to an area of low concentration; more to less (i. e. , down their concentration gradient)

Electrochemical Gradient § Electrochemical gradient is established by Na+/ K+ ATPase pump § Na+/ K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP § Establishes separation of charge: positive outside/ relatively negative inside § sodium-potassium pump

Sodium-Potassium Exchange Pump 11 -8

Electrochemical Gradient § Electrochemical gradient is established by Na+/ K+ ATPase pump… § and maintained by the Na+/ K+ ATPase pump + Na+ and K+ leaking back in via leak channels § Na+ and K+ move depending on their electrochemical gradients (charge + conc. ) and amount of leak channels present for each ion § Which way does K+ go? § Which way does Na+ go?

Electrochemical Gradient § The cell has different amounts of leak channels for each ion that affects the permeability of each ion § K+ is the most leaky/permeable because K+ has most leak channels

Resting Membrane Potential (RMP) § Measures the inside of the cell

Resting Membrane Potential § Resting membrane potential (RMP)= membrane potential of the cell due to: Na+/ K+ ATPase pump + Na+ leaking in + K+ leaking out § RMP approaches K+ equilibrium potential (-90 mv) because there are so many K+ leak channels that a large amount of K+ is leaking out of the cell § But Na+ is also leaking in so the resting membrane potential= -70 mv (MORE POSITIVE)

Four Phases for Excitable Cells 1. Resting membrane potential (-70 mv) 2. Depolarization Threshold (-55 mv) Action Potential 3. Repolarization 4. Hyperpolarization back to resting membrane potential

http: //apbrwww 5. apsu. edu/thompsonj/Anatomy%20&%20 Physiology/2010%20 Exam%20 Reviews/Exam%203%20 Re view/CH%2011%20 Impulse%20 Transmission. htm

Resting Membrane Potential RMP is= -70 mv § Equilibrium constant of K+ is = -90 mv § Gets close to this # because more leak channels for K+ § Na+ is leaking in and K+ is being transported in § RMP never reaches -90 mv because of Na/K ATPase

Depolarization § To make the inside of a cell more positive § What happens? § Na+ voltage gated channels (Na+ vgc) open at a certain voltage

Na+ Voltage Gated Channels Na+ voltage gated channels open at a certain voltage and open for a set amount of time after the set amount of time they become inactive after a certain amount of time they become closed again and can be stimulated to open

Depolarization § Which way does Na+ go? § How would that look on a graph?

Depolarization § Na+ rushes into the cell making inside more positive § Opening Na+ channels allows + charges to enter thereby making interior less negative (-70 m. V -60 m. V)

§ Once depolarization has started it can die down OR § If the internal part of the cell membrane reaches -55 m. V then it has reached THRESHOLD and an action potential will happen § If depolarization does not reach THRESHOLD no action potential will happen § action potential animation

Action Potential § Action potentials operate on an all-ornone principle: § Stimulus has to open up enough Na+ v. g. channels to reach threshold § If threshold is reached AP § If threshold is not reached no AP § Once an AP has been generated the amplitude of the depolarization wave will be the same for all action potentials generated.

Depolarization § End of depolarization—start of repolarization § Na+ v. g. channels become inactive § (no Na+ in) § K+ v. g. channels open

http: //apbrwww 5. apsu. edu/thompsonj/Anatomy%20&%20 Physiology/2010%20 Exam%20 Reviews/Exam%203%20 Re view/CH%2011%20 Impulse%20 Transmission. htm

Repolarization § To make inside more negative—how? § Na+ v. g. channels become inactive (no Na+ in) § K+ v. g. channels open – Which way does K+ go? – How does that affect the ICF voltage? § K+ v. g. channels remain open for a certain amount of time then close

http: //apbrwww 5. apsu. edu/thompsonj/Anatomy%20&%20 Physiology/2010%20 Exam%20 Reviews/Exam%203%20 Re view/CH%2011%20 Impulse%20 Transmission. htm

Hyperpolarization § To make inside excessively negative § K+ v. g. channels close at various times during hyperpolarization § The ones that opened first close first § The ones that opened later close later § K+ still rushing out and membrane potential dips down past resting. § All K+ v. g. channels close preventing K eq § Na+/ K+ ATPase brings membrane back to resting potential

http: //apbrwww 5. apsu. edu/thompsonj/Anatomy%20&%20 Physiology/2010%20 Exam%20 Reviews/Exam%203%20 Re view/CH%2011%20 Impulse%20 Transmission. htm

undeemedstudentnotes. files. wordpress. com/2012/04/untitled-pjbkgicture 17. png

Action Potential Propagation The illustration shows continuous propagation of a nerve impulse down an unmyelinated axon.

Propagation down a nerve axon occurs in one direction because Na+ v. g. channels are inactive for a certain length of time.

Refractory Period § Two types of refractory periods in excitable cells: § Absolute refractory period Na+ v. g. channels are inactive no matter how much voltage you zap them with ensures one way propagation of charge § Relative refractory period some Na+ v. g. channels are now closed/ some still inactive can be stimulated to open with a greater charge (stronger than threshold stimulus)

Na v. g. channels become inactive Na v. g. channels inactive/ closed

http: //www. d. umn. edu/~jfitzake/Lectures/DMED/Neural. Communication/Membrane. Potentials/Refractory. Periods. html

Change in Ion Concentrations § How would an action potential be affected by: § High/low K+ outside cell § High/low Na+ outside cell § To determine this figure out what phase K+/ Na+ is responsible for and how more or less would affect the membrane potential