Conduction of Nerve Impulse - Action Potential and Resting Potential
The conduction of nerve impulse is one of the sub groups of biomedical engineering. Nowadays, nerve impulse and its conduction play a vital role in bio medical engineering. In nerve conduction, both the electricity and the chemical molecules are together involved as electro chemical process. Electrical signals are carried by nerve cells to transfer signals from one nerve cell to another nerve cell. During this time between the inside of axon and its surroundings, small electrical difference occurs as like a tiny battery. If nerve is activated at that time there is a sudden change in voltage across the wall of the axon. This occurs due to the movement of ions in and out of the neuron. It makes to induce a wave of electrical activity that passes from the body of the cell along the length of the axon to the synapse.
It is one aspect of bioelectricity. The electrical effects are created by biological systems of a human body like in brain. The electricity is not produced by electrons flowing the way they do through a household electrical wire. Instead, the brain’s electricity is caused by the movements of electrically charged molecules through the neurons’ membranes. The speed of movement varies enormously in different types of neurons. The fastest travel speed is at about 250 mph. They result from a difference in electrical charge across the plasma membrane of a neuron.
The concept of conduction of the nerve impulse can be easily understood by its two sub divisions, they are resting potential and action potential. The time in which the neurons are not conducting any nerve impulse is said to be at rest. The time in which neurons are conducting the nerve impulse is said to be active.
The resting potential is the resting state of the neuron, during which time the neuron has an overall negative charge. During the resting state of the cells, the sodium-potassium chemical combinations retain a difference in the charge across the cell membrane. It makes the inner side of the neuron to become negatively charged as compared to the extracellular fluid which is surrounding the neuron. It is due to the availability of more positively charged ions outside the cell as compared to the inside of the cell. This kind of difference in electrical charge is known as resting potential. The resting potential in neurons is approximately -70 milliVolts (mV). In that the negative sign indicates the presence of negative charge inside the cell relative to the outer side of the cell.
The following points are the reasons for the overall negative charge of the cell:
• Some of the cells have potassium-selective ion channel proteins that remain open all the time. The potassium ions move down the concentration particles (passively) by both the potassium channels and outer side of the cell, which results in a build-up of excess positive charge outside of the cell. There are a number of large, inversely charged molecules, such as proteins, inside the cells that are available.
• The combination of sodium-potassium leads to the removal of Na+ ions from the cell by active transport. Now, the net negative charge inside the cell is due to the higher concentration of sodium ions outer side the cell than inside the cell.
Here, one more concept now begins which is known as Action potential, which is also called as nerve impulse when the electrical charges that travels besides to the membrane of a neuron. At the time of an action potential, the cell membrane energy changes quickly from negative to positive as like sodium ions flow into the cell through ion channels. At the mean time, potassium ions flow out of the cell. This process will start once when the neuron receives a chemical signal from another cell. The signal becomes the reason for the gates in sodium ion channels to open allowing the positive sodium ions to flow back into the cell. Finally the inside of the cell becomes positively charged on comparing with the outer side of the cell. This reversal of charge ripples down the axon very rapidly as an electric current. The potential involves with that current and is called as action potential. The action potential moves towards the nerve cell membrane from one of the node to another node, instead of spreading smoothly towards the entire membrane. This increases the speed at which it travels inside the nervous system of the body. The sudden conduction of impulses is the wide requirement for allowing the nervous system to moderate short duration and very quick communication and control between entire body systems. Axons (neurons) are specialized so that at one of the end there is a burst structure called as dendrite. At the dendrite, the axon can process chemical signals from other nerves and endocrine hormones. If the signals received at the dendrite final end of the neuron are of a required strength, and properly timed, they are changed into action potentials that sweep down the neural cell body (axon) from the end of the dendrite to the other end of the neuron. The pre-synaptic portion of the axon ends at the next synapse (the extra cellular gap between neurons) in the neural pathway. The arrival of the action potential at the presynaptic terminus causes the release of ions and chemicals (neurotransmitters) that travel across the synapse, the gap or intercellular space between neurons, to act as the stimulus to create another action potential in the next neuron, and thus perpetuate the neural impulse. The, action potential is created when the neuron receives a chemical signal from another cell. It can also be created by applying external threshold voltage called as stimulus. This value of the threshold potential continuously varies, but is generally about 15 millivolts (mV) more positive than the cell's resting membrane potential.
If membrane depolarization does not reach the minimum level, an action potential will not occur. The action potential will occur through two channels. One of the channels is sodium ion channel and the other channel is potassium ion channel. In that the first channels to open are the Na+ channels, which allow sodium ions to enter the cell. This causes an increase in the level of the positive charge inside the cell (up to about +40 mV) which starts the action potential. K+ channels then open, allowing potassium ions to flow out of the cell which ends the action potential.
Action potentials can move along the nerves at a speed rate of 0.1-100 m/s. This means that nerve impulses can get from one part of a body to another part of a body in a very few milliseconds, which allows for very fast responses to stimuli. The speed is affected by two factors. These factors are as follows:
• Nerve diameter - The larger the diameter, the faster the speed. So, marine invertebrates, which live at temperatures nearby 0°C, have developed thick nerves to speed up their responses. This is the reason behind squids having very large size nerves.
• Temperature - The faster the speed, the higher the temperature. So, homoeothermic (warm-blooded) animals have very faster responses than the poikilothermic (cold-blooded) ones.
The vast development in bio-medicals will progress even further in the coming decades by the evaluation of conduction of nerve impulse.