Mechanism of Generation and Propagation of Nerve Impulse.docx
- 1. A PROJECT WRITE-UP ON Mechanism of Generation & Propagation of Nerve Impulse For course B.Sc. (H) Biotechnology Submitted By Abhinav Baranwal 202110902120035 Supervised By Dr. Gurminder Kaur Assistant Professor Faculty of Bioscience, Institute of Bioscience and Technology, Shri Ramswaroop Memorial University, Lucknow- Deva Road, Barabanki, UP, India, 225003 SHRI RAMSWAROOP MEMORIAL UNIVERSITY
- 2. Acknowledgement I would like to take this opportunity to express my profound gratitude and deep regards to my teacher Dr. Gurminder Kaur for providing a wonderful opportunity to do this project on the topic Mechanism of Generation & Propagation of Nerve Impulse. The opportunity helped me improve my research skills and helped me to learn new things about Nervous Coordination. I would also like to show my utmost gratitude towards my friends for their exemplary guidance, monitoring and constant encouragement throughout the course of this project. At last I would like to thank Dr. Gurminder Kaur for her blessing, help and guidance given time to time that shall carry me a long way in the journey of life.
- 3. Contents Page No 1 INTRODUCTION 1 2 GENERATING NERVE IMPULSE 2 2.1 The Nerve Impulse 2 2.2 Resting Membrane Potential 3-4 3 INITIATION OF IMPULSE 5 3.1 Action Potential 5-6 3.1.1 All or None Phenomenon 6 3.2 Depolarization 6 4 CONDUCTION OF NERVE IMPULSE 7 5 REPOLARIZATION 8 6 REFRACTORY PERIOD 9 7 MYELIN AND PROPAGATION OF THE ACTION POTENTIAL 10 7.1 Special Faster Connections 12 BIBLIOGRAPHY 13
- 4. 1 | P a g e Introduction The form of the action electric-potential in nerve membranes in the nerve cell-membranes are first described, it is, in essence, a nerve impulse and can be formed by virtue of periodic and ruled changes of non-uniform distribution of the sodium and potassium ions in the inner and surface of nerve cell-membranes. The nerve impulse can be also transported along the nerve fibre membranes under action of periodic works of sodium pump and potassium pump arising from the bio- energy released from the hydrolyses reaction of ATP molecules, which is transported by Pam’s split on along the protein molecules. The experiments verified that there is not the nerve impulse, or the action electric- potential without the works of sodium pump and potassium pump, or the bioenergy. We investigated further the properties of transport of the nerve impulse along the Nerve Fibre membranes. The unit of the nervous system in animals is the nerve cell or the neuron. It is the unit of structure and function of nervous system, its main function is to accept process and transfer the nerve information, to complete the functions of the nervous system. It consists of nerve cells having different sizes and forms.
- 5. 2 | P a g e Generating Nerve Impulses Fig: Lightning - This amazing cloud-to-surface lightning occurred whena difference in electrical charge built up in a cloud relative to the ground. When the build-up of charge was great enough, a sudden discharge of electricity occurred. A nerve impulse is similar to a lightning strike. Both a nerve impulse and a lightning strike occur because of differences in electrical charge, and both result in an electric current. Neurons are excitable cells because their membranes are in a polarized state. The Nerve Impulse A nerve impulse, like a lightning strike, is an electrical phenomenon. A nerve impulse occurs because of a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules. We can also say that Nerve Cells have polarized membrane i.e., have electrical potential difference or membrane potential. This is because of variety of ion channels (pores formed by proteins) specific for particular types of ions.
- 6. 3 | P a g e Resting Membrane Potential Fig: Establishment of resting membrane potentials in nerve fibres when the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+ pump. In a resting nerve fibre, the cytoplasm just beneath its membrane is electronegative relative to the layer of extracellular fluid (ECF) just outside the membrane. The inner side of membrane is seen to possess a negative potential of about 70 mV relative to the outer side. This is called resting membrane potential. This result due to two factors: 1. The resting membrane has a poor permeability for Na+ although it has a higher permeability for K+. Therefore, K+ can cross more easily while Cl- and Na+ have more difficulty in crossing.
- 7. 4 | P a g e 2. Negatively charged protein molecule inside the neuron cannot cross the plasma membrane. Fig: Establishment of resting membrane potentials, whenthere is efflux of 3Na+ ions and influx of 2K+ ions. The differential flow of the positively charged ions and the fact that the negatively charged organic ions within the nerve fibre cannot pass out cause an increasing positive charge on the outside of the membrane and negative charge on the inside of the membrane. This makes the membrane of the resting nerve fibre polarized (i.e., its outside being positively charged with respect to the inside.) Such electrochemical gradients are maintained by the active transport of ions involving Na+ – K+ ion transmembrane pump. It pumps out 3Na+ for every 2K+ ions passed inwardly. K+ concentration is 30 times more inside neuron than outside and Na+ concentration is 10 times more in interstitial fluid as compared to inside of neuron.
- 8. 5 | P a g e Initiation of Impulse When stimulated voltage gated Na+ channel open which causes a rapid, very localized, temporary inflow of Na+ into the cell which causes development of net positive charge on the inner side of the membrane in that area. This is called depolarization. It occurs at a particular region of neuron called trigger zone. Voltage gated ion channels are clustered in the area of triggers zone. Stimulus of threshold value causes stoppage of Na+ – K+ ATPase pump. Continued passage of Na+ ions into inside of neuron creates a reverse potential of +20 millivolt to +30 millivolts, rarely to +60 millivolts. The total change occurs in spike-like fashion which is also called spike potential. It creates a potential that sets in a wave of depolarization through the nerve fibre. The membrane potential which sets in a wave of depolarization is called action potential. ACTION POTENTIAL Fig: Diagram depicts opening of Na+ channel whensignal is received and marks start of Action Potential.
- 9. 6 | P a g e An action potential, also called a nerve impulse, is an electrical charge that travels along the membrane of a neuron. It can be generated when a neuron’s membrane potential is changed by chemical signals from a nearby cell. In an action potential, the cell membrane potential changes quickly from negative to positive as sodium ions flow into the cell through ion channels, while potassium ions flow out of the cell. The change in membrane potential results in the cell becoming depolarized. All or None Phenomenon An action potential works on an all-or-nothing basis. That is, the membrane potential has to reach a certain level of depolarization, called the threshold, otherwise, an action potential will not start. This threshold potential varies but is generally about 15 millivolts (mV) more positive than the cell's resting membrane potential. If a membrane depolarization does not reach the threshold level, an action potential will not happen. Depolarization You can see in the figure that signal or stimulus is received and the first channels to open are the sodium ion channels, which allow sodium ions to enter the cell. The resulting increase in positive charge inside the cell (up to about +40 mV) starts the action potential. This is called the depolarization of the membrane. Fig: Stimulus received and Depolarization caused.
- 10. 7 | P a g e Conduction of Nerve Impulse Fig: Diagrammatic representation of impulse conduction through axon (at points A to B) In the area of depolarization, the potential difference across the membrane is small while its nearby region has large difference in membrane potential. This produces a small local current in the area. The local current becomes a stimulus and causes the voltage gated Na+ channels of next region to open and depolarized the area to produce fresh action potential. This process will continue till the impulse reaches the end of neuron. A nerve impulse is defined as an electric signal that goes through the dendrites to create an action potential or a nerve impulse. An action potential results from the movement of ions in and out of a cell and it particularly includes sodium and potassium ions. They are transferred in and out of the cell via sodium and potassium channels and sodium- potassium pumps. Conduction of nerve impulses happens because of the presence of active and electronic potentials with conductors. Internally, the transmission of signals amidst the cells is done via a synapse. The electrical synapse got its application in escape reflexes, the heart, and the retina of vertebrates. Nerve conductors consist of relatively higher membrane resistance and low axial resistance. They are significantly utilized when there is a need for fast response and timing being important. The ionic currents pass across the two cell membranes when the action potential nears the stage of such synapse.
- 11. 8 | P a g e Repolarization Fig: An action potential graph of membrane potential over time. A neuron must reach a certain threshold in order to begin the depolarization step of reaching the action potential. The figure also shows the change in potential during the repolarization and refractory periods of the axon. As the Na+ channels close, the membrane becomes extra permeable to K+ ions which leads Potassium ion channels then to open, allowing potassium ions to flow out of the cell, which ends the action potential. The inside of the membrane becomes negative again. This is called repolarization of the membrane. Both of the ion channels then close, and the sodium-potassium pump restores the resting potential of -70 mV. The action potential will move down the axon toward the synapse like a wave would move along the surface of the water. Above figure shows the change in potential of the axon membrane during an action potential. However, K+ ion channels remain open for a bit longer period so that the membrane potential becomes more negative than -70mV. This is called Hyperpolarization.
- 12. 9 | P a g e Refractory Period A new action potential cannot occur in an excitable fibre as long as the membrane is still depolarized from the receding action potential. The reason for this is that shortly after the action potential is initiated, the sodium channels (or calcium channels, or both) become inactivated and no amount of excitatory signal applied to these channels at his point will open the inactivation gates. The only condition that will allow them to reopen is for the membrane potential to return to or near the original resting membrane potential level. Then, within another small fraction of a second, the inactivation gates of the channels open and a new action potential can be initiated. So, the nerve now goes through a brief refractory period before racing resting potential. During the refractory period, another action potential cannot be generated in myelinated neurons, ion flows occur only at the nodes of Ranvier. As a result, the action potential signal "jumps" along the axon membrane from node to node rather than spreading smoothly along the membrane, as they do in axons that do not have a myelin sheath. This is due to a clustering of Na+ and K+ ion channels at the Nodes of Ranvier. Unmyelinated axons do not have nodes of Ranvier, and ion channels in these axons are spread over the entire membrane surface. The period during which a second action potential cannot be elicited, even with a strong stimulus, is called the absolute refractory period. This period for large myelinated nerve fibres is about 1/2500 second. Therefore, one can readily calculate that such a fibre can transmit a maximum of about 2500 impulses per second.
- 13. 10 | P a g e Myelin and Propagation of the Action Potential Fig: Propagation of action potentials in both directions along a conductive fibre. For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon, increasing the speed of action potential conduction. Diseases like multiple sclerosis cause degeneration of the myelin, which slows action potential conduction because axon areas are no longer insulated so the current leaks.
- 14. 11 | P a g e Fig: Action potential travel along a neuronal axon: The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes. A node of Ranvier is a natural gap in the myelin sheath along the axon. These unmyelinated spaces are about one micrometre long and contain voltage gated Na+ and K+ channels. The flow of ions through these channels, particularly the Na+ channels, regenerates the action potential over and over again along the axon. Action potential “jumps” from one node to the next in saltatory conduction. If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly; Na+ and K+ channels would have to continuously regenerate action potentials at every point along the axon. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon.
- 15. 12 | P a g e Fig: Nodes ofRanvier: Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K+ and Na+ channels. Action potentials travel down the axon by jumping from one node to the next. Special faster connections Electrical synapses that are fast are used in reflexes, the retinas of different vertebrates, and also the heart. This is because they are much faster as they do not require the slow diffusion process of neurotransmitters through the synaptic gap. Hence, electrical synapses are utilized when the fast response and coordination of timing are important. These synapses directly join the presynaptic and postsynaptic cells directly. When an action potential reaches this kind of synapse, the ionic currents cross the two cell membranes and move inside the postsynaptic cell using the pores termed as connexions. Hence, presynaptic action potential stimulates the postsynaptic cell directly.
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