phases, depolarization, repolarisation and hypo polarization. Depolarization – Reversal of Polarity When a nerve fibre is stimulated, sodium voltage-gate opens and makes the axolemma permeable to Na + ions; meanwhile the potassium voltage gate closes. As a result, the rate of flow of Na + ions into the axoplasm exceeds the rate of flow of K + ions to the outside fluid [ECF]. Therefore, the axolemma becomes positively charged inside and negatively charged outside.
This reversal of electrical charge is called Depolarization . During depolarization, when enough Na + ions enter the cell, the action potential reaches a certain level, called threshold potential [-55mV]. The particular stimulus which is able to bring the membrane potential to threshold is called threshold stimulus . The action potential occurs in response to a threshold stimulus but does not occur at subthreshold stimuli .
This is called all or none principle. Due to the rapid influx of Na + ions, the membrane potential Nodes of Ranvier Depolarized membrane Membrane at resting potential Axon Myelin sheath + + + + + + + + + + + + – – – – – – – – – – – – Figure . Conduction of nerve impulse Figure . Graph showing Action potential in Neuron - - - Depolarization Hyperpolarization Reapolarization Resting potential Resting potential Threshold potential Action potential Refractory period Time (milliseconds) Membrane Potential (mV) XI Std Biology-Zoology Chapter- fuses with it.
In the neurilemma, the vesicles release their neurotransmitters into the synaptic cleft by exocytosis. The released neurotransmitters bind to their specific receptors on the post-synaptic membrane, responding to chemical signals. The entry of the ions can generate a new potential in the post-synaptic neuron, which may be either excitatory or inhibitory . Excitatory post-synaptic potential causes depolarization whereas inhibitory post-synaptic potential causes hyperpolarization of post-synaptic membrane (Figure .
). Presynaptic neuron Nerve impulse Synaptic end bulb Synaptic clef Neurotransmitter receptor Ligand-gated channel closed Postsynaptic neuron Neurotransmitter Synaptic vesicles Cytoplasm Voltage-gated Ca + channel Ca +