Chapter 451. Principles of Electrical Signalinga. Two types of nervous systems1. Diffuse arrangement of cells called the nerve net is found in jellyfish and other creatures2. Central Nervous System-large numbers of neurons aggregated into ganglia2. Types of Neurons in the Nervous Systema. Sensory receptors in skin, eyes, ears, and nose-transmit data about external environmentb. sensory receptor cells transmit information by means of a sensory neuronc. Nerves-long tough strands of nervous tissue and have projections-that carry information to and from the brain and spinal cordd. Cells in the CNS are called interneuronse. motor neurons-nerve cells that send signals to effector cells in glands or musclesf. Parts of nervous system outside the CNS are part o the peripheral nervous system3. The Anatomy of a Neurona. Most neurons have the same 3 parts: Axon, dendrites, cell body w. a nuclusb. Axons can be over a meter in lengthc. Dendrites receive signals from axons of other cells-axons send signals to dendrites and cell bodies of other neuronsd. Cell body: where signals are integrated and an outgoing signal is sent to the axone. Membranes of axons and dendrites meet at junctions called synapsesf. Most interactions occur where plasma membranes of two neurons meet at synapses4. Introduction to Membrane Potentialsa. If positive and negative charges on ions on two sides of plasma membrane dont balance-the membrane will have an electrical potentialb. Large separation of charges across membrane--membrane potential largec. membrane potentials are measured in millivolts-volt is the standard unit of electrical potential5. Electrical Potential, Currents, and Gradientsa. when membrane potential exists-the ions on both sides of membrane have potential energyb. membrane potential also includes energy stored as the conc. gradient-of charged ions on the 2 sides of the membrane6. How is the Resting Potential Maintaineda. Resting potential: When neuron is not transmitting an electrical signal-but is just sitting fluid at rest, the membrane potential is called its resting potentialb. 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 dominate7. The K Leak Channela. Resting neurons are most permeable to K ions bec. most open channels are for Kb. K channels involved are often called leak channels-bec. they allow K to leak out of the cell8. Role of the Na/K ATPasea. Na/K ATPase pumps Na out of the cell and K into the cellb. Na/K ATPase ensures that the conc. of K is higher inside the cell-while the conc. of Na is lower inside the cellc. Na/K ATPase also makes interior of membrane more negative than outsided. resting potential repr. energy stored as conc. and electrical gradients in a series of ions9. What is an Action Potential Figure 45.5a. The action potential has 3 phases1. Depolarization: membrane becomes less polarized than before-membrane potential changes from highly negative to positive2. Repolarization: changes the membrane potential from positive back to negative3. Hyperpolarization: membrane is slightly more negative than the resting potentialb. 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 enterc. When membrane potential reaches about 40mV repolarization phase beginsd. action potential occurs bec. ion channels in membrane open or close -in response to changes in voltage10. All or None Signal that Propagatesa. No such thing as a partial action potentialb. all action potentials in a neuron are identical in magnitude and durationc. Action potentials are propagated down the axond. Neurons have excitable membranes: they are capable of generating-action potentials that propagate rapidly along the axonse. the freq. of action potentials is the meaningful signal1. Dissecting the Action Potentiala. If Na channels opened early in action potential-then Na should flow into the neuron until the membrane potential was 40mVb. This had to be tested2. Distinct Ion Currents are Responsible for Depolarization and Repolarizationa. Action potential begins when Na flows into the neuron-sodium ions are responsible for the depolarization phaseb. There is strong flow of potassium ions out of cell during repolarization phasec. Action potential consists of a strong inward flow of Na ions-followed by a strong outward flow of K ions3. How Do Voltage-Gated Channels Work Draw 45.6a. Action potential depends on voltage-gated channels-membrane proteins that open and close in response to changes in membrane voltageb. Behavior of the ion channels depends on voltage4. Patch Clamping and Studies of Single Channelsa. How current flows through individual channels in active potential1. Voltage-gated channels are either open or closed-current flow starts and stops instantly and the size of current is always the same2. Sodium channels open quickly after depolarization3. Potassium channels open w. a delay after depolarization-they continue to flip open and closed until the membrane repolarizes5. Positive Feedback Occurs During Depolarizationa. Na channels are more likely to open as a membrane depolarizes-initial depolarization leads to opening of more Na channels and keeps doing thisb. Positive feedback: occurrence of an event makes the same event more likely to recur6. Using Neurotoxins to Identify Channels and Dissect Currentsa. Neurotoxins: poisons that affect neuron function-often results in convulsions, paralysis, unconsciousnessb. puffer-fish toxin blocks the voltage-gated Na channel-probably by binding to a specific site on the channel protein7. How is the Action Potential Propagated Draw Figure 45.8aa. In first step: influx of Na at start of an action potential-causes charge to spread away from sodium channelsb. Signal in active potential doesnt diminish as it moves bec. the response is all or nothingc. Na channels are refractory: once they have opened and closed-they are less likely to open again for a short periodd. Action potentials are propagated in one direction -bec. sodium channel's downstream of site are not in refractory state8. Axon Diameter Affects Speeda. Compared to small axons, large axons have relatively few Na channels-so less current leaks back out of a large axon than a small axonb. Large axons and large-diameter neurons transmit action potentials-much faster than small axons do.9. Myelination Affects Speeda.
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