Negative potential in axon has to be changed from -70mv to about -50mv (action potential threshold)bysodium ions in axon before action potential is triggered.Graded potential:changes in negative resting potential that do not reach the -50mv action potential threshold. Under certain circumstances, graded potentials caused by several neurons, can add up to triggeran action potential.
-For neuron to function prop, sodium and potassium ions enter and exit membrane at just the right rate. Drugs alter this which can decrease or prevent neural functioning.The Myelin Sheath:fatty insulating substance on axon of some neurons, inc speed of neural transmission. Derived from glial cells.Nodes of Ranvier: gap in the insulating sheath, cause nerve impulsesUnmyelinated axons: AP travels down axon length. Myelinated axons: Electrical conduction can skip from node to node.-myelin sheath is not formed until after birth, ind neural transmission infant’s muscular coordinationMultiple sclerosis:damage to myelin sheath, disrupting nerve impulses, resulting in eventually paralysis.Synaptic Transmission (How Neurons Communicate)Synapse: a functional connection between neuron and its target. neurons do not make physical contact.-Otto Loewishowed neurons release chemicals that carry messages (chemical neurotransmission-CN)Synaptic cleft: space between axon terminal of one neuron and dendrite of next neuron.Neurotransmitters:chemicals that carry messages across synapse to either cause or prevent activity of next cell.Chemical communication includes 5 steps: 1. SYNTHESIS: chemical molecules are formed inside neuron. 2. molecules are STOREDin chambers called synaptic vesicles. 3. when AP comes down axon, vesicles move to surface and RELEASECN in synaptic cleft. 4. Neurotransmitters BINDto receptor sites:protein molecules embedded in neuron’s cell membrane. 5. they deactivate, occurs in 2 major ways:(1) transmitter molecules deactivated by chemicals located in synaptic space that break them down(2) deactivation is reuptake: transmitter molecules are reabsorbed into axon terminal. Bindingstep, has 1 of 2 effects on neuron, making it either more or less likely that neuron will gen AP.Excitatory transmitters:neurotransmitters that create depolarization.Action of inhibitorytransmitter from a presynaptic neuron may prevent postsynaptic neuron from reaching AP threshold. Balance excitatory and inhibitory processes must be maintained for nervous system to function properly. Inhibitionallows prevents uncoordinated discharge of nervous system .-when receptor molecule is vacant, neuron returns to former resting state, awaiting next chemical stimulation. Drugs can also alter synaptic transmission by influencing how transmitter is cleared from synaptic cleft after it has been released.