Nerve impulse:
Electrochemical signal developed by a neuron for communication is called nerve impulse. Most of the neurons develop an impulse in resting membrane potential and action potential.
(1) Resting membrane potential (RMP): It was first studied in squids that unstimulated inactive neurolemma (Plasma membrane of neuron) maintains a charge of -65 mv. This electrical potential is called resting membrane potential. This polarity is due to unequal distribution of ions across neurolemma. It was found that there is greater concentration of sodium ions outside than inside the membrane. Similarly Potassium ions are concentrated much inside than outside the membrane. This is attributed to the activity of ATP driven Sodium Potassium Pump in neurolemma. Towards the inner surface of the membrane, negative organic ions such as proteins, organic acids, are more concentrated which make the polarity of neurolemma negative.
(2) Action Potential: It is temporary and local reversal of the polarity of neurolemma which arises when neurolemma is stimulated (e.g. electric shock, touch, sound, vibrations etc.). During the action potential, the polarity of neurolemma first changes to +40 mv (i.e. depolarization) and then restores to -65 mv again i.e. repolarization.
This action potential is extremely rapid as it takes only few milliseconds to happen.
The change in potential across membrane is due to the presence of Sodium and Potential channels in the neurolemma. Upon stimulation sodium channels open to allow the pushing in of Na+ ions inside the cell, so the membrane potential changes from negative to zero and then proceeds upon +40 m. At this stage sodium channels are automatically shut down but simultaneous potential channels open to allow their outward movement. As a consequence the + vehy charged membrane gradually restores its negative charge of -65 mv. Since sodium ions accumulate inside and potassium outside at this stage sodium potassium pump becomes operational to restore the initial accumulation of sodium outside and potassium inside.
Action potential transmission between cells:
After an action potential travels along an axon it reaches the end of a branching axon terminal called the end bulb. The synapse in the junction between axon of one neuron and he dendrite of another neuron or effector cell. The space between the end bulb and the dendrite of the next neuron is the synaptic deft. The neuron carrying the action potential toward a synapse is presynaptic neuron. It initiates a response in the receptive segment of post synaptic neuron leading away from the synapse. The presynaptic cell is always a neuron but the post synaptic cell can be a neuron muscle cell or gland cell.
Types:
(1) In an electrical synapse, nerve impulses transmit directly from neuron to neuron when positively charged ions move from one neuron to the next. These ions depolarize post synaptic membrane as two neurons were electrically coupled. An electrical synapse can rapidly transmit impulses in both directions. Electrical synapses are common in fishes and partially account for their ability to dart swiftly away from a threatening predator.
(2) In chemical synapse, two cells communicate by means of chemical agent called neurotransmitter. While the presynaptic neuron releases. A neurotransmitter changes the resting potential in the plasma membrane of post synaptic cell, creating an action potential in that cell continues the transmission of the impulse when a nerve impulse reaches an end bulb, it causes storage vesicles to fuse with the plasma membrane. The vesicles release the neurotransmitter by exocytosis into synaptic cleft acetyl chlorine or norepinorphrine.
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