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Neuronal conduction.pptx
- 1. NEURONAL CONDUCTION Dr. M. Ramya Maheswari Assistant Professor and Head Department of Psychology Ethiraj College For Women
- 2. HOW DO NEURONS SEND AND RECEIVE SIGNALS ?
- 3. THE NEURAL COMMUNICATION AN OVERVIEW Neural communication actually involves both electrical and chemicalor electrochemicalcommunication. The signal usually starts in the neurons dendrites, and then travels down the length of the cell until it reaches the terminal buttons at the very tips of the neurons axon. A single neuron normally has many dendrites. The number of dendrites being stimulated may vary from one moment to the next. Thus, there may be more or less electrical activity traveling down into the soma at any given time. If there is too little electrical activity, nothing happens. If, however, the electrical activity reaches a certain critical amount, or threshold, the axon hillock activates the axon. If the axon hillocks threshold is reached and the axon is activated, an electrical signal travels down the axons length until it reaches the very end of the neuron. This activation of the axon is referred to as the neuron firing. From the terminal buttons , the neuron will pass the signal on to another cell, such as another neuron or muscle cell.
- 4. THE NEURON
- 5. THE NEURAL IMPULSE CYCLE The nerve impulse is the reversal in the charge of the cell membrane, which spreads along the cell membrane forming an electrical current. Resting Potential Tells us about what happens when the neuron is at rest. Action Potential - Occurs when a neuron sends information down an axon i) Depolarisation ii) Repolarisation
- 6. THE MEMBRANE POTENTIAL The key to understanding how neurons work is the membrane potential. The membrane potential is the difference in electrical charge between the inside and the outside of a cell. Recording resting membrane potential. when the tip of the intracellular electrode is inserted into a neuron, a steady potential of about 70 millivolts (mV) is recorded. This steady membrane potential is called the neurons resting potential, and the neuron is said to be polarized.( electrical difference across the membrane)
- 7. RESTING POTENTIAL Recording the membrane potential: difference in electrical charge between inside and outside of cell Inside of the neuron is negative with respect to the outside. Resting membrane potential is about 70mV. Membrane is polarized (carries a charge)
- 8. TERMS TO KNOW Concentration Gradient: A concentration gradient occurs when the concentration of particles is higher in one area than another. In passive transport, particles will diffuse down a concentration gradient, from areas of higher concentration to areas of lower concentration, until they are evenly spaced. Electrical Gradient: In biological solutions, electrical gradient refers to the electrical potential that acts on an ion to drive the movement of the ion in one or another direction
- 9. IONIC BASIS FOR RESTING POTENTIAL Why are resting neurons polarised? Salts in neural tissue separate into positively and negatively charged particles called ions. The resting potential results from the fact that the ratio of negative to positive charges is greater inside the neuron than outside. Resting potential results from (1) the concentration of Na+ is higher outside, (2) the concentration of Cl is higher outside, (3) the concentration of K+ is higher inside, and (4) various negatively charged protein ions are trapped inside
- 10. FOUR FACTORS THAT UNDERLIE RESTING POTENTIAL Factors contributing to even distribution of ions. Random motion ions in a solution are normally under random motion. particles tend to move down their concentration gradient. Electrostatic pressure like repels like, opposites attract. It disperses accumulation of any positive or negative charges in any area. Factors contributing to uneven distribution of ions. Selective permeability to certain ions pass through the neural membrane at specialised pores called ion channels. When neurons are at rest, the membrane is: a) totally resistant to the passage of protein ions, b) extremely resistant to the passage of Na+ ions, c) moderately resistant to the passage of K+ ions, d) and only slightly resistant to the passage of Cl ions
- 11. SODIUM POTASSIUM PUMP Sodium Potassium Pump- energy consuming process involved in the maintenance of the resting potential.
- 12. SODIUM POTTASSIUM PUMP Sodium ions tend to be driven in as a result of both concentration gradient and the negative internal resting potential of 70 mv. About 120 mv of electrostatic pressure forces sodium ions into the cell. Potassium ions tend to move out of the neuron because of their higher concentration inside the cells, although this tendency is partially offset by the internal negative potential . However the sodium potassium pump pumps out sodium ions as rapidly as they pass in and pumps in potassium ions as they pass out . For every three sodium ions in it pushes into the cell it two ottassium ions it send out
- 13. GENERATION OF POST SYNAPTIC POTENTIALS . How are neural signals created? When neurons fire, they release chemicals called neurotransmitters These chemicals diffuse across the synaptic cleft and bind with the post synaptic receptors in a lock and key fashion.
- 14. WHAT ARE THE EFFECTS? When neurotransmitter molecules bind to post synaptic receptors, they typically have two effects. a) They may depolarise the receptive membrane (decrease the resting potential, from - 70 to -67 mv) b) They may hyperpolarise ( increase the resting membrane potential from -70 to -72 mv). Post synaptic depolarizations are called excitatory post synaptic potentials (EPSP).They increase the likelihood of neuronal firing. Post synaptic hyper - polarizations are called inhibitory post synaptic potential (IPSP). They decrease the likelihood of neuronal firing. Both EPSP and IPSP are graded potentials : ie., the amplitudes of E PSPs and IPSPs are proportional to the intensity of the stimulus
- 15. CONDUCTION OF POST SYNAPTIC POTENTIALS EPSPs and IPSPs travel passively from their sites of generation at synapse, usually on the dendrites or cell body in much the same way that electrical signals travel through the cable. Transmission of post synaptic potentials has two characteristics. 1) It is rapid, almost instantaneous irrespective of whether they are brief or enduring. 2) They are decremental, ie., they decrease in amplitude as they travel through the neuron.
- 16. INTEGRATION OF POST SYNAPTIC POTENTIALS AND GENERATION OF ACTION POTENTIALS A neuron's action potentials are triggered at the axon hillock when neuron is depolarized to the point that the membrane potential at the hillock reaches about -65 mV. This is the threshold of excitation for many neuron. Action potential is a massive momentary reversal of the membrane potential from about -70 to about +50 mV. This last for 1 millisecond. Unlike EPSPs and IPSPs, Action potentials are not graded. They follow the all or none law. Most neurons receive hundreds of synaptic contacts. Whether or not a neuron fires is determined by the adding together (integration) of what goes on at many presynaptic neuron synapses
- 17. INTEGRATION OF POST SYNAPTIC POTENTIALS AND GENERATION OF ACTION POTENTIALS There are two kinds of neural integration: Spatial summation (EPSPs + EPSPs; IPSPs + IPSPs; EPSPs + IPSPs) - It shows how local EPSPs that are produced simultaneously on different parts of the receptive membrane sum to form a greater EPSP . Temporal Summation: (EPSPs + EPSPs; IPSPs + IPSPs) It shows how post synaptic potentials produced in rapid succession at the same synapse sum to form a greater signal
- 20. CONDUCTION OF ACTION POTENTIALS - IONIC BASIS Conduction of action potential takes place through the action of the voltage activated ion channels the ion channels that open and close in response to the changes in the level of the membrane potential.
- 21. CONDUCTION OF ACTION POTENTIALS - IONIC BASIS i) Voltage activated gates present in the axon membrane become more permeable to sodium ions. ii) Sudden influx of NA+ ions iii)Reversed Polarity- -70 mV to +50 mV iv)Opening of voltage activated potassium channels. Potassium driven out of cell because of the high internal concentration and at the peak of the action potential due to positive internal charge. v)Sodium channels close marking the end of the rising phase of the action potential and beginning of repolarisation by continuous outflow of potassium ions to the extent that the membrane stays in a state of hyperpolarisation for a brief period of time.
- 22. REFRACTORY PERIODS A brief period of about 1 or 2 milliseconds after the initiation of an action potential, during which it is impossible to elicit a second one , This is called absolute refractory periods. This is followed by relative refractory periods - the period during which it is possible to fire the neuron again, but only by applying higher than normal levels of stimulation . After which the amount of stimulation necessary to fire a neuron returns to the baseline.
- 24. THANK YOU