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Excitable Tissues, Resting Membrane Potential & Action.pptx

Excitable tissues are capable of generating and transmitting electrochemical impulses along cell membranes. The resting membrane potential in most neurons is around -70mV due to uneven distribution of ions like potassium and sodium across the cell membrane. When a threshold stimulus is reached, voltage-gated ion channels allow rapid sodium influx and potassium efflux, causing a brief reversal of the potential known as an action potential. This propagates the electrochemical signal along the membrane.

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Excitable Tissues, Resting Membrane Potential & Action Potential DR ANWAR H. SIDDIQUI PHYSIOLOGY, JNMC, AMU
What is an excitable tissue? • Tissues which are capable of generation and transmission of electrochemical impulses along the membrane. • An ability of specialized cells to respond to certain stimuli by producing electrical signals.
Excitable tissues in the human body  Nerves  Muscles Cardiac Muscle Smooth Muscle Skeletal Muscle
Understand excitable tissue  Suppose you have a dead frog. (Yes, that's kind of gross, but let's just imagine it for a second.)  What would happen if you applied an electrical stimulus to the nerve in the frog's leg? Creepily enough, the dead leg would kick!  The Italian scientist Luigi Galvani discovered this fun fact back in the 1700s, somewhat by accident during a frog dissection.  Today, we know that the frog's leg kicks because neurons (nerve cells) carry information via electrical signals.
Resting Membrane Potential • A potential difference exists across all cell membranes • This is called – Resting Membrane Potential (RMP)
Resting membrane potential explained  Imagine taking two electrodes and placing one on the outside and the other on the inside of the plasma membrane of a living cell.  If you did this, you would measure an electrical potential difference, or voltage, between the electrodes. This electrical potential difference is called the membrane potential.
Resting Membrane potential  For a cell’s membrane potential, the reference point is the outside of the cell. In most resting neurons, the potential difference across the membrane is about 70 to 90 mV (1 mV is 1/1000 of avolt) with the inside of the cell more negative than the outside.  That is, neurons have a resting membrane potential (or simply, resting potential) of about - 70 to -90 mV.
Resting Membrane Potential  Because there is a potential difference across the cell membrane, the membrane is said to be polarized.  If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.  If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.
Where does the resting membrane potential come from?  The resting membrane potential is determined by the  uneven distribution of ions (charged particles) between the inside and the outside of the cell, and  by the different permeability of the membrane to different types of ions.
Distribution of ions inside and outside the cell K+ and organic anions (such as those found in proteins and amino acids) are present at higher concentrations inside the cell than outside. In contrast, Na+ plus, Cl− usually present at higher concentrations outside the cell.
How ions move across the cell membrane  Because they are charged, ions can't pass directly through the hydrophobic ("water- fearing") lipid regions of the membrane. Instead, they have to use specialized channel proteins that provide a hydrophilic ("water- loving") tunnel across the membrane.  Some channels, known as leak channels, are open in resting neurons. Others are closed in resting neurons and only open in response to a signal.
• Ion channels that mainly allow K+ to pass are called potassium channels, and ion channels that mainly allow Na+ to pass are called sodium channels. • The resting membrane potential depends mainly on movement of K+ through potassium leak channels. Howevr both Na+ and K+ contribute to resting potential.
• Potassium concentration intracellular is more • Membrane is freely permeable to K+ • There is an efflux of K+ Flow of Potassium K+ K+ K+ KK + + K+ K + K + K+ K+
Entry of positive ions in to the extracellular fluid creates positivity outside and negativity inside Flow of Potassium K+ K+ K+ KK + + K+ K+ K+ K+ K+
• Outside positivity resists efflux of K+ • (since K+ is a positive ion) • At a certain voltage an equilibrium is reached and K+ efflux stops Flow of Potassium K+ K+ K+ KK + + K+ K+ K+ K+ K+
Nernst potential (Equilibr ium potential) The potential across the cell membrane at which the net diffusion of ions across the cell membrane due to concentration gradient stops. • Nernst equation determines this potential Where R= Universal Gas constt T = Absolute Temp, z = ion Valence F = Faraday, an electrical Const
Nernst potential for K+ ions Nernst Equation: EMF = (RT/zF) x log (Cin / Cout) RT/zF = -61 Conc of K+ ions inside the cell=140 mEq/l Conc of K+ ions outside the cell= 4 mEq/l EMF(mv)= - 61 log 140/4 = -61 log 35 = - 94mv
Nernst potential for Na+ ions Nernst Equation: EMF = (RT/zF) x log (Cin / Cout) RT/zF = -61 Conc of K+ ions inside the cell=14 mEq/l Conc of K+ ions outside the cell= 142 mEq/l EMF(mv)= - 61 log 14/142 = +61 mv
The Goldman Equation • When the membrane is permeable to several ions the equilibrium potential that develops depends on – Polarity of each ion – Membrane permeability – Ionic concentration • This is calculated using Goldman Equation (or GHK Equation)
 In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium.  Goldman equation gives a potential inside the membrane of −86 millivolts, which is near the potassium potential
Contribution of Na/K PUMP:- - This is a powerful electrogenic pump on the cell membrane. - It Pump 3 Na to outside & 2 K to inside, causing → loss of +ve ions ,loss of + ve charge from inside , negativity about - 4mV inside -4mv
 Nernst potential for Potassium -94mv  Nernst potential for Sodium +61mv  Putting these values in Gold man equation, gives a value of - 86mv Which is nearer to K+ diffusing potential  Na- K pump provides - 4mv  i.e adding -86 and -4mv= -90mv  Resting membrane potential in nerves is -90 mv
Resting Membrane Potential in Various Excitable Tissues  Large Myelinated Nerve fibers  Skeletal Muscle Fibers = - 90mv  Ventricular Muscle fibers  Smooth Muscle fiber & } = -55 to -60 mv  Self Excitatory Tissues
Action potential  Definition:  Abrupt / sudden Change (reversal) in resting membrane potential in response to a threshold stimulus.  Stimulus: “Any Change in the environment”  TYPES: a. Electrical b. Mechanical c. Chemical
Action Potential (A.P.) • When an impulse is generated – Inside becomes positive – Causes depolarisation – Nerve impulses are transmitted as AP
RMP -90 +30 Hyperpolarisation 0 Threshold potential 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Time (millisecond)
Inside of the membrane is • Negative – During RMP • Positive – When an AP is generated -90 +30
• Initially membrane is slowly depolarised • Until the threshold level is reached – (This may be caused by the stimulus) Threshold level -90 +30
• Then a sudden change in polarisation causes sharp upstroke (depolarisation) which goes beyond the zero level up to +30 mV -90 +30
• Then a sudden decrease in polarisation causes initial sharp down stroke (repolarisation) -90 +30
• When reaching the Resting level rate slows down • Can go beyond the resting level – hyperpolarisation -90 +30
• Spike potential – Sharp upstroke and downstroke • Time duration of AP – 1 msec -90 +30
 Ion channels called volted gated channels responsible for action potential  Two types of channel:  Na+ channel  K+ channel Physiological basis of AP
 N+ channel  K+ channel
• When the threshold level is reached – Voltage-gated Na+ channels open up – Since Na conc outside is more than the inside – Na influx will occur – Positive ion coming inside increases the positivity of the membrane potential and causes depolarisation – When it reaches +30, Na+ channels closes – Then Voltage-gated K+ channels open up – K+ efflux occurs – Positive ion leaving the inside causes more negativity inside the membrane – Repolarisation occurs
• Since Na+ has come in and K+ has gone out • Membrane has become negative • But ionic distribution has become unequal • Na+/K+ pump restores Na+ and K+ conc slowly By pumping 3 Na+ ions outward and 2 K+ ions inward