HUMAN PHYSIOLOGYBME2001Week#3Canan Bağcı, PhDAssistant Profesor1
HUMAN PHYSIOLOGY BME2001Week#3–Outline•Action Potential•General Overview of The Nervous System•Muscle Physiology2
Action Potential•Rapid change in the membrane potential due to rapid changes inpermeability to ions (Na+and K+for the neuron).5
Initiation of Action Potential6
•If the stimulus that reach the axon hillock isgreat enough, the neuron depolarizes by about15 mV and reaches a trigger point,threshold.•Atthethreshold,anactionpotentialisgenerated.•Weak stimuli do not produce an action potential.•Action potential (AP) is anall-or-none event.•AP always have the same amplitude and sameduration•Threshold for this neuron is -55mVInterActive Physiology 10-System Suite. (Cummings Publishing Company), 20067Threshold
8DepolarizationInterActive Physiology 10-System Suite. (Cummings Publishing Company), 2006At the threshold,depolarization opens morevoltage-gated Na+channels.Thispositive feedback loopproduces the rising phase ofthe action potential.This causes more Na+to flow into thecell, which causes the cell to depolarizefurther and opens more voltage gatedNa+channels.
The rising phase of the action potential ends when thepositive feedback loop is interrupted by:•Inactivation of the voltage-gated Na+channels•Opening of the voltage gated K+channelsoVoltage-gated Na+channels are time-sensitive, theyclose after for a certain period of time.oVoltage-gatedK+channelsrespondslowlytodepolarization. They begin to open as the membranedepolarizes, but they respond very slowly.9Beginning of RepolarizationInterActive Physiology 10-System Suite. (Cummings Publishing Company), 2006
10Characteristics of voltage-gated Na+and K+channelsNa+channelsK+channels
•The slow voltage-gated K+channels remain open after the cell membrane has repolarized. K+ionscontinue to move out of the cell, causing the membrane potential to become more negative than theresting membrane potential.•By the end of the hyperpolarization all the K+channels are closed.11RepolarizationInterActive Physiology 10-System Suite. (Cummings Publishing Company), 2006
All-or-none principle•The amplitude of an action potential is independent of the amount ofcurrent that produced it.•In other words, larger currents do not create larger action potentials.Therefore, action potentials are said to be all-or- none signals, sinceeither they occur fully or they do not occur at all.•Greater intensity of stimulation does not produce a stronger signalbut can produce a higher frequency of firing.13
•The action potential is followed by a brief period of diminished excitability, orrefractoriness, which can be divided into two phases:1.The absolute refractory:during this period it is impossible to excite the cell no matter howgreat a stimulating current is applied.