OUTLINE Neurons Glia Resting Membrane Potential Action Potentials The Synapse Ingredients of Intracellular and Extracellular Fluid Water H2O Ions Charged particles Potasium K Sodium Na Calcium Ca2 Chloride ClProtein anions A Ion Concentrations Based on distribution of ions and other charged particles the inside of the neuron is negatively charged relative to the outside Resting Membrane Potential The difference in charge between the inside and outside of the membrane of a neuron at rest At rest the inside of the cell is about 70 mV lower than outside of cell Potential Voltage Diffusion Molecules will move from areas of high concentration to areas of low concentration Diffusion Diffusion pressure moves molecules along a Concentration Gradient Electrical Force Charged molecules or ions will be attracted to areas of opposite charge and repelled by areas of like charge Like charges repel each other Opposite charges attract each other Diffusion and Electrical Force Diffusion and Electrical Force Selective Permeability Different channels and receptors gate specific ions i e they are selectively permeable Resting Membrane Potential Summary The neuron is polarized in it s resting state Resting membrane potential is about 70mV Resting membrane potential due to Selective permeability of membrane Uneven distribution of ions on the inside vs outside of the cell Action Potential Action Potentials method by which neurons communicate At rest membrane potential is approximately 70 mV When the axon hillock region becomes more positive to about 65 mV an AP is generated Depolarization membrane potential becomes less negative Hyperpolarization membrane potential becomes more negative Action Potential properties once threshold reached Rising phase Na enters neuron Depolarization Overshoot Neuron positive inside relative to outside Falling phase K exits neuron Hyperpolarization Properties of Action Potentials All or None AP amplitude speed is constant Each AP followed by refractory period Voltage Gated Channels Voltage gated Na and K channels open and close as a function of the neuronal membrane potential They are located along axon hillock axon membrane and terminals Their rapid opening and closing is responsible for AP initiation and propagation Refractory Periods Absolute Refractory Period Neuron can NOT fire again Limits how frequently a neuron can fire Accounts for unidirectional nature of action potential Refractory Periods Absolute Refractory Period Neuron can NOT fire again Limits how frequently a neuron can fire Accounts for unidirectional nature of action potential Na channel can only open again once membrane potential hyperpolarizes Refractory Periods Relative Refractory Period Membrane potential becomes more negative than resting membrane potential Neuron can fire again but only with strong stimulus Plays a role in intensity coding i e stimulus intensity coded by firing rate Action Potentials Convey information over distance in nervous system Neural information code Pattern temporal code Frequency rate code Action Potentials Convey information over distance in nervous system Neural information code Pattern temporal code Frequency rate code Action Potential Propagation Remember once a voltagegated Na channel opens and closes it can only be opened again once the membrane potential has hyperpolarized In this way the AP cannot flow backwards Action Potential Propagation Action Potential Propagation How Fast Speed depends on Myelination myelinated unmyelinated Axon diameter large small Invertebrate axon 11 mph Human axon 268 mph Axon Terminal AP invades axon terminal signal changes from electrical to chemical OUTLINE Neurons Glia Resting Membrane Potential Action Potentials The Synapse The Synapse Synapse point of contact between axon terminal and another neuron Information passed directionally from presynaptic to postsynaptic cell 1897 Charles Sherrington coined term Synapse Soups vs Sparks Physical nature of synaptic transmission Chemical vs Electrical transmission Neurotransmitters regulate information transfer Slowed heart beat Same effect found when applied to second heart Vagusstoff Acetylcholine Otto Loewi Synapses Steps in Synaptic Transmission Figure 3 21 Exocytosis the Release of CalciumResults theinSynapse Neurotransmitters Voltage gated calcium channels open in response to AP Calcium must be cleared prior to arrival of next AP Removes one calcium ion for every three Na ions brought into terminal Vesicular Release Exocytosis Entering calcium releases vesicles from protein anchors and stimulates fusion with membrane Endocytosis Excess membrane pinches off to form new vesicle Synaptic Cleft Vesicular Release Calcium channels Collapsed vesicles Exocytosis Neurotransmitters endogenous chemicals that transmit signals from a neuron to a target cell across a synapse Small molecules serotonin norepinehprine epinephrine dopamine acetylcholine Amino acids GABA glutamate Neuropeptides secretin oxytocin Soluble gases nitric oxide carbon monoxide Activation of Receptor Sites Neurotransmitter molecules diffuse into and throughout the synaptic clef Neurotransmitters bind to specific receptors in a lock and key fashion Post synaptic receptors Pre synaptic receptors autoreceptors Autoreceptors NT synthesis and release regulated usually inhibited by presynaptic autoreceptors Terminal autoreceptor reduce NT synthesis and release Somatodendritic autoreceptor hyperpolarizes neuron reducing AP spiking rate Terminating NT Signal Postsynaptic Effects Two Receptor Types Voltage gated Receptors activated based on changes in the membrane potential Ligand gated Receptors activated by the binding of specific molecule or neurotransmitter Postsynaptic Receptors Ionotropic Direct and fast Metabotropic Indirect and slow Ionotropic Receptor Receptor is activated and opens within milliseconds Metabotropic Receptor 2nd messenger Activate kinases or enzymes Gene transcription Receptor is activated and downstream response occurs in hundreds of milliseconds to seconds Contrasting Receptor Types Ionotropic Metabotropic Opens channels directory Relatively fast Relatively short Effects are localized Opens channels indirectly Uses chemicals called second messengers Relatively slow acting Relatively long lasting effects Effects are more widespread and varied Local Effects of Receptor Activation Excitatory Postsynaptic Potential EPSP Opens sodium channels Depolarizes dendrites and cell body Facilitates likelihood of Action