Chapter 45 1 Principles of Electrical Signaling a Two types of nervous systems 1 Diffuse arrangement of cells called the nerve net is found in jellyfish and other creatures 2 Central Nervous System large numbers of neurons aggregated into ganglia 2 Types of Neurons in the Nervous System a Sensory receptors in skin eyes ears and nose transmit data about external environment b sensory receptor cells transmit information by means of a sensory neuron c Nerves long tough strands of nervous tissue and have projections that carry information to and from the brain and spinal cord d Cells in the CNS are called interneurons e motor neurons nerve cells that send signals to effector cells in glands or muscles f Parts of nervous system outside the CNS are part o the peripheral nervous system 3 The Anatomy of a Neuron a Most neurons have the same 3 parts Axon dendrites cell body w a nuclus b Axons can be over a meter in length c Dendrites receive signals from axons of other cells axons send signals to dendrites and cell bodies of other neurons d Cell body where signals are integrated and an outgoing signal is sent to the axon e Membranes of axons and dendrites meet at junctions called synapses f Most interactions occur where plasma membranes of two neurons meet at synapses 4 Introduction to Membrane Potentials a If positive and negative charges on ions on two sides of plasma membrane dont balance the membrane will have an electrical potential b Large separation of charges across membrane membrane potential large c membrane potentials are measured in millivolts volt is the standard unit of electrical potential 5 Electrical Potential Currents and Gradients a when membrane potential exists the ions on both sides of membrane have potential energy b membrane potential also includes energy stored as the conc gradient of charged ions on the 2 sides of the membrane 6 How is the Resting Potential Maintained a Resting potential When neuron is not transmitting an electrical signal but is just sitting fluid at rest the membrane potential is called its resting potential b Interior of membrane has low conc of Na and Cl ions and large conc of K ions In the extracellular fluid sodium and chloride ions dominate 7 The K Leak Channel a Resting neurons are most permeable to K ions bec most open channels are for K b K channels involved are often called leak channels bec they allow K to leak out of the cell 8 Role of the Na K ATPase a Na K ATPase pumps Na out of the cell and K into the cell b Na K ATPase ensures that the conc of K is higher inside the cell while the conc of Na is lower inside the cell c Na K ATPase also makes interior of membrane more negative than outside d resting potential repr energy stored as conc and electrical gradients in a series of ions 9 What is an Action Potential Figure 45 5 a The action potential has 3 phases 1 Depolarization membrane becomes less polarized than before membrane potential changes from highly negative to positive 2 Repolarization changes the membrane potential from positive back to negative 3 Hyperpolarization membrane is slightly more negative than the resting potential b If membrane depolarizes less than 65 or 55 mV action potential doesnt occur if this threshold potential is reached certain channels in axon open and ions enter c When membrane potential reaches about 40mV repolarization phase begins d action potential occurs bec ion channels in membrane open or close in response to changes in voltage 10 All or None Signal that Propagates a No such thing as a partial action potential b all action potentials in a neuron are identical in magnitude and duration c Action potentials are propagated down the axon d Neurons have excitable membranes they are capable of generating action potentials that propagate rapidly along the axons e the freq of action potentials is the meaningful signal 1 Dissecting the Action Potential a If Na channels opened early in action potential then Na should flow into the neuron until the membrane potential was 40mV b This had to be tested 2 Distinct Ion Currents are Responsible for Depolarization and Repolarization a Action potential begins when Na flows into the neuron sodium ions are responsible for the depolarization phase b There is strong flow of potassium ions out of cell during repolarization phase c Action potential consists of a strong inward flow of Na ions followed by a strong outward flow of K ions 3 How Do Voltage Gated Channels Work Draw 45 6 a Action potential depends on voltage gated channels membrane proteins that open and close in response to changes in membrane voltage b Behavior of the ion channels depends on voltage 4 Patch Clamping and Studies of Single Channels a How current flows through individual channels in active potential 1 Voltage gated channels are either open or closed current flow starts and stops instantly and the size of current is always the same 2 Sodium channels open quickly after depolarization 3 Potassium channels open w a delay after depolarization they continue to flip open and closed until the membrane repolarizes 5 Positive Feedback Occurs During Depolarization a Na channels are more likely to open as a membrane depolarizes initial depolarization leads to opening of more Na channels and keeps doing this b Positive feedback occurrence of an event makes the same event more likely to recur 6 Using Neurotoxins to Identify Channels and Dissect Currents a Neurotoxins poisons that affect neuron function often results in convulsions paralysis unconsciousness b puffer fish toxin blocks the voltage gated Na channel probably by binding to a specific site on the channel protein 7 How is the Action Potential Propagated Draw Figure 45 8a a In first step influx of Na at start of an action potential causes charge to spread away from sodium channels b Signal in active potential doesnt diminish as it moves bec the response is all or nothing c Na channels are refractory once they have opened and closed they are less likely to open again for a short period d Action potentials are propagated in one direction bec sodium channel s downstream of site are not in refractory state 8 Axon Diameter Affects Speed a Compared to small axons large axons have relatively few Na channels so less current leaks back out of a large axon than a small axon b Large axons and large diameter neurons transmit action potentials much faster than small axons do 9 Myelination Affects Speed a Oligodendrocytes specialized accessory cells that wrap
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