Communication Within a Neuron PDF

PSYC 2210 Biological Psychology COMMUNICATION WITHIN A NEURON Prof. Dr. Hakan Çetinkaya Yaşar University – Department of Psychology  COMMUNICATION WITHIN A NEURON Neuron Structure: Neurons have a distinctive structure consisting of a cell body (soma), dendrites, and an axon....

PSYC 2210 Biological Psychology COMMUNICATION WITHIN A NEURON Prof. Dr. Hakan Çetinkaya Yaşar University – Department of Psychology  COMMUNICATION WITHIN A NEURON Neuron Structure: Neurons have a distinctive structure consisting of a cell body (soma), dendrites, and an axon. Dendrites receive signals from other neurons or sensory receptors. The axon carries signals away from the cell body.  COMMUNICATION WITHIN A NEURON Resting Membrane Potential: Neurons have a resting membrane potential, an electrical charge across the cell membrane when the neuron is not actively transmitting signals. This resting potential is primarily maintained by the selective permeability of the cell membrane to ions like potassium (K+) and sodium (Na+).  COMMUNICATION WITHIN A NEURON Action Potential: The basic unit of communication within a neuron is the action potential, an electrical impulse that travels along the axon. When a neuron receives a strong enough stimulus, it depolarizes, causing voltage-gated sodium channels to open, allowing sodium ions to enter the cell, leading to a rapid change in membrane potential.  COMMUNICATION WITHIN A NEURON Propagation of Action Potential: The action potential travels down the axon in a process called saltatory conduction, jumping from one node of Ranvier to the next, which significantly speeds up the transmission of the signal.  COMMUNICATION WITHIN A NEURON A Withdrawal Reflex The figure shows a simple example of a useful function of the nervous system. The painful stimulus causes the hand to pull away from the hot iron. 6  COMMUNICATION WITHIN A NEURON The Role of Inhibition Inhibitory signals arising from the brain can prevent the withdrawal reflex from causing the person to drop the casserole. 7  COMMUNICATION WITHIN A NEURON Squid or "Kalamar" is not only a delicious culinary delight but also holds scientific significance. 8  COMMUNICATION WITHIN A NEURON Measuring Electrical Potential of Axons ▪ The giant axon of a squid is about 0.5mm in diameter (hundreds of times larger than the largest mammalian axon). ▪ This axon controls an emergency response (sudden contraction in danger). 9  COMMUNICATION WITHIN A NEURON Terms ▪ Resting potential The resting potential refers to the electrical potential difference across the membrane of a neuron when it is not actively transmitting signals. ▪ In a resting giant axon membrane, such as the giant axon of a squid, the resting potential is typically around -70 millivolts (mV). This negative potential is primarily due to the uneven distribution of ions across the neuronal membrane. ▪ The key ions involved in establishing the resting potential are sodium (Na+), potassium (K+), and chloride (Cl-) ions. 10  COMMUNICATION WITHIN A NEURON Measuring Electrical Potential of Axons: Membrane Potential ▪ Microelectrode: a very fine electrode, generally used to record activity of individual neurons. ▪ Since glass will not conduct electricity, the glass microelectrode is filled with a liquid (a solution of potassium chloride) that conducts electricity. ▪ Inside of axon is negatively charged with respect to outside. The difference is 70mV, so the inside of the membrane is -70mV. ▪ This electrical charge is called the membrane potential. ▪ This electrical charge across the membrane is called Measuring Electrical Charge resting potential. The figure shows (a) a voltmeter detecting the charge across a membrane of an axon and (b) a light bulb detecting the charge across the terminals of a11battery.  COMMUNICATION WITHIN A NEURON Studying the Axon The figure shows the means by which an axon can be stimulated while its membrane potential is being recorded. 12  COMMUNICATION WITHIN A NEURON Terms ▪ Depolarization Reduction (toward zero) of the membrane potential of a cell from its normal resting potential. ▪ Hyperpolarization An increase in the membrane potential of a cell, relative to the normal resting potential. ▪ Action Potential The brief electrical impulse that provides the basis for conduction of information along an axon. ▪ Threshold of excitation The value of the membrane potential that must be reached to produce an action potential. 13  COMMUNICATION WITHIN A NEURON The Action Potential ▪ They increased the strength of depolarizing stimulus. ▪ Each stimulus briefly depolarizes the membrane potential a little more. ▪ At 4, the membrane potential suddeenly reverses itself. Thus, inside becomes positive and the outside becomes negative. ▪ It quickly turns to normal, but after it hyperpolarized for a short time. ▪ The whole process takes about 2 msec. ▪ This phenomenon, a very rapid reversal of the membrane potential is called the action potential. ▪ The voltage level that triggers an action potential is called the treshold of excitation. 14  COMMUNICATION WITHIN A NEURON Terms ▪ Diffusion: Movement of molecules from regions of high concentration to regions of low concentration. ▪ Electrolyte: An aqueous solution of a material that ionizes—namely, a soluble acid, base, or salt. ▪ Ion: A charged molecule. Cations are positively charged, and anions are negatively charged. ▪ Electrostatic pressure: The attractive force between atomic particles charged with opposite signs or the repulsive force between atomic particles charged with the same sign. ▪ Intracellular fluid: The fluid contained within cells. ▪ Extracellular fluid: Body fluids located outside of cells. in the intracellular fluid. 15  COMMUNICATION WITHIN A NEURON What Causes the Action Potential ▪ Why there is membrane potential This electrical charge is the result of a balance between two opposing forces: 1. The force of diffusion 2. The force of electrostatic Pressure 1. The force of diffusion: Movement of molecules from regions of high concentration to regions of low concentration. Molecules are constantly in motion and their rate of movement is proportional to the temperature. Only at absolute zero [0 K (kelvin) = -273.15 C] do molecules cease their random movement. 2. The force of Electrostatic Pressure: When some substances are dissolved in water, they split into two parts, each with an opposing electrical charge (ions). This aqueous solution of a material that ionizes are called electrolyte. The ions are two types: Cations (with a positive charge) and anions (with a negative charge). Therefore, what the electrostatic pressure is the attractive force between atomic particles charged with opposite signs, or the repulsive force between atomic particles charged with the same sign. 16  COMMUNICATION WITHIN A NEURON Ions in the Extracellular and Intracellular Fluid ▪ The fluid within cells (intracellular fluid) and the fluid surrounding them (extracellular fluid) contain different ions. ▪ Thus, the forces of diffusion and electrostatic pressure contributed by these ions give rise to the membrane potential. There are several important ions: 1. Organic anions (negatively charged proteins) (A-), found only in the intracellular fluid. 2. Chloride ions (Cl-), found in the intracellular and extracellular fluids, but predominantly in the extracellular fluid. 3. Sodium ions (Na+), found in the intracellular and extracellular fluids, but predominantly in the extracellular fluid. 4. Potassium ions (K+), found in the intracellular and extracellular fluids, but predominantly in the intracellular 17 fluid.  COMMUNICATION WITHIN A NEURON The relative concentration of some important ions inside and outside the neuron and the forces acting on them. 18  Action Potential A sodium-potassium transporter, situated in the cell membrane. 19  Action Potential Ion channels: When ion channels are open, ions can pass through them, entering or leaving the cell. ▪ The forces of both diffusion and electrostatic pressure tend to push Na+ into the cell. However, the membrane is not very permeable to this ion, and sodium-potassium transporters continuously pump out Na+, keeping the intracellular level of Na+ low. ▪ What if the membrane suddenly becomes permeable to Na+? ▪ This sudden influx of positively charged ions would drastically change the membrane potential. ▪ Then, Na+ ions rush into the cell (by FD&EP), and K+ ions rush out of the cell (by FD&NoEP). ▪ Action potential: The brief electrical impulse that provides the basis for conduction of information along an axon. 20  Action Potential Ion channels: When ion channels are open, ions can pass through them, entering or leaving the cell. ▪ The membrane consists of a double layer of lipid molecules in which are floating many different kinds of protein molecules. ▪ One class of protein molecules provides a way for ions to enter or leave the cells. ▪ These molecules constitute ion channels. ▪ Each open sodium channel can admit 100 million ions per second. ▪ There are at least 75 different types of ion channels. 21 Action Potential The Movements of Ions 1. Na+ rushes in (propelled by the forces of diffusion and electrostatic pressure. Because these channels are voltage-dependent ion channels, they are opened by changes in the membrane potential. Inflow of positively charged sodium ions produces a rapid change in teh membrane potential, from -70 to +40 mV. 2. The membrane contains voltage-dependent potassium channels, but these channels are less sensitive than voltage-dependent sodium channels. That is, they require a greater level of depolarization before they begin to open. Thus, they begin to open later than the sodium channels. 3. At the peak (in 1 msec), sodium channels cannot open again until the membrane once more reaches the resting potential. At this time, then, no more Na+ can enter the cell. 22 Action Potential The Movements of Ions 4. The voltage-dependent potassium channels in the membrane are open, letting K+ ions move freely through the membrane. At this time, inside of the axon is now positively charged, so K+is driven out of the cell by diffusion and electrostatic pressure. This outflow of cations causes the membrane potential to return toward its normal value. As it does so, the potassium channels begin to close again. 5. Potassium channels close, and no more potassium leaves the cell. 6. The accumulation of of K+ ions outside the membrane causes a temporary hyperpolarization. These extra ions soon diffuse away, and membrane potential returns to -70mV. Eventually sodium- potassium transporters remove the Na+ that leaked in and retrieve the K+ that leaked out. 23