Desrvibe how an action potential propagates along a myelinated and an unmyelinat

Question

Desrvibe how an action potential propagates along a myelinated and an unmyelinated axon, including how the potential is spread along the membrane. what TWO benefits do myelinated axon is similar to an electrical wire and one way in which an axon is different than a wire. How are these two processes similar and how are they different? why does multiple sclerosis (MS) prevent transmission in myelinated axons (that is why can't these axons just propagate the action potential in the sam way that an unmyelinated axon does)?

Explanation / Answer

Generation and propagation of action potential - Myelinated axons contain a myelin sheath. In non-myelinated nerve fibres action potential travels along the whole length of the neuron but in myelinated nerve fibres, action potentials are generated only at the node of Ranvier where the Na+ voltage gated channels are located. Action potential passes as a wave of depolarisation from dendrite to cell body, then passes along the axon if the fibre is non-myelinated to reach the axon terminal.

Let's see, how the nerve impulse is generated in a nerve fibre.

At resting stage, the inner part of the nerve fibre is negatively charged because of the presence of potassium ions and outer part is positively charged because of the presence of sodium ions. The resting memebrane potential is -70 mV. When a nerve fibre is stimulated by mechanical, chemical, thermal or electrical stimulus of adequate strength ( equal or higher value than threshold value), the sodium gates open and sodium ions rapidly diffuse inside. As a result of this movement, the inner side of axon becomes positively charged with respect to the outer side. This reversal of polarity is called depolarisation. Due to influx of sodium, a very rapid change occurs in the axon membrane potential from -70mV. to a positive peak of +40mV. This potential difference is called action potantial. At the peak of action potential the sodium gates close and the voltage gated potassium channels open. Now potassium moves from inside to outside. But so much potassium flows that the membrane becomes hyperpolarized i.e the membrane potential goes down beyond -70mV. the voltage sensitive potassium gates now close and the sodium -potassium pump brings the resting potential by transporting sodium out and potassium in.

The above described process is localized and occurs across the membrane where a stimulus or impulse is present. When one part of the membrane is depolarized, the adjacent area is still polarized. So the impulse travels to the adjacent (in non-myelinated neuron) part due to mutual attraction of positive charge and negative charge. At this time, the negative charge present on the outer surface of a depolarized area attracts the positive charge from the outer surface of the neighbouring polarized area and the positive charge present on the inner side of the depolarized area is attracted by negative charge on the inner surface of the neighbouring area . As a result of which, the polarized area become depolarized. In myelinated nerve fibre, same thing happens but here the impulse jumps from one node to another node where myelin sheath is absent and where the intracellular fluid is in contact with extracellular fluid. The movement of impulse from node to node is called saltatory conduction.  

Similarity of myelinated axon and an wire - Conduction of impulse through a myelinated axon is faster and more energy efficient as that of an elrctric wire.

Difference between an axon and a wire- A wire leads a current in one spot and in one direction only. A neuron on the other hand can stimulate other neurons which send signals to different parts of the body. An axon has its own regulatory mechanism. It can transport ions, generate potentials and again can restore the normal condition. An electric wire can only conduct eectricity.

Difference between the two processes - Electrons move through a wire. Sodium ions move through a nerve fibre. Electricity is conducted through the wire at a speed of 150,000 km. per second while the nerve impulse travels at a maximum speed of about 100 metres per second.

Multiple Sclerosis is supposed to be an autoimmune disorder in which the immune system attacks the myelin sheath and destroys it. Loss of myelin sheath cause hardening or sclerosis of nervous tissue in spinal cord, brain and optic nerve. Lack of myelin sheath and formation of multiple scars causes slowing down of impulse conduction resulting in weakness, numbness, pain and vision loss.

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