Biol118 1st Edition Lecture 27 Outline of Last Lecture I. Nutritional RequirementsII. Structure & Function of MouthpartsIII. How Are Nutrients Digested & Absorbed IV. Nutritional Homeostasis- Glucose as a Case StudyOutline of Current Lecture I. IntroductionII. Principles of Electrical SignalingIII. How Does Action Potential Work?Current LectureIntroduction- Animal movements are triggered by electrical signals conducted by nerve cells (neurons)to muscle cells- Complex processes are based on the simple flow of ions across plasma membranes- All animals except sponges have neurons & muscles Principles of Electrical Signaling- Two basic types of nervous systemso Nerve net: diffuse arrangement of cells Found in cnidarians (jellyfish) & ctenophores (comb jellies)o Central nervous system (CNS) Includes large number of neurons aggregated into clusters called ganglia Consists of brain & spinal cord Most animals with CNS have large ganglia or brain- Neuron Anatomyo Dendrite: receives electrical signals from axons of adjacent cellso Cell body (soma): integrates the incoming signals and generates an outgoing signalo Axon: sends the signal to the dendrites of other neurons- Types of Neuronso Sensory neuron: sends information to the brain via nerves Long, tough stands of nervous tissue containing thousands of neuronsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. Sensory receptors: transmit streams of data about the external environment using the sensory neuron- Sensory cells respond to stimuli - Monitor conditions that are important in homeostasiso Motor neurons: nerve cells that send signals to effector (response) cells in glands/muscleso Interneurons: make connections between sensory neurons & motor neurons- Peripheral Nervous Systemo Anything that isn’t in the CNSo Receptors from PNS sends it to CNS to be processedHow Does Action Potential Work?- Membrane potentialo Cells are electrical by nature Ions on both side of plasma membrane create electrical potentialo When there is an electrical potential on either side of the membrane, membranepotential= separation of charges Membrane potentials are measured in millivolts (mV) Generally more negative ions on the inside of the plasma membrane- membrane potential is usually negative Usually about 70-80 mV- Electrical potentialo Ions on both sides of membrane have potential energyo Ions normally spontaneous move from area of like charge to the area of unlike charge= flow of charge (electric current)o Ions move across membranes in response to concentration & charge gradients Electrochemical gradient: combination of electric gradient & concentration gradient- Resting potentialo Resting potential: neuron is at rest in extracellular fluid & is not communicating with other neurons Represents energy stored as concentration gradients in a series of ions o Exists because of high intracellular concentration of K+ & low Na+ & Cl- Organic negatively charged molecules are found in cell along with the K+o Ions can only cross plasma membrane in these ways (b/c of selective permeability): Along electrochemical gradient through an ion channel Carried via a membrane cotransporter protein or antiporter protein Pumped against an electrochemical gradient by a membrane protein that hydrolyzes ATPo Maintaining resting potential is tied to movement of K+ out of the cell Most permeable to K+ ions Leak channels are involved because they allow K+ to leak out of the cell As K+ moves out of the cell, the inside becomes more negatively charged relative to the outside- Equilibrium potentialo Membrane eventually reaches a voltage where there is equilibrium between concentration gradient that move K+ out & electrical gradient that move K+ ino Each ion has its own equilibrium potential- Sodium Potassium Pumpo Imports K+ ions and exports Na+ ions, resulting in the concentration of K+ ions being higher inside the cell & Na+ being higher outside o Results in the inside of the neuron being negatively charged Neuron has negative resting potential- What is Action Potential?o Action potential: rapid temporary change in a membrane potential Depolarization: phase in which the membrane becomes less negative andmoves toward positive charge Repolarization: changes the membrane back to negative charge Hyperpolarization: membrane becomes more negative than it was duringresting potentialo Threshold potential: negative charge that will not allow the action to proceed- if reached, the action is deniedo All 3 phases of action potential occur in about a millisecondo Occurs because specific ion channels in the plasma membrane open or close in response to changes in voltage Always has 3 phase form even if size and peak may vary among specieso Is an all-or-none event No partial action Action potentials are propagated down the length of the axon Na+ channels are more likely to open as a membrane depolarizes, leadingto the opening of additional Na+ channels, further depolarizing the membrane Neurons have excitable membranes because neurons are capable of generation action potentials that propagate rapidly along the length of their axons Information is coded in the form of action potentials that travel along axons- frequency is more meaningful than sizeo Depends on voltage-gated channels Voltage-gated channels: ion channels that open & close in response to changes in membrane voltage- Shape changes in response to the charges present at membrane surface Voltage-gated channels use technique called voltage clamping- allows researchers to hold an axon at any voltage & record the electrical currentsthat occur Patch clamping: allows isolation of a single channel- Voltage-gated channels are either open or closed- Sodium channels open quickly after depolarization- Potassium channels open with a delay after depolarization- How is action potential propagated?o When Na+ enters a cell at onset of action potential, positive charges in the cell are repulsed & negative charges are attracted Charge spreads away from sodium channelso As positive charges are pushed farther from the initial sodium channels, they depolarize adjacent portions of the membraneo Nearby voltage-gated Na+ channels pop open in response to depolarization Positive feedback occurs & a full-fledged action potential results- Refractory
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