GENERATION AND PROPAGATION OF ACTION POTENTIAL

Transcription

1 GENERATION AND PROPAGATION OF ACTION POTENTIAL LEARNING OBJECTIVES: Define some basic electrophysiological terms Discuss post synaptic potentials (PSP s) Discuss of Resting Membrane Potential (RMP) Define Ionic equilibrium Potential Define Action Potential Key properties of Action potentials Phases of Action Potentials Characteristics of Action Potentials Conduction of Action Potentials BASIC ELECTROPHYSIOLOGICAL TERMS Excitability: The ability of the cell to generate the action potential Excitable cells: Cells that generate action potential during excitation. in excitable cells (muscle, nerve, secretery cells), the action potential is the marker of excitation. Some scholars even suggest that in excitable cells, action potential is identical to the excitation. BASIC ELECTROPHYSIOLOGICAL TERMS Stimulus: a sudden change of the (internal or external) environmental condition of the cell. includes physical and chemical stimulus.

2 The electrical stimulus is often used for the physiological research. Threshold (intensity): the lowest or minimal intensity of stimulus to elicit an action potential (Three factors of the stimulation: intensity, duration, rate of intensity change) Types of stimulus: Threshold stimulus: The stimulus with the intensity equal to threshold Subthreshold stimulus: The stimulus with the intensity weaker than the threshold Suprathreshold stimulus: The stimulus with the intensity greater than the threshold. POSTSYNAPTIC POTENTIALS EPSP o Excitatory o Depolarization o Na+ influx o AP more likely IPSP o Inhibitory o Hyperpolarization o K+ efflux o AP less likely EXCITATORY POSTSYNAPTIC POTENTIALS (EPSPS) Opening of ion channels which leads to depolarization makes an action potential more likely, hence excitatory PSPs : EPSPs.

3 o Inside of post-synaptic cell becomes less negative. o Na + channels (NB remember the action potential) o Ca 2+. (Also activates structural intracellular changes -> learning.) INHIBITORY POSTSYNAPTIC POTENTIALS (IPSPS) Opening of ion channels which leads to hyperpolarization makes an action potential less likely, hence inhibitory PSPs : IPSPs. o Inside of post-synaptic cell becomes more negative. o K + (NB remember termination of the action potential) o Cl - (if already depolarized) INTEGRATION OF INFORMATION PSPs are small. An individual EPSP will not produce enough depolarization to trigger an action potential. IPSPs will counteract the effect of EPSPs at the same neuron. Summation means the effect of many coincident IPSPs and EPSPs at one neuron. If there is sufficient depolarization at the axon hillock, an action potential will be triggered. RESTING MEMBRANE POTENTIAL (RMP) A potential difference across the cell membrane at the rest stage or when the cell is not stimulated.

4 Property: It is constant or stable It is negative inside relative to the outside Resting potentials are different in different cells. GENERATION OF RMP (-70MV) Plasma membrane Selective permeability, permeable to K, not Na Unequal distribution of ions across membrane o Due to open potassium channels and closed sodium and chloride channels Action of ion pumps 3Na/2K ATPase IONIC EQUILIBRIUM POTENTIAL Membrane Potential (potential difference across the plasma membrane) at which the net flow of an ion type = zero The number of ions moving into the cell = the number of ions moving out of the cell for a particular species of ion ACTION POTENTIAL Action Potential in the Nervous System o Conveys information over distances o Cytosol has negative charge relative to extracellular space o Neural code - frequency and pattern o Action potential Spike Nerve impulse Discharge

5 Changes in Ion Permeability allows inward Na flux and triggers an increased outward K flux through voltage gated ion channels Causes transient change in Membrane Potential The change in ion permeability is triggered by transient depolarization of the membrane KEY PROPERTIES OF ACTION POTENTIAL o Threshold o Rising phase o Overshoot o Falling phase o Undershoot o Absolute refractory period o Relative refractory period PHASES OF ACTION POTENTIAL

6 MECHANISM OF PHASES Rising phase (depolarization): o The membrane voltage change triggers the opening of voltage-gated sodium channels o The influx of sodium depolarizes the neuron o (up to V=+65mV) Falling phase (hyper-polarization): o The sodium channels become inactive at around 1ms after their opening o Meanwhile, the potassium channels open the efflux of potassium ions overshoots the resting potential (down to V=-80mV). Returning phase o The hyper-polarization causes the close of potassium and sodium channels o Ion bump may be called to retain the concentration gradients of o potassium and sodium ions

7 ACTION POTENTIAL CHARACTERISTICS Voltage-gated channels All or none Slow

8 Non-decremental Self Propagated regenerated ~ ACTION POTENTIAL CONDUCTION Initiation of synaptic transmission Propagation of the action potential o Down axon to the axon terminal Orthodromic: Action potential travels in one direction Antidromic (experimental): Backward propagation o Typical conduction velocity: 10 m/sec o Length of action potential: 2 msec o Passive conduction will ensure that adjacent membrane depolarizes, so the action potential travels down the axon. But transmission by continuous action potentials is relatively slow and energy-consuming (Na + /K + pump). A faster, more efficient mechanism has evolved: saltatory conduction.

9 Myelination provides saltatory conduction. Factors Influencing Conduction Velocity o Spread of action potential along membrane Dependent upon axon structure o Path of the positive charge Inside of the axon (faster) Across the axonal membrane (slower) o Axonal excitability Axonal diameter (bigger = faster) Number of voltage-gated channels Factors Influencing Conduction Velocity o Myelin: Facilitates current flow o Layers of myelin sheath Myelinating cells Schwann cells in the PNS Oligodendroglia in CNS SALTATORY CONDUCTION Nodes of Ranvier o action potentials

10 Myelinated o like electricity through wire o decremental but triggers AP at next node Safety factor - trigger AP across 5 nodes o.2-2 mm apart larger neurons farther apart ~ REFRACTORY PERIOD

11 There are two types of refractory period: Absolute Refractory Period Na+ channels are inactivated and no matter what stimulus is applied they will not re-open to allow Na+ in & depolarise the membrane to the threshold of an action potential. PROPERTIES OF THE ACTION POTENTIAL The Generation of an Action Potential o All-or-none : Cross threshold value for action potential o Chain reaction e.g., Puncture foot, stretch membrane of nerve fibers Opens Na + -permeable channels Na + influx depolarized membrane reaches threshold action potential MEMBRANE CURRENTS AND CONDUCTANCES Depolarization (influx of Na + ) and repolarization (efflux of K + ) Membrane Currents and Conductances o Current The net movement of K + across membrane o Potassium channel number Proportional to electrical conductances o Membrane potassium current Flow and driving force MEMBRANE CURRENTS AND CONDUCTANCES THE ACTION POTENTIAL The Ins and Outs of an Action Potential

12 o Rising phase: Inward sodium current o Falling phase: Outward potassium current THE ACTION POTENTIAL Key properties of the action potential (Cont d)

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