Brain Mechanisms and Behaviour III (University of Nicosia) PDF

MED 104-Medical Psychology Brain Mechanisms and Behaviour III Dr Stelios Georgiades, AFBP’sS, C.Psychol, Professor of Clinical Psychology Dr. Achilleas Pavlou, Lecture...

MED 104-Medical Psychology Brain Mechanisms and Behaviour III Dr Stelios Georgiades, AFBP’sS, C.Psychol, Professor of Clinical Psychology Dr. Achilleas Pavlou, Lecturer HOW DO NEURONS COMMUNICATE? The messages that are sent through the axon from the body to the terminal button are called action potentials Synapsis  An Introduction to Action Potentials – The Preliminaries Intracellular Fluid is negatively charged because, A -= Organic Anions K+ = Potassium Ions  An Introduction to Action Potentials – The Preliminaries Extracellular Fluid is positively charged because, , Na+ = Sodium Ions Cl- = Chloride Ions An Introduction to Action Potentials – The Preliminaries A sodium-potassium transporter, situated in the cell membrane  An Introduction to Action Potentials – The Preliminaries Resting Potential:  Resting and Action Potentials This electrical charge is called the membrane potential. The term potential refers to a stored-up source of energy, and in this case energy is electrical energy.  Resting and Action Potentials Lets see what will happen, if resting potential is disturbed with the aid of an electrical stimulator which allow us to alter the membrane potential at a specific location. The stimulator will pass current through another microelectrode that we have inserted into the axon and because the inside of the axon is negative, a positive charged will be applied to the inside of membrane producing depolarization.  The Resting Potential That is, it takes away some of the electrical charge across the membrane near the electrode, reducing the membrane potential.  Resting and Action Potentials The diagram shows what happens to an axon when we artificially change the membrane potential at one point. We deliver a series of depolarizing stimuli, starting with a very weak stimulus(number 1) and gradually increasing the strength. Each stimuli briefly depolarizes the membrane potential a little more.  Finally Resting and Actionwe after Potentialspresent depolarization number 4, the membrane potential suddenly reverses itself, so that the inside becomes positive and the outside becomes negative. The membrane potential quickly returns to normal, but first it overshoots the resting potential, becoming hyperpolarized, more polarized than normal for a brief period of time. The whole process takes approximately 2 msec (1msec=1/1000 of a sec).  Resting and Action Potentials This phenomenon, a very rapid reversal of the membrane potential, is called the action potential. It constitutes the message carried by the axon from the cell body to the terminal buttons. The voltage level that triggers an action potential, which was only achieved by depolarizing shock number 4 is called the threshold of excitation.  Resting and Action Potentials Lets see in some detail what actually happens in neurons. As it was shown, both diffusion and electrostatic pressure tend to push the Na+ into the cell. However, the fact that the membrane is not very permeable to this ion and sodium potassium pump pumps Na+ out, keeps the intracellular level of Na+ low. Now imagine, what will happen if suddenly the membrane become permeable to Na+. The forces of diffusion and electrostatic pressure would cause Na+ to rush into the cell. This sudden influx of positively charged ions would drastically change the membrane potential.  Resting and Action Potentials Experiments show that indeed this mechanism is precisely what causes the action potential. A brief increase in the permeability of the membrane to Na+. (allowing these ions to rush into the cell) is immediately followed by a transient increase in the permeability of the membrane to K +.  Resting and Action Potentials A question that we can ask at this point, is what is responsible for these transient increases in permeability? As we have already seen, one type of molecule embedded in the membrane, the sodium potassium transporters, actively pump sodium ions out of the cell and pumps potassium ions into it. Another type of protein molecule provides an opening that permits ions to enter or leave the cells. These molecules provide ion channels which contain passages (pores) that can open or close. When an ion channel is open a particular ion can flow through the pore and thus can enter or leave the cell. Resting and Action Potentials  Each of these millions of sodium channels admit up to 100 millions ions per second when it opens. Therefore, on the basis of the above we can conclude that the permeability of the membrane to a particular ion at any given moment is determined by the number of ion channels that are open.  Resting Description of and Action Potentials movement of ions through the membrane during the action potential. Resting and Action Potentials  Resting and Action Potentials Understanding resting membrane and action potential allows us to study axon conduction using the giant squid axon. Attach an electrical stimulator to one end of the axon and place recording electrodes connected to oscilloscopes at various distances. Apply a depolarizing stimulus to trigger an action potential, recording it at each electrode along the axon. The action potential travels consistently along the axon, maintaining its strength as it moves. Resting and Action Potentials  Resting and Action Potentials The experiment demonstrates the all-or-none law of axonal conduction. This law means that an action potential either occurs fully or does not occur at all. Once triggered, it is transmitted down the axon to its end. The action potential remains the same size throughout its journey—without growing or diminishing. When the action potential reaches a branch point in the axon, it splits but does not lose strength. Action potentials move in one direction because they always start at the end of the axon that is attached to the soma (cell body), ensuring one-way traffic along the axon. Saltatory Conduction – Due to myelination of axons action potentials as described occurs only at parts ofand the unmyelinated Resting the Action Potentials axon (Nodes of Ranvier) Depolarizing Myelin sheath stimulus Just like a fighter jet refueling midair, Decremental conduction Action potential is the action potential regenerates at the under myelin sheath regenerated at Nodes of Nodes of Ranvier, allowing for fast and Ranvier efficient transmission along the axon! Advantages of Saltatory Conduction 1. Economic Resting and Action in terms of Potentials Energy (save the energy required to get rid of sodium entering the axon since it enters only at the Nodes of Ranvier) 2. Speed. Conduction of an action potential is faster in myelinated axon because the transmission between the Nodes is very fast. Increased speed enables fast reaction times. Depolarizing Myelin sheath stimulus Decremental conduction Action potential is under myelin sheath regenerated at Nodes of Ranvier