Electrical Signals in Animals Exam V Study Guide Bio 1107 Electrical signals conducted by nerve cells or neurons Complex processes such as moving seeing thinking are based on flow of ions across plasma membranes 200 m sec 450 mph Principles of Electrical Signaling o Muscles can respond to electrical signals by contracting o Two types of nervous systems Diffuse arrangement of cells called a nerve net found in cnidarians jellyfish hydra and ctenophores comb jellies A central nervous system CNS that includes large numbers of neurons aggregated into clusters called ganglia Types of Neurons in the Nervous System o Sensory receptors in skin ears and nose transmit stream of data about the environment Monitor conditions and homeostasis o Sensory receptor transmits the info it receives from environment by means of a nerve cell called a sensory neuron Sensory neuron sends info to neurons in spinal cord called nerves long tough strands of nervous tissue containing thousands of neurons o The central nervous system CNS made up of the brain and spinal cord integrating information from many sensory neurons Cells in here are called interneurons that make connections between sensory neurons and motor neurons which are nerve cells that send signals to effectors cells in glands or muscles o All components of the nervous system outside of CNS are part of the peripheral nervous system PNS Anatomy of a Neuron o Dendrite receives electrical signals from axons o Cell body or soma which includes nucleus integrates incoming signals and generates an outgoing signal o Axon sends the signal to dendrites of other neurons An Introduction to Membrane Potentials o A different of electrical charge between any two points creates a difference in electrical potential or a voltage o When an electrical potential exists on either side of a plasma membrane the separation of charges in called a membrane potential Units and Signs o Membrane potentials are form of electrical potential are measured in millivolts mV Inside relative to outside Electrical Potential Currents and Gradients o When a membrane potential exists the ions on both sides have potential energy which is based on position o Electrochemical energy How is Resting Potential Maintained o When a neuron is at rest in extracellular fluid its membrane has a voltage called resting potential Neurons have high intracellular K and low intracellular Na Cl o Plasma membrane is selectively permeable o Ions move in three ways Along electrochemical gradient through ion channel Carried via cotransporter or antiporter protein Pumped against electrochemical gradient by membrane proteins that hydrolyzes ATP The K Leak Channel o Potassium channels involved called leak channels because they allow K to leak out of the cell o Eventually membranes reaches voltage at which there is equilibrium between concentration gradient that moves K out and electrical gradient that move K in this is called equilibrium potential for K Na K ATPase o Imports 2 K and exports 3 Na o Negative resting membrane potential Microelectrodes to Measure Membrane Potentials o Squid axon o Resting disrupted by action potential Action Potential o An action potential is a rapid temporary change in membrane Three phrases Depolarization o Membrane potential must shift from resting to about 55 mV o If this threshold potential is reached certain channels in axon membrane open and ions rush into the axon following electrochemical gradient Repolarization o When membrane potential reaches about 40 mV closing of the channels and opening of others o Ions flow out of axon changing back to negative Hyperpolarization An all or none signal o No such thing as partial action potential o Neurons have excitable membranes because neurons are capable of generating action potentials that propagate along their axons o Frequency of action potentials rather than size is meaningful How Do Voltage Gated Channels Work o Voltage gated channels are ion channels that open and close in response to changes in membrane voltage Shape changes in response to the charges present at the membrane surface Exists due to technique called voltage clamping which allows researchers to hold an axon at any voltage and record the electrical current Patch Clamping and Studies of Single Channels o A technique called patch clamping allows researchers to document the currents that flowed through individual channels Using this determined that Voltage gated channels are either open or closed Sodium channels open quickly after depolarization Potassium channels open with a delay after depolarization Na channels are more likely to open as a membrane depolarizes an example of positive feedback the occurrence of an events makes the same event more likely to occur Neurotoxins are poisons that affect neuron function and have been used to explore the dynamics of voltage gated channels o Tetrodotoxin Prevented action potentials Blocked Na channel by binding Action potentials do not propagate back up the axon because Na channels are refractory once they open and close they are less likely to open again for a short period of time o Channels downstream of the site are not in refractory resulting in one way propagation Axon Diameter Affects Speed o Compared to small axons large axons have relatively few sodium channels per unit of membrane surface Myelination Affects Speed o In vertebrates the membranes of specialized accessory cells are wrapped around axon collectively called glia Schwann cells in PNS and oligodendrocytes in CNS o Where glia wrap around an axon they form a myelin steath that acts as electrical insulation o Myelination is the process of making a myelin steath The Synapse o Influx of charge from action potential spread unimpeded until it hits an unmyelinated section of the axon called a node of Ranvier Dense concentration of voltage gated Na channels Electrical signals jump down myelinated much faster than unmyelinated o The disease multiple sclerosis MS develops as damage to myelin increase and electrical signaling is impaired o The interface between two neurons is called a synapse and just inside the axon contains synaptic vesicles that store neurotransmitters o The sending cell is the presynaptic neuron and the receiving cell is called the postsynaptic neuron o The action potentials arrive at the end of the axon and triggers the entry of calcium ions in presynaptic membrane from outside which then release neurotransmitter in response into the synaptic cleft