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BIO271H Lecture 1.doc

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University of Toronto St. George

BIO271H – Lecture 1: Structure and Function of Neurons; Action Potentials - This is basically an introduction to fundamental neuroscience. It is not too scary but you have to pay attention. We will also talk about the respiratory system (both anatomical and the physiological aspect of it). - The nervous system is divided into two parts: the central nervous system (brain and the spinal cord) and the peripheral nervous system (spinal nerves that innervate the muscles and these are motor neurons (relay nerve impulses away from the CNS); it is composed of 12 cranial nerves that leave straight from the brain and not through the spinal cord). - We will not go in too much detail of the anatomy. But we do need to know roughly the definition of the nervous system. We will mostly focus on how each of the neurons is functioning. Organization of the Nervous System - Made up of neurons and glia (general name for non-neuronal cells in the nervous system) - Neurons can be further subdivided: - Sensory neurons: relay nerve impulses toward the CNS - Interneurons: perform computational processing - Motor neurons: efferent fibres; provide output Neurons - Vary in structure and properties - Use same basic mechanisms to send signals - In the diagram are 3 types of neurons. The signal reception area is where in a sensory neuron there is input from the outside world. It could be light coming from the outside or sound waves. For motor neurons, it could be signal receptors from other neurons. The signal integration area will be discussed later on. The signal conduction area: sometimes the cell body is within a signal conduction area and sometimes it is within the signal reception area (compare motor vs. sensory neurons). Often times signals can arrive to the cell body as well. The signal transmission area: the neuron transmits signals to the effector (the muscle) or to the next neuron. Neural Zones - There are four functional zones. - Signal reception: dendrites and the cell body (soma) are involved. The incoming signal is received and converted to change in membrane potential (difference in the number of ions from the outside of the neuron to the inside of the neuron). - Signal integration: the axon hillock is involved. This is the “trigger zone”, where electrical signals are generated and then propagated away from the cell body along the axon. So the strong signal is converted to an action potential (AP) because there is a change in membrane potential. - Signal conduction: the axon is involved (some of them are wrapped in myelin sheath). The action potentials travel down the axon. - Signal transmission: occurs in axon terminals, which release neurotransmitters from the axon. Neural zone – summary: - Soma = cell, body; responsible for metabolic maintenance of cell - Nerve processes = dendrites and axon - Dendrites: extend from the soma; are branched; receive signals from other neurons and carry them to the soma. Extensive dendritic branching is often referred to as dendritic arbor or tree. Dendrites receive and integrate the signal (sometimes the soma also receives the signal directly). - Axons: conduct signals away from the soma; can be very long (metres); carry information with high fidelity and without loss of signal strength; and terminate into axon terminals, allowing the neuron to communicate with other tissues. - Action potentials (APs) are then generated in the spike-initiation zone (located near the axon hillock). AP = spike = nerve impulse - APs travel down the axon to the terminals where they cause neurotransmitter release. Membrane Potential - Like all cells, neurons have a resting membrane potential, which is negative at rest (before any activity or signal arrives at the cell). - What determines the membrane potential? - Differences in ion concentrations (concentration of particular ions going in vs. concentrations of the ions going out) - Selective permeability of the membrane The Membrane Separates Charges - The membrane acts as a capacitor – it can maintain the separation of charges. Ions pass to the other side of the membrane through ion channels, and are transported or pumped across by other proteins. - The cytosol is more negative than the extracellular fluid. So there are more positive ions outside. Usually the normal value of membrane potential at rest is -70 mV. This is only in the vicinity of the membrane. - Ion channels allow passive transport of ions across the membrane. They exhibit ion selectivity. Some ion channels are open most of the time, while others are gated by stimuli (ligand-gated channels, voltage-gated channels, etc.) Electrochemical Equilibrium - Let’s say we have potassium in one side of the tank and chloride on the other side. Let’s say we put an ion-selective membrane in the middle (only potassium can go through this membrane). We will only get a steady state (we will not have an active flow of potassium). Steady state means there are few ions going to the other side and some are coming to this side; we don’t have a net flow of ions. So we won’t have any electrical potential. - Let’s say there is a 10-times difference in the concentration of potassium in relation to chloride. In this case, we will have a flow of potassium from the high concentration to the low concentration. After a certain amount of potassium ions has flown to the other side, we will have an electrical potential across the membrane. Each potassium ion carries a charge with it, and we will end up with more potassium ions on the right side. - At a certain point, the potassium ions will repel other potassium ions from coming to the right side because there are too many positive charges. We will reach a new steady state. Few ions are coming to the right and few are going to the left. It will take time until we reach this electrochemical potential and then there will be no net flow of potassium. We can calculate what potential we arrive at. Nernst Equation - Calculates the potential required to balance the concentration gradient EK= RT ln [K]o zF [K]i R =thermodynamic gas constant T = absolute temperature Z =valence of the ion F = Faraday’s constant At 20 degrees, RT/F = 25.26 mV - So we can take the intracellular [19 mM] and extracellular [3 mM] and we get E asK -47 mV. Membrane Potential - Factors contributing to membrane potential: - Distribution of ions across the membrane - Relative permeability of the ions - Charges of the ions - Goldman equation allows us to determine the membrane potential by taking into account all of the ions that permeate through that membrane. - We can talk about 3 different ions and their relative permeability. Who is more permeable? Sodium, potassium, or chloride - The relative permeability: potassium will govern the membrane potential. Sodium and chloride absolutely have an effect. The distribution of sodium is the exact opposite of potassium. We have high intracellular potassium (19 mM) with low extracellular concentration (3 mM). Sodium has 120 mM extracellular [] and 30 mM intracellular []. Chloride is a bit more complicated. We still have high extracellular [], but keep in mind that chloride is negatively charged. - Membrane potential is usually close to the equilibrium potential of potassium. - All other ions (calcium, magnesium, etc.) are ignored in this simplified form of the equation because their permeabilities are low at resting membrane potential. Gated Ion Channels - Neurons depolarize or hyperpolarize by selectively altering permeability. - Gated ion channels open or close in response to a stimulus. An example of a stimulus: neurotransmitter. Changes in membrane potential - Depolarization is caused by the opening of sodium or calcium channels. Sodium flows from the outside to the inside. The intracellular voltage is brought close to the extracellular voltage. - Hyperpolarization is caused by potassium channels. When we open a potassium channel, potassium flows from the inside to the outside. This is when the membrane potential is increased from the resting value (-60 mV). Gated Ion Channels - Channels only allow specific ions to pass through the membrane. The ions move down their electrochemical gradient. Only a small number of ions move across (concentration gradient usually remains the same). - As permeability to a specific ion increases, membrane potential will approach that ion’s equilibrium potential (Nernst equation). – See diagram below – Changes in Membrane Potential - Using the Goldman equation, we can see the relative permeability of sodium is higher than it was now before. Signals in the dendrites and the cell body - The incoming signal is usually a neurotransmitter, a small molecule released by another axon (from a neighbouring neuron). - Membrane-bound receptors bind to the neurotransmitter. - The receptors transduce the chemical signal to an electrical signal by changing ion permeability of the membrane. - Change in ion permeability causes change in membrane potential (graded potential). The graded potential will trigger the action potentials. They are called “graded” because the magnitude of the potential change can vary. - Sodium will flow in and depolarization of the cell body will occur. Graded Potentials - Vary in magnitude depending on the strength of the stimulus - More neurotransmitter  more ion channels open  larger magnitude of graded potential - Then depolarization occurs as sodium (Na ) or calcium (Ca ) channels are opened. - Hyperpolarization occurs when potassium (K ) and chloride (Cl ) channels are opened. Stimulus Strength and Graded Potentials - In the first example, we have closed ion channels. When we have a low concentration of the neurotransmitter, only one of the five receptors has the neurotransmitter bound to it and the ion channel is open. Similarly, if we have a high concentration of the neurotransmitters, we will have many ion channels open and thus ions would be able to pass through the membrane and result in a greater magnitude of graded potential. What else could affect the magnitude of the potential? - The number of channels - The type of channels activated (different channels can be activated with the same neurotransmitter) Conduction with Decrement - Remember that the change in membrane potential decays while the ions go along the cell body. Why does it decay? We have a current of ions flowing from one side to the other; some of them leak out. - So the change in membrane potential from resting level also decreases with the distance from the potential’s site of origin. This is just like how water flow decreases the farther along the leaky hose you are from the faucet. - The magnitude of the current decreases with the distance from the initial site of the potential change. Graded Potential s travel Short Distances - Conduction with decrement: magnitude of graded potential decreases with increased distance from opened ion channel - Decrement occurs due to: - Leakage of charged ions across the membrane - Electrical resistance of the cytoplasm - Electrical properties of the membrane: the membrane is a capacitor and it needs to be charged. - Electrotonic current spread: positive charges spread throughout the cytoplasm and cause depolarization of the adjacent membrane. Action Potentials travel Long Distances - Characteristics of action potentials: - Triggered by net graded potentials at the axon hillock (trigger zone) - Do not degrade over time or distance - Travel long distances along membrane - All-or-none - Must reach threshold potential to fire: depolarizations that occur below the threshold will not initiate an action potential. Threshold is the minimum membrane potential that the cell has to arrive at in order to get an action potential. The sodium channels have a specific voltage at which they open; before that they wi
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