بنام یزدان

Membrane Potential

At rest  * A membrane potential at rest is defined as the potential at which the flow of ions out (mostly K + ) is equal to the flow of ions in (mostly Na + )  * To maintain this in a steady state, the cell must utilize pumps

agents most important in membrane potential ions electrical Charge membrane permeability ion concentration in both side membrane ion channels and pomp Na-K K+ “leak” channels Na K K+ “leak” channels

ACTION POTENTIAL 1. RESTING PHASE - Voltage gated Na+ channels are in resting state (CLOSED Activation gate and OPEN inactivation gate i.e. NO Na+ (CLOSED Activation gate and OPEN inactivation gate i.e. NO Na+ passing through) passing through) - Voltage gated K+ channels are closed 2. DEPOLARISATION -Membrane depolarizes ( e.g. from an incoming stimulus) to threshold (~ -55mV) and opens Na+ activation gate Na+ inflow (down its concentration gradient) -Na+ inflow further depolarizes membrane until polarity reverses. 3. REPOLARISATION BEGINS -At reverse membrane polarity (more depolarization), voltage gated K+ channels opens K+ outflow (down its concentration gradient) -Na+ inactivation gates close 4. REPOLARISATION CONTINUES -K+ outflow restores RMP. -Na+ inactivation gates open and K+ gates closes. NOTE : At this stage (4) RMP is restored, but not the electrochemical gradient.

Calculating the equilibrium potential of an ion across a membrane Calculating the equilibrium potential of an ion across a membrane We use the Nernst equation to determine this value for each ion (X) : E x = (RT/zF) log e {X o /X i } –R=Gas constantT=°Kelvin –z=valence of ion (X)F=Faraday’s constant Reduces to: E x =+_ 61 (log 10 {X i /X o })

Examples of normal concentration values and equilibrium potentials Ion outside inside ratio(o/i) E x (mV) Na mM 12 mM K + 4 mM 155 mM Cl mM 4.2 mM Ca mM10 -7 mM 15, (the net effect is a membrane potential of -90mV - how?)

Membrane potential. Membrane potential.  *Membrane potential is the ‘sum’ of all of the different ions and their concentration differences across the membrane  *Generally, we focus on the dominant ions for purposes of simplifying this calculation –Na +, K +, Cl - (sometimes calcium is included)

Action Potentials  Phases –Depolarization  Inside plasma membrane becomes less negative –Repolarization  Return of resting membrane potential  All-or-none principle –Like camera flash system  Propagate –Spread from one location to another  Frequency –Number of action potential produced per unit of time

ALL-OR-NONE PRINCIPLE -i.e. Means that each time an AP arises, it always have a constant and maximum strength and will therefore will be conducted along the axon. ANALOGY – A long row of dominoes, once you push the first domino, all the dominos along the row will fall. So it either falls or remains standing.

Calculating the membrane potential Goldman equation  Vm=+_ 61 log (X)  X = P K [K] o +P Na [Na] o +P Cl [Cl] i  P K [K] i +P Na [Na] i +P Cl [Cl] o P permeability P permeability

Frequency of Action Potentials Figure 8-13: Coding for stimulus intensity

Generator Potentials  In response to stimulus, sensory nerve endings produce a local graded change in membrane potential.  Potential changes are called receptor or generator potential. –Analogous to EPSPs. Phasic response: Generator potential increases with increased stimulus, then as stimulus continues, generator potential size diminishes. Tonic response: Generator potential proportional to intensity of stimulus. Figure 10-2

cardiac action potential : Fast slow

Figure 44.2 dendritddndrite receives information cell body contains nucleus & organelles axon transmits nerve impulse axon terminal transmits to next neuron synapse junction between two neurons Direction of nerve impulse dendrite Cell body Axon Axon terminal synapse

Two types Absolute When Na+ channels close, at peak of AP, they do not reopen for a time Relative Membrane hyperpolarized Some Na+ channels still refractory Refractory Period

ACTION POTENTIAL PROPERTIES OF ACTION POTENTIAL; PROPERTIES OF ACTION POTENTIAL; 1. REFRACTORY PERIODS (Resting periods) 2. CONDUCTION PATTERN

PROPERTIES OF ACTION POTENTIAL REFRACTORY PERIOD – is the period of time that an excitable tissue can not generate another AP REFRACTORY PERIOD – is the period of time that an excitable tissue can not generate another AP 2 types of refractory period; 2 types of refractory period; 1. ABSOLUTE REFRACTORY PERIOD 2. RELATIVE REFRACTORY PERIOD

REFRACTORY PERIODS 1. ABSOLUTE REFRACTORY PERIOD Refers to the time period during which a 2 nd AP can not be initiated even with a very strong stimulus REASON; Na+ inactivation gates still closed – they must first return to resting state before they can get activated.

REFRACTORY PERIOD 2. RELATIVE REFRACTORY PERIOD -Refers to the period during which a second AP can be initiated, but only by a stimulus larger than threshold strength. REASON: -Voltage gated k+ channel are still open -The inactivated Na+ gates have returned to their resting state – so they are ready to be opened again as long as the stimulus is strong enough (i.e. larger than threshold ~55mV)

Regulating the AP Figure 8-12: Refractory periods

برای انسان های بزرگ بن بست وجود ندارد ، چون بر این باورند که : یا راهی خواهند یافت ، یا راهی خواهند ساخت. بسیاری در پیچ وخم یک راه مانده اند و همواره از خویشتن می پرسند : ما چرا ناتوان از ادامه راهیم. بدانها باید گفت می دانی در کجا مانده ای؟ همانجای که خود را پرمایه دانسته ای.

SALTATORY CONDUCTION i.e. Conduction pattern where current leaps from node to node as each nodal area depolarizes to threshold. nodal area depolarizes to threshold.REASON: -When a nerve impulse propagates along a myelinated fiber the current is carried by the flow of ions through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node to the next. surrounding the myelin sheath and through the cytosol from one node to the next. -But current flows across the membrane only at the nodes.

Synapse site of communication between two cells formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell

NERVE CELL PARTS OF A NERVE CELL; 1. CELL BODY; - Contains the nucleus of the nerve cell. - Control center of the nerve cell 2. AXON; - longest cytoplasmic extensionof nerve - Conducts impulses away from nerve 3. DENDRITES; - Short cytoplasmic extensions coming off the nerve cell body. the nerve cell body. - Receives incoming impulses 4. MYELIN SHEATH - Neuroglia around nerve - Provides metabolic, structural support to nerve fiber to nerve fiber