Objectives Define graded potential and action potential Describe

Objectives • Define graded potential and action potential. • Describe characteristics of graded potential • Compare between the different phases of excitability during a nerve action potential( relative refractory period, absolute refractory period) • step-by-step, explain how an action potential is produced • Describe the characteristics of action potential.

Two major regulatory systems of the body: that ensure survival of the body: The Nervous System: Neural Communication Accomplished by Nerve Cells( signals) The Endocrine System: Hormonal Communication Accomplished by Hormones

Neural Communication • Nerve and muscle are excitable tissues • Can undergo rapid changes in their membrane potentials. • Can change their resting potentials into electrical signals – Electrical signals are critical to the function of the nervous system and all muscles

Neural Communication • Electrical Signals are produced due to changes in ions movement across the membrane. • 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 10 mv 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.

Current Flow During a Graded Potential

Graded Potentials • The Stronger a triggering event is, The larger the resultant graded Potential • Graded Potential spread by passive Current flow. • Graded potentials die over short distances

Graded Potential • Occurs in small, specialized region of excitable cell membranes • Magnitude of graded potential varies directly with the magnitude of the triggering event

GRADED POTENTIAL ‘Important Points’ • It is localized. • It can be summated. • Longer the stimulus – longer the duration of graded potential. • Graded Potential die down over short distance. • Example of Graded Potential: - Receptor Potential, Pace-maker Potential, end plate potential

Action Potentials • Brief, rapid, large (100 m. V) changes in membrane potential during which potential actually reverses • Involves only a small portion of the total excitable cell membrane • Do not decrease in strength as they travel from their site of initiation throughout remainder of cell membrane

Changes in Membrane Potential During an Action Potential

ACTION POTENTIAL (cont) • AP is referred as Spike potential because it appearance looks like spike. • When excitable membrane produces AP, it is said it is Firing. • Therefore Action potential, Spike, Firing all refer to same thing.

Channels & Local Potentials • The ionic basis of the action potential qmembrane permeability qion channels q. Voltage gated Na+ and K+ Channels

Action Potentials

Action Potentials Permeability Changes and Ion Fluxes During an Action Potential

Action Potentials The Na+/K+ pump gradually restores the concentration gradients disrupted by action potentials. – Sodium is pumped into the ECF – Potassium is pumped into the ICF

Action Potentials • Additional characteristics – As the action potential develops at one point in the plasma membrane, it regenerates an identical action potential at the next point in the membrane. – Therefore, it travels along the plasma membrane undiminished.

ACTION POTENTIAL (cont) • 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.

Refractory period during AP • Refractory period is that period , during which no new action potential can be initiated. • Refractory Period – Two Types: 1– Absolute Refractory period 2– Relative Refractory

Refractory Period v Absolute Refractory Period • It is that period of action potential during which no new action potential can be initiated even by strong stimulus. v Relative Refractory period • It is that period during which second action potential can be produced by very strong stimulus.

Refractory Period

The action potential (AP) An action potential is: A regenerating depolarization of membrane potential that propagates along an excitable membrane. • Action potentials: Øare all-or-none events qneed to reach threshold Øhave constant amplitude qdo not summate Øare initiated by depolarization Øinvolve changes in permeability Ørely on voltage-gated ion channels

Comparison of Graded Potentials and Action Potentials Graded Potential 1. Stimulus does not reach threshold level. 2. Stimulus causes local change in membrane potential e. g. -70 to -60 mv 3. It dies down over short distance. 4. Can be summated. 5. Does not obey all or none law. Action Potential 1. Stimulus reaches threshold level therefore causes AP. 2. Stimulus causes depolarization to threshold level. 3. It is propagated. 4. Can not be summated. 5. Obeys all or none law.

References • Human physiology by Lauralee Sherwood, seventh edition • Text book physiology by Guyton &Hall, 11 th edition • Text book of physiology by Linda. s contanzo, third edition