Biol118 1st Edition Lecture 27 Outline of Last Lecture I Nutritional Requirements II Structure Function of Mouthparts III How Are Nutrients Digested Absorbed IV Nutritional Homeostasis Glucose as a Case Study Outline of Current Lecture I Introduction II Principles of Electrical Signaling III How Does Action Potential Work Current Lecture Introduction 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 systems o 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 Anatomy o Dendrite receives electrical signals from axons of adjacent cells o Cell body soma integrates the incoming signals and generates an outgoing signal o Axon sends the signal to the dendrites of other neurons Types of Neurons o Sensory neuron sends information to the brain via nerves Long tough stands of nervous tissue containing thousands of neurons These 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 homeostasis o Motor neurons nerve cells that send signals to effector response cells in glands muscles o Interneurons make connections between sensory neurons motor neurons Peripheral Nervous System o Anything that isn t in the CNS o Receptors from PNS sends it to CNS to be processed How Does Action Potential Work Membrane potential o Cells are electrical by nature Ions on both side of plasma membrane create electrical potential o When there is an electrical potential on either side of the membrane membrane potential separation of charges Membrane potentials are measured in millivolts mV Generally more negative ions on the inside of the plasma membranemembrane potential is usually negative Usually about 70 80 mV Electrical potential o Ions on both sides of membrane have potential energy o 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 potential o 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 ATP o 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 potential o Membrane eventually reaches a voltage where there is equilibrium between concentration gradient that move K out electrical gradient that move K in o Each ion has its own equilibrium potential Sodium Potassium Pump o 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 and moves toward positive charge Repolarization changes the membrane back to negative charge Hyperpolarization membrane becomes more negative than it was during resting potential o Threshold potential negative charge that will not allow the action to proceed if reached the action is denied o All 3 phases of action potential occur in about a millisecond o 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 species o 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 leading to 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 size o 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 currents that 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 channels o As positive charges are pushed farther from the initial sodium channels they depolarize adjacent portions of the membrane o Nearby voltage gated Na channels pop open in response to depolarization Positive feedback occurs a full fledged action potential results Refractory States o Action potentials do not propagate back up the axon because Na channels are refractory o Once they have opened and closed they are less likely to open again for a short period of time refractory
View Full Document
Unlocking...