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Mechanism of Neuron Action (Depolarization, Repolarization, Hyperpolarization)

Neuron is the functional unit of nervous system. Neurons are specialized to produce signals. These signals can be communicated over short or long distances. They can be sent from one part of the body to another.
Changing the resting Membrane Potential into the Action Potential (Nerve Impulse)
The transmission of an action potential takes place by wave of depolarization and repolarization.
1.      Depolarization
The loss in membrane polarity is called depolarization. Nerve impulse is conducted by change in electric potential. A stimulus of minimum intensity that can initiate an impulse is called a threshold stimulus. Threshold stimulus is applied on the membrane. The permeability to Na+ ions increases at that point. Na+ ions flow inside the membrane. The potential changes from -70mV toward 0. It causes depolarization. The depolarization reaches certain level. Special Na+ channels (voltage gated) are sensitive to changes in membrane potential. Thus, they quickly open. So, more Na+ ions rush inside of the neuron.

2.      Repolarization
The rebuilding of positive charge outside is called repolarization. Shortly after the Na+ gates close. Now voltage gate of K+ channels are opened. K ions rapidly diffuse outward. It again develops positive charge outside the cell. Therefore, the membrane becomes repolarized.
Similar cycle is repeated in an adjacent region of the membrane. The wave of depolarization moves down the axon. Thus the action potential travels forward.
3.      Hyperpolarization
The state of neuron in which more negative charge is developed inside the membrane is called Hyperpolarization. There is an interval of time after each action potential. Large number of K+ ions rushed out during this time interval. Thus more negative ion developed inside. (more negative than -70 mV). Therefore, the membrane becomes hyperpolarized. More action potential cannot occur during this interval of time. This brief period is called the refractory period. Resting potential is restored during this period. Afterward, the neuron is repolarized and ready to transmit another impulse.
A minimum stimulus is necessary to initiate an action potential. But an increase in stimulus intensity does not increase the strength of the action potential.
All-or- none law
The principle that an axon will fire at full power or not at all is called all-or- none law.
Factor affecting the speed of nerve impulse
       I.            Diameter of axons
Increase of the diameter of axon increase the speed of nerve impulse. Axons with a large diameter transmit impulse faster than smaller ones. Large diameter axons are common among many invertebrates e.g., crayfishes, earthworms. The squids have largest diameter. The diameter of its axon is over 1 mm. the axons have a conduction velocity greater than 36 m/s. the giant squid axons can easily escape from predators. A single action potential cause maximum contraction of the mantle muscle in squids. Mantle contraction rapidly expels water like a jet. Thus , the squid moves away from the predator. Most vertebrates axons have a diameter less than 10 pm. They extend from the brain. They pass down the spinal cord and activate skeletal muscles.
    II.            Addition of Myelin
Addition of sheath also increase the speed of conduction of a nerve impulse. Myelin is an excellent insulator. Thus, it stops the movement of ions across it. Therefore, it increases the rate of conduction of nerve impulse. Action potentials are generated only at the neurofibril nodes. Thus action potential jumps from one node to the next node. Therefore, conduction along myelinated fibers is known as salutatory conduction. It takes less time for conduction of nerve. Myelin sheath allows rapid conduction in small neurons. Thus, evolution of nervous system in animals took place. It does not occupy large spaces.
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